Miami Pedestrian Bridge, Part I
Miami Pedestrian Bridge, Part I
(OP)
Multiple Fatalities After Pedestrian Bridge Collapses Near Florida International University
https://www.nbcmiami.com/news/local/Pedestrian-Bri...
John R. Baker, P.E. (ret)
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https://www.nbcmiami.com/news/local/Pedestrian-Bri...
Quote:
As investigators continue to search the site of a deadly collapse involving a 950-ton pedestrian bridge near Florida International University in Miami Thursday, officials say the death toll has risen.
Early Friday morning, the Miami-Dade Police Department confirmed that six people have died as a result of the collapse....
John R. Baker, P.E. (ret)
EX-Product 'Evangelist'
Irvine, CA
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RE: Miami Pedestrian Bridge, Part I
John R. Baker, P.E. (ret)
EX-Product 'Evangelist'
Irvine, CA
Siemens PLM:
UG/NX Museum:
The secret of life is not finding someone to live with
It's finding someone you can't live without
RE: Miami Pedestrian Bridge, Part I
Link
RE: Miami Pedestrian Bridge, Part I
RE: Miami Pedestrian Bridge, Part I
RE: Miami Pedestrian Bridge, Part I
This may a case where the team thought that the existing deck structure was strong enough to self-span the roadway while the cables and tower were erected. I wonder if the warm Florida sun hit it, made it expand, and the temporary construction load + expansion broke loose a metal-diagonal-to-concrete-web-chord connection...
http://www.miamiherald.com/news/local/community/mi...
"We shape our buildings, thereafter they shape us." -WSC
RE: Miami Pedestrian Bridge, Part I
It appears to be shallow... but, not unreasonably so.
Dik
RE: Miami Pedestrian Bridge, Part I
The horrific part is that the bridge fell on a large number of cars that must have been stopped for a traffic light. It seems that at least 4 and possibly as many as 10 vehicles were under the bridge.
One witness reports that a support failed sideways, but I think that must have been in reaction to a failure elsewhere.
The lack of symmetry and the irregular columns is unsettling. See http://www.miamiherald.com/news/local/community/mi... Obviously without knowing the reinforcement making a call about sufficiency is impossible, but I am unnerved by the second column on the campus side; the side that fell on the cars.
Edit:
I see that this was to be cable supported. That explains the irregular beams and columns. Someone didn't do a good job of analyzing the partial construction strength.
RE: Miami Pedestrian Bridge, Part I
Dik
RE: Miami Pedestrian Bridge, Part I
RE: Miami Pedestrian Bridge, Part I
RE: Miami Pedestrian Bridge, Part I
Were there some warnings or concerns expressed on this?
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RE: Miami Pedestrian Bridge, Part I
I suppose titanium dioxide in the concrete wouldn't perhaps have any affect on strength, bond, etc. but I don't know.
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RE: Miami Pedestrian Bridge, Part I
Wonder if there was lateral movement from an eccentric loading causing the abutment to shift and the bridge to lose support on that end.
Mike McCann, PE, SE (WA)
RE: Miami Pedestrian Bridge, Part I
We'll see.
Mike McCann, PE, SE (WA)
RE: Miami Pedestrian Bridge, Part I
RE: Miami Pedestrian Bridge, Part I
JAE, sorry, I didn't mean to imply that there were warnings regarding this specific structure; indeed, in my case this is the first I've heard of it. Rather, I was referring to the various discussions of bonded vs. unbonded post-tensioning that have occurred over time on the structural forum. I probably should have withheld speculation, though.
RE: Miami Pedestrian Bridge, Part I
RE: Miami Pedestrian Bridge, Part I
RE: Miami Pedestrian Bridge, Part I
It will be interesting to learn what happened.
RE: Miami Pedestrian Bridge, Part I
https://www.wunderground.com/history/airport/KMIA/...=
RE: Miami Pedestrian Bridge, Part I
RE: Miami Pedestrian Bridge, Part I
http://www.miamiherald.com/news/local/education/ar...
RE: Miami Pedestrian Bridge, Part I
RE: Miami Pedestrian Bridge, Part I
Dik
RE: Miami Pedestrian Bridge, Part I
Dik
RE: Miami Pedestrian Bridge, Part I
https://www.google.com/amp/abcnews.go.com/amp/US/p...
RE: Miami Pedestrian Bridge, Part I
https://heavy.com/news/2018/03/florida-internation...
RE: Miami Pedestrian Bridge, Part I
RE: Miami Pedestrian Bridge, Part I
What a horrible tragedy. It could it have been prevented.
I am not a civil engineer, so bear with me as restate what is being discussed here, and what I have seen in reports, photos, videos and Google street view.
Here is the artist's rendering showing the cable supported pedestrian bridge.
Here is the main bridge span resting on two supports on either side of the highway. Notice NO cable supports.
The collapsed bridge shows no evidence of the tower. No tower = no cables. So, the span crosses 8 or 9 lanes of traffic with no supporting cables.
So maybe someone had built a temporary support from median strip to the bridge span? I'm looking through the pictures and video and seeing no evidence of any temporary support structure.
How could they have opened the road to traffic when the bridge had no cables? It's a cable supported bridge, for God's sake! And without any temporary support structure?
Am I missing something here?
Roopinder Tara
Director of Content
ENGINEERING.com
RE: Miami Pedestrian Bridge, Part I
Mike McCann, PE, SE (WA)
RE: Miami Pedestrian Bridge, Part I
Here is a good shot of the post-tensioning Link
Notice the VSL Corp truck/van under the bridge Link
If VSL isn't working on this bridge, then they will for sure get to the bottom of what went wrong.
Notice also in the last photo the tensioning equipment. I wonder if they were tensioning some strand when something went wrong?
RE: Miami Pedestrian Bridge, Part I
I hate this situation for everyone--the injured, their families, the engineers, and our profession in general.
RE: Miami Pedestrian Bridge, Part I
TLDR: Construction workers are a lot more tolerant of L/180 deflections than the public is.
Unfortunately, getting phased construction loading right is harder than it looks, so I agree that's a likely root cause. I'm glad to see the reputable names of Figg and VSL mentioned here. I think both those companies will be motivated to iron out what happened (rather than just rushing to rebuild).
----
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RE: Miami Pedestrian Bridge, Part I
It's a terrible tragedy that (as you have said) could have been avoided.
RE: Miami Pedestrian Bridge, Part I
If not, I bet it's coming!
----
The name is a long story -- just call me Lo.
RE: Miami Pedestrian Bridge, Part I
RE: Miami Pedestrian Bridge, Part I
It didn't appear to have any of the cable stayed structure in place. It may be that part of the procedure for erection was missing or missed; it failed 'way too soon', and it may have required temporary support that was 'missed'. In all my years... I've never done a concrete truss. I've seen a few concrete trusses used for old railway bridges, and, that's it.
If it was designed for cable stayed, then the members should be light to reflect the closeness of the stays. That feature may have provided a weak structure if not properly supported.
Dik
RE: Miami Pedestrian Bridge, Part I
RE: Miami Pedestrian Bridge, Part I
RE: Miami Pedestrian Bridge, Part I
RE: Miami Pedestrian Bridge, Part I
RE: Miami Pedestrian Bridge, Part I
Also, video is out. The collapse was brittle.
RE: Miami Pedestrian Bridge, Part I
Video in link
[This 5 second video from a random cell phone in a building across the street captures the actual collapse of the structure]
RE: Miami Pedestrian Bridge, Part I
RE: Miami Pedestrian Bridge, Part I
I wonder if the reason so many people were standing just above the failure point was they had spotted a crack and were trying to figure that out - otherwise it's an even more horrible coincidence.Edited - so they were apparently performing some work as noted by later posts.
RE: Miami Pedestrian Bridge, Part I
Future stay-cable tower to the left of the photo:
Photo looking along the longitudinal-axis of the bridge:
General view during erection:
RE: Miami Pedestrian Bridge, Part I
The column at the end of the bridge where the tower was to be built, was only half of its final width. Once the other half of the bridge was in place the two half columns were to be joined together & the column would have had an octagonal shape.
Here is an interesting angle Link
Couple other large scale reference photos
Link
Link
RE: Miami Pedestrian Bridge, Part I
RE: Miami Pedestrian Bridge, Part I
RE: Miami Pedestrian Bridge, Part I
RE: Miami Pedestrian Bridge, Part I
RE: Miami Pedestrian Bridge, Part I
RE: Miami Pedestrian Bridge, Part I
Most of interest would be the details of the anchorages within the concrete elements. Were they castings?
An issue with a concrete truss is that it doesn't want to act only like a truss, but also like a rigid frame, so there are substantial moments at the joints. Central stressing would not cope well with these moments, so a lot of deformed bars would be required. Does anyone other than me think that the photos show very little in the way of non-prestressed reinforcement?
RE: Miami Pedestrian Bridge, Part I
If the bridge was designed for 100psf of traffic on it, that is an additional 340 tons approximately. So, dead load is by far the dominant load.
Without knowing project details, it seems that the simple answer is that the bridge probably wasn't designed for simple span in construction phase, so the engineer would have assumed shored construction until cables installed. The contractor didn't understand that.
Its possible also that the strand tensioning work at bottom chord was assumed to have already been finished, prior to releasing the shoring towers.
RE: Miami Pedestrian Bridge, Part I
... that had been extended with, er, locally sourced used food, if you catch my drift.
The statuettes had broken in shipment, in a suitcase, protected by dirty laundry.
Impromptu testing caused further fractures, simply by shaking the object gently while cantilevered from a hand's grip.
Those fractures looked a lot like the TV images of the broken bridge.
So as to infinitesimally reduce my colossal ignorance of related subjects, I ask:
Does Accelerated Bridge Construction normally include moving a concrete structure before the post-tensioning strands had been fully stressed and locked?
Is 'de-stressing' a normal procedure in PT construction, ever?
Is 'de-stressing' or 're-stressing' or 'stressing' a PT tendon such a trivial undertaking that it's often carried out on a structure with civilian traffic passing beneath?
Mike Halloran
Pembroke Pines, FL, USA
RE: Miami Pedestrian Bridge, Part I
So that puts it at 6ksi for the dead load (24 inch x 24 inch drops it to just over 3ksi.) That seems to be in the middle to a bit above middle end of the capability of typical concrete.
If the load was eccentric that would make this even worse.
RE: Miami Pedestrian Bridge, Part I
It will be interesting when the report on the collapse is completed, how many of the comments in this thread 'ring true'.
The bridge was under construction and it is not known what component was contractor's 'means and methods'.
Dik
RE: Miami Pedestrian Bridge, Part I
I don't know how many times I've had to hear about the Hyatt Regency Failure in getting my Ethics hours...but (and I am assuming here) If there were problems with slipping of tendons...and concrete cracking observed and they were trying to re-stress a tendon...then the Traffic should have been shut down....but then there is the other side...the Pride of our design and the Fear of letting others know we made a mistake...
I also wonder what happens to Figg as a company...there are post-tensioned bridges in our state done by them...otherwise a fine, outstanding company I'm sure...but they get work through State DOTs...Do state bridge engineers hire them as consultants...the public will already be concerned about any bridge done by Figg...With the lawsuits and damage of their image...If I was working for them...I'd by looking for a new employer....my $0.02
RE: Miami Pedestrian Bridge, Part I
Yes, No, No.
Yes.. it's normal to under stress the strands during the construction process and tensioning more after the instalation of beams for example.
No.. Haven't heard of de-stressing... re-stressing is a common method.
No.. these are dangerous activities and must be done without traffic.
RE: Miami Pedestrian Bridge, Part I
It looks from the video like the bottom slab/chord buckled in flexure between the first and second diagonal.
RE: Miami Pedestrian Bridge, Part I
https://facilities.fiu.edu/projects/BT_904/MCM_FIG...
RE: Miami Pedestrian Bridge, Part I
RE: Miami Pedestrian Bridge, Part I
RE: Miami Pedestrian Bridge, Part I
On Sheet 115/173..the first compression strut is labeled #11...It looks like there is no P.T. strand for that member...but there has been talk of that member Tensioned in the field???
Member #10 shows 280k and is in tension...I see the connection detail at the top...but what about the bottom where it ties into the tendons running thru the deck?
Member #9 (in compression)and doesn't indicate any PT Tendons...Was there mild reinforcing in the compression member???
RE: Miami Pedestrian Bridge, Part I
Collapsed FIU Bridge Was Funded by Federal Grant Program Criticized for Shoddy, Politicized Review Process
The TIGER grant program has come under fire for putting politics ahead of technical concerns.
By Christian Britschgi
March 15, 2018
TIGER was created as an economic stimulus measure under President Barack Obama and morphed into a permanent program. It has awarded $5.6 billion in nine rounds of grants since 2009. Members of Florida's congressional delegation publicly lauded the TIGER award to FIU.
"Thanks to this TIGER funding, FIU students will be able to walk from their student housing to class through a pedestrian bridge across Southwest Eighth Street," Rep. Debbie Wasserman-Schultz (D-Fla.) said in 2013. "More jobs will be created in our community thanks to this grant, and I look forward to celebrating the project's success with everyone in South Florida."
Rep. Mario Diaz-Balart (R–Fla.) made similar comments on Saturday. "FIU has come a long way since the TIGER grant that funded this pedestrian bridge was awarded in 2013," he said. "This project represents a true collaboration among so many different partners at local, state, and federal levels, and in both the public and private sectors."
RE: Miami Pedestrian Bridge, Part I
RE: Miami Pedestrian Bridge, Part I
I think the tendons being de-tensioned are TEMPORARY PT bars in the top of the top flange- the wheeled trailer support was centered about the first interior node point, so maybe that 'overhang' necessitated the temporary PT.
And this would explain the stressing operations occuring in the collapse photo - the photo above I posted may be the crew de-stressing the top bars (NOT the compression diagonal, as I first stated) - there is a large stressing stool in front of the ram - that is often used to access the nuts from threaded PT bar and therefore enable destressing.
RE: Miami Pedestrian Bridge, Part I
RE: Miami Pedestrian Bridge, Part I
We're all guessing at the reasons....could be all or none, but certainly interesting though incredibly tragic. Unfortunate for all involved and their families for their loss. This will gone on for years to come.
RE: Miami Pedestrian Bridge, Part I
Link
* They were supposed to tension the strands in the longitudinal direction prior to setting main span and removing shoring.
* It was planned to set the main span, and 2nd span, and only then install the guys (which were 16" pipe).
Presumably they would have checked the engineering of the "main span as simple span", as its shown in their drawings and also in their construction schedule.
Its not clear to me whether they were re-tensioning the long strands or not. If they were, then this was either a sequencing screw-up (extremely unlikely in my opinion), or, when they removed the shoring they must have had structural problems and were trying to fix it by re-tensioning strands etc.
So based on the above, its possible that the construction sequence was followed, but the performance of the single span action was lacking, whether it was an engineering screw up or a materials screw up.
RE: Miami Pedestrian Bridge, Part I
RE: Miami Pedestrian Bridge, Part I
Looking thru the drawings I am confused when we needed to make our bridges into an art project? At one time the chicken merely needed to get to the other side of the road, but now it seems he needs to do so very elegantly. We could put in twice as many simple structures for what silly things like this cost.
RE: Miami Pedestrian Bridge, Part I
RE: Miami Pedestrian Bridge, Part I
The photos Ingenuity posted clearly show a tendon protruding from member #11. I'm assuming that was a change in the design sometime after that document in the pdf was created on 9/30/15.
Marco Rubio tweeted this last night: “The cables that suspend the #Miami bridge had loosened & the engineering firm ordered that they be tightened. They were being tightened when it collapsed today.” So it seems like it was either over-tensioned and the concrete crushed or a cable snapped and the jolt lead to a failure. The eccentric loading theory from tensioning sequence seems plausible too, particularly since based on the details in that pdf document, they'd have to go to the underside of the bridge to get to the other tendon(s) in the diagonal.
It's very obvious in hindsight they should have closed traffic for that since it's a heavily loaded compression diagonal, but they were touting their construction methods as causing minimal traffic impact, so I'm sure there was pressure to avoid any closures. This will probably be added to the list of failures covered in ethics courses or least to safety protocol for post-tensioning.
RE: Miami Pedestrian Bridge, Part I
His basic premise was that bridge design in particular was susceptible to a unique phenomenon and that is that since the vast majority of them are financed using public funds, that the pressure to continue to optimize the design is enormous, trimming away at safety factors and the costs of materials used, in order to win the bidding process, and that this will continue until we learn what constituted going just a bit too far. At that point, the consensus will be that the last successful project will be the standard by which all future ones will be be modeled. That is until a radical new design is introduced and the process will start over. In this case, it might not be the design of the bridge itself which was the victim of this phenomenon, but rather the method and processes used to construct it.
John R. Baker, P.E. (ret)
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RE: Miami Pedestrian Bridge, Part I
I believe what was optimized was political pressure; specifically, the pressure not to close down a road. It brings to mind a topic that was discussed on the structural forum, preventing a high-incidence / low-consequence failure (in this case preventing traffic impediments) leading to a low-incidence / high-consequence failure. And here we are.
RE: Miami Pedestrian Bridge, Part I
The way I read it, this was a concrete truss bridge with stays added for vibration control, or to look like a cable stayed bridge.
The proposal committed to an independent peer review. Wonder if that was done.
Concrete truss/frame, bad idea. Unbonded PT, bad idea. Very little bonded reinforcement, bad idea.
RE: Miami Pedestrian Bridge, Part I
RE: Miami Pedestrian Bridge, Part I
Are you sure about that? Could it be the tendon in the photo is actually Member #10...a tension member? Member #11 is under the greatest compressive force already...why would you add additional compressive force...if we assume this is a purely axial compressive force in a truss?...The only reason I would want to further compress this member, is if I thought the member had bending forces in it..and I needed additional moment capacity of member.
And the drawing shows that they were to apply the jacking force from the bottom of the truss...but the photo shows the stressing ram from the top...so the design is different than shown on the proposal (but that is not surprising).
RE: Miami Pedestrian Bridge, Part I
I just read this article, which cites previous safety violations against Figg and MCM.
Link
RE: Miami Pedestrian Bridge, Part I
structuralengr89, I think it was likely #11. On page 115 of that pdf, it shows the tendon arrangement for the panel point on the opposite end. If you mirror that, the #11 tendon, assuming #11 has tendons, would protrude in the spot that matches the tendon in the photos with the jack attached. It also appears to be pointing toward the end bent in the photos. I agree it makes no sense to be tensioning #11 when it's already under compression. Maybe you're right and it is #10, though. That would make the failure more difficult to explain, but would make a lot more engineering sense.
RE: Miami Pedestrian Bridge, Part I
I read somewhere where someone heard a 'snapping or pinging' sound a few hours before the collapse.
Does not bode well if the engineering firm ordered that the strand be tightened. Also, did anyone question why the strands loosened?
Dik
RE: Miami Pedestrian Bridge, Part I
RE: Miami Pedestrian Bridge, Part I
Please read the last sentence in my post, where I acknowledged that possibility.
John R. Baker, P.E. (ret)
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RE: Miami Pedestrian Bridge, Part I
RE: Miami Pedestrian Bridge, Part I
He says the bridge should have had a centre support. Duh! That would have been a different bridge.
https://olivermcgee.org/fiu-bridge-collapse-why-mu...
RE: Miami Pedestrian Bridge, Part I
RE: Miami Pedestrian Bridge, Part I
Thought there wasn't a safety issue, although repairs were going to be needed.
https://www.google.com/amp/amp.miamiherald.com/new...
"We shape our buildings, thereafter they shape us." -WSC
RE: Miami Pedestrian Bridge, Part I
Yes, it was spanning full length.
RE: Miami Pedestrian Bridge, Part I
Makes sense. Maybe they messed up the sequence, or maybe the sequence had errors meaning the strut had to carry the bridge and the stressing load - which was actually only intended to apply when the thing was hanging.
RE: Miami Pedestrian Bridge, Part I
Pretty sad the way everyone bunkers down and tries to pass the buck rather than honestly appraising the situation. Reminds me of our professor's parting words: "When something goes wrong on one of your projects - and something will - the person who's fault it ends up being is the person who the blame ends up landing on."
No doubt the designer and builder will already have worked out why it's not their fault.
RE: Miami Pedestrian Bridge, Part I
Cheers
Greg Locock
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RE: Miami Pedestrian Bridge, Part I
Compressing more may have caused the cables to loosen and tensile forces to happen where they weren't expected (the observed cracking).
The jacking the cables down more would have just added more load in that member, croaking the connection.
To the post-tensioners in the group- does that happen?
"We shape our buildings, thereafter they shape us." -WSC
RE: Miami Pedestrian Bridge, Part I
Yes, the portable red lifting rigs were only used to move the bridge segment from its (nearby) fabrication site to the installation point. They were NOT under the bridge when it collapsed - which had a full traffic flow: 3x lanes each way.
I have driven under that location several times while down in Miami for AWS and power plant work. Sobering to know I was at that exact traffic light - waiting to be crushed later.
RE: Miami Pedestrian Bridge, Part I
Easy to say with benefit of hindsight, but concrete cracks, tendons detension, tendons snap, anchorages fail, and getting on with things is part of life.
RE: Miami Pedestrian Bridge, Part I
RE: Miami Pedestrian Bridge, Part I
Based upon what I have read, the engineering firm and the GC have worked together on several projects. Can this type of relationship lead to questionable decisions?
I am not making accusations, but I have been part of design build teams. The pressure exists.
RE: Miami Pedestrian Bridge, Part I
Engineer warned of cracking on bridge two days before collapse
https://www.cnn.com/2018/03/16/us/bridge-collapse-...
John R. Baker, P.E. (ret)
EX-Product 'Evangelist'
Irvine, CA
Siemens PLM:
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The secret of life is not finding someone to live with
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RE: Miami Pedestrian Bridge, Part I
Yes, obviously in retrospect the traffic should have been stopped, the area cordoned off etc. But no-one knew that at the time. Presumably in their minds it was some ordinary cracking and misbehaving tendons, and they made that assessment, the same assessment all sort of engineers make everyday in response to issues - including engineers on this forum.
In this case it was the wrong call, no doubt.
That's assuming the cracking was in fact related. Do we know that yet?
RE: Miami Pedestrian Bridge, Part I
Frame Prior to Collapse:
Not sure if it is the video or if the first diagonal is buckled and maybe the section of top chord between the first diagonal and end of the bridge:
First diagonal appears buckled and the bottom chord has collapsed:
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RE: Miami Pedestrian Bridge, Part I
Sorry for the rant.
RE: Miami Pedestrian Bridge, Part I
The original proposal had both sets of the Self-Propelled Modular Transporters spaced apart. Starting on the south end (FIU) the first rig was planned to be positioned directly under the south end of the span & the next rig 24 ft to the north. At the opposite end, north (Sweetwater)/Pylon, one rig was again to be positioned directly under the north end of the bridge with the final rig 27 ft to it's south. As it turned out this plan was too wide for the road way and they may have encountered a hiccup with uniformly supporting the bridge when driven over the center divider. It looks like they still had trouble driving over the center divider, even with the tandem Self-Propelled Modular Transporter arrangement. They stopped half way across and just sat their for a long time. Since the Self-Propelled Modular Transporter was no longer under the ends of the bridge, I wonder if they decided to PT the No. 11 Truss, with the intent to De-Stress the truss once the bridge was in place. It sure seems like it took a lot longer than 6 hours to position the bridge. It looked close to midday when they we finished. Link
RE: Miami Pedestrian Bridge, Part I
RE: Miami Pedestrian Bridge, Part I
(Also, regarding that professor, absolute garbage writing. Unfortunately, it seems he's rather effective at self-promotion. Same way we get our politicians)
OhioMatt -- absolutely. Design-build projects inherently have more aligned interests, which can mean more trust and fewer checks. That's part of the efficiency, but can also be dangerous if people work outside their competency or back down too early in the face of peer pressure. It requires the individual engineers involved to be that much more careful to act ethically.
----
The name is a long story -- just call me Lo.
RE: Miami Pedestrian Bridge, Part I
And another "exactly" from me too.
16" CHS/tubes with bolt on connections - hardly your high-tech strand stay cables!
For those that wish to see the "PRELIMINARY 2015 - NOT FOR CONSTRUCTION" details, see the following:
RE: Miami Pedestrian Bridge, Part I
"Easy to say with benefit of hindsight, but concrete cracks, tendons detension, tendons snap, anchorages fail, and getting on with things is part of life."
...said NASA management. Sorry, if you are designing stuff that risks people's lives then 'shit happens' is not an acceptable standard of care.
Cheers
Greg Locock
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RE: Miami Pedestrian Bridge, Part I
On page 115 of the submittal the PT bar schedule details ZERO PT for compression member #11, which I assume is for the final spanning condition:
I think this is most probable. During transportation/erection there was tension in member #11 so maybe they provided some temporary PT for this condition, and whilst this temporary PT was being de-stressed the collapse occurred. It is in keeping with the location of the stressing ram and de-stressing stool seen in the collapse photos.
RE: Miami Pedestrian Bridge, Part I
If the cable stays are just for show, when would the top flange ever be in tension to require PT?
RE: Miami Pedestrian Bridge, Part I
RE: Miami Pedestrian Bridge, Part I
For during lifting etc? as per diagram immediately above.
Also for bending of the top flange.
RE: Miami Pedestrian Bridge, Part I
I'm not saying that. Just pointing out that cracks and tendon issues are pretty common in stressed decks and people don't generally hit the panic button unless it's out of the ordinary. Evidently this guy didn't think it was.
RE: Miami Pedestrian Bridge, Part I
RE: Miami Pedestrian Bridge, Part I
The so-called media reports of "loose cables" having to be re-tensioned is highly suspect. All the photos that I have reviewed of the collapsed bridge show black-colored grouted end-caps (at the ends of the bottom and top flanges), and none that I have found to show any with 'stressing tails' exiting the ends, EXCEPT for the PT bar being de-stressed to diagonal member #11.
RE: Miami Pedestrian Bridge, Part I
I don't interpret that as zero PT in #11, but rather as "ditto", so 280 kips.
I find all those numbers to be incongruous. Member #1 will have more force than Member #11 once the truss is erected, as it is not as steep.
adamewood,
Tension members are not the only axial elements which are prestressed. Think prestressed columns and piles.
RE: Miami Pedestrian Bridge, Part I
It looks like zero to me. Same symbol as #1.
Do you mean #2 instead of #1?
RE: Miami Pedestrian Bridge, Part I
If that was true then how much PT was to be applied to Member #1 - the first scheduled member and with a "-" for all PT data?
Member #1 is the end-vertical - I think you mean Member #2 - the opposite-end, first diagonal, that is flatter than #11. However, Member #2 [in the final 'faux' stay-cable configuration] is in direct line-of-action of the last stay cable [so may see some tension under some load cases ?] - whilst #11 is always in compression in the final configuration.
RE: Miami Pedestrian Bridge, Part I
RE: Miami Pedestrian Bridge, Part I
No mild steel reinforcement drawings/details that I could find.
RE: Miami Pedestrian Bridge, Part I
This makes some sense as to the buckling of the concrete strut - the rods in the preliminary plans are off-center. If one fails and the other is still under tension there is a moment introduced into the member.
Maybe they were trying to pull hard enough to close a crack and popped one of the rods instead, leading to buckling of the member.
Per the original plan - there was no PT to be applied to #11. Not only is the tension "-", there is no member type (A, B, or C) specified to tell where the tensioners are to be placed in the member.
In this picture
it looks like a trench was cut on the anchor blister over member 10-11. This trench is not obvious in the post-collapse images. There is a similar trench into the anchor blister over member 2-3. Both align within the body of the most steeply sloped member. This is the same image as
the post epoxybot (Structural) 17 Mar 18 04:22 linked to; it's easier to zoom in on epoxybot's linked image.
RE: Miami Pedestrian Bridge, Part I
A county spokeswoman, Karla Damian, said the county's Traffic Management Center in Doral, which monitors more than 100 intersection cameras, found about the collapse through the media. Damian said it too late to preserve the incident before it was erased by new footage in the video system's rolling 30-minute recording loop.
"By the time that staff at the TMC attempted to access the video camera, it was already beyond the 30-minute threshold and our staff could not go back to the time of the collapse," Damian said in a statement Friday
http://www.miamiherald.com/news/local/community/mi...
RE: Miami Pedestrian Bridge, Part I
https://www.youtube.com/watch?v=Ucflj-MsJBI
RE: Miami Pedestrian Bridge, Part I
https://www.youtube.com/watch?v=BSq8295GFk4
RE: Miami Pedestrian Bridge, Part I
RE: Miami Pedestrian Bridge, Part I
RE: Miami Pedestrian Bridge, Part I
"Didnt think" isnt an excuse when it comes to safety matters bc safety is process driven per engineering ethics. These circumstances should've been considered in a FMEA, circumstances outside the FMEA require the pause button being hit until the process docs are reviewed and updated. If this truly is a matter of someone "didnt think" then hopefully those folks end up in jail and out of the profession.
RE: Miami Pedestrian Bridge, Part I
"Well Harry, how are we supposed to take the nut off that bar?"
"Nothing to it Joe. We use the hydraulic jack to stretch the bar a little and then spin the nut off by hand."
"Okay Joe, the nut should turn off now."
"Gosh Harry, I can't turn it. It seems to be stuck somehow."
"Okay. I'll stretch it a little more and see if it will turn."
"Oops. Too much."
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Miami Pedestrian Bridge, Part I
Thanks for the link. I had seen links to a dashcam, the one in the semi, but it was too low resolution, making the image a blur. In Russia there would might have been 40 videos. It's a shame the crane blocks the view of the anchor plate.
It is either reassuring (privacy) or disappointing (evidence) that the traffic-cam footage is automatically deleted. Good on the operators for having some presence of mind to record it before it was deleted. It had the advantage of not changing perspective, making frame-by-frame comparisons clearer.
RE: Miami Pedestrian Bridge, Part I
RE: Miami Pedestrian Bridge, Part I
RE: Miami Pedestrian Bridge, Part I
I can't help but think that maybe there are things going on in a truss web/chord connection that maybe we don't quite fully understand. Did "graceful" top "robust" in this design?
RE: Miami Pedestrian Bridge, Part I
RE: Miami Pedestrian Bridge, Part I
This will be winding its way through the courts long after I’m gone. In addition to the fatalities, there are numerous other things. A lot of questions, so far unanswered. What really caused the failure; the video is poor and may not show the initial failure. Did the University really have an agreement with the FDOT regarding review? With construction going over a busy roadway, this seems reasonable. Was the FDOT aware of the construction; it appears to be widely publicised, and maybe a dozen others, but, not suitable for this forum.
If that voice phone message is correct, then, there may have been a precursor to the collapse that was not followed through. There is also a reference to a ‘pinging’ sound a couple of hours before the collapse. If acted on, several lives may have been saved.
I’m not a big fan of design-build because the owner is ‘short changed’ in the QA/QC of the project. Additional review may have been useful.
The cable stayed structure, from the renderings, appears to be an attractive structure. A lot of work went in to providing these asthetics. I don’t know why it was chosen. It is likely a very costly undertaking. I think steel would have been more economical and would offer greater ductility. The spans are not excessive. The depth to span appears to provide sufficient depth to use a 2-span structure, in concrete or steel, without cable stays. I’m not familiar with the quakes and/or hurricanes that Florida can expect. It may be that construction has to be more robust than what I’ve normally encountered. For hurricanes, I would consider that a massive, albeit ‘brittle’ structure, would have merit; I’m not so sure about a seismic environment.
I don’t know what the intended support at the ‘free’ ends of the cable stayed span. I couldn’t gather this from the drawings and it has a major impact on forces/stress throughout the structure. The sequence of stressing and relaxing the strand or bars has a major impact on the resulting stresses in the structure. As noted, some of the details don’t make sense for a cable stayed structure, in particular, how will it be ‘adjusted’ after erection.
[Added] I didn't know at the time of this post that the 'cable stays' were decoratively only.
I’ve taken a cursory look at the structural information. Some of it makes little sense. Not knowing the intent of the design, it may be that this could be rationalised. I just don’t know. As I noted earlier the compression force in the end diagonal may very well be a tension member in the final configuration. Just so many unknowns.
In these environs, a ‘dash’ in a table is regarded as ‘nothing’, not a ‘ditto’ or a “do”. Having no information in these spaces makes little sense, IMHO. It could be that the ‘dash’ means to repeat. One thing about-design build is that this type of notation may mean ‘whatever it should mean’ and can be easily interpreted. This reasoning applies to sections and details. ‘As Built’ drawings or record drawings may be a little different.
Will the city proceed with this work? It looks like the schedule is ‘shot’.
RE: Miami Pedestrian Bridge, Part I
The bridge was built over Southwest 8th St. which is US Hwy 41. During the bridge move the site would have been crawling with FDOT engineers. The call from FIGG was simply good conduct to the Chief FDOT Engineer for the project.
Since the first line in the table of PT members has dashes, I would interpret the dashes to me NO PT.
According to the initial proposal, the bridge over Hwy 41 was essentially structurally complete when it was moved. The faux stay cables would add stiffness that would dampen harmonic ~play~ caused synchronous pedestrian foot traffic.
The project was a ridiculous amount of money for the FHWA to be spending on a pedestrian bridge. The entire idea of making this area into a public space was delusional. Who in their right mind wants to hang out next to 40 mph noisy traffic on a road that has little to none in the way of sound damping foliage? All the hard surfaces would have made it even worse from a noise perspective.
The bridge is, for all intents, on the back side of the university. There is nothing over in this area but parking garages. The city of Sweetwater is only 15000 residents. 74% foreign born, it is known as "Little Managua". When you do the projections, it really doesn't pencil out to spend this kind of money for a pedestrian bridge, so why?
Here is why: Link
A private development, that's why. This whole project stinks of politics, using Federal money to build a bridge for the benefit of private investors. The poor people of Sweetwater are going to see their city bulldozed in the next decade. The developer/s have already torn down the two story condos that were adjacent to the bridge, to build the tower. They call the tower "University Bridge" and it was originally suppose to be half condo half apartments. With the tax changes it is now to be all apartments.
They obviously would need a pedestrian bridge BUT if the university & Miami County wanted a signature bridge, then they should have been on the hook for at least HALF the cost, instead of the FHWA paying for 80% of the project.
RE: Miami Pedestrian Bridge, Part I
RE: Miami Pedestrian Bridge, Part I
Dik
RE: Miami Pedestrian Bridge, Part I
RE: Miami Pedestrian Bridge, Part I
https://www.youtube.com/watch?v=KtiTm2dKLgU
BA
RE: Miami Pedestrian Bridge, Part I
I have very little experience with concrete, and none with pre/post tensioned concrete structures, but it appears to me from the dashcam video and the method of collapse that the first compression member's connection (where the alleged de-tensioning or tensioning operation was occuring) to the upper chord failed in shear.
If AVE's assertion is correct that the PT rod and ram being ejected some distance out of the conduit is evidence that the construction crew tensioned beyond the Fu of the rod, why would this cause a collapse of the entire structure? (being as it is a compression member and all)
What levels of redundancy are typical for a structure of this type?
RE: Miami Pedestrian Bridge, Part I
That's how it goes when things go badly wrong. You do someone else over so you don't get done over (that's an observation, not an endorsement). Evidently the strategy is working given the comments even in this forum here.
RE: Miami Pedestrian Bridge, Part I
I had the same thought. Maybe the rods clamping stress was in fact keeping the member and joint confined, keeping the member and node intact.
RE: Miami Pedestrian Bridge, Part I
RE: Miami Pedestrian Bridge, Part I
It appears to be a single rod hanging out the ends. That would make sense if it was just there to carry the weight of the overhangs during lifting.
RE: Miami Pedestrian Bridge, Part I
I agree.
RE: Miami Pedestrian Bridge, Part I
https://www.reuters.com/article/us-florida-bridge/...
RE: Miami Pedestrian Bridge, Part I
<iframe width="560" height="315" src="https://www.youtube.com/embed/Ucflj-MsJBI?rel=0" frameborder="0" allow="autoplay; encrypted-media" allowfullscreen></iframe>
RE: Miami Pedestrian Bridge, Part I
Here in the Dallas area, they have built some major bridges with big arches and whatnot. There's nothing below that would prevent them from putting piers down every 10' if they chose, and the older bridges were more along that line. I deduce that whether the big arches and cable supports serve a structural function or not, they were selected primarily for aesthetics, not because that was the ideal way to build something. I suspect the Florida bridge has similar concerns going on.
RE: Miami Pedestrian Bridge, Part I
Tomfh (Structural)17 Mar 18 21:25
It appears to be a single rod hanging out the ends. That would make sense if it was just there to carry the weight of the overhangs during lifting.
Detail shows one pulled from the top and one pulled from the bottom....but it could have only been one in #11
Video from foreman with MOM stated that there were 2 tendons to detension (after bridge is set on abutment) and they can reopen traffic
I wonder if these 2 tendons were detensioned....traffic was reopened...cracking was observed so FIGG decided to retension the strands due to cracking...then when pulling the top one....it either snapped or the node busted
RE: Miami Pedestrian Bridge, Part I
That would make sense.
RE: Miami Pedestrian Bridge, Part I
Dik
RE: Miami Pedestrian Bridge, Part I
For safety reasons you want supports to be well clear of the driving lanes. Clear span is good.
Dik
RE: Miami Pedestrian Bridge, Part I
Yep, there would be photos. I hope someone leaks that.
RE: Miami Pedestrian Bridge, Part I
That is a bit of a callous characterization, IMO. One member of the stressing crew died in the collapse, and two of his colleagues suffered injuries, meanwhile the engineer sat in his/her 'ivory tower'.
PT stressing operations are undertaken by experienced and specialized personnel, using calibrated and serviced equipment, and bridge projects even moreso (compared to building structures). VSL (the PT subcontractor on this project) have a worldwide history of post-tensioning that exceeds 50 years.
Don't run the field guys 'under the bus' - I know of several 'stressors' who have saved the engineer's a$$ a bunch of times.
RE: Miami Pedestrian Bridge, Part I
First, these TIGER grants are always associated with some questionable projects. For example, back in 2009 the I-44 bridge over the Arkansas River in Tulsa, Oklahoma got a TIGER Grant that funded its replacement. The reason it got the grant? Because it was a MULTIMODAL bridge. It was a double decker bridge, the top deck for cars and trucks and the bottom deck for future light rail and also a pedestrian trail on the side. That the bridge needed to be replaced had nothing to do with getting the grant, it had the ability to support light rail in the future (that doesn't exist and will never happen by the way) and people could also walk across it. Therefore, it got the Federal Grant to be built. It was all politics.
This looks to be the same way. Link
Why in the world would you spend all this money ($11.4 million) for a bridge that would have tables and chairs with ceiling fans built into it? Elevators with glass walls. Solar technology, "eco-friendly" concrete, programmable LEDs. A large center pylon with fake pipe stays that don't serve any real purpose. This one span weighed 940 tons, how does it have a vibration problem? I don't buy that the pipe stays were for vibration, they were just there for looks. All of this just added to the cost. You could have built a much simpler steel truss (with redundant members) and still used ABC with SPMTs to set it in place and saved a lot of money.
Second, why in the world would you build a non-redundant concrete truss over 6 lanes of traffic? Too dangerous in my opinion.
From what I gather in all of these posts it looks like the contractor was doing something with the post tensioning on the diagonal member (#11) when the truss failed. I've never designed anything that was post tensioned, but my hunch is that they did something that caused that diagonal member to buckle. It's pretty clear from the video that P205 posted that the source of the failure came from that member.
RE: Miami Pedestrian Bridge, Part I
Dik
RE: Miami Pedestrian Bridge, Part I
RE: Miami Pedestrian Bridge, Part I
Dik
RE: Miami Pedestrian Bridge, Part I
Following from that, something has happened to #11, and it no longer aligns with the node. Pity that crane is in the way.
Here's the relevant frames from the CCTV. Not a great video, however it appears something's happened to #11.
RE: Miami Pedestrian Bridge, Part I
The work on member #11 happens after cracking is reported.
I think the construction crew was working with faulty information and tried a fix for a condition they didn't understand.
RE: Miami Pedestrian Bridge, Part I
RE: Miami Pedestrian Bridge, Part I
RE: Miami Pedestrian Bridge, Part I
RE: Miami Pedestrian Bridge, Part I
RE: Miami Pedestrian Bridge, Part I
RE: Miami Pedestrian Bridge, Part I
RE: Miami Pedestrian Bridge, Part I
The design of this pedestrian bridge was apparently driven by the appearance, not the structural efficiency and safety. The truss could serve without stays, but single diagonals and posts were not sufficient for the wind loads and required redundancy.
As usual, the devil is in the details - lack of mild reinforcement at the joints, where the stresses are almost unpredictable, post-tensioning of the compression member lead to this disaster.
Eugene Figg was a great engineer - but apparently the "successors" do not have the same capabilities. And even greatest legacy do not do the design - it's just the designer in charge of the project.
So, the recipe for for disaster is rather simple - great legacy company, inept design, and no response to apparent overstress signs.
Just bunch of idiots running the project.
RE: Miami Pedestrian Bridge, Part I
RE: Miami Pedestrian Bridge, Part I
RE: Miami Pedestrian Bridge, Part I
It will be interesting to see if the project goes ahead, and, if there is any 'political spin'. It would be nice to see if the funding is challenged.
I was really surprised that the collapse was 'instantaneous', without warning, other than maybe the earlier crack. The EoR will likely carry his statement about the insignificance of it to his grave...
Dik
RE: Miami Pedestrian Bridge, Part I
BA
RE: Miami Pedestrian Bridge, Part I
Have I missed something along the way, or do we not actually know that yet?
I wonder if the cracks might eventually turn out to have played no part in the ultimate failure. Too early to tell.
A.
RE: Miami Pedestrian Bridge, Part I
RE: Miami Pedestrian Bridge, Part I
RE: Miami Pedestrian Bridge, Part I
BA
RE: Miami Pedestrian Bridge, Part I
"An eyewitness reported that at the moment before collapse, a blue box fell loose from a crane hook, dropping onto the roof of the bridge very near where the roof and span then immediately broke apart."
RE: Miami Pedestrian Bridge, Part I
RE: Miami Pedestrian Bridge, Part I
BA
RE: Miami Pedestrian Bridge, Part I
RE: Miami Pedestrian Bridge, Part I
Now there was a crane there, and you do wonder why. What was its roll during the tensioning/detensioning operation that was going on? Could something have fallen from the crane? I suppose so, but why would something fall from the crane, and what would be large enough to contribute to the bridge collapse? What are the chances of 2 catastrophic failures (the crane and the bridge) happening at the same time, or I suppose one catastrophic failure resulting an an immediate second catastrophic failure?
Of the two blue things in the photos of the collapsed bridge, one (the cylinder) is the stressing ram/jack and is still attached to the PT rod which has clearly erupted from the diagonal about 5 or 6 feet, and the other (the box) I think is the hydraulic gage to monitor the jacking force. I think this gage would have been on the deck right next to the workers stressing the pt rod, not suspended from a crane.
RE: Miami Pedestrian Bridge, Part I
RE: Miami Pedestrian Bridge, Part I
I don't apologize for the basic content of my post.
I have seen too many versions of that play out over the years.
Sometimes with injuries, often with property damage and sometime with firings.
Who is responsible?
Sometimes the engineer in his ivory office.
Sometimes the engineer on the ground personally directing the work, possibly against the advice of experienced tradesmen.
Sometimes the crew is taking a shortcut.
Sometimes the crew does not follow directions.
Sometimes the crew does not understand the instructions.
Sometimes people get fired.
Sometimes the right people get fired but not always.
Sometimes the accident report is accurate and true.
Sometimes not.
And sometimes s
htuff happens.Why are cameras banned on many construction sites?
There is nothing like a photograph to screw up a perfectly good coverup.
Extraneous thought: After the Columbia bridge failure and now this one;
Maybe all cracks in important structure should be treated as serious until proven otherwise.
Inconvenient in many cases yes.
How much inconvenience is more important than a fatality?
Design/build?
Sometimes it works, but most of the old timers here have seen the results of profit driven decisions made on design build projects.
One that comes to mind is a pulp mill that was profitable for over twenty years.
Then came the design/build expansion and a newsprint mill was added.
There is nothing like improperly driven foundation pilings to mess up a newsprint drier.
In a few short years the company was out of business and the entire mill was dismantled and shipped to the third world.
The design/build company is still in business.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Miami Pedestrian Bridge, Part I
Just curious...
Dik
RE: Miami Pedestrian Bridge, Part I
RE: Miami Pedestrian Bridge, Part I
Original 'Powerteam' orange that was re-painted in VSL 'blue':
Full of oil, these pumps weight about 65 lb.
Stressing is always a two-person (minimum) operation: pump operator and jack 'stressor'. If you look at the videos that Tomfh and epoxybot post you can see a guy fall as his lanyard tie-off breaks, so likely that the pump and operator were in a 'dog box' suspended from the crane.
RE: Miami Pedestrian Bridge, Part I
RE: Miami Pedestrian Bridge, Part I
I cannot see a 'third blue object'. And with a PE55 pump the 4-way control valve (for double-acting ram) and oil reservoir are all contained within the same box, as my photo above shows (the orange pump).
RE: Miami Pedestrian Bridge, Part I
RE: Miami Pedestrian Bridge, Part I
Don't let some engineering get in the way of some bridge 'art'. It is in PINK for sarcasm!
RE: Miami Pedestrian Bridge, Part I
Having said that, something to consider: If you have never worked for a contractor or on a project where you are responsible for project execution, then consider that something that appears black and white from the design office "ivory tower", is quite a bit grey-er in the field.
RE: Miami Pedestrian Bridge, Part I
Dik
RE: Miami Pedestrian Bridge, Part I
https://i.imgur.com/9o6zVT1.gif
RE: Miami Pedestrian Bridge, Part I
I agree, but the whole situation is still puzzling. Was the span designed as a truss or as an "I" section with web cutouts? If we assume it was designed as a determinant truss, Members 2 and 11 would have been in tension while the bridge was moved into place, based upon the position of the SPMT's in the photos, thereby requiring PT. Also,the deck or bottom chord of the first and last panels would have gone suddenly from compression to tension once the SPMT jacks were lowered. Perhaps in the final position, Member 11 had too much compression, which resulted in the "de-tensioning" operation.
IMHO, if it were designed as a truss it was the wrong approach. Forgive me if I'm rambling.
RE: Miami Pedestrian Bridge, Part I
The PT bar appears to be Williams Grade 150 ksi: Link.
For the 175' span truss, member #10 had its 4 PT bars stressed to 280 kips each. Min. UTS is 390 kips, so max was 72% of ultimate tensile. PTI (and manufacturer's) recommendations stipulate max of 80% of MUTS for test loads (usually for rock or soil anchor type projects where proof testing is undertaken), but there is also a requirement that the lock-off load should not exceed 70% of the specified minimum tensile strength. Threaded PT bar does not undergo significant seating losses like strand/wedge systems, so the stressing load is the lock-off load, for all intents-and-purposes.
The other PTI requirement is that the design load is not more than 60% of the specified minimum tensile strength of the prestressing steel.
The other PT bars to members of the 175' truss were stressed to 52% and 62% of MUTS.
I have not seen photos to show evidence of a tensile failure of the PT bar. The 10 foot 'projection' of the PT bar and jack from the canopy 'blister' could be explained by 'nodal' failure at the bottom of member #11 diagonal, where the bottom chord appears to have ripped away from the support 'node'.
RE: Miami Pedestrian Bridge, Part I
RE: Miami Pedestrian Bridge, Part I
Just wondering...of all the engineers here, has anyone ever designed or had knowledge of a concrete truss?
Bridgebuster, you have it right, a truss made of concrete was the wrong approach.
RE: Miami Pedestrian Bridge, Part I
This thread is getting long. Discussion continued in new thread:
thread815-436699: Miami Pedestrian Bridge, Part II