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Miami Pedestrian Bridge, Part XII
34

Miami Pedestrian Bridge, Part XII

Miami Pedestrian Bridge, Part XII

(OP)
A continuation of our discussion of this failure. Best to read the other threads first to avoid rehashing things already discussed.

Part I
thread815-436595: Miami Pedestrian Bridge, Part I: Miami Pedestrian Bridge, Part I

Part II
thread815-436699: Miami Pedestrian Bridge, Part II: Miami Pedestrian Bridge, Part II

Part III
thread815-436802: Miami Pedestrian Bridge, Part III: Miami Pedestrian Bridge, Part III

Part IV
thread815-436924: Miami Pedestrian Bridge, Part IV: Miami Pedestrian Bridge, Part IV

Part V
thread815-437029: Miami Pedestrian Bridge, Part V: Miami Pedestrian Bridge, Part V

Part VI
thread815-438451: Miami Pedestrian Bridge, Part VI: Miami Pedestrian Bridge, Part VI

Part VII
thread815-438966: Miami Pedestrian Bridge, Part VII: Miami Pedestrian Bridge, Part VII

Part VIII
thread815-440072: Miami Pedestrian Bridge, Part VIII: Miami Pedestrian Bridge, Part VIII

Part IX
thread815-451175: Miami Pedestrian Bridge, Part IX: Miami Pedestrian Bridge, Part IX

Part X
thread815-454618: Miami Pedestrian Bridge, Part X: Miami Pedestrian Bridge, Part X

Part XI
thread815-454998: Miami Pedestrian Bridge, Part XI: Miami Pedestrian Bridge, Part XI

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RE: Miami Pedestrian Bridge, Part XII

MikeW7 (Electrical)
What's the best way to download a video from facebook?
Thanks

SF Charlie
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RE: Miami Pedestrian Bridge, Part XII

Quote (RAB678 (Mechanical) 13 Jul 19 19:20)

...Bridge photogrammetry 17 by Zac Doyle also on youtube. On my monitor, I can see the end of the crane, vibrate what looks like twice.
:
Also notice a black lifting strap? suddenly shoot upwards out of the picture.

  1. When you talk about a URL or video, attach a link so we can read/see exactly what you are talking about.
  2. If you look at the Collapse - N View video you will see that the crane cables are slack and not carrying any weight, right up to the collapse at the end of the video.
  3. The Zac Doyle video is a heavily processed and interpolated version of the original 1280x720 dash cam video. Whenever there is motion, such as approaching the bridge as shown in the dash cam video, a thin object such as the crane mast will seem to shimmer, shake, etc. because the interpolation area of interest (a small group of pixels) is shifting one row/column of pixels in the direction of motion. As an example of what you were actually looking at, here is a cropped area of the crane mast from dash-cam video Frame 75:
  4. Also note that the tip of the crane mast has a small eastward extension (see the N- view video) and the dash-cam truck is approaching from a direction where this extension is just coming into view from behind the mast, so there are only a few pixels visible to begin with, and one or two more pixels become visible in each new frame.
  5. The lifting strap, as you call it, is visible in all frames (see the N-view video), but since it is so thin (only one pixel wide) it may seem to lighten in color (disappear) as it moves from one column of pixels to the next as the dash-cam truck moves closer. See Frame 75 (above) as an example - in some places there doesn't appear to be any cable at all, but if you look closer some pixels in the column containing the cable are a slightly different color than the background.
  6. For these and other reasons, the way we usually examine videos here is frame-by-frame using the original source. I "enlarge" the frames using an algorithm that replaces each original pixel with a 4x4 block of identical pixels, because just about any other enlargement method will introduce significant interpolation artifacts (stuff that really isn't there). We may have disagreements over what a group of pixels actually represent, but at least we are confident we are all look at exactly the same thing, but just have different interpretations.
  7. The Doyle video and others are heavily processed, usually with an intent to "enhance" something the author imagines is there. Something similar is done here, on occasion, but the author can explain exactly what was done and why, so everyone else understands the motive and the result, and so they can ask informed questions.
  8. Don't rely solely on one video when there are more available. Most of your questions are answered (and claims dismissed) by simply looking at the N-View video, for example. The bridge installation and collapse was monitored continuously by at least 3 time-lapse cameras. These are labeled N-view, SW-view and SE-view in my PlayList of Bridge Timelapses videos. Other viewpoints also exist for parts of the move - see my Dropbox Playlist.

RE: Miami Pedestrian Bridge, Part XII

Quote (SFCharlie (Computer) 13 Jul 19 19:25)

What's the best way to download a video from facebook?

FaceBook Video Downloader

For everything else, I use 4k Video Downloader, a desktop app that you just paste links into. Been using it for a couple of months with no issues.

RE: Miami Pedestrian Bridge, Part XII

Quote (RAB678)

IF you were lifting the canopy to reverse the sagging and close the cracks.
You would be lessening the friction of 11 base area. Meanwhile.... tensioning the bars. NOW the rods would be under apparently MAXIMUM tension. Basically cocking a gun. Then SNAPPED 11b. The force unloaded would decimate the end. The crane becomes more loaded, cable lift device lets go. Crane vibrates. 11 sends the end outward in pieces. Falls down. Simple conclusion really.

Any thoughts on all this?

The rod did not snap. They were in tact after the collapse. The rods are also checked with predicted elongation with actual load and elongation (standard practice for PT).

Tightening the #11 bottom rod was the final blow but the stresses and cracks were unacceptable before that point.

Tightening the rods would have never fixed the cracks. It added a little bit of shear friction capacity but also substantially increased the horizontal shear stress at the joint. The overall effect was to make the whole situation worse.

RE: Miami Pedestrian Bridge, Part XII

Quote (SFCharlie)

His web presence has included several channel names. One of his websites was for wedding photography.
He is very enterprising, searching out info on the collapse, including feedback from his subscribers, monitoring this site... He refuses to provides links to his sources. That's how I found this site, search for a link to one of the photos he used in a clip. He has long held that the collapse started somewhere else than 11.12. He holds the participants on this forum in disdain.

Yes, he and some his followers appear to have a real disdain for structural engineers yet they really don't appear to understand the failure, statics, vectors, physics, terminology etc. He has been resourceful for tracking down information but his interpretation is a long stream of nonsensical and illogical gibberish. He tries to act like an expert but clearly doesn't understand fundamentals like equilibrium, FBDs, weight vs mass etc. He is clearly not a numbers person either. That would be critical for an engineer.

In any case, he is just a conspiracy theorist with a stack of information, and severe lack of comprehension. He would do well to at least try to understand the bare minimum fundamentals but it sounds like he never studied any kind of physics so that would be hard for him to do.

RE: Miami Pedestrian Bridge, Part XII

Quote (RAB678 (Mechanical) 13 Jul 19 19:20)

If you were lifting the canopy ...
  • If you were lifting the canopy, you'd be using a BIG-ASS crane (500T minimum by my uneducated guess) - pictures on the internet would indicate something about the size as a semi-trailer with 12 wheels.
  • You would not be doing it with the boom extended all the way out to infinity.
  • And you sure as heck wouldn't be doing it with humans on top of the lift.
[Post Deleted]

RE: Miami Pedestrian Bridge, Part XII

I did not mean to steal your idea of the jbar. I've read so much I can't remember everything, sorry bout that. Shout out to Vance one of your longer posts was exactly spot one. I trust you and your thoughts.

Thanks for the video and feedback. I agree crane not lifting bridge.
Although video does seem to have some frames missing.
Are we in agreement that what they did made it fall?
ALTHOUGH NOT their fault but the design side.
May I present to you some thoughts?
Here's something I see.
While on scaffolding all is good. Supports one end to other. Balance. Yea....
Set it on piers.
Now its a Warren bridge truss I-beam. Concrete style.

We all know if the base of 11 or 2 fails its over.
Geometry is paramount.

So....Canopy is in compression, being concrete always expanding and contracting.
But the compression factor makes it always be shorter.
Deck would be opposite. Always elongating. Due to temperature curing.......same reasons.
ADD: No roller to allow for expansion / contraction and the rigidity of the end columns to deck also help prevent the deck from expanding as needed. If it can't expand it WILL sag.
(if you use a simple 4 beam Warren truss, you can see this)

The lack of HANGERS (whatever... a consistent vertical separation type device) on north side allows the parallels of canopy to deck to alter.

Because there are no vertical columns/hangers/risers to stop this.
11 to 10/9 base elongates. 10/9 to 8 base elongates. Sag develops in the deck.
Seeing how concrete does not do well in tension. We hope the rebar will stop it from elongating. Yea...

The canopy is sagging. MORE weight is on 12 and 1. Specifically 12.
12 should not be holding up the bridge. 11 and 2 that's their job.
This weight WILL force the rigid canopy to be preloaded and absolutely snap and project itself downward.
The deck isnt exactly a decent truss with 2/3 down bars. Basically a long wide sidewalk.
Deck cannot ever support itself and is HUNG from the canopy. Which is another issue in itself.....

Geometry is changing. The height to depth ratio isn't great and getting worse.
(for a 950 ton bridge probably quite questionable.)
The forces on 11 and 2 increasing.

1 and 12 were never meant to support this bridge. For additional (flop over to the side) (which is ANOTHER issue in itself) stability only.

All of the cracks seen in pics would be noted.

Oh the rebar in 11 keeps it from buckling down and maybe some friction. But if they would have beefed up 11 as in 2 and added some significant support for the base then maybe it would still be up. Well except for the other issues.

So.....allow deck to expand, fortify 11, fix symmetry, somehow don't allow canopy to rest prematurely on end columns(that's a good one right there)(to allow settling/expansion), maybe slight upward bow on canopy and deck before piers. blah blah....

The whole center section could have been an endless? loop of cable....parts and pieces......yea...

[Post Deleted]
[Post Deleted]
[Post Deleted]

RE: Miami Pedestrian Bridge, Part XII

nop. it just seemed to be a common practice when I joined the forum. didn't mean to put on airs.

SF Charlie
Eng-Tips.com Forum Policies

RE: Miami Pedestrian Bridge, Part XII

You kinda seemed like a mod because you asked me several times for clarifications, fix my links, etc. (Which I appreciated, by the way!) Just wondering how things were going to pan out trying to answer all the "usual" questions that were asked this afternoon. I tried my best with answering the video stuff, but I feel sorry for whoever tries to help with the structural stuff....
--- This post will self-destruct in 5 minutes....


[Post Deleted]
[Post Deleted]

RE: Miami Pedestrian Bridge, Part XII

MikeW7 (Electrical) I just ask for links so I can follow up. Thanks.

SF Charlie
Eng-Tips.com Forum Policies

[Post Deleted]
[Post Deleted]

RE: Miami Pedestrian Bridge, Part XII

Quote (Rab678)

I did not mean to steal your idea of the jbar. I've read so much I can't remember everything, sorry bout that. Shout out to Vance one of your longer posts was exactly spot one. I trust you and your thoughts.

Thanks for the video and feedback. I agree crane not lifting bridge.
Although video does seem to have some frames missing.
Are we in agreement that what they did made it fall?
ALTHOUGH NOT their fault but the design side.
May I present to you some thoughts?
Here's something I see.
While on scaffolding all is good. Supports one end to other. Balance. Yea....
Set it on piers.
Now its a Warren bridge truss I-beam. Concrete style.

You have a lot of stuff in your post but I just want to comment on a few things.

Contrary to popular opinion, I don't think the problem was the concrete truss. It was the design of the concrete truss that was the issue. It has its merits. Maintenance of steel bridges is costly. PT bridges are very durable when designed and constructed correctly. Functionally the truss made sense for road clearance and reduced the height to the deck. The design issues are all well understood and have been for many decades.

There was plenty of tension capacity in the deck for the truss to work. The deck is part of the truss. It is also a beam/slab structure that spans between the nodes of the truss. That can all be designed for with standard code requirements.

There are bearing pads that allow for contraction and expansion. There is also a little bit of give in the abutments.

Even though the deck has a net tension, the concrete is in compression (at service load levels). The deck concrete goes into less compression as more load is added to the bridge. The member is the concrete deck plus the PT. The truss force from the analysis is the member force. So the net tension is the tension in the PT minus the compression in the concrete. Even though the concrete is in compression, the net force is still tension.

All these behaviours are well understood.

RE: Miami Pedestrian Bridge, Part XII

Yes and like I said lets backtrack and analyze these statements...........
(my responses will be in parenthesis)

You said.......

Contrary to popular opinion, I don't think the problem was the concrete truss. (weight had nothing to do with it)
It was the design of the concrete truss that was the issue. ( i do agree)It has its merits. Maintenance of steel bridges is costly. PT bridges are very durable (durable....)when designed and constructed correctly (correctly.....). Functionally the truss made sense for road clearance and reduced the height to the deck. (ok but the height could have been increased to fulfill the height to depth ratio...)(with consequences) The design issues are all well understood and have been for many decades. (except for this one, then we got stupid)

There was plenty of tension capacity in the deck for the truss to work. (maybe it couldnt expand) The deck is (hanging from the canopy)part of the truss. It is also a beam/slab structure that spans between the nodes of the truss. That can all be designed for with standard code requirements.(apparently which didnt work)

There are bearing pads that allow for contraction and expansion. (1 million pounds on plastic pads isnt exactly bearings)There is also a little bit of give in the abutments.(you want the piers to lean?)

Even though the deck has a net tension, the concrete is in compression. (deck always in tension)(UNDERSTAND at this point the deck needs to BE ABLE to resist UP TO 5 million pounds of force)(we do need a safety factor dont we?)(AND the footing of 11 and 2 also need to constrain this force)(which was....arghhh)

(ALWAYS be quite sure if 11 and 2 foot are NOT ABLE to withstand this force the structure is done. BASIC BASIC BASIC design parameters.)(THIS is by far the MOST significant aspect of this design.)(WHY IS THIS THE MOST OVERLOOKED AND MISUNDERSTOOD PART OF THIS STRUCTURE?)(No way in heaven this design could ever resist this force)(FAIL 101)(This obvious part disturbs me so much.............)(IDK)(IS THIS US?)

The deck concrete goes into less compression as more load is added to the bridge. (sag develops) The member is the concrete deck plus the PT. (ok..)(it's still basically a 175 foot long wide sidewalk that in no way can never <and yes two negatives makes more negatives> support itself. never. it hangs from the canopy)(period) The truss force from the analysis is the member force. So the net tension is the tension in the PT minus the compression in the concrete. Even though the concrete is in compression, the net force is still tension. (right deck in tension)

All these behaviours are well understood. (except for the bridge failed)(well understood went negative.)
(next)

From me: I don't mean to be a .... but the obvious is the obvious. graphs, slide rules, cad, and blah blah blah sometimes just cant beat the obvious. There is just no way in heaven you can put a 2 x 4 on an angle drive in a 16 penny nail and hope it will hold up your car. I dont know what else to tell you. I outlined the basics for you. You want more flaws I got em. I havent even started yet.

Meaning this in a nice Southern charm kinda way. RAB678(period)

Here's one for you.............I need a solid soft steel strand approximately 40 thou in diameter,
about 2.5 feet long. Purpose.... prefabrication for form and fit.

What product currently for sale would fulfill this requirement?
STOP reading look away, answer.
Stuff to read look away look away look away.
more stuff for distraction.
and the answer?

The use of a common wire coat hanger would satisfy this. Thank you. Mr. Obvious.
Please feel free to use on know it alls.



RE: Miami Pedestrian Bridge, Part XII

@MikeW7 - In frame 74 there is a spray of stuff at the north end of 12. Until now, I've taken that as artifacts. It only seems to be present for one frame with nothing significant appearing below it in the following frames. What is the general consensus? I haven't chased the other video copies yet.

RE: Miami Pedestrian Bridge, Part XII

Quote (Sym P. le (Mechanical) 14 Jul 19 03:20)

@MikeW7 - In frame 74 there is a spray of stuff at the north end of 12.
Are you talking about the new WSVN video? I noticed the one frame appearance/disappearance of the whitish cloud also, but didn't want to say anything that could color someone's impressions. Like I said when I posted the ZIP file, the WSVN video seems to much closer to an original copy than the GenFK version - larger file size usually means less compression. When a video is encoded, each frame is compressed much like the way a JPEG version of an original image is created. The dimensions remain the same, but fine details and subtle color differences are lost as more and more compression is applied.

ADD: The fact that it came from a news source rather than a random FaceBook account also means its more likely to be an original, or at least a high quality duplicate.

RE: Miami Pedestrian Bridge, Part XII

Sym P. le (Mechanical) Are you speaking of the white vertical lines? I think I see something in the same position relative to the tree in the previous two frames. If so, then it would obviously be part of the background.

SF Charlie
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RE: Miami Pedestrian Bridge, Part XII

This single column of pixels.

RE: Miami Pedestrian Bridge, Part XII

Here are two ZIP files that include earlier material from the ZIP file I uploaded in my post of 13 Jul 19 18:54 - the methodolgy is the same, except I've included frame 55 to 90 instead of 72 to 90. This allows you to watch the north end of the bridge as the background tree and foreground manlift/lightpole combine to created multiple distractions at frame 74.

EDIT: Well, the video actually starts at about frame 10, just as the bridge comes into view.

RE: Miami Pedestrian Bridge, Part XII

I tried different interpolation methods to upscale the video by 400%, and they all show the same thing: a big white & gray cloud that is partially obscured at the bottom by the infamous triangle that very much appears to be the NW corner of the deck. The upper part of the cloud is also visible in frame 73.

RE: Miami Pedestrian Bridge, Part XII

Quote (RAB 678)

Yes and like I said lets backtrack and analyze these statements...........
(my responses will be in parenthesis)

Obviously the designers made very significant errors and that lead to the collapse. These failure mechanisms were understood for decades now and the appropriate design checks are in the concrete codes. It was a serious oversight in the design and there were mistakes the contractor and engineers made when the cracks appeared. However, the fundamentals are that the bridge could have been designed properly as a concrete truss.

I could spend hours on each item you bring up but the best thing to do is a proper analysis and all the required design checks along with a full independent review. There are aspects to the design that would have to change (obviously) but it is feasible and in rare circumstances could make some sense. I doubt now that any one will be jumping in line to design a concrete truss bridge.

Don't forget that there have been many other bridges with other materials that have failed suddenly but that doesn't stop us from having bridges.

RE: Miami Pedestrian Bridge, Part XII

Quote (MikeW7 (Electrical))

The upper part of the cloud is also visible in frame 73
I notice that also. EDIT I also note a white stripe to the left of the black lower arm of the cherry picker that continues through most of the video

RE: Miami Pedestrian Bridge, Part XII

Quote (SFCharlie (Computer) 14 Jul 19 14:41)

I also note a white stripe to the left of the black lower arm of the cherry picker

The photo posted by Tomfh (Structural) 11 Jul 19 11:52 doesn't show anything on the north side of the lift arm, but other chopper pics pics show a white stripe on the north side of the arm, but this wouldn't be visible from the truck dash cam.

EDIT: no stripes on either side:

RE: Miami Pedestrian Bridge, Part XII

In saikee119 (Structural)8 Jul 19 16:38 post, it is visible in frames 76, 77 EDIT 78, 79 also. Gone in 80.
I did not intend to imply it was real...

RE: Miami Pedestrian Bridge, Part XII

My guess is that it's a specular highlight (reflected sunlight) on the nearly upright bottom arm from the nearly noon sun. Just one more spooky coincidental alignment to mess with our eyes.

RE: Miami Pedestrian Bridge, Part XII

Quote (RAB678 (Mechanical)13 Jul 19 23:35 So....Canopy is in compression, being concrete always expanding and contracting. But the compression factor makes it always be shorter. Deck would be opposite. Always elongating. Due to temperature curing.......same reasons.)

Concrete under long term compression does creep, resulting in changes in member lengths.
One of the benefits (?) of prestressing concrete is the ability to "tune" the structure, much like a piano or guitar. Like a stringed instrument, talent is required in that tuning.
This structure had PT forces in both the deck and canopy. Tuning these forces to produce higher compressive stresses in the deck than in the canopy under self weight conditions can actually result in upward camber over the long run. Adding live loads would cause elastic reactions which vary with load intensity and likely exist over short periods. Predicting the effect of the "open web" conditions which must transfer the horizontal shears between the canopy and deck is much more difficult.

Overall, with both the canopy and deck being under compression basically all the time, the structure will shorten. The immediate shortening of the deck under the PT forces is about 5/8 inch, if I did it right.
It might shorten another inch over its expected service life. That is a guess, without any calc. Thermal expansion is within the elastic range and will basically increase the compression a bit or reduce it a bit, with basically elastic reactions in the concrete. Because the PT strands are well within their elastic range, they will respond accordingly, and basically be a constant force applied at the ends of the deck and canopy, maintaining compression in the concrete and preventing and cracking from tension stresses.
To insert a 'pun' here, the 'key' is in the tuning.
Because the canopy has a greater surface to mass ratio than the deck, I would expect the canopy to react a bit faster to changes in solar exposure and air temperatures. This could cause some different expansions in the canopy compared to the deck, and initiate some rise and fall cycles on a daily basis. I will let the Mechanicals advise on this effect. If the result is significant movements, the cycling strains on web members could cause cracking and a cause for concern,

Your comment "it hangs from the canopy)" is perhaps a bit too limiting. The canopy is in effect the top chord of the truss, and provides the added benefit of some shading. The deck is the bottom chord of the truss, with the added benefit of being the walk surface supporting people. And the webs connect those elements, creating a "truss". In effect, the structure depends on each and all of these pieces doing their job, and therefore "hangs" on each and every one of those components. Thus the concern for a lack of redundancy.
Thank you for your comments.




RE: Miami Pedestrian Bridge, Part XII

Quote (Earth314159)

Even though the deck has a net tension, the concrete is in compression (at service load levels).

Hi Earth 314159 -- Your statement above is 100% incorrect and wrong. In the vicinity of the connection between deck and node 11/12 the concrete in the deck is under a fantastic amount of tension under all load conditions.

This is because the deck in that vicinity is under fantastic shear load, and as we know (see, Mohr's circle), shear is a combination of compression in one direction and tension in the orthogonal direction. It is that tension that killed off this bridge, and the six human beings underneath it. There was no steel in place to take that tension.

RE: Miami Pedestrian Bridge, Part XII

Quote (Fortyyears experience)

Hi Earth 314159 -- Your statement above is 100% incorrect and wrong. In the vicinity of the connection between deck and node 11/12 the concrete in the deck is under a fantastic amount of tension under all load conditions.

This is because the deck in that vicinity is under fantastic shear load, and as we know (see, Mohr's circle), shear is a combination of compression in one direction and tension in the orthogonal direction. It is that tension that killed off this bridge, and the six human beings underneath it. There was no steel in place to take that tension.

I am talking about the deck as a whole and not the disturbed region at the joint. I have written multiple times about the shear/tension etc. at the joint and I am well aware of it. I have also done the calculations for myself.

If we could take the comment "the prestressed concrete is all under compression" literally and without exception, we could build out of sand rather than concrete. You have to put the comment in context.

BTW, Concrete in shear is not necessarily accompanied by tension stress. You can have shear with 0 tension or even two different compression stresses from orthogonal directions. The mohr circle is simply shifted to the right.

RE: Miami Pedestrian Bridge, Part XII

Quote (Earth)

BTW, Concrete in shear is not necessarily accompanied by tension stress.

Not necessarily, but almost always. And it's not really true to sat the shear is "accompanied" by tension. The tension is part of the shear.

RE: Miami Pedestrian Bridge, Part XII

Quote (Tomfh)

Not necessarily, but almost always. And it's not really true to sat the shear is "accompanied" by tension. The tension is part of the shear.

The tension is the components of the shear stress and vice versa. It is just a matter of perspective. I get that and that is part of my point. I am not too sure exactly what wording you would like me to use. I really didn't want to be pedantic about it. The mohr circle is just derived from the statics of a stress tensor at a point.

In concrete, the principal stresses at 45 degrees are commonly not in tension. If you have a crack at 45 degrees in the web, as soon as the concrete cracks, the mohr circle taken at the edge of the crack shifts to the right and will have zero tension (it has too since tension cannot cross a crack)in the principal direction. This is where compression field theory comes from. If the mohr circle is taken right at a tie/stirrup location, the stresses change again. Having a mohr circle with equal magnitude principal tension and compression (a circle with centre at the origin (x,y)=(0,0)) is not the common condition. The mohr circle varies from point to point.

RE: Miami Pedestrian Bridge, Part XII

2
ADD: The vertical movements and distortions shown in the frame images and GIFs are real, but they are probably enhanced to an unknown degree by the 1600% enlargement process. Just because something seems to move 6" (even in the raw pixel data) doesn't mean it actually moved that much. The important thing to glean from this data is that observable movement occured, but it's magnitude is unclear.

Been working on this for two days. I'm tired. There are probably still a few typos....

THE SMOKING GUN



So ........ I found a "smoking gun" late Saturday night, hidden in plain sight and already viewed countless times. I spent the last two days figuring out details and writing this post. It turns out a small cluster of pixels can tell quite a story if the right questions are asked...

The GIF above shows the last 20 frames of the North View timelapse, zoomed in by 16x. The order of events are as follows:
  1. Kevin Hanson is standing in shadow to the right of member 12, looking at the 11-12-deck joint.
  2. Hanson moves to the NW deck corner (possibly to talk by phone with his canopy crew).
  3. Hanson moves in for a close up view, then returns to the NW corner.
  4. Just as he leaves, member 12 starts moving up and down and the diaphragm to its west lifts upward - watch the area to the right of the 8" drain.
  5. A short pause is followed by the collapse of the bridge.
The GIF was made from individual JPEG frames using the GIF Maker at https://ezgif.com/maker

It turns out a large section of the North View timelapse, about 170 frames to be exact, documents Hanson's movements as his tensioning crew works on the canopy, and the warping of the 11-12-deck joint that resulted from their actions. Most of these structural movements are much more subtle than the coup de grace that finished the bridge, but they seem to follow a predictable pattern of adjust and wait.

HISTORY

This all started with the Saturday discussion about a dust cloud that was observed (in the truck dash cam video) at the north end just before the bridge collapsed. I began with the North View timelapse source video and zoomed in by a factor of 8x, then 16x, to observe the north end itself, and what went on as Kevin Hanson was actively checking the cracked areas while his tensioning crew was making adjustments on the canopy.

What I saw was unexpected and remarkable. The bridge collapse was not an instantaneous event. The 11-12-deck joint and surrounding area was experiencing repeated cycles of distortion and relaxation as Hanson's crew worked above. I spent most of Sunday experimenting with deciding how much enlargement to use, what type of interpolation to apply, whether other filters might show clearer results, and documenting when critical events occured. Today I worked on this writeup, which took a lot longer than expected.

METHODOLOGY

Editing tools: Open source video editor VirtualDub2 and freeware file viewer IrfanView

I started with the 1920x1280 North View timelapse (Bridge-109 Mar 8-19 2018 600X-1080.mp4) and used VD2 to crop out a work area that included both corners of the deck, the top of the north pier, and enough of the deck area to locate Kevin Hanson as he paces from NE to NW deck corners. I settled on a 120x67 crop that enlarges to 1920x1072 at 1600% magnification, which is almost identical to the standard 1080p video size. This enlargement is big enough that everyone should be able to see exactly what I produced without the addition of artifacts created by their viewer's interpolation routine.

For the time span, I decided to start at the point where Hanson ends a face-to-face conversation with a mystery person. After this meeting he appears to be solely devoted to checking the 11-12-deck joint, and making periodic phone calls to his canopy crew. This start time corresponds to 1:59 in the Collapse - N View YouTube video, and Frame 26252 (14:35.942) of the source timelapse video. I stopped at source Frame 26425, a couple frames after the bridge collapsed.

In the past I've enlarged using the "nearest neighbor" interpolation because it retains the original blocky nature of the source video so details are preserved and artifacts are avoided. It works by replacing each original pixel with a block of identical pixels - for 400% enlargement, each pixel is replaced by a 4x4 block.

For this project I wanted to enlarge by a much larger factor, and I wasn't concerned with preserving detail because there really wasn't any because the area was so small, so I used bilinear interpolation, a method that spreads each 2x2 block of source pixels apart then uses simple linear interpolation (in 2-dimensions) to calculate intermediate pixel values, in this case 252 "new" pixels to completely fill a 16x16 grid. There are several variations of this algorithm, but what I used is the Precise Bilinear interpolation method that's a native feature of the VirtualDub2 filter kit.

A comparison of Nearest Neighbor (NN) and Precise Bilinear (PB) for a 120x67 crop are shown below:

16x enlargement of cropped Frame 26293 using Nearest Neighbor (NN):


16x enlargement of cropped Frame 26293 using Precise Bilinear (PB):

As you can see, PB is useful to "guesstimate" the shapes hidden within blocky pixels, and at 16x it was very effective at magically converting square pixels into believable representations of the 8" drain pipe and tendon sleeve openings in the north end of the deck, as well as the proper shapes of member 12 and Kevin Hanson.

Much more important, however, is the fact that PB can detect subtle motion. As a sharp feature moves within the area covered by two source pixels, the pixel colors change value to provide an indication of the changed location. If 2x interpolation is used, movement can be detected at approximately 0.5 pixel resolution. 4x interpolation yields 0.25 pixel resolution, and so on. Motion detection at 16x is down to 6% of a pixel width, which means very subtle motions can be detected, like the displacement of member 12 and diaphragm warping.

ZIP FILES

For the image analysts, here's a 16MB ZIP file that contains:
  • A folder containing all the images for the period when I believe the tensioning was occuring, frames 26252 through 26425 of the North View timelapse video (174 frames) saved as JPEGS with 80% compression.
  • A bonus folder containing the NN versus PB comparison images.
ADDS:
  1. If you decide to analyze the "smooth" Precise Bilinear (PB) interpolated images, be sure to have a look at the "chunky style" Nearest Neighbor (NN) versions before you make any decisions about the reality of what you think you see. Here's a ZIP file of the same 174 images as above, at the same 16x enlargement, using Nearest Neighbor interpolation.
  2. Here's another ZIP file containing 100 control frames (21960 to 22060) from the previous day, taken at the same time, so you can understand what the undisturbed North View looks like. There's a lot of wind buffeting, but the spatial relationship between all components remains very stable, an indication that the distortions seen on the collapse day were, in fact, distortions. NOTE: I forgot to put them is a folder before they were zipped.
  3. ZIP file of last 44 frames of North View closeup, cropped to include the entire north end (canopy, member 11, deck, and pier top). This time I remembered to put them in a folder.

ANALYSIS NOTES

The frame rate is not constant. It appears that an intervalometer was used to make the camera shoot a short burst of three frames followed by a multi-second time delay. A good estimate of the frame intervals can be found by observing traffic movement in the source video, and noting that a car moving at 30mph is moving at 44 feet per second, but that's a project for another day....

High power enlargement is hampered by many factors:
  • The change in one source pixel value affects the interpolated values of a surrounding 32x32 block of calculated pixels.
  • Motion artifacts can be caused by camera shake (wind gusts, nearby auto and foot traffic, birds, etc.)
  • The absolute location of the image sensor can shift by several pixels due to thermal expansion of the camera body, it's mount, and the structure the mount sits on.
  • Sun motion alters indirect illumination, shadows and reflections.
  • Imaging sharpness can be diffused by dust kicked up by nearby construction activity.
  • ADD: As discussed below in my posts of, 16 Jul 19 15:38 and 16 Jul 19 16:39, objects that extend toward the camera, or edge surfaces that are viewed from the side, may appear to be flattened and have no 3-dimensionality if they are represented by a small number of pixels.
Making determinations required a disciplined plan:
  • Since the tensioning process was performed in steps, I looked at surrounding frames to insure the motion I observed was confined to a small number of frames, surrounded by a larger number of more stable frames.
  • Then I quick-stepped back and forth between frames (using IrfanView) to ensure that only a specific area was showing movement.
I discovered the GIF Maker tool just recently, but I think it would make a useful tool to make permanent loops of frames that contain suspected motion, allowing you to just sit and stare at all parts of the image before making a decision. The GIF frame rate is also adjustable as a percentage of 1.00 second.

ADD: As an example, here's a GIF made from 85x30 crops (enlarged to 1360x480) of the last 7 frames, with 1 second between frames:



FRAME-BY-FRAME COMMENTS

The frame sequence begins with the departure of a mystery person who was talking with Hanson. After that person leaves, Hanson actively begins checking the cracked area at the 11-12 node, walking from one side of 11 to the other, and at times just crawling over 11. He stops often in the NE or NW corner where he appears to be having conversations with his work crew using a phone, since his arm is held up to the side of his head instead of his mouth as would be the case with a walkie-talkie. In one or two cases there appeared to be lengthy conversations, possibly with an outsider. (Any guesses??)

Here are my notes on what I THOUGHT I saw, using the last 3 digits of the frame number. Your opinion may differ....

252 The west edge of the canopy shadow has already moved several feet in from the deck edge. The actual NW deck corner is defined by a post at the upper RH corner of frame, and the NE deck corner is defined by a post at the center LH edge. The deck corners will be easier to locate in later frames as Hanson moves from corner to corner, apparently having phone conversations.
252-263 - Hanson is standing with an unknown person in the canopy shadow to the right of member 12.
264 - With the mystery person gone, I assume the tensioning process is now beginning.
266-267 - The diaphragm appears to distort while 12 does not move.
276 Camera shake.
281 - 12 drops.
282 - 12 and the west diaphragm clearly drop with respect to the pier (after slowly being raised?)
285 - 12 and east diaphragm rise.
286 - 12 drops.
299-301 - East diaphragm distorts.
306-307 - Camera shake.
313 - 12 moves

314-415 - The diaphragm and 12 are in almost constant motion against a stationary pier and background.

314 - A dark gap appears to the east of the 8" drain. The 12-deck boundary appears to lighten and darken throughout the video, but in this case it is more pronounced.
336 - Camera shake.
348 - The dark gap reappears.
354-374 - Hanson appears to have a lengthy phone conversation. (Outside call??)
390-395 - Hanson has another (shorter) conversation.

NOTE: The descriptions below also apply to the 7 frames that make up the opening GIF.

416-420 - Member 12 moves significantly down-up-down-up as the surrounding deck area warps dramatically.
421-422 - A short pause.....
423 - And the bridge is gone.

RE: Miami Pedestrian Bridge, Part XII

I'm going to skip over the rag in the wind theory for the puff of magic dust out the end.

and.......let us jump into another fine moment to analyze on this bridge.
The latest comments revolve around tension so lets go there.

If we examine the absolute center of the deck.
2 things should be quite obvious.
It should literally be trying to split apart at the middle.
The top center of canopy is a hinge point, there are two deck ends on piers.
A big triangle. The center of deck would be in full tension. See the picture?
It's merely? trying to split into 2 pieces.
Seems easy enough to understand.

Here is the problem.................

When I said the deck hangs from the canopy.....it does.
Specifically at the bases of 5/6 7/8 deck junctions or nodes if you must.
These two points the deck would love to fall straight down.

But......something holds it up.

Now gentlemen and ladies...............what is holding it up?

Just how much weight are 4 rods, 4 squares? of steel, 2 cold joints and some rebar.....
embedded into 4 to 8"? of concrete at the surface of the deck, holding up?

Let's not forget to add some dynamic loading to this.

Most certainly if "WE WILL... WE WILL......ROCK YOU!" plays.............we'll need another bridge.

I would like this addressed. Anyone else see this? Am I just being a nervous kitty kat?

RE: Miami Pedestrian Bridge, Part XII

MikeW7
WOW !!
That is revealing. Will take a week to digest. Awesome work. So much to see.
Has NTSB called you yet? EOR should hire you to keep others from doing so.

RE: Miami Pedestrian Bridge, Part XII

Vance

I'm retired. This has been a side diversion hobby while I've been putting solar panels on my Aliner camper. Finished yesterday. Everything appears to work (fingers crossed). Off to Utah in a couple weeks, I hope.

RE: Miami Pedestrian Bridge, Part XII

you said...
416-420 - Member 12 moves significantly down-up-down-up as the surrounding deck area warps dramatically.

can you elaborate on significantly?
somewhere between 50 thou and a foot? Like an earthquake?
end response

overview......
I've gotten to where the weakness of 11 12 junction is so pathetic does anything else matter?
Somebody got played...............leg sweep, shoulder throw, foot stomp.... faceplant.

Fortunately for me, other than comments on ideas and observations, I got nothing to do with this memorable display of crapola.

oh.....a certain video site not mine has some content of this discussion.
In the title should be included Propaganda 101.

and I am so done with this............

RE: Miami Pedestrian Bridge, Part XII

@MikeW7 - Great work! I didn't imagine finding more in those videos.

RE: Miami Pedestrian Bridge, Part XII

Quote:

RAB678 (Mechanical)16 Jul 19 02:39 If we examine the absolute center of the deck. 2 things should be quite obvious. It should literally be trying to split apart at the middle. The top center of canopy is a hinge point, there are two deck ends on piers. A big triangle. The center of deck would be in full tension. See the picture? It's merely? trying to split into 2 pieces. Seems easy enough to understand.
It does not "split in the middle" because the PT forces prevent that.
It is not really "under full tension" because the concrete is under compression from the PT forces which are tension.

Here is the problem................. When I said the deck hangs from the canopy.....it does.
That statement implies that the canopy is the only thing working and the canopy has the strength to support this structure. I hope it is a matter of semantics and not understanding. The canopy actually sits atop the truss (cast integrally).
Specifically at the bases of 5/6 7/8 deck junctions or nodes if you must. These two points the deck would love to fall straight down.
The deck does hang from the truss at nodes 3/4 thru 9/10, with the bottom chord cast integrally with the deck.
No comment on the amorous intentions of the deck.

RE: Miami Pedestrian Bridge, Part XII

Quote (RAB678)

Just how much weight are 4 rods, 4 squares? of steel, 2 cold joints and some rebar.....
embedded into 4 to 8"? of concrete at the surface of the deck, holding up?

There is a lot more rebar than you suggest going through the cold joint (see cage rebar on the drawings). What you are talking about is punching shear of the deck at the nodes. This is just a quick ball park calc for you. 36'x30'x350psf (estimated factored DL+LL)=380Kips (1700kN). That is about 10-25M or #8 bars required. There is a cage of anchored rebar, PT rods with anchors. Longitudinal bar anchored with 180 bends (more than 8" embed that you suggested).

It is something that has to be considered but was not the cause of the incident. Although I would say the joints look lightly reinforced (especially with interface shear and punching forces combined together).

RE: Miami Pedestrian Bridge, Part XII

MikeW7 (Electrical)16 Jul 19 02:17 OMG A whole lot of work, but the payoff!
Thank you, thank you, thank you

SF Charlie
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RE: Miami Pedestrian Bridge, Part XII

Quote (MikeW7)

It appears that an intervalometer was used to make the camera shoot a short burst of three frames followed by a multi-second time delay.

The frame delay can be estimated by comparing this video with the dash-cam video since the delivery truck is seen leaving the yard and then coming back into view with the collapse.

Regarding the apparent deck/12 movement just before the collapse, again relying on the three frame bursts, in the last three frames prior to the collapse, 420-422, the deck is steady. In the previous three frames, 417-419, there appears to be movement at the lower end of 12 and adjacent diaphragm. More specifically, the drain pipe drops down one or two pixels in frames 417 and 419. Frame 418 finds the drain at the same height as in the preceding and succeeding sets. This puts the attention on frames 417-419. Sad to say I don't have any theories. Insert more info here --> 3eyes

RE: Miami Pedestrian Bridge, Part XII

Sym P. Le
The FIU video with SE View (Bridge-PG5-Mar-1-19-2018-1000K-1080.mp4) has a running clock in upper LH corner that advances 1 minute each frame. It only shows members 1-10, however. The collapse occurs at 11:38.

The SW View video (Bridge-PG6-Mar 1-19-2018-1080.mp4) doesn't have a clock, but it is almost the same length, 14:54 versus 14:47, so it was probably shot at 1 fpm as well.

It wouldn't take much effort to use them figure the North View frame rate, but it's bedtime for me so I'll have a look tomorrow.

RE: Miami Pedestrian Bridge, Part XII

I added a ZIP file of North View closeup images enlarged by Nearest Neighbor interpolation to my post of 16 Jul 19 02:17, highlighted with tan background. You should use both the smooth and chunky versions to guide your assessments.

ADD: I also added a second ZIP containing a batch of 100 control images taken at the same time on the previous day so you can see what the North View closeup looked like when it wasn't being worked on.

RE: Miami Pedestrian Bridge, Part XII

MikeW7, basic question. In your posted GIF, why does the back side of member 12 have a very large jog in it about 6' up from deck?

RE: Miami Pedestrian Bridge, Part XII

After spending 2 days wrapping my head around all the grotesque torture the bridge endured in its final minutes, I have a few questions:
  • Is it practice at all to photograph or record structures as they are being tensioned/detensioned? That is, take a reference "before" shot followed by a continuous sequence (or recording) of the structure as work is done. As I've found out, it is very easy to see displacements with no special processing, so it should be a simple matter to make "heat maps" of the changes, especially if physical markers are applied before hand, or if a laser pattern is shone continuously on the structure.
  • Shouldn't it be mandatory to do this type of recording when unplanned work is done? I know it's an added expense (extra guy, equipment, etc.) but holy crap after watching and re-watching the closeups all weekend I was constantly asking myself why in the hell did they keep going when it was so, so obvious that they were literally tearing the bridge apart.
  • In a case like this where traffic is present, should a crew chief like Hanson have the legally protected authority (moral or otherwise) to jump the chain of command and make a phone call directly to a traffic agency like FDOT?

RE: Miami Pedestrian Bridge, Part XII

Quote (TheGreenLama (Structural) 16 Jul 19 14:2)

... why does the back side of member 12 have a very large jog in it ...
This "bulge" is present in the North View timelapse all the way back to when the bridge was installed, but photos around that time don't show any sort of protusion other than the 8" drain pipe that extends out about a foot.

My guess is that the "bulge" is an interpolation artifact related to the drain pipe. If you look at the PB-NN comparison images in my 16 Jul 19 02:17 post, you'll see that the horizontal extent of the drain is not apparent in the black-gray pixels, so the interpolation routine saw those pixels as a flat oval shape and probably just conformed some of the tan background pixels to surround it. This bulge is formed by a small area of about 5x10 raw pixels, so it should be easy to understand that when each of those pixels is replaced by a block of 16x16 interpolated pixels you're sometimes going to see the appearance of wierdly shaped "details" that don't really exist.

High power enlargements can be enhanced to "guesstimate" the shapes hidden in a small group of pixels, but not the details, and sometimes the "guesstimations" are incorrect. That's why I've been stressing that you should only look at original pixels when you're making critical judgements, because that's the only real data that exists. If you can't see it in the pixels, it ain't real....

RE: Miami Pedestrian Bridge, Part XII

MikeW7 Great work, Thank you!
I seem to see the east edge of the top of the deck slightly tip down away from the diaphragm in the last frame or two before the collapse? I moved the right half of my browser window off screen, so the motion there would not distract or influence me. Can that be real?

SF Charlie
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RE: Miami Pedestrian Bridge, Part XII

Quote (SFCharlie (Computer) 16 Jul 19 16:07)

I seem to see the east edge of the top of the deck slightly tip down away from the diaphragm ... Can that be real?

If you read my 16 Jul 19 15:38 reply to TheGreenLama you'll see that the raw pixels representing the extended drain pipe were recorded in such a way that its North-South extension appears as a flat, right-left horizontal oval. I just looked at the canopy area by simply moving the crop window up, and did not observe any kind of vertical movement. Using an analogy similar to the drain pipe mystery, I think the apparent vertical movement we are "obviously" seeing is in fact an interpolation artifact of what is actually the northward kickout of the 11-12-deck joint and diaphragm area that OSHA described as a blowout.

We are all in uncharted waters here. We have to understand that what we are seeing is very "real" in some way, but the actual reality behind it is subject to interpretation, so we need to develop a set of interpretation standards before this devolves into a quagmire of competing SWAGs.


ADD: ZIP file of last 44 frames of North View closeup, cropped to include the entire north end (canopy, member 11, deck, and pier top). This time I remembered to put them in a folder. I also added this ZIP file link to my original post.

ADD: I'm editing my original post with ADDs as new insights are discovered, such as this "Is it vertical or flat?" conundrum.

ADD: GIF of canopy frames 26412-26423

RE: Miami Pedestrian Bridge, Part XII

Quote (MikeW7 (Electrical)16 Jul 19 14:56 Is it practice at all to photograph or record structures as they are being tensioned/detensioned? That is, take a reference "before" shot followed by a continuous sequence (or recording) of the structure as work is done. As I've found out, it is very easy to see displacements with no special processing, so it should be a simple matter to make "heat maps" of the changes, especially if physical markers are applied before hand, or if a laser pattern is shone continuously on the structure.)

In my experience , limited to slabs, beams and building components, record photographs have not been taken. Measurements were taken to compare elongation with ram force as standard procedure, and measurements of relaxation and elongation were required by my drawings when PT strands were to be relaxed and restored for some purpose.
But, like just about everyone in the US, I have no experience with posttensioned concrete trusses. To my knowledge, no one has seen a PT concrete truss succeed, but many have now seen the one example fail. In this case, I have seen no evidence of an attempt to monitor crack progressions with more than a felt tip pen and a tape measure. It is common practice to monitor and quantify cracking using transparent overlapping plastic grids and recording movements to quite small increments. That is from 20 years ago, and valid today, but no doubt supplemented by electronic strain recording today.
Without verification on my part, I would estimate that the majority of PT work at construction sites is in slabs of buildings and girders of highway structures. The problems encountered in this work are pretty well worked out by now. I see no indication of problems with the PT in the canopy or deck in this structure, probably because, at least in part, it is so similar to the countless thousands of miles of PT installed in similar conditions. The web members are a bit more unusual in their application of PT. Relatively short members, dramatic changes in force directions at joints (nodes), members under PT forces carrying tension working with common reinforced members in compression, joints with rigidity, PT anchor plates with high bearing pressures in joints loaded by normal reinforced compression from a different direction - and the list goes on. Trusses work - the devil is in the details.
Having had VSL as the PT contractor on a project, I can say the personnel encountered were efficient, worked carefully, and understood well their work. As a piece at a time, there is nothing unusual in the PT of this structure. As a complete assembly, it is far more complicated. I am not sure that those complications are a problem that the PT contractor is supposed to solve.
But I have strayed from the focus.
A question - is there some "sharpen" filter that can now be applied to your enlarged images? Without creating new information that did not exist ?

RE: Miami Pedestrian Bridge, Part XII

MikeW7 You have done so much work, I can't complain. The new crops do not show as much of the east edge of the deck as the old ones. This is where I though I saw the slight rotation downward about the east-west axis.
ADD Oh, I guess that I was looking at the shadow of the top of the orange safety barrier, not the edge of the deck. My bad!

RE: Miami Pedestrian Bridge, Part XII

Quote (Vance Wiley (Structural) 16 Jul 19 18:02)

is there some "sharpen" filter that can now be applied to your enlarged images? Without creating new information that did not exist ?

Viewing these interpolated images is the equivalent of trying to figure out what's behind a wall of glass bricks. There is only so much you can do to make it look more normal.

You have to remember that the PB (smooth looking) enlargements are representing the exact same detail information that's present in the NN (chunky looking) enlargements, and no amount of filtering is going to recover the details that were lost when the original scene was digitized into chunky pixels. It's frustrating because the smooth edges look more realistic than the chunky blocks, but it's an illusion...

RE: Miami Pedestrian Bridge, Part XII

Quote (SFCharlie (Computer) 16 Jul 19 18:15)

I guess that I was looking at the shadow of the top of the orange safety barrier

Yeah, it's the shadow of the safety netting, which appears as a fat line because the sun is at high noon. I had problems with it as well until I realized that the shadow was just starting to creep onto the west edge of the deck when the bridge collapsed.

Another spooky coincidence, like the flash of reflected sunlight off the manlift arm just as the north end blew out.

CSI work is fun!

ADD: The fact that the North View images were shot in burst of 3 followed by a delay can be confusing as heck also. You're watching something change slowly over 3 frames then BAM. This is especially true with shadow movements.


RE: Miami Pedestrian Bridge, Part XII

MikeW7, I hate to throw cold water on your lengthy and very detailed post, but what I think we're seeing in the GIF is not real. Member 12 moves up and down WITHOUT any associated noticeable horizontal movement. I don't see how this is possible. Particularly the upward movement--and seen from a camera that's how far away?

RE: Miami Pedestrian Bridge, Part XII

MikeW7, TheGreenLama Mike, does the canopy go up and down with 12?

SF Charlie
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RE: Miami Pedestrian Bridge, Part XII

Quote (TheGreenLama (Structural) 16 Jul 19 20:30)

Member 12 moves up and down WITHOUT any associated noticeable horizontal movement. I don't see how this is possible.
:
... and seen from a camera that's how far away?

Quote (SFCharlie (Computer) 16 Jul 19 21:10)

...does the canopy go up and down with 12?

If it were some kind of artifact, why is it visible only in this region, and why would similar but smaller disturbances be visible in the same region, and only during the time period that the canopy crew was working?

The camera distance is not a factor because the entire image is uniformly focused and rock steady. There's no heat shimmer, focus drift, etc. There are a couple of frames with camera shake in this video, but they affect the whole frame. The 100 control frames I posted (from the previous) day show some wind buffeting and small random movements of member 12, the drain pipe, and the tendon sleeves, but they don't show the non-random up-down motion of member 12, or the repeated distortions visible in the west diaphragm.

As for the up-down motions: from my post dated 16 Jul 19 16:39

Quote:

...the raw pixels representing the extended drain pipe were recorded in such a way that its North-South extension (of the pipe) appears as a flat, right-left horizontal oval. I just looked at the canopy area by simply moving the crop window up, and did not observe any kind of vertical movement. Using an analogy similar to the drain pipe mystery, I think the apparent vertical movement we are "obviously" seeing is in fact an interpolation artifact of what is actually the northward kickout of the 11-12-deck joint and diaphragm area that OSHA described as a blowout.

What we are seeing is real, but what exactly it represents is still being determined. I've included as much information as I can, including a set of control images from the previous day, so people can make informed decisions.

ADD: See the new GIF of the entire north end at the end of my post dated 16 Jul 19 16:39

RE: Miami Pedestrian Bridge, Part XII

Quote (MikeW7)

Viewing these interpolated images is the equivalent of trying to figure out what's behind a wall of glass bricks

This is true, but the purpose of examining the videos is to explore them for clues and to determine what evidence they provide to support theories or dismiss theories. To dismiss the videos out of hand as junk is to do a disservice to any investigation. The example of the width of column 12 is an example of the difficulties in relying solely on the video record. We all know that 12 is of fixed width for the full height, yet the video displays a variety of representations dependant upon adjacent lighting of the canopy shadow. On the other hand the rag hanging off the end of the slab offers the best of what the video can offer, i.e. we could see that there was a blemish displayed on the video, we investigated the source and found a cause.

The work of isolating and refining the video is valuable, even if it does not hit the jackpot by which we would measure that today.

RE: Miami Pedestrian Bridge, Part XII

Quote (MikeW7)

Awesome work with the images.
Because I have limited equipment and greatly limited image manipulation talents, I can't see what you are describing as vertical movement in 12, but that member is supporting about 45 kips of structure weight. It would take a lot of energy to lift member 12 - even slightly. Enough, I think, to announce to anyone on this structure that it is just about over.
And my question about sharpen filters is indicative of my image limitations. Thanks for responding.
Now about this vacation to Utah - you did fill out form 42ImOuttaHere-2, revised March 15, 2018, right? The deadline is long passed. What are we gonna do in your absence? Have you arranged a replacement?
Travel safe and enjoy.


RE: Miami Pedestrian Bridge, Part XII

Quote (Sym P. le (Mechanical) 16 Jul 19 23:27)

We all know that 12 is of fixed width for the full height

Quote (Vance Wiley (Structural) 17 Jul 19 00:16)

I can't see what you are describing as vertical movement in 12

Yet is seems to be wide at the bottom, which implies that the west edge of 12 is almost perfectly located between two columns of sensor, but the camera is probably not exactly vertical so the top of 12 only strikes the left column of sensors while the bottom part strikes both the left column and the right column. As a result the top of 12 only gets interpolated toward the left, while the bottom gets interpolated to the left and right, making it look like 12 suddenly widens at the bottom. This sideways interpolation carries over to affect up to 16 other columns of interpolated pixels, so the effect is much more dramatic that you might first expect.

This effect might also explain the dramatic rise and fall of 12 - its bottom edge might be almost perfectly located between two rows of sensors, and a small vertical displacement de-activates the bottom row of sensors, and removes its presence from up to 16 rows of interpolated pixels, making it look like 12 rises up dramatically.

Easy explanations, once you figure them out. There will be many more of these types of puzzles to solve before we get a good handle on what's going on....

I'm not an artist, so it would be nice if someone can illustrate these concepts, then I'll figure out how to incorporate them into a "user Guide" at the end of my lengthy introduction.

ADD: Sped-up GIF of North End from predawn to collapse. Watch the right side of member 12 and how changes in lighting/shadow affect it's apparent width. One more reminder: The purpose of this 16x enlargement is to make motion easier to detect, which it does well. It was never expected to enhance details.


RE: Miami Pedestrian Bridge, Part XII

Quote (Vance Wiley (Structural) 17 Jul 19 00:16)

you did fill out form 42ImOuttaHere-2, revised March 15, 2018
I filed my "TakeThisJobAndShoveIt" paperwork the spring of 2015, took all my remaining vacation days the end of June, then went on permanent extended vacation July 1, 2015 (yet somehow still got paid a vacation day for the 4th!).

I'll have a laptop, and will try to check in from time to time.

RE: Miami Pedestrian Bridge, Part XII

No interpolation, just enlarged pixels (Nearest Neighbor)

RE: Miami Pedestrian Bridge, Part XII

Quote (Report Edit Delete MikeW7 (Electrical)17 Jul 19 00:30 Yet is seems to be wide at the bottom, which implies that the west edge of 12 is almost perfectly located between two columns of sensor, but the camera is probably not exactly vertical so the top of 12 only strikes the left column of sensors while the bottom part strikes both the left column and the right column. As a result the top of 12 only gets interpolated toward the left, while the bottom gets interpolated to the left and right, making it look like 12 suddenly widens at the bottom. This sideways interpolation carries over to affect up to 16 other columns of interpolated pixels, so the effect is much more dramatic that you might first expect. This effect might also explain the dramatic rise and fall of 12 - its bottom edge might be almost perfectly located between two rows of sensors, and a small vertical displacement de-activates the bottom row of sensors, and removes its presence from up to 16 rows of interpolated pixels, making it look like 12 rises up dramatically.)

Thanks for that explanation. I have encountered similar results with digital measurements - tapes and levels - with a minimum increment of one "pixel" or "graduation", it either is or it ain't.
Perhaps the camera should be oriented at 45 degrees or the pixel rows staggered?

RE: Miami Pedestrian Bridge, Part XII

Regarding the aliasing explanation above, It’s worth remembering that very few lenses have the ability to resolve that finely. Typically each point of light is blurred across several pixels. You can see this in the images above. At light/dark edges, it take several pixels to go from black to white.

Except for the best lenses (or perhaps the crudest sensors) it is very rarely the case that each point in the image is triggering one pixel and one pixel only.


That’s why it’s such a joke to hear small sensor cameras with “20 mp resolution”. The lens isn’t capable of resolving anything like that.

RE: Miami Pedestrian Bridge, Part XII

Guess I'm not quite done.
I can't seem to find inner truce with the crane vibrating question.
The little pixel blow up video was interesting and gave me an idea.
I went back and looked at the video dash cam video link that was provided here.

https://www.youtube.com/watch?v=Sb4npaRtPfQ I believe that is the link to the dash cam video.
Sorry, I don't know how to embed it as well as MikeW7 did. Thanks Mike.

It made me bust out a BIG magnifying glass.
I also placed a movable wire in front of the screen to help track the cranes movement.
Amazing.... these new high tech gadgets they have today.

And then.......I saw something I missed before.

It would seem there is a large ball weight on the end of the crane cable. as expected.
and....It does swing back and forth about 15 feet? left to right.
Coincidentally like right after the bridge begins to fall.
There is a strap on it, traveling with it.
The ball also somewhat disappears and reappears. A couple of times.

I know...... that's because of camera angles, frame rates, pixelation and
the awkward gravity system in Florida.....

Using the wire as a reference.............
The crane boom top does move left, then waits, then seems to move a little faster. (Jumps a few pixels.)
It's not quite matching what I would expect from a stationary boom. Standard progression of pixels.
And repeats a few times. The streetlight does seem to progress as expected for reference.

That's not bouncing back and forth, is it......it is...is it.......it is...
Care to pixelate that one?

Almost forgot....while someone mentioned....you would need a larger crane to lift the bridge.
You are not exactly lifting the bridge. Just helping it, while cables are tensioned.

Then again I guess the size crane that was used is certainly needed to lift a
200 lb tensioner and support a 200 lb man in case something happens.......

Don't they use 2 cables actually? One for the main lift and maybe another for a lighter reason......
And what exactly would make a 200? lb steel ball go wildly swinging in the air like that?
Maybe.....it got a leak like a balloon...and went pffffffffffff..... all around.

Sorry to waste your time. I just think I see that.


RE: Miami Pedestrian Bridge, Part XII

The PT rod was adjusted with a hollow hydraulic jack which was supported by a crane. It would have to be moved from upper to lower PT rod nut occasionally.

SF Charlie
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RE: Miami Pedestrian Bridge, Part XII

Quote (RAB678 (Mechanical) 18 Jul 19 09:39)

Sorry to waste your time.
:
It would seem there is a large ball weight on the end of the crane cable ... There is a strap on it, traveling with it.
:
Don't they use 2 cables actually?

I gave you links to a couple of YouTube Playlists a couple of days ago. They contain 3 videos of the collapse. If you'd also watched the Collapse - N view and Collapse - SW view videos, they would have answered most of your questions and clarified many of your guesses. Then you wouldn't have had to write such a long post, and I wouldn't have had to spend so much time answering it....

The crane block (pulley and lifting hook - looks like a ball from a distance) is located about halfway between the end of the crane mast and the work area. Attached to it are 2 slings:
  • [EDIT to add more sling info] One sling supports the heavy equipment the workers are using. This equipment has to be repositioned multiple times as the workers adjust different tensioning rods. My guess is that they used a nylon lifting sling that is much wider than a thin wire sling of the same rating, making it much easier to grap and tug as the workers are positioning their equipment. When the bridge falls the bottom sling loop broke, and the crane cable tension caused the sling to shoot up in the air, as seen in the dashcam video. I'm sure there are news pictures somewhere that can shed more light on this.
  • The other strap/cable is used as a connection point for a safety harness. You see it pulled to the side numerous times in the N View video so it's out of the way. One of the workers tried to put the harness on just before the bridge started to fail. When the bridge collapses you see the arms of the "Falling Man" shoot up over his head as he falls out of this harness, then the harness itself falls.

[Post Deleted]

RE: Miami Pedestrian Bridge, Part XII

Quote (RAB678 By far the worst aspect of this structure was the use of an inferior duct tape.)

DAMMIT !! Someone figured it out! Someone call Mike Roman and tell him it's over - we can't cover it any longer. By the way - I'm selling my 3M stock tomorrow.

RE: Miami Pedestrian Bridge, Part XII

Quote (Vance Wiley (Structural)19 Jul 19 06:24 [/quote)

Someone figured it out! Someone call Mike Roman and tell him it's over - we can't cover it any longer.
I saw that too, but decided to let somebody else take the bait. Either Eng-Tips is testing the professionalism and overall quality of this thread, or we've been trolled. If the former, we've done a fine job. If the latter, I'd guess he's of the under-bridge variety, and I hope his home falls down....

RE: Miami Pedestrian Bridge, Part XII

I think the Russians are beta testing a new bot and using eng-tips to feed its neural network.

RE: Miami Pedestrian Bridge, Part XII

Funny, does the affected foreign sentence structure disappear when he gets excited?

SF Charlie
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RE: Miami Pedestrian Bridge, Part XII

The discussion about lines and pixels and smallest unit of measurement digitally leaves me wondering what the smallest increment of measurement was for the instrumentation during the move? I may have skipped over it if it has been presented, so a reference is appreciated.
Thanks

RE: Miami Pedestrian Bridge, Part XII

Quote (Vance Wiley (Structural) 21 Jul 19 00:24)

...what the smallest increment of measurement was for the instrumentation during the move?

The Barnhart Movement Plan (Sheet LLT-01) says the following BDI instruments were used:
  • STRAIN TRANSDUCER BDI ST350
  • ROTATION SENSOR BDI T500-030
  • DISPLACEMENT SENSOR BDI STRING POT
Here's the link to the BDI sensor page. The page for each instrument includes PDFs for a Spec Sheet and Operations Manual.

RE: Miami Pedestrian Bridge, Part XII

Thank you, MikeW7
The ROTATION SENSOR BDI T500-030 has a sensitivity of 0.0003 arc degrees. If I did it right, that is 0.002 inches (2/1000) across a 30 foot wide deck.
That should be adequate. I suspect distortion from solar heating as the sun angle changes would be readable at that resolution.
I have no criticism of the capability of that instrumentation. sensor.

RE: Miami Pedestrian Bridge, Part XII

Quote (MikeW7)


I've been thinking about this reality and roughly calculated that 3.75" needs to be gained on the long side, as per my mark-up below. This could be gained by various distortions or by movement at the bearing points. The south end has a 1.5" expansion joint to the landing and the north end has play around the hold down through the plastic duct. Needless to say, 900 tons does not need my permission to do what 900 tons does.



I also wondered if there was evidence of contact by the deck at the south end landing. Google Maps says, "Maybe ..."


RE: Miami Pedestrian Bridge, Part XII

Quote (Sym P. le)

roughly calculated that 3.75" needs to be gained on the long side
I am dense tonight, so I am having trouble following you. And the sketch/drawing is quite light on my monitor, but I see a green line (I think) from lower left to upper right. That seems reversed to me -
Question - were the PT anchor rods installed thru the deck and into the pylon at the time of the collapse? Did they shear off?

RE: Miami Pedestrian Bridge, Part XII

Mike showed that the long south end of the span rotating about its base would push the top of 12 north. This has to be factored into the collapse sequence or another explaination provided. Given the 175' of span and 20' of height, I was curious how much distance the sweep would require. I picked three points that seem to be indicated by the videos, the bearing point of the south diaphragm, the upper canopy surface at the 10/11 blister, and the lower canopy surface at the 12/canopy node. I then scaled the two long sides off of the dwg. with the angle between them (easy with GIMP) and calculated the balance. The sum of the two short sides is 3.75" longer than long side.

The hold downs were loosely attached and seem to be sheared. They are visible in various OSHA report photos.

RE: Miami Pedestrian Bridge, Part XII

Pushing the top of 12 North a few inches seems immaterial compared to the bottom of 12 being shoved about 10 feet, if the bridge was to stay together that long, before yanking the bottom of 12 back as the canopy heads for the ground. Obviously not all of 11 or 12 made the reverse trip intact.

RE: Miami Pedestrian Bridge, Part XII

Quote (3DDave)

... a few inches seems immaterial compared to ...

We didn't have a number so I gave it one. Also until something breaks free, it all has to move together and that makes any number significant. It's neccessary to explain how the bottom of 12 ended up hanging four feet off of the pylon.

RE: Miami Pedestrian Bridge, Part XII

I made it a 4-bar linkage, which explains how the bottom of 12 ends up off the pylon. Make the 12-pylon a slider, which is the motion allowed, a pin joint near 9-10, and a pin at 1-deck. The top of 12 cannot move at all until the bottom is already broken; the top is part of a rigid frame.

RE: Miami Pedestrian Bridge, Part XII

Based on what I think I see, from frame to frame in the dashcam video, the first movements seem to show:
12 and the deck to 9 move as a unit;
11 and the canopy to 12 move as a unit; and
10 and the canopy and deck back to 1 move as a unit.
The angle between 11 and 10 appears to me to change.
If this is correct, 11 has to effectively change length and the canopy has to hinge or slide at 12.
I know that the forces on 11,canopy,10 are less than the forces at 12,11,deck by the weight of 11; but somethin's gotta' give at 11,canopy,10

SF Charlie
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RE: Miami Pedestrian Bridge, Part XII

New cut-and-paste mashup. Now in Color!
(Old mashup posted 11 Jul 19 20:46)

EDIT ADD: Not a structural guy! I made this diagram to visualize the spatial relationship of the components based on how they seem to appear in videos. I will adjust the captions based on group suggestions.

Apologies for the tilty north pier...


ADD: At this point both the canopy and deck have hinged and the 11-12-deck joint has been compromised - the order of occurance is not posited.
EDIT: Next 2 pictures: reworded to change description of picture 2 to an "IF" scenario and moved a "pinch point" reference from pic 3 to pic 2.
IF the base of 11 wasn't immediately destroyed and the canopy-11-12 triangle managed to remain intact for a bit, the bottom end of 11 would have slid over the lip of the north deck and been crushed to rubble in the pinch point between 12 and the deck edge.


With the base of 11 gone, either by it's initial failure or after it was crushed in the pinch point, 12 drops off the deck and onto the pier. Member 11 is removed from this and following pictures.


12 drops behind the pier.


At this point the deck hinge has already impacted the highway and the 1-10 truss is starting to flatten out.
The deck hinge pulls the north deck section off the pier.
The canopy hinge drags 12 back onto the north pier.

RE: Miami Pedestrian Bridge, Part XII

Quote (MikeW7)

I assumed the canopy-11-12 triangle remained intact for a bit, at least until the end of 11 slid past the end of the north deck.

I guess this is where I disagree. I don't see 11 sliding past the end of the deck. It just pancakes or disintegrates while the canopy and 12 struggle with the extra load until 12 is yanked downward by the canopy. I see that in both the dashcam video and the traffic cam video.

RE: Miami Pedestrian Bridge, Part XII

Quote (Sym P. le (Mechanical) 22 Jul 19 06:53)

I don't see 11 sliding past the end of the deck.
I seem to remember that the canopy-11-12 triangle was one of the initial theories about the likely failure of the 11-12-deck joint. I reworded the captions so the second picture describes this theory as an IF scenario.

RE: Miami Pedestrian Bridge, Part XII

Quote (MikeW7)

... was one of the initial theories ...

It's quite possible there was an event at the 11/12/deck node at the onset of the collapse but as I stated earlier, everything has to move together so I can't see a wholesale departure. That would just relieve the stress on 11 and 12 and leave them in tact.

I see the 11/12 team breaking out of the deck/diaphragm pocket in steps, giving a loud crack each time, starting when the false work was removed. This in turn allows the deck to sag, members to deflect as stress/strain builds up, various tell cracks to show etc.. What if another event caused the lower PT rod to kink or something like that?

RE: Miami Pedestrian Bridge, Part XII

2
I've always said that when you watch this collapse in real time it all occurs frighteningly fast and trying to work out which piece failed exactly when is not going to be of importance.

It is very clear that the 11/12 connection to the deck was suffering huge cracking and impending failure before they assaulted it with the tendon tightening exercise. The real issue in that node could easily be the presence of those two PE tubes which moved from an early design further away from no 12 to the final design where they are right next to the 12 member plus a load of smaller tubes appear as well. saikee199 in part V of this thread provides more details and the OSHA report photos have more also. If those PE tubes were cast in a packet then even more shear resistance disappears.

So maybe as has been posited in the last few parts - the 11/12 joint failed by degree during the rod tightening, a bit here then a bit more. All the time the stress on the other nodes, especially 9/10, continued to climb. Maybe 9/10 or 10/11 did fail first leading to blow out of 11/12, but it was wholly due to the failing of that 11/12 joint. otherwise why would the deck or the canopy just decide to fail? The evidence has been pointing at 11/12 being the root cause since about 2 weeks after the collapse and everything just keeps coming back to it.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Miami Pedestrian Bridge, Part XII

I find interpreting frames of the dashcam video difficult due to the changing angle between the cam and the north end of the bridge. Given that, I tried to measure the apparent end of 12 to the north of the pier in frame 80. The distance from the apparent end of 12 to the bottom of the north end of the canopy appears to be 16% bigger than the original length of 12. Are we seeing a chunk of 12, 11, and/or deck fall off the pier? It seems to be gone in the next frame.

SF Charlie
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RE: Miami Pedestrian Bridge, Part XII

Quote (SFCharlie (Computer) 22 Jul 19 19:43)

Are we seeing a chunk of 12, 11, and/or deck fall off the pier?

In the chopper photo posted by Tomfh (Structural) 9 Jul 19 22:19 there appears to be a considerable amount of rubble immediately north of the pier, but no large chunks. Can't really identify the large object hidden in the manlift shadow (between the canal wall and the red beam). Anybody know if there are other shots of that area?

In my N View crop 26423 posted 9 Jul 19 21:45 (just above Tom's post) there isn't a splash ring, bubbles, or cement colored swirls in the canal, so if the image was taken within 15-30 seconds of the collapse it doesn't appear a large chunk went in the canal. Even if it did, by comparing 26422 and 26423 you can see that any evidence on the water surface would have been disrupted as it was swept east (left) by the mini-tsunami caused by the excavator.

RE: Miami Pedestrian Bridge, Part XII

Quote (MikeW7 (Electrical)22 Jul 19 03:24 New cut-and-paste mashup. Now in Color!)

Great work.
It helps me just to know not everyone uses CAD or PowerPoint today.
The cut ups capture very well the collapse. Your second image, with member 11 still shown, may predict what happened to the bottom of 11 - it is trapped on the corner of the deck for that brief period and was already split when it becomes loaded from its lower side. Half the canopy from 10/11 to the end plus 12 weigh about 35 kips. Add the pull down at 10/11 and that could significantly damage member 11, perhaps causing it to shatter in its lower section.
I did a spreadsheet of dimensions from the bearing at 1 to 10/11(canopy) , then from 10/11 to top of 12, and one of deck dimensions with segments 1 to 9/10 and from 9/10 to the end at the pylon. It calculates the total length as each point (10/11 and 9/10) drop in increments of one foot. It could supplement your cutouts.



RE: Miami Pedestrian Bridge, Part XII

Quote (Vance Wiley (Structural) 22 Jul 19 20:46)

It helps me just to know not everyone uses CAD or PowerPoint today.
:
I did a spreadsheet of dimensions

I started re-learning a CAD program I was once very familiar with, and also working out a table of dimensions, but gave up because other things in my life have a higher priority. CAD (cardboard-assisted design) seems to be very popular with the YouTube kids nowadays (see Project Binky and Acorns to Arabella for example) so it was kind of an obvious choice for me.

I remember when I was younger I used to work with all sorts of puzzles and building projects (even bound into magazines) that relied on printed cardboard, scissors, tape, and "insert tab A into slot B." Does that sort of stuff exist anymore, or has everything gone "virtual" with no more "hands on"?

RE: Miami Pedestrian Bridge, Part XII

Spartan5 (Civil/Environmental) 23 Jul 19 13:10

Cool! Free toys AND a slick marketing ploy to sell Canon printer ink by the gallon.

What I seem to remember are puzzles printed on thick paper, bound into a magazine. I also remember buying the whole series of The Incredible Machines games back in the DOS days.

RE: Miami Pedestrian Bridge, Part XII

When did 12 depart its perch on the deck?
I would like to see comments by all as to amount of damage visible to the north section of the canopy at member 12 and damage to the remaining upper section of member 12. Perhaps good zoomed images - I cannot find severe cracking of the canopy or member 12 at the joint with the canopy.
The canopy is a pretty stiff element. I am guessing maybe 1.5 to 2.5 times a flat section. If anyone has AutoCad and can draw it I think AC can return the section properties - I, S, A, CG, etc. Did a bit of math on a 16 foot segment of a 24.5 ft radius 12” thick, ignoring the oblique orientation of the 1' X 1' segments I think the NA is about 24.153 ft above the Layout Pt, or 0.153 above the bottom of the canopy at the center. Rough “I” is 30616 in^4. Add something for the oblique issue and use maybe 32000 in^4.
The column is easy. 21X34^3/12=68782 in^4. We can use those values later.

Some discussion now. If everything remains connected successfully, members 11 and 12 and the canopy form a stable and well braced system, and 11 protects the joint of 12 to the canopy. But what if 11 loses its grip at either end? Or fails somewhere along its length?
Lets explore conditions when 10/11 drops 3 feet and take member 11 out of the picture. How much stress or deformation can the canopy at 12 and 12 at the top withstand?
When node 10/11 drops 3 feet there are three possibilities with regard to joint canopy/12.
1. No damage visible - the joint remained basically at 90 degrees, and its base departed the top of the deck and moved 2 feet north. 16'/24' X 3' = 2 feet.
2. Complete failure of joint canopy/12, - bottom of 12 could have remained in position on deck. I do not see this as the result when looking at joint canopy/12.
3. Something in between - then some damage would be visible. How much?



To find the possible load conditions at the joint of 12 to the canopy, the canopy is isolated at 10/11 and member 12 is pinned at the base at the top of the deck. The pin simplifies the analysis and allows rotation but prevents lateral movement to the north, simulating a condition in which 12 remained connected to the deck. The canopy is isolated at 10/11 and allowed to move vertically without moving to the south. This is not an acual condition but it allows the approximation of moment capacities and stresses in the relatively undamaged joint of the column 12 to the canopy.
So lets load this 2 member bent with 100 kips at the free end at 10/11. The deflection is about: Delta=PL^2(L+C)/(3EI). L+C is about 24+5 (5 being the equivalent length of canopy section to be same stiffness as the 16 foot column ).
Defl=100K*24^2*29*1728/(3*6x10^6*32000) = 5.01 inches. Pretty stiff.
The moment in the deck and column top is 100K*24'=2400 ft-kips.
Canopy stress is 2400*12*(10.156)/32000=8.14 ksi. That is about failure, but we have not included reinforcing.
Column stress is 2400 'K*12*34”/2/68782=7.11 ksi. The column could survive this moment.
But that is at a deflection of only 5 inches.
If joint 10/11 dropped 3 feet, the stresses on the canopy and column would be much greater. Like 35”/5”=7 times greater. Or in the order of 56 ksi in the canopy and 49 ksi in the column. That is failure levels, and did not happen. We can tell that by observing the canopy and column area of the collapsed structure.
So the column could not have been on top of the deck when joint 10/11 dropped 3 feet. From the dimension results, reading 3 feet drop in joint 9/10, the deck has moved south only 0.15 feet or about 2 inches.
These results are linear, assuming everything remains elastic. So even at the point when 10/11 dropped one foot, the results are 2.4 times the 5” result and stresses, making failure of the canopy/12 joint a certainty - if 12 remained atop the deck.

I suggest 11 and 12 departed the deck as the initial event of this collapse. It must have decoupled from the deck before joint 10/11 dropped a foot or so.
Note - I hope I got the numbers right - I welcome peer review.
EDIT ADD: One more possibility - if member 11 failed by bursting or because of the splits captured in the photos, thereby allowing joint 10/11 to drop 5", the above calc shows the canopy and member 12 could have delivered about 150 kips northbound to the 11/12/deck joint. That additional 150 kips may have triggered or hastened the blow out and collapse. And the retensioning of the PT rods in 11 may have further damaged 11 to the point of failure.

RE: Miami Pedestrian Bridge, Part XII

I've tried to view this collapse incrementally, i.e. three foot increments are too large, even when you back it up to one foot, it is too large. Nonetheless, you moved in the right direction. I was thinking more along the lines of fractions of inches to envision the implications of any one element having some freedom to move. This is how I came to the awakening that 11/12 already had separation from the slab and was merely leaning against the rebar that was running the width of the diaphragm through 12, among other.

Your numbers however support my earlier theory that as the span sagged, the load on 11 could grow until it was overwhelmed. Also, as 11 pushes 12 northward, it is necessary that 9/10/deck sags and so doing pulls the deck toward the centre of the span, opposite to the movement of 11/12 and also dropping out from beneath 11, tearing it longitudinally. So, in short, 12 didn't have to leave its perch completely, it just had to shift a little bit more w.r.t. the slab.

The damage to 12/canopy is best seen in these two images. It's hard to tell whether the damage is from the initial collapse or from the collision with the ground. In the sequence, you can see the vertical rebar leaning south but six of the nine rebar are only associated with the canopy, i.e., if the joint is crimped, the rebar won't align with the column. From my previous gif's, I tend to think that the joint initially crimped.




Frame 76 gives the first indication of the canopy sagging and the vertical rebar is already leaning south. It's hard to tell if 11 starts to pancake because the truck hasn't stopped yet. There is no significant lateral or vertical movement of 12.

By Frame 77, 11 is already pancaking and in Frame 78, 12 drops and moves independently of 11/slab. Thus, my position is that 12 left its perch in 78, though there was a likely event at the 11/12/deck/diaphragm connection at Frame 75. As I indicated earlier, careful inspection of 75 indicates a puff arising from the joint that was not visible in the earlier frame. That really could be the precipitating event.




The "event" could be the rebar finally letting go. But again, I don't see any giant step movements starting the collapse. It seems likely to me that an impulse from a sudden jarring shattered the node and 11 while 12 was yanked down by the canopy. I don't know how else one could conjure up so much clean rebar.




RE: Miami Pedestrian Bridge, Part XII

I see FIU's 2019 International Accelerated Bridge Construction Conference will be in Miami Dec 11-13. https://abc-utc.fiu.edu/conference/

RE: Miami Pedestrian Bridge, Part XII

epoxybot (Structural) 24 Jul 19 20:53 - Now I understand why the "falling man" had his safety harness clipped to the crane hook....

Have a gander at the "safety line" these guys were using:
  • A nylon sling cinched around rebar through its own end loop
  • A light-weight pulley clipped to the other sling loop
  • 13mm climbing rope pulled tight through the pulley and tied off with what appears to be a simple slip-knot. (This was at the north end, so I assume the tightening occured there.)
Crop of expoxybot's image, rotated 90 degrees:

RE: Miami Pedestrian Bridge, Part XII

Quote (epoxybot)

How much is it 12 rebar & how much is 11?

Roughly speaking, all of the rebar you highlighted belongs to 12.

RE: Miami Pedestrian Bridge, Part XII

Sym P. le
Thank you for the clear close-up photos. The first one shows damage I had not seen before. There it appears the concrete of the lower deck surface is failed, indicating rotation between the canopy and column. The second photo shows cracking in the raised diaphragm at the north end of the canopy, suggesting the beginning of a punch thru failure. I can no longer support the concept that the joint is undamaged and it likely failed to hold the original angle between the canopy and column. After viewing the photos, I do not think the canopy/column joint can resist the 2400 ft-kips presented in my post of yesterday.

as the span sagged, the load on 11 could grow until it was overwhelmed

I am not sure this fits, unless 11 fails internally and effectively shortens. Because the support of the north end of this trussed structure depends on member 11 successfully supporting its loads and holding node 10/11 in its original position, there should be no "sag" until 11 had failed to support its loads. If 11 failed to support the loads under static conditions, it seems unlikely that member 11 would support greater loads than those that caused its failure. The same can be said of member 10, but to a lesser extent, and the failure was not in 10.

If the cracking of the deck photographed at the base of 11 and 12 on March 13 represent a movement of one inch northward, the corresponding drop in node 10/11 would be 1.5 inches. The change in geometry would not increase the loads on member 11 significantly (approximately 1%). From the calc presented yesterday, and which assumed the joint canopy/12 remained intact, the 1.5 inch drop would be 1.5"/5" X 100kips =30 kips load on the bent comprising the canopy and 12. That is possibly within the capacity of the joint, and could have introduced 45 kips force to the north at the deck joint. That could have been the added load that triggered the collapse.
I agree that there is much to be revealed at very small distortions. I jumped to 3 feet because that would have moved the bottom of 12 two feet north and basically off the deck, introducing a different condition. That is assuming joint canopy/12 remained as cast, of course.
Thank you for the photos and discussion.

RE: Miami Pedestrian Bridge, Part XII

Quote (epoxybot (Structural)24 Jul 19 20:53 This is the largest image)

Thank you.

RE: Miami Pedestrian Bridge, Part XII

I'll post these for consideration. They are images of the failed 12 rotated with a structural dwg. imposed over it. The scale works well as the discoloration of the concrete on the tab at the base of 12 indicates the deck height. The height from T.O.S. to underside of canopy at the column is approx. 15'. The formwork imperfection seen just above the number 12 would be at eight feet above the slab.

Ignoring the mess at the base is not for the feint of heart but these images give an idea for the location of significant diagonal cracks that occurred in 12.


RE: Miami Pedestrian Bridge, Part XII

While looking for images taken immediately after the collapse I noted that only 3 people are responsible for most of them:
  • All chopper pictures were by Pedro Portal
  • Ground images were taken by Joe Skipper and Wilfredo Lee
If you're still looking for good pictures, search for those names, images, and restrict the search dates to 3/15/18 to 3/31/18.

I found this shot by Portal that might be new

ADD: In the Portal image, the two short beams at base of the north pier have been there since at least March 1st - they are visible at the very beginning of the N. View timelapse dated Mar 1-19.

RE: Miami Pedestrian Bridge, Part XII

Quote (Sym P. le)


Wonderful graphics. Outstanding. I have long thought the use of real images overlaid with drawings created the most informative representations.
Now we can see how the damaged member 12 fits into the picture.
I wonder if NTSB saved the piece you show? I do not recall seeing any pieces in the rubble big enough to reconstruct the part of 11 and 12 just above the deck.
The photo link just posted by MikeW7 to a pic by PORTAL has good detail, but mostly small rubble visible at the pylon.
Thanks for the graphics.

RE: Miami Pedestrian Bridge, Part XII

Vance, thanks for your response, please excuse me for not taking enough time to consider all of your approach. It certainly made sense and ensured that we consider a broad range of scenarios.

The model that I suggest is a dire predicament for 11. To be sure, 12 is firmly committed to the diaphragm below the drain pipe but this is no relief for the structure as a whole as the deck/upper diaphragm are not picking up the longitudinal forces from 11 as intended. The key to my failure model, is that the distance between 12 and 10 at the deck level is free to increase with no functional restraint and 11 does not need to shorten as the initiating event of the catastrophe (as I initially thought). The more the structure sags, the worse the incident angle becomes for 11 while also enduring induced bending moments, longitudinal tearing at the base, and PT rod tensioning. 11/12 just need to free themselves of the last vestige of restraint in the pocket and its game over.

At the risk of being an arrogant fool, I'll repost two images that best explain my model once again. The chevrons indicate surfaces of ineffective load transfer, essentially creating a pocket which allowed 11/12 to move independently of the deck/upper diaphragm. I struggle to concede that this failure can be referred to as shear failure or punch out because the unintended reality does not rise to that level of sophistication. Clearly there was a straw that broke the camels back, but all I can do is speculate. This structure was far more robust than I thought but it wasn't enough.








RE: Miami Pedestrian Bridge, Part XII

[quote Sym P. le][/quote]
My thought is "don't ignore the obvious" - the members 11 and 12 were causing serious cracking in the deck, indicating an imminent failure. So I think that is where it all started - and I think that agrees with you.
From there to the ground it is a progressive collapse with a pretty clear path, and luckily we have real time video and some great enhancements by members here. We do not have a black box, but we have dash cams.
As diagonal 11 lost its ability to support node 10/11 adequately, node 10/11 began to move downward, as you state. With 11 compromised, the load path to the support pylon relied more and more on the canopy from 10/11 north to member 12, and on the deck, from node 9/10 north to the pylon. I suspect the geometry of the truss was basically intact from the south end to nodes 9/10, member 10, and node 10/11 until the bending failure of the canopy and deck (they folded), with some allowance here for the effect of the moment induced by 11 at node 10/11.
The geometry of the 11-12-canopy element requires node 10/11 to drop 2 to 3 feet before the base of 11 and 12 can slip over the end of the deck (assuming 11 is sufficiently intact and can maintain the dimensional relationships. Node 9/10 drops about the same distance as 10/11, and the spreadsheet posted earlier indicates dropping node 9/10 by 3 feet pulls the north end of the deck maybe 2 inches.
IF 11 and 12 remained connected while being pushed over the end of the deck, their weights and that of the canopy and the canopy portion of the truss weight from the shifting load path were supported by the underside on 11 and the corner of the deck (or were sliding thru the "yoke" slot). Another 5 to 7 feet of fall and the deck slips off the pylon, dropping 11 across the corner of the pylon, and destroying the lower part of 11 and 12. Member 11 almost lay on the deck surface as node 10/11 dropped and as the deck tilted down on its way off the pylon. See the "paste-ups in color" previously posted.
That's my story and I'm sticking to it - - - for now.
Of course when 9/10 hit the roadway, more bays of the truss pancaked.

RE: Miami Pedestrian Bridge, Part XII

Can someone please explain this “yoke” concept? What is the yoke?

RE: Miami Pedestrian Bridge, Part XII

The "yoke" concept is related to the same principles as involved in the kitchen.
If an egg is cracked and dropped to far the "yoke" splits wide open upon hitting the pan.
Hence the egg is ruined. Similar and applicable, in laymen's terms.
Sorry. Just had to..................

RE: Miami Pedestrian Bridge, Part XII

Quote (Tomfh (Structural)25 Jul 19 22:13 Can someone please explain this “yoke” concept? What is the yoke?)

As I recall, it was first coined by Sym P. le (Mechanical)28 Jun 19 07:07 and seemed rather descriptive to me. It basically refers to the north end of the deck as seen in the NTSB yard with a slot missing where 11 and 12 once were. I see it as referring to the slot left behind.deck on each side of the slot.
Origins probably relate to ox drawn carts.

RE: Miami Pedestrian Bridge, Part XII

Mr Vance I have to disagree with your statement. It must be flawless.
Saying that " With 11 compromised, the load path to the support pylon relied more and more on the canopy from 10/11 north to member 12, and on the deck, from node 9/10 north to the pylon."

I think you are stating the canopy at 12 is holding up the bridge.
I beg to differ.
Why? Well because it doesn't.

(Alright, I'll give you this............Probably 90% of the bridge's weight should be held up by 11 and 2.
Some per cent goes to deck on piers and 1 and 12 supporting canopy at ends! only. I am allotting 10% for ends only. Actual numbers....IDK......?
BUT 12 and 1 should be considered as a form of 2 horizontal stabilties. Now getting this right after moving and settling would be an engineers nightmare. This is a part of the problem.)

And if this was the case. Isn't South end...1 to 2 diagonal longer?
Shouldn't it fail first? Meaning if the South canopy is being held up by 1 being the main support.
Its length should produce a quicker failure?
But 2's base is most secure and it's connection is what holds up the canopy.
Consider...why would you want to hold up a 175 foot long 975 ton bridge at the ends.....
when we could cheat and hold it up more inwards....30 feet or so. On either end.
175 now becomes roughly 125 feet.
Would it not be a better bet to support such bridge at 125 feet?
BUT...........your diagonals better not fail.
South end did not fail. 2 was more capable of this. 11 was not.
Obviously other considerations are in order. Ones that we know thru pictures, drawings and the obvious.

11 base was lacking. It failed first. Compare and contrast the two ends. Why not South end first?
Simple, look at the failed side. What was different?

A: 11 undersized, inadequate footing strength (MAIN CONCEPT see note 1:), lack of symmetry (also MAIN CONCEPT), cold joint. Just bad design. Get over it.

Note 1: There is just no way the transfer of forces involved in 11 could EVER be resolved at the deck and 12. 12 should not even be considered as a block.

Picture this... deck longitudinal cables running around a steel bar, 6 inches" ? in diameter, running width of the deck. A steel box possible able to slightly pivot would then capture 11. 11 would be inserted into it.
This would certainly be a more ideal way to capture these longitudinal forces.

Basically you are making a huge door hinge with a box welded to it for 11.

You can't do this as designed. Why not just throw the egg in the air and see how your yoke responds.

Over and over same same same outcome.....splat..........see A:

There's is probably at least 1.5 million pounds of force on 11 base. Play if you want....but.....
This park is closed for renovations.

Reword it. Try again. When I can't see errors in it. And your story is beyond critique, I'll let you know.
Put it out there, one end to the other. Describe it all. I'll pick it apart for you.
If you want, but then again......who am I?
Any questions? just press submit.

RE: Miami Pedestrian Bridge, Part XII

Btw Sym p le your drawing is good and concepts are good. Why not trust yourself?
Vance is good also.
Personally I find the canopy as the better part of the bridge.
The curves, wings and rigidity of it are interesting.
By far of the models I made... canopy.
Truss connections at deck lacking.
Torsional aspects of deck to truss during dynamic loading questionable.
Cat 5 resistance....futile.

RE: Miami Pedestrian Bridge, Part XII

How do you think this bridge would have performed under earthquake? Failed node aside, would those web members have been ok resisting the lateral load from the canopy? Seems like a decent lump of concrete sitting atop some sticks.

RE: Miami Pedestrian Bridge, Part XII

Quote (RAB678 (Mechanical)26 Jul 19 00:44 Mr Vance I have to disagree)

I think you are stating the canopy at 12 is holding up the bridge.
Only for the brief instant before gravity overtook inertia and it began its downward travel. But there were two load paths available after 11 failed. The Canopy on column 12 and the Deck on the pylon were paths for load because they continued to the support. There was no way they had the capacity to support the structure, so that is now 3 failures - first 11, then canopy, then deck. If they had enough capacity tp support the structure after 11 failed, they would have provided redundancy. And I recall your post saying the deck hung from the canopy.

Isn't South end...1 to 2 diagonal longer?Shouldn't it fail first?
I think you know this, but every comic needs a straight man.
The flatter angle provides a greater length to transfer shear to deck. Member 2 is much larger. The south Diaphragm is over 3 feet thick, while the north diaphragm is 2 feet.

Would it not be a better bet to support such bridge at 125 feet?
Sure - right in the center of the second lane of traffic. Nice bridge. Did you study at Texas Tech? On another forum they have a professor touting a dead German named -Muller-Breslau- from about 1850 who discovered putting a column in the center of a span helped. So he espoused that as a cure. I was cut off after I asked if one column helps, it then creates 2 spans, requiring 2 more columns, and so forth until it is a wall and not a bridge. https://olivermcgee.org/fiu-bridge-collapse-why-mu...

Note 1: There is just no way the transfer of forces involved in 11 could EVER be resolved at the deck and 12.
It would probably still be in the air if 11, 12, and the north diaphragm were as robust as their counterparts at the south end and if the reinforcing was adequate and the construction joint properly prepared. Formed sockets and strut/tie design have been discussed. Check posts by FortyYearsExperience.

Straight man wants a raise. Close the curtains.


RE: Miami Pedestrian Bridge, Part XII

Quote (Vance)

It would probably still be in the air if 11

Yes if the diaphragm was 3 feet thick instead of 2 feet thick it probably wouldn't have punched out. Much bigger failure surface, which would also have intersected PT cables.

RE: Miami Pedestrian Bridge, Part XII

Vance - the irony is that the FIU bridge did have a support in the middle of the span. That's where it failed. I hope those students develop a healthy skepticism for what their instructors tell them.

RE: Miami Pedestrian Bridge, Part XII

Quote (3DDave (Aerospace)26 Jul 19 04:41 Vance - the irony is that the FIU bridge did have a support in the middle of the span. That's where it failed. I hope those students develop a healthy skepticism for what their instructors tell them.)

Before I was blocked I reminded them that the Golden Gate bridge did not have a column in its center. I was told the suspension cables provided support for the center. I guess they did not comprehend that the truss was providing center support in this case.


RE: Miami Pedestrian Bridge, Part XII

Quote (Tomfh (Structural)26 Jul 19 04:29 Quote (Vance) It would probably still be in the air if 11 Yes if the diaphragm was 3 feet thick instead of 2 feet thick it probably wouldn't have punched out. Much bigger failure surface, which would also have intersected PT cables.)

Thank you.

RE: Miami Pedestrian Bridge, Part XII

Quote (Tomfh (Structural)26 Jul 19 04:06 How do you think this bridge would have performed under earthquake?)

Forces in the longitudinal direction should not be much of a problem. Not so for forces/motions perpendicular to the length. The canopy is a single mass structure atop columns which cantilever from the deck. The deck is not a particularly stiff element in torsion about the longitudinal axis. Serious concerns about the natural frequency of the system.
The deck should be adequate as a flat beam to span the 174 feet.
What I do not understand, even in non-EQ areas, is failing to form a "cradle" at the south end and north end, to prevent any perpendicular movements at those supports. It would have been so simple to do, without defeating the freedom the bearing pads provided.
I would not recommend this structure in Seattle.

This is from 4 years ago - This you gotta watch. From drilling the ocean floor they have learned that it has happened 40 times over the last 10,000 or so years. That is every 240 years on average. It has been about 320 years since the last one. Listen for the size of the tsunami the last time.

https://video.foxnews.com/v/4356513070001/?fbclid=...

RE: Miami Pedestrian Bridge, Part XII

Tomfh (Structural)26 Jul 19 04:06
"How do you think this bridge would have performed under earthquake?"
Having been through an earthquake in '89, NO WAY. With cold joints at the top and bottom of the web? Earthquakes just keep accelerating back and forth for what seems an eternity, this increases the gaps every cycle. The highest energy in a large quake is at about 1/2 to 1 cycle per second at about 1g. Imagine tipping it on its side. They didn't want it to tip more than 1/2 degree.

SF Charlie
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RE: Miami Pedestrian Bridge, Part XII

Quote (SFCharlie (Computer) increases the gaps every cycle)

So correct. If the seismic motions fit with the natural frequency of the structure, creating resonance, it is all over. This structure has no dampening available to absorb the energy - except for the concrete being ground up in failure zones.

RE: Miami Pedestrian Bridge, Part XII

Quote (Vance Wiley (Structural) 26 Jul 19 08:35)

That is every 240 years on average.

Lots of hyperbole in the video - here's a list of many, many more videos. The RANGE of years figured in the 240 year average is something like 200-900 years, so the futurecast for an earthquake is "more likely" not ASAP. As more and more people are educated about the risks, the PNW has been legislating code upgrades in the area expected to be worst hit by the earthquake, and designating tsunami zones which include evacuation routes and restrictions on certain type of new structures (schools, fire houses, etc.) In many areas none of this will matter because the combination of earthquake damage and soil liquefaction will render all escape routes useless before the tsunami, a miles-wide wedge of displaced water, is pushed onshore and overflows everything left standing.

I grew up in an area that's also "overdue" for a major earthquake that will cause massive destruction and possibly a much larger disruption to the transportation network, the New Madrid Seismic Zone that's centered near Memphis TN. The area surrounding the Mississippi River rests on thousands of feet of sediment deposits, and a major earthquake will liquify this sediment and possibly destroy every structure near the MS River from St. Louis to SE Arkansas, including rail lines and highways. There isn't a lot of education or prep work being done because when a large quake happens, there will be no escape... I remember a discussion of some sort, possibly in high school, that basically said we're letting you know about this so you'll understand what's happening as you die, or why you'll wish you were dead if you somehow survive.

I sometimes wonder how much different this country would look like if early settlers (and especially insurers) were aware of all dangers zones: active seismic faults, Tornado Alley, areas prone to major flooding, forest fires, hurricanes, and so on.



RE: Miami Pedestrian Bridge, Part XII

Quote (RAB678 (Mechanical)27 Jul 19 03:05 Vance Wiley (Structural)26 Jul 19 04:25 in your first paragraph why did you add...And I recall your post saying the deck hung from the canopy.?)

I guess my reading comprehension and interpretation skills need a little work.

I thought you said
Here is the problem.................
When I said the deck hangs from the canopy.....it does.

That wasn't you? Must have been that silver tongued devil on your shoulder.

Both are ABOUT 25 feet No - you just asked about 2, saying it is longer than 11 so why didn't it fail. So the canopy over 2 has to be longer too. Simple geometry.

when we could cheat and hold it up more inwards....30 feet or so. On either end. Read posts re:FDOT

para 3 not a column in the road ja. Perhaps you would decode this term "ja"?

I just said some of your comments were acceptable. Oh - it's YOUR bridge badge that arrived early?

Do your homework before coming to class.


RE: Miami Pedestrian Bridge, Part XII

Vance - RAB678 Please stop sniping at each other. That's not what we're here for. Yes, some diagonals (10,3,8,5, I believe) hang from the top chord of a truss. Even a computer nerd knows that. Let's just get on with what happened to the bridge.
Added (and don't 9,4,7,6, rest on the deck and support the canopy?)

RE: Miami Pedestrian Bridge, Part XII

Apologies to the Forum.

RE: Miami Pedestrian Bridge, Part XII

Apologies yes indeed are in order.
Sorry, bridge and its discussions sometimes can be aggravating.
Please accept my apology.

I shall place myself in timeout. Listen but do not speak.

"ja" is a term from Thailand. It roughly translates to....fight but fight with good reason.

RE: Miami Pedestrian Bridge, Part XII

Quote:

Let's just get on with what happened to the bridge.

Ya, it fell down. Cause was a design issue where 11/12 met the deck. Most everything about the details of exactly what happened is speculation.

RE: Miami Pedestrian Bridge, Part XII

Quote (LionelHutz)

Ya, it fell down. Cause was a design issue where 11/12 met the deck. Most everything about the details of exactly what happened is speculation.

I think there is a lot more to learn from this collapse. There were certainly multiple design flaws that should have never have happened. However there are many more ethical and procedural questions and changes that need to be made to the profession.

I would like to know the fee structure and how hours were allocated. Other collapses have been linked to too few hours being spent by senior engineers checking the work. I can't help but suspect that a junior engineer took the results from the computer analysis to design the members but then totally forgot to check the transfer of the loads at the joints. The connection design is the hard part that takes many hours. The senior engineers would then assume the joints were designed correctly. I don't know how this bridge passed the independent review process. Again, what were the hours allocated for this procedure? Doing an independent review of the connection design would have taken many more hours than a review of the member sizing. Was that why the connections were ignored? I can only assumed it was ignored since I don't see how else this could have been missed.

There are also ethical components to this disaster and all those issues have to be reviewed as well.

We don't yet have enough information to fully explore the ethical and procedural issues.

I would also suggest that we use a limit states stress design approach rather than a limit states force design method. This is my own opinion but understanding stress more easily and quickly identifies problem areas. It also gives the designer a far greater feel for the behaviour of structures.

Mentoring of young engineers is also lacking and I also suspect this is due to fee structures. We also need to encourage more people into the profession so that engineers do not have to do so much overtime. More engineers means that there will be more hours for reviewing the work that does get done (of course fees also have to go up to do all this).

RE: Miami Pedestrian Bridge, Part XII

Quote (SFCharlie)

May I post a dissertation of truss action and design? It is about a page and a half, and contains no sniping. It does not address directly the collapse and details of that, but may clarify some concepts about trusses.
If it is considered off course, I will not post it.

RE: Miami Pedestrian Bridge, Part XII

Vance Of course you can, but you don't need my permission. In fact, I love the long posts where experienced engineers explain their work...

SF Charlie
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RE: Miami Pedestrian Bridge, Part XII

Quote (Vance Wiley (Structural) 28 Jul 19 18:54)

May I post a dissertation of truss action and design? It is about a page and a half...
Just upload it as a PDF file and no one should have any issues with it. Even if it's hand-written, you can scan it with a "save-to-pdf" option.

RE: Miami Pedestrian Bridge, Part XII

Vance Just post it. That way we don't hav'ta download it, open it in AcrobatReader, copy 'n paste it here to comment. Sorry Mike.

SF Charlie
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RE: Miami Pedestrian Bridge, Part XII

OK - here goes.

The FIU Pedestrian Bridge is basically a concrete truss. It has the added complexity of being cast monolithically, albeit there were three basic phases to that casting. The deck was cast, then the diagonals, then the canopy. But the construction joints were intended to create a monolithic state. The diagonals were cast in one phase, and could likely be considered monolithic without conditions, provided they were cast quickly enough to prevent cold joints.

The “added complexity” of being monolithic creates the possibility of bending moments being induced in members by rotation of the joints. So while this structure is not a pure truss, I suspect the designers used a truss analysis to determine the axial forces in the members, and then checked or analyzed the effects of joint rigidity on the diagonal members. And I would concur with that procedure. FEA analysis may make the distinction a moot point and have included everything in one analysis. I think it would be appropriate to not reduce axial forces in members due to continuity, and the simple truss analysis should be used to determine primary forces in the members.

If it can be considered as a truss, here is a definition for that from Wikipedia:

“A truss is an assembly of beams or other elements that creates a rigid structure.[1] In engineering, a truss is a structure that "consists of two-force members only, where the members are organized so that the assemblage as a whole behaves as a single object".[2] A "two-force member" is a structural component where force is applied to only two points. Although this rigorous definition allows the members to have any shape connected in any stable configuration, trusses typically comprise five or more triangular units constructed with straight members whose ends are connected at joints referred to as nodes.
In this typical context, external forces and reactions to those forces are considered to act only at the nodes and result in forces in the members that are either tensile or compressive. For straight members, moments (torques) are explicitly excluded because, and only because, all the joints in a truss are treated as revolutes, as is necessary for the links to be two-force members.”
(Emphasis mine).

One dictate of “truss” design is to provide concentric loads at a joint. This eliminates eccentricity and minimizes (to zero if a true pin) bending and shears in the members. This is accomplished in the layout and dimensioning by passing the neutral axis (or line of action, or centroid) of all members thru the center of the joint. A primary benefit of this concept in a truss of the configuration at FIU is that the vertical components of web members are directly passed from tension in one member to compression in the adjacent member, without passing through another member. The PT rods and anchor plates pass thru to above the canopy and transfer their loads to the blister and canopy as a means of anchorage, and could cause rotation forces which must be supported by the canopy and blister. The downward vertical component of the PT force is considered to be resisted by the compression component of the other diagonal, applied at the center of the joint. The behavior of the joints with lapping PT rods anchored beyond the extremity of the compression member causes me a bit of concern. The compression forces under the PT anchor plates are intersected by the compression in the other member at different angles (depending on the joint). I am unsure about how well this creates an ideal truss joint.

From observing the detailing of this structure, the principal axis or line of action of the web diagonals in this structure seem to comply with this concept. One condition which may not comply with concentricity is the connection of the fake pipe “stays” which align with a diagonal but deliver a vertical load quite eccentric to some joints, but these loads were not in place at the time of the collapse. Vertical loads induced by the weight of the pipes would have been distributed to the joint thru moments in the blister and canopy.

In a horizontal parallel chord truss with joint concentricity, all vertical components of loads are resisted by the diagonals, Loads from above (canopy weight, pipe stays, live load) are considered applied directly over the joint. Tension in a diagonal passes thru the “work point” of the joint, with its vertical component added to the roof load from above and becomes the vertical component of compression in the adjacent diagonal at that joint. The horizontal components are transferred to the top chord (canopy in this case) as compression (increasing the axial force in the canopy or decreasing that force, depending on geometry and force vectors). With a horizontal top chord having only compression, there is no vertical component of that force in the chord which can provide vertical support to the truss. The top chord must carry its weight and tributary loads and deliver those loads to the joint while resisting the flange force in this structure.

With that said, technically, in the case of a simple truss, no diagonals “hang” from the top chord (canopy). They receive their vertical support from the joint. In this case, the canopy is above the diagonals, and while they may appear to “hang”, I do not think the designers intended that to be the case. The vertical component of member 11 would likely have blown thru the 12” thick canopy far more easily than it blew out thru the deck and diaphragm but it did not because member 10 restrained the joint 10/11, intercepting the vertical component of member 11 below the canopy.

It is not impossible to connect a diagonal of either tension or compression some distance from the center of the joint (the lines of action might intersect above the canopy, in such a case). Doing so would apply loads of several hundred kips in this case (lets think compression) to some point away from the line of action of the vertical component of the tension member, and induce serious shears and moments in the roof canopy. That can be dealt with in the design but would require special design and detailing of the canopy in that area. Thankfully the joints appear concentric in this structure.

Every piece of this structure is necessary for the success of this structure. If one element fails, it collapses. In that sense, the success of this structure “hangs” on the proper performance of every component. The canopy is one of the elements which must not fail.

As Ben Franklin said, “We must all hang together, ....“.

Comments expected.
Thank you.

RE: Miami Pedestrian Bridge, Part XII

Does anyone have any thoughts or ideas as to why this rebar was stubbed short of column 12 (presumably column 1 as well)? Does the rest of slab rebar stop short of the diaphragm as well? What does 3 x 15 refer to in the end view dwg.? I'm not a structural type so I'm just asking.

edit: end view markup edited to be consistent with rebar indicated on OSHA figures.






RE: Miami Pedestrian Bridge, Part XII

Quote (Sym P. le (Mechanical)29 Jul 19 05:45 Does anyone have any thoughts or ideas as to why this rebar was stubbed short of column 12 (presumably column 1 as well)? Does the rest of slab rebar stop short of the diaphragm as well? What does 3 x 15 refer to in the end view dwg.?)

No idea of why the short rebar but maybe I can help on the 3 X 15 bars.
The bars are available in 60 foot lengths. The "3 x " indicates 3 sets required to make the deck length of 174 feet. Top & bottom left & right is 4 locations of 15 bars each, making 60 bars X 3 sets to make the length = 180 bars. The single bar top and bottom each side at center = 4 bars X 3 sets is another 12 bars required. 180 plus 12 = 192 bars as noted in the table. Extra lengths are used in laps. I have not decoded the "B", "C" dimensions - they usually are leg lengths of bent bars so there is probably a code somewhere designating which legs are noted.
I do not see a note or a reason for holding any of this reinforcing short of the end - 3" clear of the formed end, of course.

RE: Miami Pedestrian Bridge, Part XII

Quote:

I think there is a lot more to learn from this collapse. There were certainly multiple design flaws that should have never have happened. However there are many more ethical and procedural questions and changes that need to be made to the profession.


Sure, but not much of that is learned by the speculating here.

RE: Miami Pedestrian Bridge, Part XII

FDOT STANDARD BAR BENDING DETAILS.
https://fdotwww.blob.core.windows.net/sitefinity/d...
Non standard bends are shown on FIGG drawings.
The FDOT details show leg designations, etc.

EDIT ADD:
Bars 4501 were intended to be TYPE 2 =straight with laps of 1'-9" and total length of 175'- 1". That is 1'- 1" longer than the deck.
That means with maybe starting layout at 8" from south end (larger diaphragm) the bars could extend 1'-9" at the north end for a lap to the back span.
The bars 4501 near the center of the deck may have been held back to reduce congestion in the base area if member 12.

RE: Miami Pedestrian Bridge, Part XII

This discussion has been going on for a long time, and much of it is about the detailing of the "truss" to take the imposed forces. But this failure was due to neither the analysis or implementation, but rather due to conceptual design. The initial cracking occurred while the it was still on the ground. The deck shortened due to shrinkage and applied compression, and that shortening was resisted by the inclined webs. Cracking resulted. When gravity loading then opened those cracks, it collapsed. The concept was doomed.

RE: Miami Pedestrian Bridge, Part XII

Thanks Vance, I was looking for that. Regarding "held back", is this the type of detail that would generate correspondence for clarification or would the installer just wing it? Or for that matter are there inspection notices?

Hokie66, the deck shortening would be assisted by PT cable compression and would also promote sagging out of the gate. This still fits with my failure model. Other than that, difficult concepts are the essence of advancement.

RE: Miami Pedestrian Bridge, Part XII

Quote (hokie66 (Structural)29 Jul 19 19:39 The concept was doomed.)

It certainly became " A Bridge Too Far".
Do you think there is some inherent reason it could not work, or that it is a concept fraught with many conditions requiring attention "outside the box" of normal concrete construction?
The "K braced frame" was basically outlawed, at least in some jurisdictions, because it was a concept to be avoided due to the consequences of failure and the ease with which it could be misused, as I understand. Perhaps a post tensioned concrete truss should be banned for those reasons.

RE: Miami Pedestrian Bridge, Part XII

Vance,

Without a rigorous program of testing the concept, I think this type concrete truss/frame will never be used again. And I doubt that will happen, as there is simply no call for it. Structural steel works, so why change? There was an opportunity here to load test the frame while still on the ground. That didn't happen, and disaster occurred.

Sym P. le, "advancement" is in the eye of the beholder.

RE: Miami Pedestrian Bridge, Part XII

Quote (LinonelHuntz)

Sure, but not much of that is learned by the speculating here.

Some of what is discussed is speculation or educated guesses but a lot more of the information has been confirmed in reports, released photos, calculations etc.

For example, we didn't know for certain if the #11 rod was being tightened or loosened until the first NTSB report came out. We didn't know how bad the cracks were until the photos were released. This brings up ethical questions as well technical issues. As more information is released, we learn more.

In my mind, the ethical and procedural issues are more important than the technical issues. As far technical issues go, a well understood failure mechanism at the connection was missed (and other significant design issues).

RE: Miami Pedestrian Bridge, Part XII

Quote (hokie66)

Without a rigorous program of testing the concept, I think this type concrete truss/frame will never be used again. And I doubt that will happen, as there is simply no call for it. Structural steel works, so why change? There was an opportunity here to load test the frame while still on the ground. That didn't happen, and disaster occurred.

Sym P. le, "advancement" is in the eye of the beholder.

The concrete truss design is not that complicated and has multiple benefits. We used to do more complicated designs by hand (as least the truss part including the non-pinned joints). However, the uneven live load distribution, aero-elastic considerations and low torsional frequencies are not so simple to deal with (but that is not the part that failed).

Multiple steel bridges have collapsed. We didn't stop building out of steel.

RE: Miami Pedestrian Bridge, Part XII

Quote (Sym P. le (Mechanical)29 Jul 19 20:17 Regarding "held back", is this the type of detail that would generate correspondence for clarification or would the installer just wing it?)

Because the steel that appears short has the same designation as the long deck reinforcing, the construction should reflect that. An RFI (Request For Information) would normally be prepared and forwarded to the proper party if a conflict arises or if more information is needed. I would expect that to be in the project file, with a response and perhaps directions or details from the EOR to provide guidance to the field.
The contractor would be assuming unnecessary risks should they make changes without guidance or approval.

RE: Miami Pedestrian Bridge, Part XII

Thanks for putting that into words for me. I struggle with the rebar placement in this project. The dissymmetry in the diaphragm is yet another example.

RE: Miami Pedestrian Bridge, Part XII

Earth314159, I think you underestimate the complications of a concrete truss design in general, not to speak of a post-tensioned truss with construction joints. I would not attempt such a design, even though I am generally a "concrete guy".

RE: Miami Pedestrian Bridge, Part XII

Quote (Hokie66)

Earth314159, I think you underestimate the complications of a concrete truss design in general, not to speak of a post-tensioned truss with construction joints. I would not attempt such a design, even though I am generally a "concrete guy".

It might be a difficult design but I have actually done more complicated concrete designs myself (including the PT). Every concrete designer should be very well familiar with failure mechanisms that took place on this bridge. The only design aspects that concern me are the aero-elastic, unbalanced live load and torsional flexibility/frequency. The long term interaction with concrete creep and the tube stays could also be a mess to untangle. However, the component that failed was one of the more simple aspects of the bridge. You can literally get a good estimate of the horizontal shear and stress levels with quick hand calculations. Once you know that you have about 8MPa shear at the pour joint, you know there are going to be design issues.

RE: Miami Pedestrian Bridge, Part XII

I found it compelling that T Y Lin, a name synonymous with prestressed concrete, originally proposed a steel bridge. This was in the request for proposals. When the proposals came in, FIU gave the design-build contract to the contractor using Figg's design, based on aesthetics. Whether T Y Lin had any further input is unclear.

RE: Miami Pedestrian Bridge, Part XII

Earth314159 Any thoughts on a better way to pour so the web and deck act more like a single unit? (I'm not even sure that's the right question)

SF Charlie
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RE: Miami Pedestrian Bridge, Part XII

Quote (hokie66 (Structural) 30 Jul 19 21:31)

Whether T Y Lin had any further input is unclear.

According to the OSHA report, page 14, the selection committee was made up of FIU officials, the City of Sweetwater, and FDOT. TY Lin, which prepared the design criteria, wasn't a part of the selection committee.

Is is normal practice to not include someone with technical expertise?

RE: Miami Pedestrian Bridge, Part XII

I cannot imagine a typical case where those responsible for making the selection would have technical expertise.

There have been a number of times when I wished they did; too many times where being up against someone incompetent who tells a pleasing story that is obviously wrong but the selector "doesn't have time to hear more" or "doesn't want a lecture" on the topic; they want what they want and shed responsibility to those with the pencils to paper.

It's basically how division of labor has to work.

Licensing and lawsuits handle the rest and, considering the rarity of stupid disasters like this, those countermeasures seem to work.

RE: Miami Pedestrian Bridge, Part XII

Quote (sym)

Any thoughts on a better way to pour so the web and deck act more like a single unit? (I'm not even sure that's the right question)

Square up the pour joint and make the diaphragm a foot thicker.

RE: Miami Pedestrian Bridge, Part XII

Quote (SFcharlie)

Earth314159 Any thoughts on a better way to pour so the web and deck act more like a single unit? (I'm not even sure that's the right question)

There are multiple issues with the joints. The horizontal interface shear stress is too high at the pour joints (it is also too high at other joints and just not the one that failed). You also have issues with the transfer of the vertical loads from the deck to the diagonals. The other issue is there is a functional architectural problem with the diagonals. When an architect or designer gives you a column projecting at an angle, you have to make sure people don't walk into the diagonal. It is especially important nowadays with people using cell phones. To solve all these issues, you fundamentally want a bigger base at the diagonals (a longer base in the direction of the bridge span). You flare out the diagonals parallel to the direction of the bridge (you can keep the 1'-9" width the same or perhaps make it a bit wider). People don't utilize the space below the diagonal anyways since you don't want to walk under a diagonal when you are close to the base (otherwise you hit your head). Architects think diagonal/sloped columns look cool so this is a common issue that I have encountered. We end up putting the columns on pedestals or coming up with some other solution.

There are multiple ways of designing the flares and it depends of the aesthetics that people are looking for. You can even create a raised flat surface for planters.

Once you get the stresses to a lower level, you can fit more vertical steel in through the pour joint without getting congestion. This helps for both the horizontal shear and the vertical loads from the deck. You have to add horizontal steel in the flares to transfer the load more evenly across the pour joint below the flares.

To give you a feel of design and stress level, an interface shear with a 1MPa stress is simple and straightforward with minimal steel crossing the interface plan. 2MPa is a typical stress but needs to be considered and specifically detailed. 3MPa is manageable. 4MPa is more difficult to design. 5MPa is possible but congestion has to be carefully considered and you have to start considering higher strength concretes and steel. 8MPa is damn difficult (not practical at all) with extremely high concrete strengths and steel strength with very careful consideration for congestion. This is why we should be working in stresses and not forces. A stress level corresponds to certain design expectations. There is a more immediate understanding of the design issues.

I should also mention that other stress levels are high (such as the vertical shear at the web joints). With just a bit a re-sculpting of the shape, you can make a big difference to the design. The reshaping can also look better. It is my experience that structures that are proportioned with more realistic stress levels (not too high or too low), tend to look better.

RE: Miami Pedestrian Bridge, Part XII

Typical for many engineering projects, the selection process was two-step: first review of proposals, and second an interview. The first step was short-listing the five proposals that were submitted. If you are interested, the selection committee and their votes are in the scoresheet folder here: http://facilities.fiu.edu/projects/BT-904-PRR.htm Interestingly, MCM was 3rd of the 5, and it appears only two were going to be interviewed. But after the ranking, the sealed cost proposals were opened and one (must have been GLF Construction) was over budget and was tossed out. Facchina and MCM remained and in the interview, MCM came out on top (scoresheet in the same link). Also if you are interested, the 4 other proposals are on the FIU site, but the MCM proposal can be found at the FDOT site https://www.fdot.gov/info/co/record.shtm as well as the pre-proposal presentation.

RE: Miami Pedestrian Bridge, Part XII

Quote (hokie)

The deck shortened due to shrinkage and applied compression, and that shortening was resisted by the inclined webs. Cracking resulted.

What are you basing that on? The report said cracking was noticed after shoring was removed.

RE: Miami Pedestrian Bridge, Part XII

Just throwing this out here. Looking at the truss members & the top of the canopy, both these surfaces appear to have received a topical application of a TiO2 coating. It kind of suggests that, if the client is expecting a uniform white appearance, then $300/cubic yard specialty white cement & TiO2 concrete is not really the way to get both an esthetic & environmental finish. I would hazard that a TiO2 coating of the bridge would have been cheaper and had it been applied to the underside of the bridge; the bridge would have sat on the temporary shores longer. I think the TiO2 concrete was the one contribution from FIU's ABC program.

RE: Miami Pedestrian Bridge, Part XII

Quote (hokie66)

This was reported by several sources.

Where does it say the cracks were caused by the PT shortening of the deck? I did a quick read of the article and couldn't find anything.

It doesn't make a lot of sense to me. In a determinant truss, the PT would not have introduced any member forces in the diagonals. The canopy would have to have a fairly high bending stiffness for the deck shortening to cause a lot of force in the diagonals and shear at the pour joints. The shear predominantly comes from the gravity load.

RE: Miami Pedestrian Bridge, Part XII

Quote (Tomfh)

Square up the pour joint and make the diaphragm a foot thicker.

What is your thinking on making the diaphragm a foot thicker? Perhaps I am misunderstanding what you are proposing and why.

RE: Miami Pedestrian Bridge, Part XII

Quote (Earth314159)

There are multiple ways of designing the flares and it depends of the aesthetics that people are looking for. You can even create a raised flat surface for planters.
Thank you for your thoughtful answer.
(...guess FIU would have had to issue bump caps...)

SF Charlie
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RE: Miami Pedestrian Bridge, Part XII

Quote (earth)

What is your thinking on making the diaphragm a foot thicker? Perhaps I am misunderstanding what you are proposing and why

I meant a foot wider, to increase the horizontal thickness.

In my view there was failure of the cold joint for the first 100mm or so, and beyond that it's an overall concrete punching failure thru the diaphragm. If the diagram was 3 foot thick not 2 foot thick it couldn't punch out the way it did.

RE: Miami Pedestrian Bridge, Part XII

Quote (hokie)

This was reported by several sources.

It mentions the bridge was supported each end, i.e. gravity loading.

I agree with Earth that the PT and shrinkage shortening would not induce sufficient loads to fail the node. The canopy and member 12 are not stiff enough to resist the shortening.

RE: Miami Pedestrian Bridge, Part XII

Quote (SFCharlie)

Thank you for your thoughtful answer.
(...guess FIU would have had to issue bump caps...)

You are welcome. I think Apple and Samsung should issue bump caps with each cell phone that they sell. bigsmile

RE: Miami Pedestrian Bridge, Part XII

Quote (Tomfh)

I meant a foot wider, to increase the horizontal thickness.

In my view there was failure of the cold joint for the first 100mm or so, and beyond that it's an overall concrete punching failure thru the diaphragm. If the diagram was 3 foot thick not 2 foot thick it couldn't punch out the way it did.

I see what you mean. Although if you solve the interface shear issue, you don't have an issue with the diaphragm thickness. There was a significant increase of the vertical steel through the joint at #12. That helped transfer the loads into the deck just above the diaphragm. I believe that is why the shear plan dips down above the diaphragm. The diaphragm does punch but with proper detailing, the load would be properly distributed to the deck PT before there would be any punching on the diaphragm. I hope that makes sense. It is a bit difficult to explain.

RE: Miami Pedestrian Bridge, Part XII

Deck Shortening and diagonals.
174 feet of concrete will shrink about an inch. FDOT required form mods so diaphragms could move. Of course the diagonals will shrink also. The shrinkage of different parts may not track together due to intervals between casting.
Shortening of the deck under PI is about 5/8 inch.
Some diagonals are loaded with PT which must have caused shortening. Some diagonals were not stressed and resisted shortening. The canopy had light PT at erection, with most to be added when the back span was completed.
Pretty much a scramble of conditions.
Would not shortening of the deck have moved the bottom of diagonals closer together and caused moments in the rigid joints? Maybe not enough to be of concern. In a pin connected truss, shortening of the deck would have caused the structure to increase in height.
FIGG was aware of some potential stresses, because they specified a sequence for applying PT loads.
Plenty of questions, few tested answers.
I wonder if perhaps the compression diagonals should have been precompressed with PT to create more compatible strains during curing?

RE: Miami Pedestrian Bridge, Part XII

Tomfh,

We don't know when the cracks observed in February first occurred or what form they took, but I can guess.

I wasn't suggesting that the canopy and the end vertical were stiff enough to resist the shortening. Nothing is stiff enough to prevent shortening due to shrinkage. But there were also the diagonals, member 11 in particular, operating in concert with the canopy. They all tried resisting, but found it impossible and instead cracked.

Shrinkage would be the dominant factor here, with PT adding a bit. A lot of shrinkage cracking could have occurred prior to application of the prestress.

RE: Miami Pedestrian Bridge, Part XII

Quote (earth)

Although if you solve the interface shear issue, you don't have an issue with the diaphragm thickness.

Which interface shear are you referring to?

In my opinion you need to:

A- ensure #11 cant slip across the deck (hence the need to square up the joint)

B- ensure the deck can’t punch out due to the load from #11 (hence idea to make diaphragm wider/stronger/more reinforced)



RE: Miami Pedestrian Bridge, Part XII

Quote (hokie)

We don't know when the cracks observed in February first occurred or what form they took, but I can guess.]

The OSHA report says the cracking occurred when sharing removed. Your older link also says it cracked when on ground in conditions mimicking the real conditions, ie supported at ends.

Do you have any other evidence the cracking was caused by shrinkage/shortening forces?

Quote:

But there were also the diagonals, member 11 in particular, operating in concert with the canopy. They all tried resisting, but found it impossible and instead cracked.

Yes, but how would the 11/12/canopy assembly provide sufficient restraint to a shortening to deck to fail the node. As Earth point out, it’s fundamentally a truss. Shortening a member won’t cause a node to rupture.

RE: Miami Pedestrian Bridge, Part XII

Quote (Tomfh)

Which interface shear are you referring to?

In my opinion you need to:

A- ensure #11 cant slip across the deck (hence the need to square up the joint)

B- ensure the deck can’t punch out due to the load from #11 (hence idea to make diaphragm wider/stronger/more reinforced)

Interface shear is shear friction (I don't know about the US code but it is called interface shear in the code that I use). In this case, I mean the slipping between the underside of #11 and the top of the deck.

The punching can't occur unless you first get an interface shear failure. If you stop the slip, punching would not have occurred. Once you have sufficient shear friction capacity, you have a strut ( a "fan" shaped strut) and tie model that can distribute the load to D1 and the remainder of the load to D2.

RE: Miami Pedestrian Bridge, Part XII

Quote (Hokie66)

We don't know when the cracks observed in February first occurred or what form they took, but I can guess.

I wasn't suggesting that the canopy and the end vertical were stiff enough to resist the shortening. Nothing is stiff enough to prevent shortening due to shrinkage. But there were also the diagonals, member 11 in particular, operating in concert with the canopy. They all tried resisting, but found it impossible and instead cracked.

Shrinkage would be the dominant factor here, with PT adding a bit. A lot of shrinkage cracking could have occurred prior to application of the prestress.

We have photos of the first appearance of shear cracks when the shores were first removed. The base of #11 and #2 had the first signs of shear friction cracks.

The canopy would have had to act as a strong back for the deck PT to cause shear cracking at the base of the diagonals. The bending stiffness of the canopy is orders of magnitude less than the stiffness of the truss as a whole. It is possible for deck PT to cause bending in the canopy joints but without a sufficiently stiff strong back, PT causes upward camber and gravity causes sag.

In a structural determinant truss, no matter the shrinkage or PT, the member force is zero without external loads but the truss shape does change.

RE: Miami Pedestrian Bridge, Part XII

Quote (earth)

Interface shear is shear friction (I don't know about the US code but it is called interface shear in the code that I use). In this case, I mean the slipping between the underside of #11 and the top of the deck.

Yes. I was just clarifying whether you meant the 11 to deck construction joint, or if you were adding up shear planes inside the deck too.


Quote:

The punching can't occur unless you first get an interface shear failure.

I may be misunderstanding you, but this strikes me as fundamentally wrong. The deck can grab onto the deck with infinite strength and the deck can still tear out in cone/punching failure.
The deck can simply tear out behind member 11, which it almost did (the deck tore out about 100mm into member 11)

RE: Miami Pedestrian Bridge, Part XII

One difference in our assessment is that most here consider this to be a truss, while I don't. I think it acted as a frame, with bending in the members and joints similar to a Vierendeel truss.

RE: Miami Pedestrian Bridge, Part XII

I agree without Earths’ assessment that the frame stiffness plus shortening effects are insufficient to break the node, and that the cracking is due to truss loading forces under gravity.

RE: Miami Pedestrian Bridge, Part XII

Five hundred and two days have passed since collapse. I'm guessing NTSB report will be released within 30 days.

Would someone here care to draw free body diagram with everything south of 9 just a moment and with hinges in deck and canopy at top and bottom of 9? What forces would develop in 11 resisting rotation at hinges?

RE: Miami Pedestrian Bridge, Part XII

I calculated it earlier at around 940 tons; about halfway down on page 1 of this thread.

RE: Miami Pedestrian Bridge, Part XII

Quote (jrs_87)

I'm guessing NTSB report will be released within 30 days.
NTSB has suggested the the report will be available "in the fall". A "docket" should become available 30 days before the hearing. I just checked and the docket is apparently not available yet.
In the mean time, I recomend the previous (apearantly November 2018) two updates:
Update and Update2

SF Charlie
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RE: Miami Pedestrian Bridge, Part XII

jrs_87 Thanks for the link. I wonder why Louis Berger would hold out?

SF Charlie
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RE: Miami Pedestrian Bridge, Part XII

Quote (hokie66)

One difference in our assessment is that most here consider this to be a truss, while I don't. I think it acted as a frame, with bending in the members and joints similar to a Vierendeel truss.

Even with a vierendeel, the canopy and verticals would both have to be relatively stiff to cause the shear issue when the PT in the deck is tightened. In this case, the canopy does not appear to be stiff enough to cause an issue. It is quite flexible compared to the truss as a whole.

RE: Miami Pedestrian Bridge, Part XII

Quote (Tomfh)

I may be misunderstanding you, but this strikes me as fundamentally wrong. The deck can grab onto the deck with infinite strength and the deck can still tear out in cone/punching failure.
The deck can simply tear out behind member 11, which it almost did (the deck tore out about 100mm into member 11)

It actually can't tear out if you have a enough capacity in the strut and tie model from the pour joint shear plane to the deck PT anchors. The horizontal shear at the base of #11 has to be dragged back to get a more even distribution of stress at the shear plane (and you want a flare at the base of #11 as previously mentioned). However, there is enough capacity in the compression strut/fan to carry the load to the D1/D2 PT anchors. There is also enough transverse steel and PT to resist the horizontal tension parallel to the diaphragm.

The punching that occurred took place within the depth of the deck. The diaphragm thickness did not play a significant role in the punching capacity. The distance from the size #11 south vertical bar in #12 to the north face of the deck was a more critical factor in the punching capacity. There was significant shear friction capacity at the base of #12 member and doweling action from the size #11 bars (the #11 bars were not well anchored so you only get doweling from the #11 bars but the other bars were better anchored) that allowed the punching to occur. Without these bars, the failure would have been a pure shear friction failure.

With such a short distance between the size#11 bars in 12 and the north face, you could not get a strut and tie model to work and hence the punching shear failure occurred.

RE: Miami Pedestrian Bridge, Part XII

Quote (Earth)

It actually can't tear out if you have a enough capacity in the strut and tie model from the pour joint shear plane to the deck PT anchors.

That's why I mentioned to widen/thicken the diaphragm, to provide greater length for the strut and tie forces. Same as when you widen out a pier cap beyond the piles to provide additional engagement between the struts and the bottom ties, to prevent the struts blowing out the sides before they get a chance to grab onto the ties.


Quote (earth)

The punching that occurred took place within the depth of the deck. The diaphragm thickness did not play a significant role in the punching capacity.

If the diaphragm thickness was greater the edge distance to the applied load thrust would be greater, a larger cone would have to fan out, which would also intersect the PT cables. The diaphragm was so short that the failure cone skirted around the PT.

Quote:

There was significant shear friction capacity at the base of #12 member and doweling action from the size #11 bars (the #11 bars were not well anchored so you only get doweling from the #11 bars but the other bars were better anchored) that allowed the punching to occur.

Yes, it was a combined failure. Horizontal shear friction for some distance, and then a cone failure surface fanned out thru the diaphragm. Akin to combined failure often seen in smaller anchors. PINK comments are mine.






So my suggestion was to square out the connection, to eliminate the shear friction aspect. Make the strut dig into the deck directly. And to make the diaphragm stronger/thicker to prevent any punch out failure once the load is in the deck.


In any case I think we agree that one way or another that strut #11 needed to be more strongly anchored!

RE: Miami Pedestrian Bridge, Part XII

The diaphragm is under the deck of the bridge. Making it wider places more of it under 11, where it does no good, rather than extending past 12. Changing the diaphragm governing dimension along the length of the deck while leaving it flush with the end of the deck doesn't increase its capacity. If you suggest increasing the length of the deck to produce an overhang beyond 12 then that doesn't work well with the pylon or other span.

I think the correct answer was to have steel tie the reinforcement in 11 to the reinforcement of the deck rather than depending on cement in the concrete to handle that job.

RE: Miami Pedestrian Bridge, Part XII

Quote (3DDave)

If you suggest increasing the length of the deck to produce an overhang beyond 12

Yes I am saying if the diaphragm extended further past #11 the punchout wouldn't have occurred so easily. Much bigger failure surface and the failure block has to intersect the PT cables. Maybe there are other reasons not to make the diaphragm thicker, but a thicker end block would have increased the capacity.


And yes steel reinforcement to anchor it back is very important.

RE: Miami Pedestrian Bridge, Part XII

Quote (Tomfh)

That's why I mentioned to widen/thicken the diaphragm, to provide greater length for the strut and tie forces. Same as when you widen out a pier cap beyond the piles to provide additional engagement between the struts and the bottom ties, to prevent the struts blowing out the sides before they get a chance to grab onto the ties.

As you say, the struts and fan are pushing out toward the north edge of the diaphragm but that is okay when doing a strut and tie model. As long as there is at least one viable load path and enough surrounding steel to meet the ductility requirements, there won't be an issue with the diaphragm. I don't know if you do many strut and tie models or not but there are requirements for minimum percentages of steel in each direction to ensure that there can be a redistribution of load path. The method is very flexible for designing disturbed regions like this and is well proven. In this case, the compression struts and anchors are all within the depth of the deck. Thickening the diaphragm is not harmful, it may helps a little bit but it is not necessary. It is more important to flare out the base of #11 and drag the horizontal components to the south side of the flare before it is transferred into the deck. You also want to note on the drawings to roughen the pour joint to an amplitude of 6mm.

There is a bit of coincidence in the failure in that that south side of #12 and the back side of the diaphragm are closely aligned to one another which adds to the confusion as to what the relevant parameters are.

RE: Miami Pedestrian Bridge, Part XII

Quote (3DDave)

I think the correct answer was to have steel tie the reinforcement in 11 to the reinforcement of the deck rather than depending on cement in the concrete to handle that job.

Just to be clear, we are definitely saying there has to be vertical steel through the joint. Reducing the shear stress allows the congestion of the required steel to be minimized. The percentage of vertical steel for the shear area goes down but the total vertical steel through the joint goes up.

The horizontal component in the flare has to be dragged southward with horizontal steel above the pour joint. This distributes the shear friction/interface more evenly.

RE: Miami Pedestrian Bridge, Part XII

Quote (Earth)


As you say, the struts and fan are pushing out toward the north edge of the diaphragm but that is okay when doing a strut and tie model.

In this bridge the strut is the #11 and the tie is the deck PT cables.

As constructed the strut load did not get into the ties. It simply detoured around those PT cables, and burst out of the back of the diaphragm.

If the diaphragm thicker (i.e. deck longer)that couldn't have happened.

Flaring out the base of 11 achieves much the same thing as lengthening the diaphragm. Provide a wider fan which can catch more steel.

RE: Miami Pedestrian Bridge, Part XII

Quote (Tomfh)

In this bridge the strut is the #11 and the tie is the deck PT cables.

As constructed the strut load did not get into the ties. It simply detoured around those PT cables, and burst out of the back of the diaphragm.

If the diaphragm thicker (i.e. deck longer)that couldn't have happened.

Flaring out the base of 11 achieves much the same thing as lengthening the diaphragm. Provide a wider fan which can catch more steel.

The punching occurred on the top surface of the diaphragm/deck. It doesn't mater where the south side of the diaphragm was located (it could have been set further south). It wouldn't have increased the punching capacity since the south side of the punch started at the south side of #12 where the vertical #11 bars were located. Thickening #12 and placing the vertical #11 bars further south would have increased the punching capacity.

RE: Miami Pedestrian Bridge, Part XII

Quote (Earth)

It doesn't mater where the south side of the diaphragm was located (it could have been set further south).

I'm not talking widening diaphragm to the south. I mean extending it to the north, so that the failure cone is bigger, and the PT rods extend further past #11 so as to help intercept the blow out cone.

RE: Miami Pedestrian Bridge, Part XII

Since 11 didn't have to align with a faux cable stay, couldn't the angle have been less steep? Is this what Earth314159 meant by square up 11?

SF Charlie
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RE: Miami Pedestrian Bridge, Part XII

Quote (Tomfh)

I'm not talking widening diaphragm to the south. I mean extending it to the north, so that the failure cone is bigger, and the PT rods extend further past #11 so as to help intercept the blow out cone.

That makes more sense. Thanks for clarifying that.I wasn't thinking that because of the physical constraints.

RE: Miami Pedestrian Bridge, Part XII

Quote (SFCharlie)

Is this what Earth314159 meant by square up 11

I can't remember saying "square up 11". Do you have the post available so I can read it and clarify? Thanks.

RE: Miami Pedestrian Bridge, Part XII

SFCharlie 10 lined up with a fake stay. 12 is where 12 is. 11 had to go from one to the other. Changing 11 means moving 10 and 12.

RE: Miami Pedestrian Bridge, Part XII

Quote (Earth)

I can't remember saying "square up 11".

I recently used the term "square up" in reference to the #11 to deck construction joint. I.e. make the joint perpendicular to the member, so as to eliminate the fatal shear friction interface.

RE: Miami Pedestrian Bridge, Part XII

Quote (3DDave)

SFCharlie 10 lined up with a fake stay. 12 is where 12 is. 11 had to go from one to the other. Changing 11 means moving 10 and 12.

The base of #11 and #12 don't necessarily have to intersect. You could move the base of #11 to the south and cantilever the abutment to the south. This would have helped.

RE: Miami Pedestrian Bridge, Part XII

Quote (Earth)

I wasn't thinking that because of the physical constraints.

Yes maybe the geometry can't be made to work. I don't know for sure. Maybe its impossible to make a heftier block for 11 to bear against.

RE: Miami Pedestrian Bridge, Part XII

Tomfh - Earth314159 Sorry I attributed the suggestion to the incorrect author.
As Earth314159 says, 11 doesn't need to abut 12, Couldn't the concrete between 11,12 be wider and higher?

SF Charlie
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RE: Miami Pedestrian Bridge, Part XII

Flaring out the base of 11 achieves much the same thing as lengthening the diaphragm.
Making the base of 11 much wider would not have restricted foot traffic - they intended to pour column 12 much bigger to support the pylon for the fake pipe stays.
EDIT: Sheet B-56 shopws it to be 5 feet wide. END EDIT.
Making all the connections of diagonals to the deck much longer on both sides (under the diagonal) would not have limited any usable space - a person cannot walk closer than about 5 feet to the joint without hitting the diagonal with one's head.

RE: Miami Pedestrian Bridge, Part XII

What steel arrangement would be best for tying the punched out concrete beyond #11 back into the deck? Regular longitudinal L-bars and U-bars?

RE: Miami Pedestrian Bridge, Part XII

Quote (Tomfh)

What steel arrangement would be best for tying the punched out concrete beyond #11 back into the deck? Regular longitudinal L-bars and U-bars?

Right at the punched area I would likely try to use U-bar with anchor bars at the inside corners of the bends.

At the other locations (at flares and haunches) you get the best anchorage with ties. However, if the flares vary in height, I might use a combination of ties and U-bars. You also want an arrangement that helps for deck punching for the interior joints. The joints at the canopy would need some reshaping as well (but not as bad since there is a shear plane between the blister and deck as well as the underside of the canopy). These are the kinds of details that you spend lots of time designing and thinking about. I would discuss different options of the shape of the flares with the client. The reinforcing depends on the shape of the flare. I generally try to look for patterns and consistency in a design.

RE: Miami Pedestrian Bridge, Part XII

Quote (SFCharlie)

As Earth314159 says, 11 doesn't need to abut 12, Couldn't the concrete between 11,12 be wider and higher?

Yes, absolutely. I was thinking the same thing. Moving the base of #11 to the south helps in multiple ways. You just need to rethink the design of the abutment.

RE: Miami Pedestrian Bridge, Part XII

Just a thought. Trying to stay close to original design profile. And it probably wouldn't hurt to fan out all connections. Of course to save time and money, you could build it to original specs. But I don't know how well that will work...........


Next pic shows redesign using upper pic with what I call endless cable running thru and around entire middle to capture forces at ends. You would add my concept to original design.

Sorry I ran out of crayons.
While we're at it. Why not relocate the drain pipe. Fill in the hole. And run some bolts completely thru the canopy down thru the verticals and out the bottom of the deck. So maybe like the bottom doesn't fall out. Get to work people. Let's see the new design revisions.

And would somebody please address the canopy drain pipe issue? Certainly someone will never throw a "soda" can up there and clog the system. Just what we need more weight.

RE: Miami Pedestrian Bridge, Part XII

Quote (Tomfh (Structural)1 Aug 19 01:31 I'm not talking widening diaphragm to the south. I mean extending it to the north)

I find it interesting that the longest span of 174 feet has only 2 feet on the pylon and they saved 4 feet for the shorter span of 96 feet. They could have easily made it 175 feet long with a 3 foot diaphragm and had room to support the shorter cip span.
I would caution that moving the base of 11 southward will move the "joint" of 11 to deck off the pylon and leave moments and shears in deck to deal with.

RE: Miami Pedestrian Bridge, Part XII

Yes Vance "moving the base of 11 southward will move the "joint" of 11 to deck off the pylon". In a previous post I asked if adding material at the top inside corner of the deck pier joint would violate clearances. I was thinking "Could 11 be extended through the deck to the pier?"

SF Charlie
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RE: Miami Pedestrian Bridge, Part XII

Quote (Vance Wiley)

I would caution that moving the base of 11 southward will move the "joint" of 11 to deck off the pylon and leave moments and shears in deck to deal with.

The idea would be to alter the abutment design with a cantilever to minimize the shears and bending in the deck. This does put bending and shear into the abutment but it would be manageable. The cantilever could also be thick enough to keep the shear within 45 degrees of the vertical abutment support.

RE: Miami Pedestrian Bridge, Part XII

Quote (SFCharlie (Computer)1 Aug 19 15:44 I was thinking "Could 11 be extended through the deck to the pier?")

In a sense, member 11 is considered to be extended thru the deck because it is intended to be cast EDIT: "ACT" END EDIT monolithically - if the construction joint works as intended. The line of force projects thru the deck and, in my mind, works better if that member inside the deck is defined by a cage of reinforcing and confinement ties like those along its free length. In this case, that was done but the ties were not carried into the deck, as I recall, and really were not adequate along the length of 11.
If all is working right, the center of the joint should be the intersection of the center of 11 and the PT tendons in the deck. The horizontal component of the axial force in 11 should have been intercepted by the PT tendons, leaving only the vertical reaction at the joint to be supported and distributed by the diaphragm.
That is the condition addressed at the Power Point Party.
In this case, the horizontal component of 11 was not intercepted well. I think captured was a term used at the meeting.
Making diaphragm 2 (north end) thicker and casting the main span 175 feet long would have accomplished much of what you are suggesting. Correcting the smooth surface of the cold joint was needed. And perhaps the base of 11 should have been something like 3 feet wide and 3 feet high and extended maybe 5 feet farther south than the little 8" fillet, all loaded with enough reinforcing to ensure no slip at the deck surface and adequate capturing of 11. That would provide the option to use a square end of 11 into the base above the deck

This was a pretty good start -- from JRS 87 about April - segment IX .
https://res.cloudinary.com/engineering-com/image/u...

RE: Miami Pedestrian Bridge, Part XII

Earth314159 Those are good solutions.
The variety of suggestions presented proves the point that engineers are individuals and there are many paths to a successful design. All will be well if the problem is recognized and addressed properly.

RE: Miami Pedestrian Bridge, Part XII

Quote (Vance Wiley)

The variety of suggestions presented proves the point that engineers are individuals and there are many paths to a successful design. All will be well if the problem is recognized and addressed properly.

I agree. This bridge was completely feasible but the design just had to be adjusted a bit. I don't see too many people wanting to line up to design a concrete truss bridge given what happened but the idea of a concrete truss was not the issue. It was just that the problems encountered were not identified or addressed properly.

RE: Miami Pedestrian Bridge, Part XII

How many ways to skin a cat?

RE: Miami Pedestrian Bridge, Part XII

2
I haven't been keeping up with the threads on this collapse, but I thought I'd post this (it may have been posted previously apologies if it has been posted already). Its a video/webinar from the fib/IStructE in the UK going into the collapse. The part on the FIU failure starts at ~32 minute in after some stuff on the Morandi Bridge collapse.

https://istructe.hosted.panopto.com/Panopto/Pages/...

RE: Miami Pedestrian Bridge, Part XII

Thank you Agent666,

I don't think anyone else has posted this. Always good to hear from the istructe on such matters.

RE: Miami Pedestrian Bridge, Part XII

I find the goings-on about reviewing the partial structure to be rather stupid. The truss section needed to be capable of standing by itself which means the failure was not caused by the bridge being incomplete. About the only place Berger could have failed in reviewing partial construction plans was by not reviewing the moving process. Overall, the reality is that the Berger review simply failed to find that the design of this truss section was faulty.

RE: Miami Pedestrian Bridge, Part XII

Quote (LionelHutz)

I find the goings-on about reviewing...

Good point, yet so obvious it's sad everyone including me kind of missed it.

Louis Berger is resisting perhaps because their fee was only $65,000 which by no means correlates to liability exposed.

This accident and the investigation is very peculiar.

RE: Miami Pedestrian Bridge, Part XII

Quote (jrs_87 (Mechanical)7 Aug 19 16:52 Berger’s Peer Review)

I seem to recall reading Berger's fee was $30,000 and FIGG's fee was 1.5 million. So Berger got 2% of the design fee. Not a lot.
How is it that the design firm chooses the peer review firm? It seems more objective if the Owner pays the peer review firm and proposes maybe 3 firms for designer to choose. That could prevent a malicious review - particularly if the peer review firm had proposed on the project and did not get the project. And it could prevent the design firm from choosing a shill review.
Wasn't the design past the 80% or so point before Berger was retained? Pretty much precludes any review at earlier stages.
When FDOT raised valid questions FIGG blew them off. Some seem to think since FDOT raised a question and then did not stop the project when their concerns were dismissed makes FDOT complicit.
All fodder for attorneys.
Remember the story of the lone attorney in the small western Montana town, who barely eked out a living? Then a second lawyer opened an office there, and in just a few years both were wealthy.
EDIT CORRECTION - I had recallled a post by EPOXYBOT which was later corrected to $61,000 for Berger's fee. I find that a bit more reasonable, but totally disproportional to the risk assumed.


RE: Miami Pedestrian Bridge, Part XII

Louis Berger was paid $61,000 for the review. They certified 3 PEER Reviews for the Foundation, Substructure & Superstructure "Plans", whatever that means. They also prepared "a finite element model of the bridge and estimation of demands on all elements due to different design load combinations" (not certified). The plans they reviewed were 100% plans. Who was the finite element model for?

Figg's original intent to use another branch of Figg to perform the PEER review, was kind of shady and should have caused FDOT to double down.

Maybe it is the Contract Documents & RFP that are flawed by allowing the Design Build team select a PEER Review consultant.

RE: Miami Pedestrian Bridge, Part XII

Quote (epoxybot (Structural)7 Aug 19 22:52 They also prepared "a finite element model of the bridge)

I agree, the term "plans" is too generic for a contract, and subject to mis-interpretation, particularly by attorneys.
Do we know if the FEA addressed only the completed structure or did it address the main span only and independently?

RE: Miami Pedestrian Bridge, Part XII

I could not confirm, but for a while I was suspicious that LB used a freeware FEA program.

To the structural engineers listening> What is the upper bound on the length of this type of span? And the width?

RE: Miami Pedestrian Bridge, Part XII

The idea that FDOT was hesitant for any reason is pretty suspicious, because they have apparently have a reputation for heavy handed bid-rigging in the Miami area.

Side note on this June, 2017 article - the favored bid by Fluor-Astaldi-MCM that FDOT screwed over was a design by .......... FIGG.

RE: Miami Pedestrian Bridge, Part XII

To the structural engineers listening> What is the upper bound on the length of this type of span? And the width?

Apparently less.

If the world comes to an end. Engineers are always worth more than lawyers.
ANYONE with ANY mechanical sense is worth more than a lawyer.
Sorry should have added the word "allegedly". and "ja".
"ja" in Thailand means " fight but fight with good reason".
In America "JA" is translated differently.

Maybe some of you "JA" lawyers should get a real skill. You wont make it.

Agent666...please.......can you post your video on youtube?

This stuff is ancient. A lawyers battle. fake names. bs. pfffff.......

RE: Miami Pedestrian Bridge, Part XII

and this star stuff. get real so fake.

RE: Miami Pedestrian Bridge, Part XII

RAB678, it should be accessible at that link and I am not a member of IStructE, it just works in Chrome for me. Unfortunately its all tied into that panopto platform so I can't download it directly.

If that doesn't work perhaps you can link to it from this page which is the video webinar index.
https://istructe.hosted.panopto.com/Panopto/Pages/...

RE: Miami Pedestrian Bridge, Part XII

Quote (RAB678)

To the structural engineers listening> What is the upper bound on the length of this type of span? And the width?

That question is very difficult to answer. There is not enough experience with concrete truss bridges to estimate a practical limit.

The limit would be more than 175'. The concept was not the issue. The main problem was the design details and a failure to identify or address those issues. These were some fairly basic mistakes.

RE: Miami Pedestrian Bridge, Part XII

It's like looking at the infamous Tacoma Narrows bridge and concluding that the problem was the span.

Ja. https://siamsmile.webs.com/thaiparticles/thaiparti... translations by Thai people.

In one intonation (there are two others):
"In my experience, when you hear a new Thai acquaintance of the opposite sex begin to replace the more formal KHRAP/KHA with JA (h)/ JA (f) in their speech, it's a sign they feel more at ease in your company and that a friendship (or more) may be developing.
Men don't tend to use JA with each other, unless they are gay.
Can also sometimes be used by older men and women to younger girls/boys (under 12)."

RE: Miami Pedestrian Bridge, Part XII

Quote (jrs_87 (Mechanical)7 Aug 19 23:26 To the structural engineers listening> What is the upper bound on the length of this type of span? And the width?)

Site conditions may govern, or at least weigh heavily (no pun intended) on this issue.
First, I see the forming and casting of a concrete truss with integral deck and roof as being far more intricate than the standard highway sections used everywhere, therefore it will be time consuming and expensive. But these boutique "signature" sites may justify an attempt.
The falsework for a cast in place truss structure would be much like that required for the standard bridge construction so there is little savings there.
And there is a maximum size and weight for ABC construction, as well as site suitability required.
It would have been difficult to set this 174 foot structure if it were necessary to reach, lift, and move from one end.
For those sites which require a long span, I think the cable stay system or the segment cast structures should be more efficient.
Adding 24 feet and making this structure 200 feet long would not have created demands any less known or less knowable than those in the 174 foot span. Reducing this span to 150 feet would not have made the design or importance of any member or connection less critical.


RE: Miami Pedestrian Bridge, Part XII

Re: Vance Wiley (Structural)9 Aug 19 02:09

Reducing the span to 150 feet with all other dimensions held makes a huge difference, span would be much lighter. A 200 foot span would have to be much taller. It's interesting to me the sub span of the bridge while having a much shorter span is of the same proportions as the main span. So therefore, the sub span was over-engineered.

I believe this structure was scaled within reason, but its parameters were in critical region and it was not given attention has such.

Would designing 11/12 in mindset of corbel have helped? http://sites.utexas.edu/faridkhosravikia/experimen...

RE: Miami Pedestrian Bridge, Part XII

Quote (jrs_87 (Mechanical)9 Aug 19 02:59 Would designing 11/12 in mindset of corbel have helped?)

I think it could have helped. There was discussion of this in the past two weeks or so. I see the primary influence as being the development of the "capture" of horizontal thrust from 11 and returning it some distance in the deck, maybe 40 feet, allowing the complete transfer of that thrust to the PT strands in the deck.
Had the horizontal component of the compression in 11 been properly connected, in this case as a strut and tie design, only the vertical component would remain. It was that vertical component that FIGG addressed in the Power Point show on March 15. Apparently they were assuming the horizontal thrust was adequately developed.
Even with a strut-tie design, the flat construction joint at the deck surface would be a problem and should have been handled differently - with a square construction joint, thereby avoiding the sliding present in the joint as constructed.

RE: Miami Pedestrian Bridge, Part XII

Quote (JRS)

Reducing the span to 150 feet with all other dimensions held makes a huge difference

150/175*100%=85%. Reducing the shear to 85% is not going to help that much. It would still be over-stressed. When designing a bridge, you need to be more than 15% or 20% from failing and that doesn't even including the live load or other load combinations. Designing the connections properly could have been done at 175' or 200'. That is the fundamental issue.

RE: Miami Pedestrian Bridge, Part XII

Re: Earth314159 (Structural)9 Aug 19 05:01

Thank you for correction. I was for some reason thinking factor was sixteen based on deflection.

RE: Miami Pedestrian Bridge, Part XII

Quote (jrs_87)

...Would designing 11/12 in mindset of corbel have helped?..

I certainly think so. If they'd cast the deck monolithically with a raised corbel the full width of the decorative pylon above 12, there'd have been more room for rebar to reinforce the shear connection between 11 and the deck, and especially with that part of the deck that contains the longitudinal PT tendons. The construction joint could have been normal to the compression force in 11, or at least have "captured" (Figg's words) 11 in the corner between a vertical face of the corbel and the deck.

RE: Miami Pedestrian Bridge, Part XII

Greetings to all:

I have been out of this forum for a while so I have not read most many of the post. Therefore, bear with me.

Now that the OSHA report has been published this is some of my additions:

1. The whole thing started with the absence of a note in the Contract drawings saying hat the deck concrete must be roughened to 1/4" before casting the truss struts. This changes the factor in the formula of the capacity of the connection #11strut/deck from 1.0 to O.6. This is why the pics in the OSHA report show the strut sliding on top of the deck. The calcs from FIGG indicate the assuption of the 1/4"roughening.
. Furthermore, the costruction practice and the FDOT spects does not provide that type of roughness unless it is specified in the Contract Drawings. MCM probably just cleaned the deck without chipping or adding a bonding agent (my guess)

2. The 8 #7 column bars in strut #11 can not be used in evaluating the Shear-friction capacity of the connection because they are in compression. AASHTO does not clarify that in the LRFD Code. But if you look at the shear-Friction section of the ACI code it really clarifies that. I am not sure if the OSHA report or the calculations of the members of this forum has accounted for this.

3. So the displacements (craks) pics indicte that the connection bars where mostly working as dowels and not in the shear-friction mode. #11 strut was being contained longitudinally by #12 column.


4. And now, my big guess is that when they attempted to restress the PT bars, the additional axial/bending loads rotated the strut end, additionally compressed the small fillet connection #11 to #12, broke the rebars, pushed into the side of ##12 and flew away: "it happenned"

5. Hwoever, for me it does not matter the specific set of events in the last miliseconds before the failure. It was clear from the cracks that the bridge was in serious trouble and that nothing should be under it. The death of all six people could have been avoided. So far nobody has accepted their responsability in public (as far as I know). And FIGG apparently still says publicly that it not their fault (as far as I read it in the publications out there). If anybody knows otherwise, please tell me.

6. The deaths could have been avoided by closing the road and having a bunch of engineers (including FDOT Central Office) evaluating the situation before touching the structure.

Live long and prosper.

RE: Miami Pedestrian Bridge, Part XII

"Ja" sorry, a translation. Posted in error for you. I thought meant jovial apologies. In America not same.
You say it short for donkey rear? In my country similar lingo is hard to work with. Condolences. Perhaps bridge too far.

RE: Miami Pedestrian Bridge, Part XII

Dowel action offers near zero resistance at start of slip motion.

RE: Miami Pedestrian Bridge, Part XII

Toward an Improved Understanding of Shear-Friction Behavior
Harries, Kent A; Zeno, Gabriel; Shahrooz, Bahram.ACI Structural Journal; Farmington Hills Vol. 109, Iss. 6, (Nov/Dec 2012): 835-844

ACI 318-08 / AASHTO

https://search.proquest.com/openview/a489f3425d869...

RE: Miami Pedestrian Bridge, Part XII

Does anyone remember an instruction in the signed prints to NOT use an adhesive? I think I do.

SF Charlie
Eng-Tips.com Forum Policies

RE: Miami Pedestrian Bridge, Part XII

Quote (jrs_87 (Mechanical)10 Aug 19 09:57 https://search.proquest.com/openview/a489f3425d869...)

Great link. Wish I could read all of it.
Comments on the general subject of shear friction:
I think FIGG was correct to discard cohesion as contributing. It seems to me all cohesion would be lost due to the slip necessary to mobilize cross-plane reinforcing in tension.
The 1/4" of intentionally roughened surface is commonly achieved by raking the surface before the concrete has set, using something like a heavy asphalt rake. The surface is rough - you would not want to slide into second base on it. But then comes the next pour - with maybe 1/2 inch and 3/4 inch aggregates. The aggregates cannot fit into the 1/4" rake marks, so only the paste and sand can "mate" to the roughened surface. And then there is the slump required, the vibrating of concrete to completely fill all voids, and the fact that the closest access for a tradesman is 16 feet above the joint. What could go wrong?
I have seen rock pockets at the bottom of pours - honeycombing - with little paste to lock the aggregates at the surface of a previous pour.
So with no aggregates in the toothed interlock, when does the concrete matrix simply powder and then lubricate the moving joint? Of course it has already failed if it moves that much.
Regarding the 800 psi upper bound - is that applied to the sum of all components - cohesion AND friction from clamping of reinforcing AND normal force across the joint?

RE: Miami Pedestrian Bridge, Part XII

Quote (The Mad Spaniard)

MCM probably just cleaned the deck without chipping or adding a bonding agent (my guess)

The 8 #7 column bars in strut #11 can not be used in evaluating the Shear-friction capacity of the connection because they are in compression.

The roughness can be done (In my experience, this is usually done) before curing. It is actually easier for the contractor to leave a deliberately roughened surface. It is really hard to say what a 0.25" or 6mm of roughness really is but it is difficult to have any other reasonable descriptor.

I don't know about the US codes but I know in other codes, the bars in "compression" can be used for the shear friction capacity. This is done for pour joints in shear walls. Even though the element is in compression, the bar itself is still in tension. The compression in the concrete increases as slip is induced at the joint (the compression in the concrete increases by the amount the steel is in tension). The bar is in tension even though the member is in compression. This implies that the compression stress should be limited in regions of high shear friction stress since the steel is no longer helping resist the compression stresses. The maximum shear stress is limited by a function related to the compression strength but it is not limited by the actual compression stress at the joint. It may make some sense to limit the shear in the joint by both the compression strength and stress but that is not how our code is written (maybe different in the US). The same applies to the compression zone of bending elements.

RE: Miami Pedestrian Bridge, Part XII

Quote (Vance Wiley)

I think FIGG was correct to discard cohesion as contributing. It seems to me all cohesion would be lost due to the slip necessary to mobilize cross-plane reinforcing in tension.

I am skeptical of using the cohesion component and prefer to ignore it. Unfortunately, the way the code is written is that a single lower phi factor applies to both the steel and cohesion components (I am assuming the US code equation is written the same way). The code writers need to multiple out the brackets in the resistance equation and use one phi factor for the steel and another factor for the cohesion. That way we don't have to under-count the steel component if the cohesion is ignored (set to zero). I can't figure out why the shear friction equation was written in this fashion. Any ideas?

RE: Miami Pedestrian Bridge, Part XII

Isn’t the steel supposed to be perpendicular for shear friction to be valid?

How does it work in this case?

RE: Miami Pedestrian Bridge, Part XII

Vance & Epoxybot, re peer reviews,

It seems that designers or contractors appointing independent reviewers is the norm now or at least widely accepted. Technical independence rather than commercial. I believe there are several reasons including the owner not wanting to administer another contract, not wanting to face delay claims ("YOUR review is late"), and not wanting variation claims ("YOUR reviewer requires a higher standard than your tender documents did"). All the problems are on the contractor's side of the fence.

Public authorities often have consultant panels (pre-approved consultants) from which the reviewer will have to be drawn. Not sure about FDOT.

Having another office of the design firm do the review isn't necessarily shady. In-house reviews are all that most designs get. It can work well because the discussions can be more frank and honest as they happen behind closed doors.

Some reports say the Berger review was by a single engineer. It would have been better with a number-cruncher working under the guidance of a guru.

RE: Miami Pedestrian Bridge, Part XII

Quote (steveh49 (Structural)11 Aug 19 12:48 Vance & Epoxybot, re peer reviews,)

Ahhhhhh - A voice of reason. Clearly in the Structural discipline.
Thank you.

Quote (It would have been better with a number-cruncher working under the guidance of a guru.)

Someone with their head above deck to watch where the ship is going.
Berger is being reluctant to release detailed information on their involvement but some things are known. Do we know if Berger reviewed the main span as an independent structure, without the referenced future contributions from the back span?

RE: Miami Pedestrian Bridge, Part XII

Quote (Tomfh)

Isn’t the steel supposed to be perpendicular for shear friction to be valid?

It doesn't have to be perpendicular but you do have to use the perpendicular component (I suspect it is the same in the US code).

RE: Miami Pedestrian Bridge, Part XII

Quote (Vance Wiley)

Do we know if Berger reviewed the main span as an independent structure
I thought we knew that he did not. He was specifically not paid to do that.

SF Charlie
Eng-Tips.com Forum Policies

RE: Miami Pedestrian Bridge, Part XII

Quote (SFCharlie (Computer)11 Aug 19 19:34 Quote (Vance Wiley) Do we know if Berger reviewed the main span as an independent structure
I thought we knew that he did not. He was specifically not paid to do that.)

Thank you. I missed that or forgot.
If the independent span check was not in the Berger scope of work that should ease the pressure on them. That is probably why they have not participated in a settlement.
Is there a way to search all 13 parts of this issue at one time? I have trouble doing one at a time - jumping to the nexxt sectiun is cumbersome.

RE: Miami Pedestrian Bridge, Part XII

Quote (Earth314159 (Structural)10 Aug 19 21:15 in other codes, the bars in "compression" can be used for the shear friction capacity.)

That seems counter intuitive to me. If the bars have compression then they are actually "stealing" some of the normal (as in perpendicular) force from the joint and the concrete to concrete force would be less.
Also, if the bar is in compression it must have shortened - however little - and would not this compression shortening need to be overcome before the steel can be mobilized to assist shear friction?

RE: Miami Pedestrian Bridge, Part XII

Some good can come from this. Two things...the concept of a post-tensioned concrete truss can be consigned to the dust bin. And hopefully, shear friction theory can be given better scrutiny. It has been seized on some in the industry as a convenient way to deal with construction joints without having to develop the forces in bearing. I remain a skeptic, as expressed many times in the structural forum.

RE: Miami Pedestrian Bridge, Part XII

hokie66 - hopefully, shear friction theory can be given better scrutiny.

Reinforcing for shear-friction.
Lets see - the development length of a bar is dependent on its size - the bigger the bar the longer the development length. Conversely, the smaller the diameter, the shorter the development length.
And is not the elastic lengthening dependent on the length of the bar under tension?
So smaller bars should exert greater forces (per unit area) at less elongation than larger bars, because they are fully developed in a shorter distance.
I am not sure the codes address this.
Too small an issue to be concerned? Accounted for in equations? Should it be? Is the dowel action an offset?
Too fine a point? It is a bridge, not a space shuttle.

RE: Miami Pedestrian Bridge, Part XII

SFCharlie (Computer)11 Aug 19 19:34
Quote (Vance Wiley)
Do we know if Berger reviewed the main span as an independent structure
I thought we knew that he did not. He was specifically not paid to do that.

Greetings to all:

What if the team MCM/FGG (or only FIGG because it was going to be from their part of the "pot") wanted to reduce design costs and ask Berger to only do a strength check of the main members using a FE model and not of the connections? Is it possible that the connections (the element that I am gessing created the problem) may have been designed between 60% and 100% and therefore Berger never saw the final reinforcing plans?

This is why I believe Berger is holding up in the restitution agreement.

I believe Berger had nothing to do with the failure because they were not contracted to evaluate the connections or the final plans and they never reviewed the final plans. What would you do if you were now in their position if my assumptions regarding their involvement are correct?

RE: Miami Pedestrian Bridge, Part XII

If Berger accepted a commission like that, they neglected their primary duty to the public.

RE: Miami Pedestrian Bridge, Part XII

Vance Wiley,

To many of us, shear friction remains a thought bubble. The Australian concrete code, which has often followed, and sometimes led, the ACI code, still does not recognize shear friction as a legitimate design procedure. I note that you, and many of the contributors to this particular discussion, are relatively new members of Eng-Tips. If you search the Structural Engineering general discussion forum, you will find extensive debate about shear friction.

RE: Miami Pedestrian Bridge, Part XII

hokie66 (Structural)11 Aug 19 20:35
Some good can come from this. Two things...the concept of a post-tensioned concrete truss can be consigned to the dust bin. And hopefully, shear friction theory can be given better scrutiny. It has been seized on some in the industry as a convenient way to deal with construction joints without having to develop the forces in bearing. I remain a skeptic, as expressed many times in the structural forum.


I disagree. I can not accept to be beaten as an structural engineer by the failures of others. We are suppose to be the masters of loads and strength. We control them. This bridge only needs about a few (about let's say 5) addicional cubic yards on concrete and about an additional 200 pounds of steel to be safe at all times. My recommendation is to rebuild it almost identically so nobody will know the difference except those who have added the needed concrete and rebar in the right places.

Well, maybe eliminate a few stays and struts in the short back span (by about 1/2). It does not look good now.

Heck, I will designed it for free. smile. It is 99.99% done. Just a couple of pages of additional calcs and slightly changing a couple of drawings. Just the publicity will provide me more additional bussines.smile

Live long and prosper.

RE: Miami Pedestrian Bridge, Part XII



hokie66 (Structural)11 Aug 19 22:30
If Berger accepted a commission like that, they neglected their primary duty to the public.

This is a wonderful comment and shows a philosophical item in our bussiness. Because it is a bussiness. My guess it that nobody at the time couuld believed that once you get all the forces thru analysis the structure would not be reinforced properly by any well known firm.

Now that we know more, what?

This case will be teached in engineering school forever.

RE: Miami Pedestrian Bridge, Part XII

Quote (Vance Wiley)

That seems counter intuitive to me. If the bars have compression then they are actually "stealing" some of the normal (as in perpendicular) force from the joint and the concrete to concrete force would be less.
Also, if the bar is in compression it must have shortened - however little - and would not this compression shortening need to be overcome before the steel can be mobilized to assist shear friction?

That is why I put compression in quotation marks. The member can be in compression or you could be in the compression zone of a bending element. However, the bar is actually in tension even though the member is in compression. The compression on the concrete interface increases (as the bar begins to stretch) with shear movement since the interface is forced to split apart under the shear load. The plane sections assumption is not valid when there is a large shear at a pour joint (concrete is in a compression strain and the rebar is in a tensile strain). There is a very high strain differential between the bar and the surrounding concrete.

I think you are also correct in assuming that you need more roughness for larger bars since the development length is longer. You would also have an issue with stronger steel for the same reason. I think I have only ever used a 55M a couple of times (pretty rare to get a bar that big) and I don't have my concrete hand book with me but I will assume a development length of 70" as a rough estimate (this is a development length and not a splice length). Assume a linearly increasing strain distribution along the length. 2x6mm/(70x25.4)= 0.0066 which is greater than the yield strain. It is conceivable that a 0.25" or 6mm roughness still works with big bars. A smaller and/or weaker bar would work with less roughness. High strength concrete may not necessarily require as much roughness in theory since the development lengths are less. The actual shear friction parameters are more complex than the code equations imply.

I hope this incident doesn't serve as an pretext to unnecessarily increase the complexity of the shear friction equations.

RE: Miami Pedestrian Bridge, Part XII

Quote (The Mad Spaniard)

I disagree. I can not accept to be beaten as an structural engineer by the failures of others. We are suppose to be the masters of loads and strength. We control them. This bridge only needs about a few (about let's say 5) addicional cubic yards on concrete and about an additional 200 pounds of steel to be safe at all times. My recommendation is to rebuild it almost identically so nobody will know the difference except those who have added the needed concrete and rebar in the right places.

Well, maybe eliminate a few stays and struts in the short back span (by about 1/2). It does not look good now.

Heck, I will designed it for free. smile. It is 99.99% done. Just a couple of pages of additional calcs and slightly changing a couple of drawings. Just the publicity will provide me more additional bussines.smile

I agree. There are advantages to having a concrete truss bridge. It was the design over-sight and not the truss that was a problem. I would say though that there are other issues with the bridge that are difficult to design but feasible.

Personally, I think the tube stays just confuse the aesthetics. I would use fewer stays and replace the tubes with viscous dampers. They would be more effective for dampening vibration and allow the bridge to creep up or down without effecting the loads on the truss. With the design as is, an upward creep from the longitudinal deck PT could overload the truss. This adds a compression load in the cable stays which makes the loading on the bridge worst. If the deck creeps downward, the load path is difficult to predict. Even though the stays were not "structural" they can add significant adverse loading at connections, members etc. if not properly addressed. The stays add redundancy but they also add complexity and reduce predictability.

RE: Miami Pedestrian Bridge, Part XII

Quote (earth)

I hope this incident doesn't serve as an pretext to unnecessarily increase the complexity of the shear friction equations.

In my opinion there should be more onerous checks and balances if you wish to rely on roughened surface for a critical connection. Construction joint preparation is prone to failure. It's not like concrete strength or steel strength, where 99%+ chance you get what you ask for. For deliberately roughened surfaces the chances are the concrete will be too smooth. It is a major hassle roughening concrete, and contractors won't generally do it, and if they do they often won't do it properly. It is labor intensive.

RE: Miami Pedestrian Bridge, Part XII

The Mad Spaniard,

So by your comments, I take it that you believe the rest of the bridge was just fine, and that the only problem was at the joint which failed first. That is a big assumption, but one which seems to be common in the recent discussions here.

RE: Miami Pedestrian Bridge, Part XII

Quote (SFCharlie (Computer)11 Aug 19 19:34 Quote (Vance Wiley) Do we know if Berger reviewed the main span as an independent structure I thought we knew that he did not. He was specifically not paid to do that.)

Charlie - I cannot find that in a post here -I have searched for "berger" in each section.
Do you have a reference?
Thank you.

RE: Miami Pedestrian Bridge, Part XII

Quote (Vance Wiley (Structural)12 Aug 19 03:37)

Charlie - I cannot find that in a post here -I have searched for "berger" in each section.
Do you have a reference?
Here is an example ... there are others.

Quote (epoxybot (Structural)13 Jun 19 01:54)

FIGG did not contract for 30%, 60% & 90% PEER Review as required by FDOT. They originally tried to finagle FDOT to allow one of their OTHER FIGG offices do the PEER Review. They finally finagled FDOT to agree to PEER Review when plans were at 100% Construction ready but they used that instead to contract with Louis Berger for a 100% completed structure PEER Review instead of the still required incremental PEER Review. Louis Berger submitted the PEER Review Certifications to Alfredo Renya the FDOT LAP Coordinator, bypassing FDOT Structural Reviewer: Tom Andres. This seems a bit slippery behavior by FIGG.

SF Charlie
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RE: Miami Pedestrian Bridge, Part XII

RE: BERGER Scope
I have found the scope of Berger work in the OSHA Report pages 36 and 37.
https://www.osha.gov/doc/engineering/pdf/2019_r_03...
It basically says Berger did only a completed structure review at 100% design.
And - thank you, Charlie.

RE: Miami Pedestrian Bridge, Part XII

hokie66 (Structural)12 Aug 19 00:55
The Mad Spaniard,

So by your comments, I take it that you believe the rest of the bridge was just fine, and that the only problem was at the joint which failed first. That is a big assumption, but one which seems to be common in the recent discussions here


Greetings:

Very soon this is going to be the most analyzed structure ever. We have already three FEM analysis: FIGG, BERGER and OSHA. THe NTSB is going to analize big time. I am sure that there are at least more than a couple of Master students out there in the world using this bridge to get a degree.

So, we are going to have several papers/documents showing forces in he existing structure and telling us what when wrong and where.

I believe that the connections strut/deck or strut/canopy show "little meat and bones" . I will put a little more concrete and steel there to beef-up them up and avoid "nasy cracks" that would create "masty paperwork and media attention" (We all in Florida remmember the construction of Shyway).I even would use fiber reinforcing concrete there and in the struts. And definitly, I will add some internal details to retrofit the connection #11/deck. But from practical and visual purposes the connections will be the same for a non-engineer.

Best reagrds

RE: Miami Pedestrian Bridge, Part XII

FDOT & FIGG email regarding 100% Plans PEER Review. The NBC Timeline is quite handy.



Scope of Work

RE: Miami Pedestrian Bridge, Part XII

Quote (epoxybolt)

FDOT & FIGG email regarding 100% Plans PEER Review. The NBC Timeline is quite handy.

That looks like a typical communication. Everyone one wants to get their foot in the door for permit approvals so the last thing to get submitted is the independent review documents. You have to be close to finishing the design drawings before the independent review is completed. At least now we know the drawings would have been close to finished for the review.

RE: Miami Pedestrian Bridge, Part XII

I didn't see anything there that prevented Berger from examining the whole construction scheme, including the main span standing alone. The scope required them to review the whole bridge, but not before the documents were complete. OSHA may have misinterpreted, or been misinformed, about reviewing construction stages as opposed to design stages. The main thrust of OSHA's report is that the road below should have been closed, and no one now should argue with that.

RE: Miami Pedestrian Bridge, Part XII

Peer Review by Berger
From the schedule proposed by Berger we learn they intended to spend 7 weeks on the peer review. That is 35 working days and for a $61,000 fee that is $217 per hour.
The Berger scope includes the following:
3. The Independent Peer Review will be performed for the following submittals:
b) Final superstructure plan Submittals
An hour spent studying the plans would have revealed to an engineer that the 174 foot span was required to span and support its own weight and construction loads for a period of time, until the back span was complete and cured.

I have no experience in performing FEA analysis - how much time could it have taken to remove the back span and everything except the bare 174 foot section from the model and run the 174 foot section independently with dead load and construction loads?

The ABC idea imposes different conditions than typical in-place construction - (all here understand that). In non ABC construction, the contractor is responsible for everything - falsework, materials, jobsite safety, procedures, public safety (with approval by the agencies) - the contractor "owns" the thing until it is complete and accepted by the Owner and authorities. The design of any portion subject to ABC procedures would be the responsibility of the design firm. FIGG's drawings show the steps for the ABC process in this project.

It appears that everyone thought if the finished structure works it will work at every step in the process.
But were the ABC requirements of this project not addressed clearly on the "Superstructure Plans"?
I think we would find the emails between Berger and FIGG very interesting.

RE: Miami Pedestrian Bridge, Part XII

I've asked this a couple times, and I don't believe that there has yet been posted a satisfactory answer:

What are the substantive structural differences between the as-collapsed construction stage and the final stage, and how would they have prevented collapse?

Clearly the final stage would have included the back span, the pylon, and its decorative faux "stays" that make it look like a cable-stayed bridge. But I have yet to see a convincing case that these elements would have substantially increased the capacity of the structure.

The back span's 13/14 node would probably have buttressed the 11/12 node and increased its resistance to getting pushed off the deck by the horizontal component of the compression force in 11. But there does not appear to have been any provision to structurally tie together the decks of the main and back spans. So it seems to me that the back span would have just changed the failure mode so that what happens is that the horizontal component of the force in 11 just pushes one or both decks off of their piers with basically the same result. It might have taken a bit more live load to make that happen, but that might have been a blessing in disguise: The live load could well have been in the form of a spring break party consisting of hundreds of students.

Thoughts?

--Bob K.

RE: Miami Pedestrian Bridge, Part XII

Good question, Bob K. My examination of the drawings was only cursory, but it seemed to me that the connection between the two spans was only nominal, perhaps for lateral stability. To make the bridge truly continuous over that support would have required much more robustness, and the top chord might have been deficient in tension. As well, the diagonals at the support would have taken even more load than in the simple span condition.

RE: Miami Pedestrian Bridge, Part XII

They’re simple spans aren’t they?

Why would it be a continuous bridge if they’re dropping the spans in one at a time?

RE: Miami Pedestrian Bridge, Part XII

I hope someone will answer Bob hpaircraft's question. Since 11, the canopy, and deck all failed without 12 moving, I don't see what the pylon and back span would have contributed to preventing the failure? Wouldn't the weight of the faux cables, expanding on a hot day, have only contributed to the load on 11? How the failed 11, canopy, and deck would have fallen, is a different question that I'm not prepared to hypothesize on.

SF Charlie
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RE: Miami Pedestrian Bridge, Part XII

Quote (Tomfh)

They’re simple spans aren’t they?

Why would it be a continuous bridge if they’re dropping the spans in one at a time?

There is bending continuity at the support. C1 and C4 cables run through for a factored tensile force of 2500 Kips in the canopy at the pier location. That is quite significant. This cannot be analyzed like a typical continuous span. The staging makes a big difference to the loads. The hogging moment can also be fine tuned unlike other typical continuous spans.

RE: Miami Pedestrian Bridge, Part XII

Quote (hpaircraft)

I've asked this a couple times, and I don't believe that there has yet been posted a satisfactory answer:

What are the substantive structural differences between the as-collapsed construction stage and the final stage, and how would they have prevented collapse?

Clearly the final stage would have included the back span, the pylon, and its decorative faux "stays" that make it look like a cable-stayed bridge. But I have yet to see a convincing case that these elements would have substantially increased the capacity of the structure.

I have partially answered this before. As you allude #11 and #14 buttress each other off. Now, only the difference in horizontal shear has to go through the pour joint. This substantially reduces the shear friction stress for this one joint. The forces actually go up in #14 and #11 when the C1 and C4 cables (the C1 and C4 cables are continuous over the pier and connect the canopies on the two sides together) are pre-stressed but the load transfer is still less.

There were still other issues with the design. You could argue that the tube stays would help but they were supposed to have been ignored for the strength. The tube stays would actually increase the shear loads on the joints if there was so much PT in the deck that caused an upward cambre. As concrete creeps, the camber can actually creep up. The deck may also creep down in which case the tube stays help relieve the stresses in the concrete joints.

There were also other failure mechanisms in the joints what would not have been resolved by having continuity in the spans such punching in the deck. The tie arrangements in the diagonals do not meet code (I am assuming the US code is similar to other codes in this respect). There is no easy fix to this issue.

Even if Louis Berger only reviewed the final stage, there were still many issues that they should have identified. To be fair, they may have identified all the pertinent issues for the final stage. It could be that the issues were never properly addressed in the final drawings.

RE: Miami Pedestrian Bridge, Part XII

ABC is similar in other prestressed structures. You may have ABC without prestressing or you may have prestressing without the ABC processes. If either one or both of these conditions apply, you have to review multiple stages. Prestressing behaves differently in the construction phase, short term and the long term. The loading conditions are also different in the long and short terms.

In the construction phase, the concrete is weaker and less stiff. The initial prestressing can over load the top of a beam (or truss in this case) in tension (near the ends of the span). Young's modulus is effectively different in the long term and short term (a factor of 2.5 to over 3 in stiffness). For short term and infrequent loads, you may choose to use the short term young's modulus. For dead weight and prestressing, you need to use the long term young's modulus for long term results. This all implies that you need multiple models for the different load combinations. The forces on the stays are affected by both longer term and short duration loading conditions.

RE: Miami Pedestrian Bridge, Part XII

It is not clear to me how the PT in the canopy would transfer any weight off 11. I did not see any mechanism to tighten the faux stays. 11 doesn't seem to have needed 12 to move for 11 to fail. 11 seems to become visibly shorter early in the collapse.

SF Charlie
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RE: Miami Pedestrian Bridge, Part XII

Quote (SFCharlie)

It is not clear to me how the PT in the canopy would transfer any weight off 11. I did not see any mechanism to tighten the faux stays. 11 doesn't seem to have needed 12 to move for 11 to fail. 11 seems to become visibly shorter early in the collapse.

The PT in the canopy actually increases the load on #11 and does not transfer any weight off of 11. #11 and #14 counterbalance each other so the shear at the pour joint can be reduced. The shear is the difference between the horizontal components of the two loads.

There is no mechanism to tighten the stays. Sag (if it sags) in the deck would load the stays in tension. Cambre in the deck would compress the stays and increase the loads (or a hot day as you noted above) on the truss.

11 would have to fail in compression if 12 was immovable with infinite attachment strength to the diaphragm or the joint up at the canopy would have to fail (likely the next weakest link in the load path).

There is shortening in 11 (you can't avoid compression strains)but mainly what you see is sliding and not shortening.

RE: Miami Pedestrian Bridge, Part XII

C-1 and C-4 PT effects
Help me here - From drawing B-69 I see 2 X C-1 tendons at 430 kips = 860 kips and 2 X C-4 tendons at 863 kips = 926 kips for a total delayed PT force of 1786 kips actual force.
A free body of each span about the pylon would appear to be a 1786 kip force 16 feet above the deck at each end, with the north and south ends on low friction bearing pads, unrestrained. Would that not create a moment at the pylon of 1786k X 16 feet = 28,576 ft-kips? That lifts the south end by 28,576/174=164 kips added to the reaction at the pylon. Likewise for the back span, that moment adds 298 kips reaction at the pylon.
Of course this is at a completed stage for the project. But of importance here is that it adds 164 kips vertical component to the force in member 11. That adds 311 kips axial compression to member 11 (unfactored).
In reviewing the strut reinforcing, I see member 9 has 10 -#7 bars while member 11 has 8-#7. Member 11 has much more load than member 9.
From drawing B-39 and 40 Member 9 is section A=A " Member with no PT bars" has 10 bars. Elevation of member 11 on B-40 calls for 2 bars each face for a total of 8. But member 11 has the PT bars for lifting and transportation. This looks like a failure to coordinate a condition that changed when the transporters were relocated and the PT was added to members 2 and 11.
I don't know if 10 bars could have saved it or not but 11 was splitting badly before it collapsed.

RE: Miami Pedestrian Bridge, Part XII

Quote (Earth314159)

...The forces actually go up in #14 and #11 when the C1 and C4 cables (the C1 and C4 cables are continuous over the pier and connect the canopies on the two sides together) are pre-stressed but the load transfer is still less...

Interesting. Yes, I can see how connecting the C1 and C4 tendons might have helped prevent the failure we saw. But I think that connecting the two decks as well would have been much better. From the tone of the Figg correspondence about "capturing the node," I would bet that they might have been hatching a plan to do just that, using the same connection scheme that they were planning to use to join the PT tendons in the canopies of the span and backspan. As I've written before, maybe they could have used the same coupler nuts that would have prevented the Hyatt skywalk collapse. But at this point we'll probably never know for sure.

--Bob K.

RE: Miami Pedestrian Bridge, Part XII

Quote (hpaircraft (Aeronautics)12 Aug 19 22:49 What are the substantive structural differences between the as-collapsed construction stage and the final stage, and how would they have prevented collapse? Clearly the final stage would have included the back span, the pylon, and its decorative faux "stays" that make it look like a cable-stayed bridge. But I have yet to see a convincing case that these elements would have substantially increased the capacity of the structure.)

I must agree - many questions remain. Your concerns point out the complexity of this project. You have covered most of the pertinent points. I originally challenged the concept that when finished all will be well, but now I see the next stage could have helped - but the extent is questionable. The pylon section to be cast thru the truss and wrapping 12 and 13 could have helped tie the sections together. It would need many ties and much reinforcing across a finished construction joint and into the deck surface, and I see little provision for this on the drawings. The idea to add late PT in the canopy could well serve if applied to the deck PT and provided a clamping force over the pylon at the deck level. How well the late PT force in the canopy transfers thru the webs to the deck is problematic, I think. Shear lag comes to mind. In a simple free body diagram it seems to work.

Quote (The back span's 13/14 node would probably have buttressed the 11/12 node and increased its resistance to getting pushed off the deck by the horizontal component of the compression force in 11. But there does not appear to have been any provision to structurally tie together the decks of the main and back spans. So it seems to me that the back span would have just changed the failure mode so that what happens is that the horizontal component of the force in 11 just pushes one or both decks off of their piers with basically the same result.)

The connection over the pylon appears problematic. But with members 11 and 13 opposing each other thrust wise, the net force between the superstructure and the pylon is reduced - IF the two sections are well connected together. The expansion joint at the south bent is 2". That could limit the slip IF there were adequate resistance provided by the south stairs. But the bottom of 11 had slipped less than 2" when the collapse began.

Quote (It might have taken a bit more live load to make that happen, but that might have been a blessing in disguise: The live load could well have been in the form of a spring break party consisting of hundreds of students.)

I have previously expressed those same thoughts. Sadly, the early collapse may have been the best way out. That is a poor and regrettable comment for a project.

RE: Miami Pedestrian Bridge, Part XII

Seismic Connection to PYLON
The completed superstructure is connected to the Pylon but has slip bearings at the south and north ends. The ends have no provision for restraint in the east-west or north-south direction.

FDOT section on Seismic:

The completed structure is a two span bridge and for seismic loads the entire superstructure is tributary to the pylon. As I read that, the connection of the superstructure to the pylon would be 0.12 X (950 tons X 270'/174') = 177 tons or 353 kips. That seems to be adequately resisted if the member 11 and 14 loads are otherwise accounted for.
The superstructure itself is exempt from seismic design. I presume the pylon and substructure would be designed for this horizontal force.


RE: Miami Pedestrian Bridge, Part XII

Quote (Vance Wiley)

C-1 and C-4 PT effects
Help me here - From drawing B-69 I see 2 X C-1 tendons at 430 kips = 860 kips and 2 X C-4 tendons at 863 kips = 926 kips for a total delayed PT force of 1786 kips actual force.

That is the correct idea. You should also add in the live loads. I don't have my work books, spread sheets, or favorite calculator at home but here is a quick approximation of the diagonal #11 factored loads and squash capacity (no slenderness taken into account). I will assume a tension capacity of 2500 Kips for C1 and C4. The conduits are grouted so I think this is fair but it may be a little conservative since I may not need the full 2500 Kips. I am going off memory for the capacity equations (the US equations may also vary a bit). I usually use the interaction diagrams for the axial and bending squash load capacities. The longitudinal bars are just barely 1% of the cross sectional area which is just enough where the code allows you to take the full concrete section into account. The tie arrangement does not meet code requirements which I am ignoring. I usually calculate capacities in metric and use metric sized rebar so if I made an error, let me know.

Reaction: 1.25x950Kips+1.5x30'x175'/2x90psf/1000+2500Kip*16'/175'=1190+350+230=1770 Kips
Load from #12 and end span of deck/canopy: 1.25*8kip/ft*15'+1.5*15'x30'*90psf/1000+1.25*3kip/ft*12.5'+1.25*2.9'x1.75'*14'*150/1000=150+60+47+13=270Kips
Net vertical component into #11: 1770-270=1500Kips
Total load in #11: 1500/sin(32)=2830Kips

Capacity of column: 0.8*0.65*8.5ksi*24*21+8*30.6Kips=2230+245=2475Kips<2830Kips therefor NG.

If you ignore the increase in the load due to the hogging moment, the #11 compression load reduces to 2400Kips. This only gives 75 Kips (40 Kips reaction) to play with. C1 and C4 can only be tightened to 430 Kips until the diagonal #11 is overloaded by code in compression. It is still not a likely point of failure but it doesn't meet code requirements.

In any case, it appears that the design for #11 in compression is insufficient for the full gravity load case. The tie arrangements also do not provide sufficient confinement for the concrete core of the column.

RE: Miami Pedestrian Bridge, Part XII

Quote (Earth314159 In any case, it appears that the design for #11 in compression is insufficient for the full gravity load case. The tie arrangements also do not provide sufficient confinement for the concrete core of the column.)

Thank you for confirming what I have sensed for a while.
I have to wonder what Berger found for member 11.

The term "hogging moment" is not familiar to me. It is referring to the load drawn by M/L, right?


RE: Miami Pedestrian Bridge, Part XII

Quote (Vance Wiley)

The term "hogging moment" is not familiar to me

Hogging moment is a negative moment over a support. I think it is more used in the common wealth than the US (I am not too sure how common it is in the US). You usually associate a hogging moment with negative bending and a high shear together at a support. You can use it for a cantilevered beam but it is usually intended for a continuous beam.

RE: Miami Pedestrian Bridge, Part XII

Thank you, SFCharlie . EDIT And thank you, Earth314159. Your reply came in while I was typing. END EDIT

The calculation by Earth314159 shows that member 11 should not have failed under self weight only in the independent simple span condition which existed at the completion of Stage 3.
Which seems to leave node 11/12/deck as the suspect.

Apparently the capacity of the joint as given by the code provisions represented on page 31 of the FIGG Power Point presentation of March 15 do not apply after the joint has undergone severe cracking. Not surprising.

RE: Miami Pedestrian Bridge, Part XII

I still don't trust the little bit of concrete between 11 and 12...

SF Charlie
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RE: Miami Pedestrian Bridge, Part XII

If I've followed it correctly, "hogging" also applies to ships where a wave lifts the center abeam of a long ship, leaving the bow and stern with less support.

RE: Miami Pedestrian Bridge, Part XII

Quote (3DDave)

If I've followed it correctly, "hogging" also applies to ships where a wave lifts the center abeam of a long ship, leaving the bow and stern with less support.

I am not a ship designer but it is the same concept.

RE: Miami Pedestrian Bridge, Part XII

Quote (SFCharlie (Computer)15 Aug 19 17:27 I still don't trust the little bit of concrete between 11 and 12...)

You are right on track. This is the place to have focused on. Had the top of that fillet been maybe a foot higher and the little fillet below 11 to the deck been maybe 3 feet longer there could have been room for lots of reinforcing across the construction joint.
But no one was focusing on this joint as loaded at the end of Stage 3.


RE: Miami Pedestrian Bridge, Part XII

Quote (SFCharlie)

I still don't trust the little bit of concrete between 11 and 12...

Once you get a slip failure, the compression stresses between #11 and #12 increases. If you only take the DL case, you end up with a vertical reaction on #11 of about 1100 Kips (factored). You have to factor DL by 1.4 instead of 1.25 when the DL is considered by itself. This makes a horizontal component of 1750 Kips. The concrete is 1.25' high by 1.75' wide for a stress of 5.6 Ksi (38MPa) which is pretty high. Assuming we can get 1% horizontal steel in the cage, you can go up to 30 MPa. However, even at 38 MPa, it isn't going to fail in compression unless there are other contributing factors.

I suspect the cracks parallel (more or less) to the diagonal you see are from varying stress across the slip failure surface. The pour joint is not perfectly flat. The shear stress on some parts of the surface are higher than others and #11 is not well tied together. The differential stress helps initiate the cracks you see. The compression between #11 and #12 makes the cracks worse. In any case, that is what I suspect.

No mater how you cut it, this joint is stressed far beyond acceptable levels.

RE: Miami Pedestrian Bridge, Part XII

I should also mention that the rotation of the base of #12 increases the compression stress of the bit of concrete between #11 and #12. The compression causes a slight rotation at the base of #12 that causes the vertical crack on the south side of #12. This focuses the compression lower to the base of #12.

RE: Miami Pedestrian Bridge, Part XII

Quote (Earth314159)

Capacity of column: 0.8*0.65*8.5ksi*24*21+8*30.6Kips=2230+245=2475Kips<2830Kips therefor NG.
So having 10 bars in member 11 like I think was intended (Section A-A Drawing B-39) would have increased the capacity by 61 kips and the member would still lack capacity from a code standpoint.
It certainly had design capacity for loads when the collapse developed.
Again, thank you for running the numbers.

RE: Miami Pedestrian Bridge, Part XII

Earth314159 (Structural)15 Aug 19 18:28
I should also mention that the rotation of the base of #12 increases the compression stress of the bit of concrete between #11 and #12. The compression causes a slight rotation at the base of #12 that causes the vertical crack on the south side of #12. This focuses the compression lower to the base of #12.


If the fillet (wedge) is compressed from #11 and from #12, and the PT bar is stressed, isn't it possible that it would "pop-up" vertically? Leaving #11 free to press more to #12. Also, given the sudden failure, don't we have additional forces due to dynamic effects (even from the sudden break of the reinforcing bars)?

Questions, questions...

But at the end on the day, the reinforced concrete was telling from March 11 2018 : I am cracking in a bad place. Please, get the hell out of here.

Live long and prosper.

RE: Miami Pedestrian Bridge, Part XII

I still don't understand how being in the finished configuration helps 11. If I use a derick to lift a weight, the wire cable to the cab lifts the weight, but the boom is in more compression?

SF Charlie
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RE: Miami Pedestrian Bridge, Part XII

Quote (SFCharlie (Computer)15 Aug 19 18:49 I still don't understand how being in the finished configuration helps 11.)

Again, you are on the right track.
Actually, by developing the "hogging moment" through the addition of PT C-1 and C-4 in the canopy AFTER the backspan is complete, more load is added to member 11. That additional load is about 311 kips, unfactored.
So the completion of the structure does not help member 11. It may have added capacity to node 11/12/deck by supporting/resisting some of the blow out forces.

RE: Miami Pedestrian Bridge, Part XII

Quote (SFCharlie)

I still don't understand how being in the finished configuration helps 11. If I use a derick to lift a weight, the wire cable to the cab lifts the weight, but the boom is in more compression?

It actually makes #11 worst (more compression on a highly stressed #11) but it makes the shear transfer at the base better. The shear transfer is the horizontal component of #11 minus the horizontal component of #14. This puts less shear across the pour joint. This assumes you could have gotten to the finished state without over stressing the joint.

I am not a lawyer but I think the issue was that the shear was the cause of the failure. There are all kinds of issues with the bridge. When comes to damages, it doesn't mater what the other issues were. I suspect the legal question is did the negligence of Louis Berger contribute to the failure mode that killed the people (or caused other damage). They were hired to review the completed structure. They missed multiple code issues for the final configuration but I suspect they will argue, all the missed items are not relevant since it was the shear failure that caused the failure. The other omissions could have lead to a failure but they did not lead to this failure. I can speed down the highway. I may get a ticket but I won't be charged with a crime. I can speed down the highway go out of control and kill someone. I will get charged with a crime. All the other times I was speeding did not mater to the final crime and charges. Each omission is likely considered a separate incident that did not happen to have a consequence (even though there could have been future consequences).

RE: Miami Pedestrian Bridge, Part XII

Quote (The Mad Spaniard)

If the fillet (wedge) is compressed from #11 and from #12, and the PT bar is stressed, isn't it possible that it would "pop-up" vertically? Leaving #11 free to press more to #12. Also, given the sudden failure, don't we have additional forces due to dynamic effects (even from the sudden break of the reinforcing bars)?

The PT added to the shear stresses which I did not account for in the above calculations. Re-tightening the PT was the final straw.

You can not lift #11 with the PT. It is like standing on a skipping rope and pulling on it in hopes that it will lift you up. For lift up to occur, the rest of the frame would have to be strong (with an independent load path) and stiff enough to support the bridge. The bending in the deck and canopy are orders of magnitude too weak and flexible to support the bridge. If the canopy and deck were strong and stiff enough to do this, there would not have been a failure (or perhaps at the very least a very slow progressive failure would have occurred).

RE: Miami Pedestrian Bridge, Part XII

Quote (Vance Wiley)

So having 10 bars in member 11 like I think was intended (Section A-A Drawing B-39) would have increased the capacity by 61 kips and the member would still lack capacity from a code standpoint.
It certainly had design capacity for loads when the collapse developed.
Again, thank you for running the numbers.

Correct.

Adding vertical steel to a column has little effect on it's capacity. 1% is minimum to take the full section into account and anything more than 4% is not practical. When there is a compression issue in a diagonal or column, it is much better to increase the area, then the strength, then as a last resort the vertical steel.

RE: Miami Pedestrian Bridge, Part XII

Quote (The Mad Spaniard)
If the fillet (wedge) is compressed from #11 and from #12, and the PT bar is stressed, isn't it possible that it would "pop-up" vertically? Leaving #11 free to press more to #12. Also, given the sudden failure, don't we have additional forces due to dynamic effects (even from the sudden break of the reinforcing bars)?

The PT added to the shear stresses which I did not account for in the above calculations. Re-tightening the PT was the final straw.

You can not lift #11 with the PT. It is like standing on a skipping rope and pulling on it in hopes that it will lift you up. For lift up to occur, the rest of the frame would have to be strong (with an independent load path) and stiff enough to support the bridge. The bending in the deck and canopy are orders of magnitude too weak and flexible to support the bridge. If the canopy and deck were strong and stiff enough to do this, there would not have been a failure (or perhaps at the very least a very slow progressive failure would have occurred).


I was taking about the "fillet" , the concrete wedge in between 11 and 12.

Regarding having the other truss built in the final stage, it confines the node horizontally. Then it is only a matter of #11 to be able to transfer horizontal loads to #12 which is now restrainded on the other side by the shorter truss with forces (may be smaller) in the oposite direction. But again, the "wedge" has to be there to transfer the load. If the wedge pops-up, then here we go aging with the dynamic effects. Ball park, many times these dynamic effects are 2 times the force that is being let go.

RE: Miami Pedestrian Bridge, Part XII

Quote (The Mad Spaniard)

I was taking about the "fillet" , the concrete wedge in between 11 and 12.

I see.

You could get an apple seed type failure. Most compression failures are shear failures. It is a plane at an angle to the longitudinal compression axis that shears (the angle varies with the amount of ties etc). It is really a shear friction failure through a monolithic section at an angle to the axis. It is pretty much like a concrete cylinder test failure plane (except rotated 90 degrees).

The piece of concrete between #11 and #12 can shear and pop out just like any concrete columns or cylinder. However, in the final stage (assuming there was no damage in intermediates stages) the compression would not be high enough to do this. The compression stress is high but there would be some shear fiction capacity through the pour joint that would help reduce the compression.

If the intermediate stages are ignored, the whole joint design in the final stage is unnecessarily tenuous and barely meets code (perhaps a little under code). Even in the final stage, you really have to scratch to make this joint work. Anyways, I think this is what Louis Berger is clinging too. The only other factor is the tube stays that Lois Berger could argue as an alternate load path for the joint. They may argue that the joint was allowed to be over-stressed since the tube stays could add some support and prevent that one failure mechanism. It is tenuous but it is up to the prosecution to prove the shear friction mechanism in the joint would have been too weak which would then cause a failure in the final stage. Damage to the joint in the intermediate stages would be immaterial to their case (if the lawyers would make this argument).

RE: Miami Pedestrian Bridge, Part XII

Quote (Vance Wiley)

So having 10 bars in member 11 like I think was intended (Section A-A Drawing B-39) would have increased the capacity by 61 kips and the member would still lack capacity from a code standpoint.
It certainly had design capacity for loads when the collapse developed.
Again, thank you for running the numbers.

I just check the hand book. You have multiply the steel component by 0.8 as well as the concrete component. This reduces the compression resistance component from the steel. It doesn't make much difference but I thought I point it out.

RE: Miami Pedestrian Bridge, Part XII

Quote (Earth314159)

I think this is what Louis Berger is clinging too.

The settlement terms are confidential but I can't imagine the hold-out is due to so subtle a point. It is more likely that the level of responsibility assigned is seen as too great. Again, we don't know all the details, but accepting responsibility for the complete loss of the structure would be one thing. Accepting responsibility for the loss of life is another. The bridge gave plenty of warning to those responsible for the construction and there is little reason why anyone had to die that can be related a plans checker (my opinion). Even if there was a lack of expected reserve capacity in the structure, it is so far beyond reasonable that the EOR and contractor exposed the public to such risk given the signs of distress and on-going post tensioning activities.

RE: Miami Pedestrian Bridge, Part XII

Quote (charliealphabravo)

The settlement terms are confidential but I can't imagine the hold-out is due to so subtle a point.

You may be right. I am no lawyer but legal questions are often not about common sense. In most cases that I have seen or heard about from colleagues, it doesn't mater how little the responsibility lies on a consultant, they become responsible for the damages. What is shocking, they can become responsible for the full amount of the damages if the other parties are not able to pay damages due to bankruptcy or dissolution of a corporation. If you are 10% responsible, you can be responsible for 100% of the damages. Each country is different. I know that there has been some movement to change the laws but I don't know how successful the changes have been.

In my view, Louis Berger was negligent but did that negligence lead to this particular failure? I think that is the real legal question.

We have to study engineering and the law where I come from. Is this the same in the US? Our professional exam includes legal questions. Do the exams in the US include legal questions?

RE: Miami Pedestrian Bridge, Part XII

- -

Quote (Earth314159 We have to study engineering and the law where I come from. Is this the same in the US? Our professional exam includes legal questions. Do the exams in the US include legal questions?)


I had a course in legal stuff - but it did not prepare me for things like this --from June

Quote (Vance Wiley (Structural)23 Jun 19 17:47 Quote: How is this MCMs fault, not FIGGs?
Because they touched it.
You don't even have to actually touch it. When the lift slab construction in NJ or somewhere around there dropped several slabs and killed workmen maybe in the 80s or 90s the guy who operated the roach coach was hit for about $80K. Why was any part of that collapse his fault? This is a many faceted case and the lawyers are salivating and ordering GulfStreams.
ADD https://www.nytimes.com/1987/10/23/nyregion/record... 28 workers killed, over $5 million in fines, U S Dept of Labor involved. Short article here, worth a minute.)

RE: Miami Pedestrian Bridge, Part XII

Quote (jrs_87 (Mechanical)17 Aug 19 18:35 Vance is referring to this)

Thank you for that link. With a construction process like that described, what could possibly go wrong?

RE: Miami Pedestrian Bridge, Part XII

Also not a lawyer. I think being 10% responsible for the problem but potentially 100% liable for the bill is called 'joint and several liability', which Florida got rid of before this bridge collapse. It's a nice idea but apparently doesn't work as intended for various reasons.

RE: Miami Pedestrian Bridge, Part XII

Abolished after a Disney World lawsuit where the plaintiff was injured in November 1971 at the grand prix attraction at Walt Disney World, when her fiancé rammed from the rear the vehicle which she was driving. The jury found the plaintiff 14% at fault, her fiancé 85% at fault, and Disney 1% at fault. Under the doctrine of Joint and Several Liability, Disney was 86% liable and ordered to compensate the plaintiff.

RE: Miami Pedestrian Bridge, Part XII

No comments over the last ten days. Is that correct ?

RE: Miami Pedestrian Bridge, Part XII

I think everybody is taking a break before the NTSB report comes out. The only recent news is a Tony Pipitone report about the creative book keeping used in MCM's bankruptcy.

RE: Miami Pedestrian Bridge, Part XII

I just assumed the dead horse had been adequately flogged for the time being.

RE: Miami Pedestrian Bridge, Part XII

Quote (JStephen)

I just assumed the dead horse had been adequately flogged for the time being.
deadhorse

RE: Miami Pedestrian Bridge, Part XII

A Florida court document recently submitted, stated.

"Internet activities, social media, forums etc... are acceptable as potential evidence.
Any sites pertaining to FIU bridge collapse could be included for reference.
Disclosure of facts true, false or subjective in clause introduced in the forthcoming legal discussions
will be accepted and challenged as deemed necessary.
Those submitting errant facts or those that are trying to sway judgement will be held accountable.
Formally announced this is a class action lawsuit.
Companies, websites and individuals submitting pictures, ideas, comments can be subpoenaed as witnesses"

WTF?!?!?
Does this include comments here?



RE: Miami Pedestrian Bridge, Part XII

Sounds like that is what it means. But I think the subpoenas would be very selectively issued.

RE: Miami Pedestrian Bridge, Part XII

I'm guessing the class action must be the civil suit not the criminal. As such I shouldn't be surprised that the criteria for evidence is much broader.

And I agree with hokie66. I'd say the eng-tips discussion over the last 12 episodes is on their top ten list of sources for "ideas, comments, facts, both true, false, and subjective".

RE: Miami Pedestrian Bridge, Part XII

Quote (charliealphabravo (Structural) 1 Sep 19 06:06)

"...their top ten list of sources for "ideas, comments, facts, both true, false, and subjective"

This guy might also be in the top ten: https://www.youtube.com/user/etiennekai/playlists

RE: Miami Pedestrian Bridge, Part XII

Quote (Mike W7)

This guy might also be in the top ten: https://www.youtube.com/user/etiennekai/playlists

They could subpoena this guy if they wanted an irrelevant incoherent rant form a conspiracy nut who is a wannabe structural engineer that lacks the ability to understand the fundamentals.

RE: Miami Pedestrian Bridge, Part XII

Quote (Earth314159 (Structural) 1 Sep 19 19:13)

...wannabe structural engineer...

But he's mentioned on page 27 of the OSHA report. Doesn't that make him a recognized expert? upsidedown

P.S. - According to his YouTube "About" he is being harrassed and stalked by Eng-Tips, so be careful.

RE: Miami Pedestrian Bridge, Part XII

Mike W7

Good point, we wouldn't want to melt the special little snow flake if he were to venture outside his safe space.

RE: Miami Pedestrian Bridge, Part XII

3
Link

G'Day mate.

Despite overwhelming evidence, apparently if you don't believe this video, you have a "conclusive file". I wonder what he believes the actual failure was? He doesn't say after hundreds of hours of previous videos. All as we know is that he believes it was not a horizontal interface shear failure and you are an "idiot" engineer if you don't believe what he believes. I would love to see the numbers and evidence to indicate another possible failure mechanism. I am truly and honestly open to any scientific evidence that shows another plausible mechanism. To date, I have seen nothing credible or even any failure mechanism presented on the YT channel.

Six people died and one person was severely injured. I only comment because someone is using pseudo science to promote conspiracy theories and personal agendas on their YT channel. Science maters and the proper use and evaluation of evidence maters. When you see pseudo science used to promote a false agenda, it is up to the rest of us to call it out. The victims and the public deserve better than this.

If you don't understand basic concepts like vectors, static, or that gravity generates a load, then you need to read up on the science and learn some of the fundamentals. The YT presenter believes it is "equilibrium" that generates the forces and not gravity and if you don't believe that, you have a "conclusive file". He has a different definition of equilibrium from engineers but he doesn't describe it or tell you how you can use it to calculate forces, stress, deflections etc. If you can't take science as a foundation for discussing or evaluating such an incident, then there is no hope in ever determining a likely cause to an incident and we learn nothing.

I invite the YT presenter to use science to describe another feasible failure mechanism and come to a forum like this one where it can be scrutinized. All scientific ideas need to be scrutinized. Being shielded on a YT channel and deleting all comments that challenge your ideas, stunts any possibility of learning. Don't be afraid to be wrong, it might hurt the ego but in the long run you will learn more.

I would also like to end by saying something positive. He does collect a lot of evidence which we can evaluate and that is a good thing.

RE: Miami Pedestrian Bridge, Part XII

After this 'youtuber' first came to my attention I watched one of his videos...for about 5 minutes...before I realized that this was a big waste of time. I'd rather spend my time, which is limited, on something more useful, like watching reruns of:

RE: Miami Pedestrian Bridge, Part XII

2
Earth314159,

He was on the forum. He started getting fired up over politics; I suspect that's why he was banned. That, or repeatedly calling Eng-tips contributors "idiots" in his videos.

There's no need to mention him any further and instead just stick to relevant engineering.

RE: Miami Pedestrian Bridge, Part XII

3DDave,

That is a good point. I didn't realize he was on the forum before. He is a waste of time. I just don't like pseudo science in principle and call it out because of the damage it causes. You are right. It is best just not to mention him.

RE: Miami Pedestrian Bridge, Part XII

I agree. We will not discuss him any further. We won’t even mention him.

RE: Miami Pedestrian Bridge, Part XII

I note istructe video linked to earlier is now avaliable on youtube, as at least one person indicated they had issues accessing it.

https://youtu.be/nbL-moHpFug

RE: Miami Pedestrian Bridge, Part XII

NTSB News Release:

Quote:

The National Transportation Safety Board announced Thursday its intention to hold a public board meeting Oct. 22, 2019, 9:30 a.m., to determine the probable cause of the March 15, 2018, FIU pedestrian bridge collapse.

RE: Miami Pedestrian Bridge, Part XII


Please discontinue posting in this thread. Go to thread815-457935: Miami Pedestrian Bridge, Part XIII
for any further posts.

This topic is broken into multiple threads due to the long length and many images creating longer load times for some. If you are NEW to this discussion, please read the following threads prior to posting to avoid rehashing old discussions.

Part I
thread815-436595: Miami Pedestrian Bridge, Part I

Part II
thread815-436699: Miami Pedestrian Bridge, Part II

Part III
thread815-436802: Miami Pedestrian Bridge, Part III

Part IV
thread815-436924: Miami Pedestrian Bridge, Part IV

Part V
thread815-437029: Miami Pedestrian Bridge, Part V

Part VI
thread815-438451: Miami Pedestrian Bridge, Part VI

Part VII
thread815-438966: Miami Pedestrian Bridge, Part VII

Part VIII
thread815-440072: Miami Pedestrian Bridge, Part VIII

Part IX
thread815-451175: Miami Pedestrian Bridge, Part IX

Part X
thread815-454618: Miami Pedestrian Bridge, Part X

Part XI
thread815-454998: Miami Pedestrian Bridge, Part XI

Part XII
thread815-455746: Miami Pedestrian Bridge, Part XII

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