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Miami Pedestrian Bridge, Part VII
16

Miami Pedestrian Bridge, Part VII

Miami Pedestrian Bridge, Part VII

(OP)

RE: Miami Pedestrian Bridge, Part VII

Quote (dbill74 (Mechanical)15 May 18 13:53 With all the talk of tensionnig of cables and fancy graphics of stresses in the members and what not, there is one aspect of this bridge I have yet to see anyone address.)

Please see:
LittleInch (Petroleum)26 Mar 18 14:18 in:
(Search (control F) for the date is easiest)
In thread 815-436924: Miami Pedestrian Bridge, Part IV

In fact I've read all six parts. I am amazed at how many of the long standing members nailed it back in Part 1.

RE: Miami Pedestrian Bridge, Part VII

Quote (SFCharlie)

I am amazed at how many of the long standing members nailed it back in Part 1.

Ya... two months ago, thereabouts... Some of the long standing members have decades of experience. not at all surprising.

Dik

RE: Miami Pedestrian Bridge, Part VII

I think now though with new testimony it is clear that the rods in no 11 were being tightened to "pull up" the column and close the crack(s) at its base.

I still don't have a clear view whether the top or bottom of no 11 failed first but my probable cause is the bottom.

The design does appear to be at best "non srandard" and at worse poorly analysed.

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

RE: Miami Pedestrian Bridge, Part VII

Hopefully when the NTSB releases their preliminary report, they will also release the actual Traffic Cam footage. I don't accept that they failed to record and make a copy of the Traffic Cam footage. Indeed, if they in fact failed to record the incident, then that itself requires an investigation. Shortly after the accident, they shut down the public feed of the traffic cam and have since, removed the cam from the public website menu.

RE: Miami Pedestrian Bridge, Part VII

I looked some, but couldn't find how to calculate/estimate self-weight point loads.

Some have assumed a percentage of total weight on upper/lower nodes (33%, 67%), then divided total weight to get kips/foot.
I tried by calculating volume of components, dividing total volume into total weight, then distributing it to each node with canopy, blister weight on upper nodes, and deck, truss, diaphragm on lower nodes. End result (12017 cf/950 kips) yielded 158 lbs/cf - heavy for concrete (150 lbs/cf), but there's steel in the span.

I'd appreciate if someone could give me guidance (if simple) or a link to read.





RE: Miami Pedestrian Bridge, Part VII

I would normally treat the top chord as a continuous beam with supports at the panel points and use the reactions obtained as the point loads.

Dik

RE: Miami Pedestrian Bridge, Part VII

Quote (epoxybot)

Indeed, if they in fact failed to record the incident, then that itself requires an investigation. Shortly after the accident, they shut down the public feed of the traffic cam and have since, removed the cam from the public website menu.

One of my concerns about the report from the NTSB.

Dik

RE: Miami Pedestrian Bridge, Part VII

Epoxybot, may I have link to similar traffic cams that still run? I would like to check frames per second. The collapse video we do have shows sub second time stamps, but very low frame rate.

Here is 2013 traffic study on the intersection (very well prepared, note nearby non-signature existing pedistrian bridge): https://facilities.fiu.edu/projects/BT_904/Traffic...

RE: Miami Pedestrian Bridge, Part VII

jrs87 - This is the best I can supply. When I first checked about a week after the bridge collapse, my recollection is that there were 3 traffic cams on US41 and now there are none. Perhaps they are all part of the same feed network. Maybe there is construction work that has disabled them but why the no longer appear on the traffic cam map is odd.

Link

RE: Miami Pedestrian Bridge, Part VII

Another possible sign poor judgement was used designing bridge. Several feet of free passage width were to be used for planters, benches, and tables? If so, to use this much width for non-function use is pure folly. We all know any increase in width for a suspended load comes at a non-linear cost. I hope to run numbers to see what a practical width would be for expected foot traffic. Decided not to bother when I discovered stairway on north end was to be less than 11 feet wide and the elevator is small. So they meant it when they said this bridge was to be "floating public space". This is a world-wide growing trend, parks and gardens on rooftops for example.

We may be a victim of our own success. Failure is so rare, people stop realizing danger is there. Building this bridge was dangerous... because building ALL bridges is dangerous.

RE: Miami Pedestrian Bridge, Part VII

Quote (epoxybot)

If the canopy isn't stressed before #2 & #11 are stressed, wouldn't the canopy crack somewhere between the 2/3 & or 10/11 blisters?

Difficult to say. Members are short so axial deflection would be small. That said, I'd be surprised if any of the diagonals were stressed prior to the longitudinal PT.

RE: Miami Pedestrian Bridge, Part VII

2


Quote (epoxybot (Structural) )

Hopefully when the NTSB releases their preliminary report, they will also release the actual Traffic Cam footage. I don't accept that they failed to record and make a copy of the Traffic Cam footage. Indeed, if they in fact failed to record the incident, then that itself requires an investigation. Shortly after the accident, they shut down the public feed of the traffic cam and have since, removed the cam from the public website menu.


That is a video camera detector for sensing vehicles in the left turn lane. Video camera detectors are a new sensor technology in use at some traffic signals. These detector systems use digital image processing to detect a change in the image at a specific location in the travel lane due to the presence of a vehicle. Video camera technology potentially allows a wider variety of vehicles to be detected than with inductive loop sensors, but the camera must be properly installed and adjusted for this to work.

Will not have a wide vision and most likely would have not picked up the bridge collapse.



A traffic detection camera

RE: Miami Pedestrian Bridge, Part VII

bimr: thanks for confirming that (and 3DDave for the link). In CA they have (old tech) sensors under the pavement to detect traffic/regulate signals. In MO they have these cameras at newer intersections - I'd wondered if they were for red light runners [at a new intersection by my house, after a year the lights were still badly timed.. been in CA 6 months so maybe they programmed them by now].
-----------------------------------------
Below is tensioning sequence for mainspan.

The 2/6/18 memo said hairline cracks only in #3 and #10 showed after stressing rods in #2 (on 1/30/18) and #11 (on 1/29/18).
The 2/28/18 memo said bad cracks at base of #11 showed after the formwork was removed - which is what triggered the cracks (cracks happened before repositioning SPMTs, cantilevering span ends, moving the span, etc - which removes most of the ABC concerns of building elsewhere/moving into position). This should have raised red flags since three of four engineers who only saw the photos said it indicated eminent failure. I'd like to know how far the #11/deck crack in Photo 4 extended toward the north end.

RE: Miami Pedestrian Bridge, Part VII

jrs87 - So many things about this project's stated objectives are nonsense. I watched an interview with an FIU student after the collapse, the student mentions during the conversation that HWY 41 traffic (5 to 6 lanes) is pretty noisy. Now imagine vehicles at a stop, under a bridge and the noise, that generates. The whole idea of a "Public Space" was silly. What above exhaust fumes?

Has anyone checked with major paint manufactures to find out if TiO2 in paint & formulated for concrete behaves in the same self-cleaning manner as these TiO2 parge coats? Parge coats are labor intensive and this bridge is a good example of how variable this kind of application coverage can be.

In the original planning proposal used for directing Design/Build bidders, T.Y. Linn's bridge recommendation was for a steel & concrete truss design because truss structures were recognized as good examples of how a structural element & missile screens could come together in function & architectural presentation. The bid request specifically called for the Missile Fence to be incorporated into the structure rather than free standing. FIGG/MCM seems to have completely ignored this concept and it makes me wonder what the proposals by the other two bidders looked like. The FIU selection committee bares some responsibility for discounting T.Y. Linn's recommendations and choosing a "Never Been Done Before" design, when the entire premise of the project was Safety. Maybe the Miami-Herald should ask to see the other bidder's proposals. Granted Facchina Construction Company the final remaining bidder is now defunct.

RE: Miami Pedestrian Bridge, Part VII

Epoxybot, yes the selection committee was up to something. MCM claims three different proposals and sleepless nights in the announcement of securing another win (Winning!)

http://www.mcm-us.com/news/mcm-awarded-fiu-pedestr...

Don't rule out existence of more dashcam video, even from crushed vehicles.

RE: Miami Pedestrian Bridge, Part VII

Clear (and blocked) view of blister. Is see nothing remarkable. According to Internet, this photo was taken on day of collapse and this worker was on canopy at time of collapse and survived. I have his full name, but choose not to disclose it at this time. My question is, can anyone here determine markings on hardhat? I'm curious who he worked for. It seems to me three people only does not add up if they were trying to mitigate cracking. Who was monitoring cracking as they changed stress?

RE: Miami Pedestrian Bridge, Part VII

I wonder if this is the worker who latched his harness when he heard a crack - I thought he may have been hidden behind the crane with harness suspended from it. Three Structural Technologies workers were there - Mr. Brown, and two went to the hospital, but I never read any more about the third. A couple comments somewhere said no one was watching the cracks, but it'd be logical to do so if rods were tightened to close them (maybe even Denney Pate) - if the road wasn't closed, no one should have thought it hazardous to stand there.
https://wcinsights.com/worker-dies-in-fiu-bridge-c...

This came out a couple days ago about the crane and explains why post-collapse photos only show the bucket lift.
https://nypost.com/2018/05/14/crane-operator-fled-...

RE: Miami Pedestrian Bridge, Part VII

Quote (chris snyder)

"latched his harness when he heard a crack"

Yes, worker in photo is the one reported to latch harness. According to his cousin, he credited it with saving his life.

About the crane operator, he jumped out the crane very quickly and seemed to understandably panic. Media reports he fled scene seem unfair.

Opinion on accident by actual forensic engineer: http://www.hgcornerstone.com/blog/expert-witness-b...

RE: Miami Pedestrian Bridge, Part VII

(OP)
jrs87, there’s way too many unnecessary sentences in that article , but a few comments:

”While the exact cause of this astonishing bridge collapse and horrific tragedy is not yet known” ….
yet:
“Secondly, no new, previously unknown, engineering principle or precept will be learned or discovered here concerning the exact cause or failure mechanism.
We will only learn how known, well-understood, engineering principles or precepts were somehow violated leading to this sudden bridge collapse.”

So the cause is not known yet – but he knows that we won’t learn anything new from it.

“The structural design concept of the bridge decks mainly involved a concrete truss reinforced by post-tensioned steel tendons. A 109-foot high central pylon, yet to be constructed, would have added more steel cables, and thus more stability, to the overall structure.”
The cables were aesthetic – as this series of threads has revealed – so this forensic engineer is discussing something he isn’t very familiar with.

“However, it is entirely fair to begin asking some very crucial questions about what, as engineers and builders, we should be focusing on during the investigation and what things we may need to modify or change as a result of this tragedy to prevent something like this from ever happening again.”
Tragedies won’t be “prevented” by “modifying things” – we only can attempt to minimize the future risk.

“But, the most important and crucial take-away is that this tragedy should never have been allowed to happen in the first place and cannot be allowed to occur again in the future.”
Let’s make a law against this sort of tragedy shall we?


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RE: Miami Pedestrian Bridge, Part VII

chris snyder, Thanks for the post-tensioning sequence. I knew it had to be spelled out somewhere, but I was just too lazy to find it.

Before you posted it I was just assuming that the deck was poured, then all deck tendons were stressed, prior to working up and casting the diagonals. But now I see this is not the case. And it raises an interesting question: Why? Since everything was supported on falsework I don't see the need to wait until entire superstructure is cast before beginning stressing operations on the deck. Does anyone have any ideas as to why you'd want to wait?

The reason I raise this question is the following:


By waiting to stress deck PT, aren't you unnecessarily introducing into the system a shear force at the base region of member 11? This would be locked in, and subsequently added to the shear loads generated from dead loading when the falsework is removed. Wouldn't it, or am I missing something?

We'd expect to see the same "shear lag" effects that have been thoroughly discussed elsewhere in this thread with respect to the dead load. And the deck is much stiffer than member 11. So most PT force will stay in the deck, but some fraction will move into member 11. It's hard to say if this is a significant load. A computer model would tell the tale of the stress history.

RE: Miami Pedestrian Bridge, Part VII

thebard3: The page you linked on MCM's website calls this a cable-stayed bridge. OOPS. Like you, I am also surprised this page is still up.

JAE: "Let’s make a law against this sort of tragedy shall we?" That's my kind of rhetorical question. Unfortunately, legislators (who are mostly lawyers) think this is actually possible. BTW, the Kellogg-Briand Pact attempted to legislate war out of existence. Except for WW2 and a few other minor skirmishes, it seems to have worked quite well. Not. Both ideas--legislating away tragedy and war--are hopelessly naive. i suspect you knew that already. smile

==========
"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

RE: Miami Pedestrian Bridge, Part VII

jrs87 - Well the hard hat doesn't look like MCM or FIGG's logo. Structural Technologies is part of Structural Group and includes a number of different companies. Their Pompano Beach office includes VSL, Structural Technologies & Structural Preservation Systems, perhaps more. Structural Group is the product licensee of VSL/Bouygues. Structural Technologies prominent use of the VSL on much of their marketing & company property tends to give people the impression they are VSL. VSL/Bouygues does contract worldwide but mostly they license their products in various regions of the world. I don't think Structural Technologies can necessarily contract as a licensed VSL installer. Their is at least one email that mentions that the company performing the Post Tensioning is not VSL.

There is this tweet from Monique O. Madan at the Miami-Herald about an hour after the collapse. It makes me wonder if the construction workers had growing reservations about the bridge as it was getting built. It looks like they spent at least half a day of crane time trying to set the rebar cage for #12.
Link

I shudder at my past experience working with Universities & Healthcare, they turn on a dime making decisions about projects already in the works without a thought to the price already given for the work contracted; while acting like they are a poor as church mice. They really do expect you to shove 10 lbs of fudge in a 5 lb bag, with your head up in the clouds, where they can talk to you.

RE: Miami Pedestrian Bridge, Part VII

JAE: I had a couple of exceptions to his comments, but, for preliminary comments they were a reasonable summary. In addition to your items, my two exceptions were; I would normally think that a forensic guy might be a little more constrained:

...whatever did go wrong here either could have been prevented, or at least could have been better accounted for such that loss of life and catastrophic damage did not ensue.

Although this appears to be a structural issue, with the bridge barely capable of supporting the actual dead load. It is possible that some of the fatalities could have been avoided. What if there had been a crew of 30 construction workers on the deck or canopy?

Secondly, no new, previously unknown, engineering principle or precept will be learned or discovered here concerning the exact cause or failure mechanism. We will only learn how known, well-understood, engineering principles or precepts were somehow violated leading to this sudden bridge collapse.

I would not be making that statement until all the 'dots were on the dice'.

RE: Miami Pedestrian Bridge, Part VII

As to the MCM page linked above still being online, I imagine there is some discord between parties now. The contractor probably prefers to emphasize the role of the engineer, and the photo with the beaming dials of Linda Figg and Denney Pate front and center serves that purpose. Or am I just being cynical?

RE: Miami Pedestrian Bridge, Part VII

How much did the length of the bridge deck change during post-tensioning? I just read several articles about PT, but didn't find anything about how much the concrete would be compressed in a typical case. I'd guess several thousandths of an inch per foot, but I'd like to know for sure.

RE: Miami Pedestrian Bridge, Part VII

If I am reading it** right, modulus of elasticity is about 5 psi/ppm. In #11, the area is nominally 500 inch^2 and the post-tension was 560 kips, so the average pressure is about 1120 psi, so the length change 224ppm. An approximation of the beam is that it is 33 feet long - so about .0074 feet; 0.09 inches. In contrast the total delta on the tension members is 1.40 inches.

I'm not doing this for the deck because I'm lazy and don't care to find the section area, the preload, and the tension that offsets the pre-load when it's installed.

**https://www.engineeringcivil.com/modulus-of-elasti...

(Feel free to check the math, but the ratio seems about right.)

RE: Miami Pedestrian Bridge, Part VII

Greetings to all

Regarding the stressing operations:

All stresssing. deck, canopy and diagonals, were made in 1 phase after the main span was cast on temporary supports at the side of the road.

Mr. Toomas Kaljas, whose analysis was posted in a previous thread, has revised his analysis incorporating the PT based on the existing published contract plans. The analysis show that almost all the load goes to the deck or the canopy because their stiffnesses. The diagonals get only a small load. For practical purposes, worthy of not being accounted for compared with dead load.

Good hunting...

RE: Miami Pedestrian Bridge, Part VII

Hokie... opens up an interesting legal issue. Was the collapse in the realm of the Contractor or the Engineering Firm.

Dik

RE: Miami Pedestrian Bridge, Part VII

Dik-- I'm sure everyone will share in the liability fest sure to follow this, including the University.

RE: Miami Pedestrian Bridge, Part VII

There seems to be a unique osmosis regarding liability in most litigation, it tends to flow toward the deeper pockets.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

RE: Miami Pedestrian Bridge, Part VII

ON: That's why I'm curious about the FDOT's involvement... deeper pockets.

Regarding Contractor and Engineer... I was thinking more about criminal proceedings...

Dik

RE: Miami Pedestrian Bridge, Part VII

The worker in photo I posted May 16 is employed by Bolton Perez and Associates. https://www.bpamiami.com/2012-bpa-employee-awards/


Quote (The Mad Spaniard)

"... deck or the canopy because their stiffnesses."

Mad, this has me wondering if the bridge was just a deck and nothing more, would it have been able to stand on its own unloaded? Maybe. Not trying to make an important point here, just a thought exercise. The whole idea of a truss is to replace columns under the deck with members in tension on top. The weight of the canopy and members would not be applied to deck.

RE: Miami Pedestrian Bridge, Part VII

Greetings jr87

Given their thickness, the canopy and the deck can not span the 175 feet by themselves. The system acts mostly as a truss for dead load and live load. Now, it can also be considered as a "beam" the same way that you take a concrete beam and the shear is evaluated using "truss" behaviour having a compression flange at the top and a tension flange at the bottom.

Now, at the ends of the span, we may have a little of "beam" behaviour because the north temporary supports are distributed transversally and we do not have a support directly under member 12. For the south, we have two bearings 5 feet away from the centerline. These two details may have some effects on the "truss" behaviour of the bridge. As I have not done a 3-D analysis, I can not say where we are.

For the longitudinal PT in the canopy and the deck, the results from Mr. Kaljas indicate that the diagonals get very little load and that most of the straight tendon compression force is absorbed directly by the canopy and deck.

Typically, the canopy should not have longitudinal PT because it is in compression. However, because the back span is attached to the main span at the end of construction. the structure acts as a "two span continuous beam" and therefore there are tension forces at the canopy near the pylon due to live load. Also, because the back span is let go after being attached to the main span, this causes negative bending near the pylon with the corresponding tension forces in the canopy. This is probably, why the designer placed PT in the canopy. Also, for symplicity, they may have decided to extend the tendons all the way to the ends. At the ends, I doubt that you need so many tendons...

Best regards

Live long and prosper.

RE: Miami Pedestrian Bridge, Part VII

Mad, thank you. I was thinking how much thicker would deck have to be with no truss or canopy at all. Would a more pronounced v-shape have helped? What kind of moments did this bridge have?

I'm beginning to suspect they really did break something while adjusting bar tension, and like a tiny pin point bursting a balloon, 11 popped. It seems to me for span to fall under dead load only, the defect must have been severe. Too bad water bottle load test was not done. https://www.youtube.com/watch?v=12s42VorvQ4

Why was T.Y. Lin's much sounder concept rejected? It seems ABC was non-negotiable. ABC as advantages, but it does not obsolete older methods. When you're holding a hammer, everything looks like a nail. https://www.youtube.com/watch?v=EfH46DTAkxo

RE: Miami Pedestrian Bridge, Part VII

jrs87: He seemed to have missed the EoR in his list; for financial reasons he may not be a target. In some jurisdictions, you can sue the engineer, the company, and, the engineer and company jointly. This adds to the permutations. Then there are party's within the group that will cross-sue other parties... it' going to be a real 'fuster cluck'.

The Spaniard: There are a few ways of looking at the framing for the bridge, but, at the end of the day, there will be a similar stress condition throughout the bridge. Treat it as a truss, treat it as a beam. A realistic model of the framing will yield similar results.


Dik

RE: Miami Pedestrian Bridge, Part VII

The prosperlaw fellow is a personal injury lawyer, otherwise known as an ambulance chaser. The other side of this is the construction defects civil lawsuit, in which I think FIU would be the plaintiff. And then there is the possibility, as dik indicated, of a criminal action.

RE: Miami Pedestrian Bridge, Part VII

In this locale, lawyers use a 'scattergun' approach... lawyers can be sued if their case fails and there is someone that was missed. Someone they should have sued, and, didn't.

I suspect FDOT will figure heavily into this. I assume they are like most government agencies who disclaim involvement... but are at the table for all morsels and scraps. Governments seem to thrive on involvement. I noticed the lawyer included them in his list of ???

They may have positioned themselves into first place.

Dik

RE: Miami Pedestrian Bridge, Part VII

Greetings jrs87 and SFCharlie

Regarding the Bridge Testing, in Spain , for instance, it is typical to perform a load test on a bridge after construction to everify its behaviour in service. See below some examples.









As you can see, it is done with trucks full of sand. Several arrangements are perform to evaluate the critical load conditions.

In the USA, well... I have not heard of any load test similar to thos that can be sen in the pics.

for the moments in the Miami structure, Mr. Kaljas has shown somenumbers.

Best regards

RE: Miami Pedestrian Bridge, Part VII

Quote (3DDave)

If I am reading it** right, modulus of elasticity is about 5 psi/ppm. In #11,.. 33 feet long - so about .0074 feet; 0.09 inches. In contrast the total delta on the tension members is 1.40 inches.

This relates to what GreenLama brought up about the deck not being tensioned before the members were poured. I (no structural background) never thought about concrete compressing. If relative to 0.09" over 33', the 174' deck would compress about 0.5" which would stress the rigid member connections (along with tensioning in the canopy), to pull in the bottom and open the ~36o angle of #11. It isn't much, but could this add to the problem of a weak 11/12/deck connection?

RE: Miami Pedestrian Bridge, Part VII

(OP)
Mad Spaniard....they must pay those truck drivers a LOT of money to load the bridge and stand their waiting to see if it collapses or not.

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RE: Miami Pedestrian Bridge, Part VII

Quote (MacGyver)

I'm reminded of a Prince Rupert's Drop

Used to make these as a kid... the neighbourhood guys just loved them. So tough and yet so fragile.

Dik

RE: Miami Pedestrian Bridge, Part VII

Quote (chris snyder)

...compress about 0.5"

Modulus of elasticity: AASHTO Load and Resistance Factor Design Manual Sec 5.4.2.4

Tendon stress can easily compress 175 foot span a visible amount. I don't know what PSI they pulled tendons at (6500?), and I don't know what happens as concrete continues to cure after post tensioning. It's correct to be concerned about the effect of this on truss system. It's not related to this accident, but concrete can also creep and stretch.

RE: Miami Pedestrian Bridge, Part VII

Harry Pepper in Florida can do load test:

RE: Miami Pedestrian Bridge, Part VII

greetings again jrs87

I am 100% sure that the designer made an analysis accounting for the time dependent properties of concrete (creep, shinkage, increased strength with time) and post-tensioning (relaxation) with a computer program that accounted for the changes in structural scheme during the relevant construction phases. This is typical of such structures in the State of Florida and there are many commertialy available programs that so so (SAP2000, LUSAS, LARSA, etc...)

We can expect that the analysis would have results at the times of casting the main span, post-tensioning the main span, moving the main span, placing the main span in the supports, casting the back span, stressing the back span tendons, stressing the continuity capony tendons, releasing the falsework, casting the pylon and attaching the stays, end of construction and 10,000 days after the end of construction (and potentially several other intermetiate time and construction phases).

The computer program would have accounted for the fact that the PT was stressed when the strength was 6500 psi and would have given very slightly different results (but not of the magnitude that would cause failure) if it was instructed to streess when the strength would have been 8500 psi.

So, let's see what the designer's calculations tell us or what some brave soul out there like Mr. Kaljas has gotten for all of us.

And BTW, I am sure that they moved the span when they were sure that the concrete strength was 8500 psi. Even if the Contract drawings do not say so in the erection sheets.

RE: Miami Pedestrian Bridge, Part VII

The Mad Spaniard,

One drawing is missing within the updated work done by Mr Kaljas. There is no drawing showing the loads in ALL members resulting from stressing of the longitudinal PT. Take member 10 for example. If I'm reading his presentation correctly:
Axial-PT = 1570 K
Axial-DL = -862 K
Axial-total = 830 K

However, 1570-862 = 708 K, which is a difference of 122 K. I don't expect Mr Kaljas to redesign this bridge for us, but I was just curious. Thanks.

Dead Loads:


PT Diagonals:


Longitudinal PT:


Dead Load + Total PT:

Link

RE: Miami Pedestrian Bridge, Part VII

Mad Spaniard, despite our picking apart this project, I fully agree designer accounted for all the core details we bring up. Conversely, I do feel the standard to criticize the concept (not the engineering) of this bridge is looser. And no computer program as an output that says you can skip empirical testing of a new concept. I believe the biggest contribution of this thread is providing strong evidence this design concept and building technique combined is untested and unproven worldwide. Kansas City and Columbia both occured while I was in college and made a lasting impression on me.

The rubble fracture is consistent with 8,500 PSI. Aggregate snaped. I have seen some analysis that states over reinforcement with re-bar can be just as bad as too little as it removes what little ductility there is. I don't know.

P.S. Sorry about the load test image.. did not mean to go there. I've tried to quit this thread many times, but it's too intriguing. My son is in high school and the events in Texas this morning have left me stunned, so I think I will quit now for a while.

RE: Miami Pedestrian Bridge, Part VII

Greetings TheGreenLama

The axial PT value (1570 Kips Compression) that you mention is the result of applying diagonal PT bars to the truss. . It does not include the effects of the longitudinal tendons in the canopy or the deck. The dead load effect (862 kips tension) Plus the effect of the PT bars (1570 kips) plus the effect of the longitudinal tendons in the canopy and the deck ( 122 kips compression) provides the total of 830 kips compression.

So what happens is that even if you prestress the heck out off the deck or the canopy , the relative stiffness between the longitudinal members (deck and canopy) and the diagonals causes the diagonals to get very small axial forces out of it.

Best regards

RE: Miami Pedestrian Bridge, Part VII

Greetings jrs87

What has been happening lately in our high schools is very sad. I never had to worry about it very much here in the South when my daughter was there 12 years ago. I understand your distress.

But keep in touch with this thread. At the end, when we learn the reasons for this failure , I am sure that our faith in math and physics (and code requirements) will be rewarded.

Be well.

Live long and prosper.

RE: Miami Pedestrian Bridge, Part VII

The Mad Spaniard,

I don't want to belabor the point about what is likely a rough computer model, but what you're saying about the change in axial load (122 k) from the application of the longitudinal PT amounts to a 15% change in loading in member 10. I don't consider that insignificant. Are there moments generated as well within the system from the PT loads? For me that would be a disastrous by-product, due to the minimal shear reinforcement provided in the diagonals.

Thanks

RE: Miami Pedestrian Bridge, Part VII

A lot about analysis is being discussed here, but not much about design, which is a different part of our discipline. This was a new and flawed conceptual design, and should have been subject to rigorous experimentation and/or load testing.

RE: Miami Pedestrian Bridge, Part VII

hokie66,

As this thread is getting so long, have you ever thought about breaking it up? Say:
1) Analysis of existing as-built structure.
2) Failure mechanisms.
3) Failed design standards.
4) ?

RE: Miami Pedestrian Bridge, Part VII

TheGreenLama: too much work, and when the NTSB report comes out, there will be another flurry of information.

Dik

RE: Miami Pedestrian Bridge, Part VII

No, I see no need to break it up. I think it will proceed in spurts, as more information becomes available, or probably as more brainstorms occur.

RE: Miami Pedestrian Bridge, Part VII

Is this duct from "upper" bar? I would not expect to see it poking out here so far away from other edge without more local damage to beam. Duct for lower bar is shown in other images still around bar going to deck.

See attachment for high resolution.

RE: Miami Pedestrian Bridge, Part VII

JRS87....
That would be a west side view. That is the bottom side #11 tendon conduit, which I believe would be a short conduit section. I believe the tendon broke somewhere near there. When the canopy hit #11 at the top 2/3, it the snapped tendon, & ejected past Mr Brown. The conduit section below this area, terminates in the rubble, & as such, appears intact.

RE: Miami Pedestrian Bridge, Part VII

I agree it's a duct from #11's lower tendon, but suspect it's a section that remained as the rod stripped from the member - not sure what held the duct - it looks like it may be split. As some others here, I suspect the rod got pushed out as the deck fell instead of breaking. I redid the rough 'before/after' diagram using ~arc of the rod (in the bottom photo) - this makes the two rods about the same length, including the portion extending from blister.

Taking a closer look, the duct (from deck) in rubble is splayed on the north side - thinking this happened when the members sheared and the rod bent while the anchor held. The duct that's against this end (in rubble photo) has a rib which appears to be missing from the duct that's still in the member. There are two holes in the duct near the rubble... these could be caused by thread friction as ~6 ft of rod are pushed out.


RE: Miami Pedestrian Bridge, Part VII

Cutterhead, Chris S., thanks. Okay, has to be lower duct. I will check how duct is manufactured to see if it is seamed.

RE: Miami Pedestrian Bridge, Part VII

Another view. A cracked edge of duct is worn through, so there is evidence duct was indeed pushed along bar towards deck. The duct still in the beam must have come from where the separation is around bar. And like Cutterhead said, the other section of duct moved in. Still, this looks odd and I see so obvious sign duct still in beam is split lengthwise. Nor do I have any idea what could have restrained it so neatly. Is there any chance this duct was for something else or is stray?

RE: Miami Pedestrian Bridge, Part VII

Greetings to all

I would appreciate your assistance.

I would like to see if the failure occurred at the interface between nodes 11-12 and the deck. For that, I would like to know if member #12 sheared at that interface. For that to happen (ball park) the length of the member after collapse should be about 15 feet from the bottom of the canopy to the top of the deck. At the back of the member the length should be 17.25 ft to account for the danopy end diaphragm.

At the back of menber 12, the distance from the top of the canopy end diaphragm to the bottom of the deck diaphragm is about (18+ 1.25 + 2 = 21.25 ft).

So if anybody can meassure the length of what remains of #12 using the pictures available, we can know where if failed at the bottom (ball park): at the deck interface (15 feet at the front, 17.25 ft at the back) or below the deck (17 feet at the front and 19.25 at the back) or at the bottom of the deck diaphragm (19 feet at the front and 21.25 ft at the back).

This info may have been noted before by one of you guys. It is just that I have not seen it.


Thanks in advance

RE: Miami Pedestrian Bridge, Part VII

The photos I've seen indicate that #12 is the same length after the collapse as it was before.

The bottom end is torn up from separating, but otherwise it's there

https://www.eng-tips.com/viewthread.cfm?qid=437029, comment timestamp 27 Mar 18 00:32

RE: Miami Pedestrian Bridge, Part VII

After considering gwideman's animated loop
https://www.eng-tips.com/viewthread.cfm?qid=437029, timestamp 28 Mar 18 07:00
and the stuck duct section, I drew out what might have happened.
There would be two causes for stripping the lower PT rod - when #11/#12 members sheer from/move off the deck, and when the deck drops.
Notes are on the drawing. I couldn't add rotation.
Bugged me that the triangle moving north in loop was dark, but it's likely in a shadow (see shadow direction of cars) - last frame shows lightening (moving out of shadow, pixelating, ???).
I estimated bottom of #11/12 would move north about 6' before moving back south - only about 2' is visible (hidden by ~2' of boom on bucket lift - right edge of boom ~lines up with north side of #12).

RE: Miami Pedestrian Bridge, Part VII

Chris Snyder,

Thank you for your postulation. Yes, this section of duct was indeed cut open by sliding bar threads, I see it now. Hope for this analysis to continue. Rotating images so member 11 is in it's precollapse position is a good idea.

RE: Miami Pedestrian Bridge, Part VII

chris snyder,

I'm not advocating one PRIMARY potential failure mechanism over another, but one thing to keep in mind, both in your photo montage as well as gwideman's photo loop, and that is that the total length of member 11 can NOT increase. As I see it, the only way for the base of member 12 to become visible is for member 11 to push it there. But the only way for member 11 to push it there, and maintain the same length, is for the base of member 11 to have, at least loosely, separated from member 12 and begin sliding up member 12. I think it's just geometry.

RE: Miami Pedestrian Bridge, Part VII

The Mad Spaniard - The back side of #12 is full length from canopy to deck. I tried last week to measure this and was not satisfied with the figures I was getting. As you can see on photos, there is only one cold joint on the back of #12. Upon collapse, #12 separates from the diaphragm just above the two rebar extending from #12 at the deck reinforcing plane.



RE: Miami Pedestrian Bridge, Part VII

For anyone interested, something from the vault: VSL Report, "Detailing for Post-Tensioning." [Link]

Too bad they don't discuss our 11-12 joint area. There's a lot of discussion about the tensile forces that develop behind, and between, anchorages using strut and tie diagrams. All this talk of possibly stressing PT in member 11 to close cracks made me think about whether adequate confinement reinforcement was detailed in the already critical zone where 11 intersects deck.

Just one more thing to consider.

RE: Miami Pedestrian Bridge, Part VII

Good reference.

Dik

RE: Miami Pedestrian Bridge, Part VII

For the non-structural engineers in this thread who are trying to follow along...

Can one of the structural gurus kindly define the term "confinement reinforcement"

I'm only about 70% sure that my understanding of that term is accurate.

RE: Miami Pedestrian Bridge, Part VII

2
The picture on page 2 and the diagram on page 8 of the reference TGL posted above might help.



RE: Miami Pedestrian Bridge, Part VII

For PT strand anchorages, the manufacturer provides spiral reinforcement directly behind the anchorage. It is still the responsibility of the engineer to check the zone situated further behind the spiral. For threaded bar systems no such spiral is provided by the manufacturer, so it is solely up to the engineer to detail this region.

RE: Miami Pedestrian Bridge, Part VII

Excellent explanation, thank you both.

RE: Miami Pedestrian Bridge, Part VII

Those picture represent confinement for the axially applied post-tensioning only. Similar to column ties, which confine the longitudinal bars when they are in compression.

In the Florida bridge truss/frame, there was also a lot of flexure and shear in the joints, and I think that was not accounted for.

RE: Miami Pedestrian Bridge, Part VII

That was my next question- so the confinement reinforcement for certain types of PT tendons is defined by the manufacturer of those tendons.

In cases where the confinement reinforcement is (for lack of a better term) 'included' with the PT strand design, how common is it that additional confinement reinforcement is required?

Or, asked another way, how easy is the oversight which appears to have taken place- no additional confinement reinforcement in the 11/12 nodes?

RE: Miami Pedestrian Bridge, Part VII

The type of connection at the 11/12 and deck slab node is unique to this world, so overlooking of development, anchorage, and confinement should not have occurred, whether there was post-tensioning involved or not.

RE: Miami Pedestrian Bridge, Part VII

I wouldn't go so far as to say no confinement reinforcement is provided. As far as I can tell, drawing B-61 [ Link] shows reinforcement details at the truss joints. To me it looks identical for all joints (except for truss member 3-4 where there actually is more). This is just a feeling, but I wouldn't expect this to be the case because loads are likely different at every joint: PT, axial, shear, bending. If anyone has been able to find in the design drawings a detail for additional steel called out for PT anchorages at these joints please post.

RE: Miami Pedestrian Bridge, Part VII

Understood. I apologize if my questions are causing us to go back over material that's been covered before- but for someone not at all versed in the minutiae of concrete design, this thread is like drinking from a firehose. It's terribly interesting but there's an overwhelming amount of information.

Re: reinforcement schedules shown in the post above:

The reinforcement shown looks to me (a non-concrete reinforcement engineer) to be remarkably light, compared to the references.

Much of it appears to be located in the 'general zone' and beyond (as described in TheGreenLama's reference graphic 2.2), especially at the critical 11/12 node.

Is localized crushing under the PT tendon fittings, where there appears to be almost no confinement reinforcement in the local zone, the potential initial failure mode the structural guys are seeing?

In the case of the upper tendon at #12, only the L-hooked bars appear to pass through the 'local zone' at that tendon's fitting, and based on the references I don't see how those bars provide much confinement since they develop tension (and appear to provide confinement) in only one axis.

When reinforcement schemes are analyzed, are bars loaded in bending considered to provide reinforcement, are are they disregarded?

RE: Miami Pedestrian Bridge, Part VII

Hay, I'm a computer scientist, so what do I know, but from what I saw in the drawings, the joint at 1 - 2 had 50% more rebar than 11 - 12. I wish I knew why they beefed up 1 - 2, and why they didn't beef up 11 - 12 at the same time? It's the human factor that throws me.

SF Charlie
Eng-Tips.com Forum Policies

RE: Miami Pedestrian Bridge, Part VII

jgKRI,

I'll take a crack at some of your questions.

1) Reinforcement does look remarkably light and uniform.

2) Crushing should never be an issue as plate size will have been designed by manufacturer to transfer specified design load.

3) Circular anchorages for PT strand transfer load to concrete over a different surface area than the flat plate, thus the need for spiral steel immediately around that type of anchor. In other words, flat plates generally don't require spiral steel immediately behind plates.

4) Overall geometry of concrete section plays a role in how load moves from anchorage into structure. In our case it's not simple.

5) Generally, you'd only consider steel perpendicular to strand/bars to contribute. If it crosses at an angle you might consider a fraction of it to be effective, but there's no hard rule on this.

6) In the 11/12 joint area we also have a very large compressive force from dead load. I haven't quite figured how this impacts PT anchorages. I would think a good practice would be to still address anchorage zone issues regardless of other loads.

RE: Miami Pedestrian Bridge, Part VII

Quote (SFCharlie)

...why they didn't beef up 11 - 12 at the same time?

Member 11 was not inline with faux stay, so making it larger would disturb visual design. In another asymmetrical aspect of the design, the deck diaphragm on north end was much smaller than the one on south end.

Concrete bridge construction reference with excellent illustrations and warnings: https://www.fhwa.dot.gov/bridge/construction/pubs/...

RE: Miami Pedestrian Bridge, Part VII

Quote (jrs87)

In another asymmetrical aspect of the design, the deck diaphragm on north end was much smaller than the one on south end.

The south end of the span was designed to float independently on bearings with an expansion joint.
The north end would be fixed to the pier and supported (to a degree) by the backspan after completion.

The south end was a lot stronger, but still only had a rebar cage (no chevron ties, steel reinforcement, etc), thicker truss, larger base for #1/2, thicker diaphragm, #1 wasn't hanging off the deck (like #12), and the 3D stress was much lower (per Mr. Kaljas's analysis). I don't know if #1 had two ~3" vertical PVC pipes on each side and other "protrusions" from the deck (visible in photos). It was strong enough to hold during the collapse.

There was a problem at #11/12/deck because it cracked after initial PT tension and shoring was removed. They moved the span anyway, and first mentioned cracking after the move (those 2/28 photos are rather damning..). No one would have wanted to say or hear the span needed to be redesigned "just" because of that #11/12 node (as said one engineer in the Miami Herald article after he saw the photos). I'm curious if someone sensed it was weak, but that the north end would hold until bridge construction was completed.

RE: Miami Pedestrian Bridge, Part VII

3
Let me just add one more thing before I'm off for vacation. Looking at the geometry of drawing B-61 more closely (detail shown here), what's to prevent a good portion of the PT force of the bottom PT bar from going into the deck slab instead of diagonal 11? (Excuse me if this has already been talked about before by someone else; maybe chris snyder.) My strut-and-tie diagram is not very good, but I think the result would be to induce a tensile force at the inside corner, potentially resulting in splitting in this area. Reinforcement would need to be detailed to prevent this. Now we don't know if they were in fact stressing or destressing this bar at the time of collapse, but as a general rule the anchor location, geometry, and existing detailing is concerning.

RE: Miami Pedestrian Bridge, Part VII

Quote (chris snyder (Electrical))

I don't know if #1 had two ~3" vertical PVC pipes on each side and other "protrusions" from the deck (visible in photos). It was strong enough to hold during the collapse.
Thanks for your insights. #1 has four vertical PT rods, seen at the top and bottom hinges. The hinges also had the side effect of providing access to the PT rod couplings.

SF Charlie
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RE: Miami Pedestrian Bridge, Part VII

And then there's that pesky construction joint just above the anchors. We don't know exactly what it looked like because the forms for the truss were in place when the deck was poured, and we don't know how much time elapsed between the pours.

RE: Miami Pedestrian Bridge, Part VII

Quote (chris snyder)

it was weak

Thanks Chris, your post is right on. They had to know north end was... let's say non-optimal while bridge was incomplete as they designed it that way. I'm not saying they had malaise or were negligent, it's more subtle than that. I'm not sure movers knew about any problems because I have seen video of them casually hanging out under span. Comments from residents of area are appearing that pre-collapse called bridge an accident waiting to happen and noted how ugly it was. I digress, the ugliness of bridge is off topic.

The pdf I linked to discusses temporary use of PT bars during assembly.

Keep going Chris, I think all of us are happy with the quality of your posts.

RE: Miami Pedestrian Bridge, Part VII

Quote (TheGreenLama)

Crushing should never be an issue as plate size will have been designed by manufacturer to transfer specified design load.

With the concentrated compression loading, due to the differences in stiffness, with post-tensioning you often develop high tensile stresses. This is one of the reasons for the added reinforcing around the anchorages. It's often not for crushing.

Dik

RE: Miami Pedestrian Bridge, Part VII

I had hoped that the NTSB would release at least photos of the several bridge structural components that they are examining, that would show for example:
the end of the deck diaphragm that had been up against the pier
the end of the 10-11 blister that had been up against the canopy

I also hope that the start date of the embargo could be moved closer to the time of the collapse.

Maybe you who have questions that should and could have been addressed; would post them please.
Thank you all very much,

SF Charlie
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RE: Miami Pedestrian Bridge, Part VII



I am puzzled that they were stressing the PT bars. Why?

Were they trying to close a weak interface plane?

If they found a problem in the design (or construction) that required fixing, why they try to fix it without closing the road until the end of construction?

What happened that morning in the meeting that resulted in doing the fix without closing the road?

This is very troublesome. More than before when I thought that they were following the Construction drawings.

If everything is based on the cracks shown in the Feb 18 memo, why they released the PT bars on March 10 to stress them again the day of the collapse? Why they moved the bridge without fixing the problem on the side of the road?

Why a bridge (non-redundant) that is not meeting code or has a problem is allowed to be above traffic from March 10 to March 15? This does not make any sense.

We need to know more. I hope the Miami Herald gets all the info up to the moment of the collapse.

Best regards

RE: Miami Pedestrian Bridge, Part VII

Nothing new there. We may have to wait a year or more. I still wonder who is actually doing the design assessment, as of the 'Party Members' listed, I don't think there is one who is both technically qualified and not conflicted.

RE: Miami Pedestrian Bridge, Part VII

Re: NTSB Preliminary Report May 23, 2018

My highlights:

the propagation of cracks in the region of diagonal member 11

Department of Transportation Office of the Inspector General (OIG)

Miami-Dade Police Department (MDPD)

Lack of detail in report and OIG involvement (if not routine) indicates to me there is a strong criminal element being investigated.

https://en.wikipedia.org/wiki/Office_of_Inspector_...

RE: Miami Pedestrian Bridge, Part VII

That NTSB Preliminary Report isn't worth the digital footprint it makes. The Pre-lim Report leads one to believe no work was carried out on the number 2 member but evidence of light chipping debris immediately outside the blister suggests otherwise, along with statements from other parties involved. Why isn't Bolton-Perez and Associates Consulting Engineers included in the list of parties assisting in the investigation? Bolton-Perez and Associates Consulting Engineers should have done a better job of documenting the cracks.

RE: Miami Pedestrian Bridge, Part VII

I sure hope the final report is better... this was a total waste.

Dik

RE: Miami Pedestrian Bridge, Part VII

2
I've read hundreds of full NTSB reports, and thousands of preliminary reports and report synopses, and this one is exactly in line with my expectations based on their past work. Their preliminary reports are never anything more than an expression of the facts available by inspection, and where practical, a statement of what resources (if any) are expected to be engaged in the ensuing investigation. They often contain substantial errors and omissions, and are rarely emended before the final report is released.

NTSB final reports are almost always of a depth commensurate with the seriousness of the accident at hand. For a non-fatal small airplane crash, the final report will usually contain only a little more than what was in the preliminary, plus a statement of probable cause. For a fatal airplane crash, the report will be quite voluminous, and may include recommendations for regulatory change in addition to the probable cause. For an accident such as the one at hand, I think we can reasonably expect that the final report will be quite thorough, will engage all necessary resources, and will evaluate a wide range of contributing factors in arriving at a probable cause.

--Bob K.

RE: Miami Pedestrian Bridge, Part VII

Quote (epoxybot (Structural))

Why isn't Bolton-Perez and Associates Consulting Engineers included in the list of parties assisting in the investigation?
Thank you for pointing that out, I missed it. What makes it seem really weird to me, is that the photos and drawing are credited to them?

SF Charlie
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RE: Miami Pedestrian Bridge, Part VII

Maybe we won't have to wait so long after all. "In the next month, the NTSB will be..." Typo maybe?

While learning the results and recommendations of the investigation will be interesting, I'd much rather see what they decide to build in place of the failed project.

RE: Miami Pedestrian Bridge, Part VII

Something in steel, I think, Retiredat46.

RE: Miami Pedestrian Bridge, Part VII

I think it would be sensible to put up a precast double tee bridge, as is commonly done to provide pedestrian access to properties that are otherwise isolated by new expressways. There seems to be a common or semi-standard design, supported by two or three hammerheads, terminated by spiral ramps or stairs, and completely covered by chain link fence.

That 8 foot tall 'projectile barrier' or whatever they call it in the plans is just silly. FIU no doubt has multiple students of both sexes who _could_ loft a bowling ball over it with ease. ... and at least an equal number of local nonstudents and random miscreants who _would_ do it, for no particular reason.

Mike Halloran
Pembroke Pines, FL, USA

RE: Miami Pedestrian Bridge, Part VII

Interesting pre-move local news video not yet seen on this thread. They refer to FIU bridge as largest pedestrian bridge in U.S. history. Note two possible safety violations at 46 and 51 seconds.

https://www.nbcmiami.com/on-air/as-seen-on/Pedestr...

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