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Miami Pedestrian Bridge, Part VIII
16

Miami Pedestrian Bridge, Part VIII

Miami Pedestrian Bridge, Part VIII

(OP)

RE: Miami Pedestrian Bridge, Part VIII

(OP)
hokie66, I'd agree with you that the article seems to be well written for its intended audience.

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RE: Miami Pedestrian Bridge, Part VIII

2

Quote (Miami-Herald)

FIU had launched and heavily promoted a center for ABC at its engineering school, and explicitly saw use of the technique for its pedestrian bridge as a demonstration of its efficiency.

Such quick-bridge construction effectively ruled out a true cable-stayed bridge. Cable-supported bridges are built in sections and in place, which requires extended road closures. Truss designs, in contrast, are ideal for the accelerated approach, engineers say.

I posted this bridge in Part IV but I'm re-posting it because it demonstrates that the above premise is not valid.
Time Lapse Video: Link
Time Lapse Video: Link



The Mary Ave. (Don Burnett Bicycle-Pedestrian Bridge) is built to withstand SF Bay Area earthquakes. As the photo demonstrates they could have built HALF of this bridge and probably have had plenty of money to dress it up & make it the deck wider, while still employing a method of ABC. This bridge would have needed support briefly. The extra unused lane on the Tamiami Trail/Hwy 41 would have served nicely.



RE: Miami Pedestrian Bridge, Part VIII

Yes, there are plenty of cable stayed pedestrian bridges built while the roadway below is in use.

And the bridge in Cupertino CA is structurally honest, and IMHO better looking than the Miami bridge was going to be.

RE: Miami Pedestrian Bridge, Part VIII

I wonder about the difficulties of anchorage to support the asymmetrical weight of a pure cable-stayed bridge and the concentration of the weight on the one central pier that would happen, given the close proximity to the canal. It's also about 12 feet wide, or about 1/3 the width of the FIU bridge and likely a lot more elastic/jouncing making for a less desirable hangout spot.

RE: Miami Pedestrian Bridge, Part VIII

Quote (hokie66)

This article in the Miami Herald, linked late in the Part VII discussion, would make a good starting point.

http://www.miamiherald.com/news/local/community/mi...

...but that 'undernourished' strut....

Brad

It's all okay as long as it's okay.

RE: Miami Pedestrian Bridge, Part VIII

Quote (thebard3)

...but that 'undernourished' strut....
I'm thinking that is a case of an overly active use of a thesaurus. (Maybe a change from underweight to undernourished?)

RE: Miami Pedestrian Bridge, Part VIII

I was quite happy to have another structural term in my vocabulary. The bridge was demonstrably understrength, and strength often relates to nourishment, so I give the journalist a bit of leeway.

RE: Miami Pedestrian Bridge, Part VIII

2
undernourished = lacking sufficient iron

RE: Miami Pedestrian Bridge, Part VIII

I quite like that description. Not enought "meat on the bones" if you like I think gets the message over very well.

I thought that was a very well written article.

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

RE: Miami Pedestrian Bridge, Part VIII

I'm feeling confident so here's my two sense on the failure progression:

a) The base of #11 fails early - essentially a column with a wedge shaped base that slips and crushes against #12. This failure is not explosive, rather it is progressive as PT is ratcheted up.
b) The displacement and crushing of the lower end of #11 results in the upper canopy being pinched downwards at the #10/#11 bubble.
c) Further PT on #11 also loads energy into #10 which bows upwards while #11 bows downwards.
d) This deformed system continues to be loaded with PT plus gravity until the upper end of #11 blows apart. This is when we first see the obvious disaster unfold.
e) As #10 is restored, the north end of the #10/#11 bubble is punched out of the failed canopy creating an illusion of time lag in the canopy collapse north/south of the bubble.
f) The lower deck fails across the #9/#10 joint.
g) The north end of the bridge is dragged down, tearing the PT cables out of #11.

Unfortunately, this disaster truly was amateur hour.

My sincerest condolences to the victims and their families.

RE: Miami Pedestrian Bridge, Part VIII

I'm with you up to d)

Looking at this extensively I think it's the base of no 11/12 which fails first or maybe at the same time as the top. This action is enough to push no 12 off the plinth. If you look closely at the top of 12 during the many slo mo videos, the canopy at the 12 end falls at the same time as the rest for about half its height before the canopy collapse drags it back over the plinth into its final resting position.

One of the survivors quotes a sudden loud noise just before the collapse. Could be either end of 11 IMHO

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

RE: Miami Pedestrian Bridge, Part VIII

OK - I understand what everyone has explained. And I am not a structural engineer, but I have to ask, what if the joint between diagonal #11 and the deck had not been a "cold joint" but a continuously poured section of concrete (not that that would even have been possible). I might have thought that critical "joints" would have been spec'ed to be completed in a single pour for this reason.
It looks a little to me as if the designer was used to building steel bridges where a properly done weld would be stronger than the base metal. It looks as if concrete bridges may be a different matter.
What does anyone think?

RE: Miami Pedestrian Bridge, Part VIII

Hello Again,
I have been working (slowly) on a frame by frame of the original dashcam images.
It is easy to draw boxes aligned with the image shapes, the hard part is to align the images frame to frame because of the movement of the truck. It is very clear that the canopy drops at the #10-11 blister before any other movement can be detected at the resolution of the dash cam. #12 appears the remain vertical at this moment. To me this implies a break at the top of #12 and requires #11 to get shorter. There is also an increase in the angle between #10 and 11, so something had to give there.
I'm working in powerpoint 'cause that's what I'm familiar with, but haven't found a way to capture the slideshow. I'll post as soon as I can convert to a reasonable format.
Thank you,

SF Charlie
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RE: Miami Pedestrian Bridge, Part VIII

SFcharlie - Hence why I think the base of 12 and 11 was initially pushed out just beyond the support (would in essence make no. 11 "shorter").

tomcat - I think many are agreed that the design of the bridge was not standard and that the analysis of the joints and connections between the different elements was a lot more complex than may have been imagined / undertaken. It has also been noted many times that the number of concrete truss like designs is very limited - probably for good reasons.

However we don't have the inside knowledge on the level of analysis and design scrutiny / review which was undertaken.

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

RE: Miami Pedestrian Bridge, Part VIII

Quote (LittleInch (Petroleum))

I think the base of 12 and 11 was initially pushed out just beyond the support
I can see the upper part of #12, and it doesn't move. I suspect that the little ~3 inch high bit of concrete between #12 and 11 failed. What happens at the top is confusing since in the aftermath, we see the blister more or less intact, but separated from the canopy, but somehow 11 became hinged re #10. Also in the aftermath, 11 is damaged at both ends. I wish NTSB would release the photos of the elements as they observed during the deconstruction of the north end.
Respectfully,

SF Charlie
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RE: Miami Pedestrian Bridge, Part VIII

Quote (IRstuff (Aerospace))

possible to screen capture the slide show into a video
Thanks!
Yes, I'm sure you're correct. I had hoped that Microsoft would provide a way to export the show as a gif or video file, but I haven't found the correct pulldown. I'll google it.

SF Charlie
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RE: Miami Pedestrian Bridge, Part VIII

Thanks for the answer. I didn't phrase the question very well. Putting bridges aside for a moment....
Does anyone know if the shear strength of a concrete "cold joint" is different than the shear strength of a solid block of concrete? Intuitively it would seem likely it would not be the same, but I expect it depends on a variety of factors including how much the first pour has cured before the second pour is made. My question stems from the excellent information in this thread about the FIU Bridge construction using a "cold joint" at the junction of the lower end of diagonal 11 and the walkway. It seems essential that the diagonal be properly bonded to the walkway and it would seem to make a difference whether these two pieces were connected with a "cold joint" or a continuous pour. There was a very large horizontal component of force on this joint (the horizontal component of the compression force in diagonal 11) and I am wondering if the engineers over estimated the shear strength of the concrete that composed that joint because they failed to recognize the field construction had to pour the walkway and diagonal in two different pours. Thanks for your patience.

RE: Miami Pedestrian Bridge, Part VIII

According to the Miami Herald article from June 14, 2018, it's apparent that the Herald's request for additional information from the parties involved was granted, as it's stated within the article that nearly 2,000 pages of calcs were reviewed by several engineers. But where are these calculations? What were the terms of the release? Are there legal restrictions to full disclosure at this time? If this was a freedom of information-like release, why weren't these calcs made public?

Could someone here please explain this to me, or could someone from the Miami Herald please speak to this issue (the Herald article was extremely vague on this point). It seems to me that a perfect way to kill, or control, a story is to request info, then once you've obtained it release it only to a select group of individuals.

RE: Miami Pedestrian Bridge, Part VIII

TheGreenLama... when this first came out, I sent an eMail to one of the people responsible for the article... no response.

Dik

RE: Miami Pedestrian Bridge, Part VIII

Andres Viglucci (aviglucci@miamiherald.com) was the guy I sent the query to...

Dik

RE: Miami Pedestrian Bridge, Part VIII

Tomkat: I can't answer your question about cold joint strength (details outside my field), but it's been discussed. I'd assumed #11/12 slid off the end of deck after breaking at cold joint, but recently someone posted a different view of #12's end that shows it broke below the cold joint (which held). The 'leaked' photo showing cracks at base of #11 after shoring was removed shows a crack traveling along the cold joint.


RE: Miami Pedestrian Bridge, Part VIII

About documentation. This was the best/detailed article. http://www.staugustine.com/news/20180616/fiu-had-g...

It sounds like they made a basic public records request for the 2000 pages of calculations.
"The Herald obtained the structural calculations and design plans through a public records request and shared them with the engineers. Howell and Beck, who both have expertise in bridge design and structural engineering, analyzed the plans and calculations independently but came to similar conclusions."

but I'm not sure if this (further down article) means the calcs are no longer accessible by the public:
"Unless you’re part of the investigative team, you may not be privy to all the information.”
"That information is no longer accessible to the public, however. The NTSB has severely restricted access to records related to the accident, leading the Herald to file a lawsuit demanding access to documents that were previously available for public review under Florida public records laws."

This is new from a lawyer. Bottom line is the NTSB is supposed to write a document to say if they want to be party to MH suit, and if not, the judge will rule on whether info between Fed 19 and Mar 15 before the span fell can be released (especially minutes from the meeting about cracks the morning before collapse). Is the same place they were weeks ago when the judge asked NTSB to show up (they didn't but wrote a letter).
https://www.roadsbridges.com/fdot-will-not-give-fi...

RE: Miami Pedestrian Bridge, Part VIII

Chris,

Thanks for the link to the July 5th article in Roads & Bridges. It reads like the Herald has received documents that were created on or before Feb. 19, and that the lawsuit is to try and gain access to material between Feb. 19 and the collapse. At least that's my interpretation. If so, it implies that they have received the design calculations from FDOT, separate from the lawsuit, and for some reason are only letting select people have access to them. What would be their motivation for doing this?

I believe that the St Augustine article is nothing but a copy of the earlier Herald article.

RE: Miami Pedestrian Bridge, Part VIII

Quote (FDOT)

DOT produced documents created on or before Feb. 19—a little less than a month before the collapse—but nothing thereafter, including during the crucial days leading up to the collapse. FDOT’s refusal to produce documents originating after Feb. 19 was the result of a written directive it received from the NTSB

Even though they claim 'no involvement'... I betcha they are in it up to their kazoo... Maybe protecting themselves?

Dik

RE: Miami Pedestrian Bridge, Part VIII

I did a quick critique on an article showing the FDOT's involvement, only to learn that it was for a different project.

Does anyone have a copy of the document that pertains to this project? I couldn't find my reply in the earlier parts of this thread.

Dik

RE: Miami Pedestrian Bridge, Part VIII

Quote (chris snyder (Electrical))

recently someone posted a different view of #12's end
This is a frame from Engineer Multiple red flags behind fiu bridge collapse
It looks to me as though the rebar for the tower (or the diaphragm for the north span?) is intact behind #12 lying on the pier?
Thank you Chris for bring this to our attention.

RE: Miami Pedestrian Bridge, Part VIII

2
It was, and still is a very interesting discussion, but somehow concentrating on the mode of the collapse, instead on the real reasons leading to this tragic accident.
It is obvious that the bridge was under designed as it failed under dead load alone. So, it’s apparent that there was a major design error overlooked by all involved parties.
As I posted before, the code and design guide used to design this bridge were leading to the seriously “undernourished” structure with unacceptable safety factor, or, as proven by the events, to failure under self-weight.
As on the typical well-founded project, a lot of studies were done such as the study of the loading. Unfortunately, no studies were conducted for the load factors to be used in the design, so typical AASHTO's were likely used, applicable to highway bridges and pedestrian crossings with light weight (steel) structures. The attached article presents some thoughts on the subject, as well as commonly used factors for different types of structures, and explains the risks associated with this approach.
Equally critical, in my opinion, was the methodology used in the design for shear stresses. The current code approach is to carry the excess stresses on the steel, using some fancy formulas, where the old approach was to carry all shear forces via reinforcement, when the allowable shear stresses where exceeded, and always use this approach in the support or anchorage zones.
Perhaps that the secret so closely guarded by NTSB? That the bridge was designed as per AASHTO and “Design Guide for Pedestrian Bridges” without major deviations?

RE: Miami Pedestrian Bridge, Part VIII

3
An important reminder: there is no "slow-motion" video. The original truck-cam video was shot at approximately 5fps, or one frame every 0.2 seconds, and had a resolution of 1280x720 pixels. This video was then processed to CREATE THE ILLUSION of zooming and slow-motion. The "zooming" was performed by CREATING in-between pixels, probably by simple linear interpolation, so a small center section appeared larger. The "slow-motion" was performed by CREATING multiple in-between frames with advanced 3D (X,Y,time) interpolation routines that recognize primative shapes and patterns in consecutive frames.

These techniques can produce a remarkably realistic interpretation of an event, as long as the event being filmed has dynamics that are relatively slow compared to the original video frame rate. When this is true, the CALCULATED in-between pixels and frames will create an ILLUSION that matches up very closely with reality.

In this scenario, however, we are dealing with with an extremely rapid series of explosive non-linear events that occurred on a millisecond or microsecond timescale in which 0.2 seconds is a lifetime. As a simple example, a car traveling at 60 mph, or 88 feet per second, travels 17.6 feet in 0.2 seconds. If you filmed that car driving into a concrete barrier at 5fps, you'd only record one or two frames of the impact, and you simply don't have enough information to process those frames into a video that portrays the crash realistically. That same reasoning should be applied to the truck-cam video.

RE: Miami Pedestrian Bridge, Part VIII

Has this been suggested yet?
The compressive strength of the diagonal member was exceeded.
The lower part of the diagonal member crumbled under compression.
With part of the diagonal member crumbled, the member ceased to be a viable structural mamber.
Gravity took over.
As the bridge collapsed, the shortened remnant of the diagonal punched into the vertical member inducing the shear failure.
At the same time, more of the diagonal member may have crumbled.
The final position of the hydraulic jack indicates that the diagonal member is considerably shorter than originally.
My main point is that the initial failure may have been compressive failure rather than a shear failure.
This suggestion is supported by reports that the initial failure occured during an operation that increased the compression stress on the diagonal member.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Miami Pedestrian Bridge, Part VIII

waross, in looking at the way the post-tension bars were installed, they acted like anchors in the deck, below the shear line of observed cracks. If one were to put a cable alongside the diagonal for the purpose of pulling it off the end of the deck one would anchor it just the same way. In an early post I took a shot that the compression load in that diagonal was approximately the full weight of the bridge; the total compression stress was from that was in the mid 4ksi range for concrete that was supposed to take 8.5 ksi minimum. The additional tension was 5-10% of the dead load (sofa-cushion estimate).

From this I have since concluded that the most likely failure was to continue to fail the connection that had already failed - the shear line between the diagonal and the deck. Only an inch of relative movement would exceed what the rebar bridging that connection would be able to take. Whether the rebar failed in pure shear or because of excess tension as the embedded ends were displaced, I don't know, but the overall failure looks to be entirely from a loss of shear capability.

RE: Miami Pedestrian Bridge, Part VIII

From earlier threads and information there are two tensioning bars in member 11, one high and one low. Apparently the top bar had been further tensioned and it was during the tensioning of the lower bar when the failure occurred.

The top bar remained in place, but the bottom bar, which was anchored into the bottom base was ripped out of the member 11 during the collapse thus showing that, at some point during the collapse, member 11 became detached from its connection to the base. This ripping out essentially shortened the bar leading to the tensioner sticking out the top.

Now whether this was cause or effect is not clear, but many here believe that the extra tension on the bottom bar added to the shear load of the joint between 11,12 and the base leading to a shear failure in that joint, perhaps initially only of a couple of inches, which then lead to a sudden failure of the rest of the structure. Now how that complex joint was designed, analysed and reviewed / approved is probably the key design issue here as wiktor states. On the design data available and evidenced by the cracks seen, it looks like the weak point and is not over encumbered with re-bar.

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

RE: Miami Pedestrian Bridge, Part VIII

The Chirajara bridge in Colombia collapsed on January 15, 2018.

On January 31, fifteen days later, a preliminary report was presented explaining the reasons for the ruling.

The report mentioned the desirability of demolishing the opposite side of the bridge.

On March 15, 2018, the FIU pedestrian bridge collapsed.

Because 120 days later there is no preliminary report?

RE: Miami Pedestrian Bridge, Part VIII

They already released a preliminary report.

The NTSB typically investigates for systemic causes into failures, not simply immediate causes and this can take longer. There is also no pressing potential of another collapse of a similar structure because there is no other structure built to these plans. I'm pretty sure the NTSB clearly stated that it would take up to a year before their final report would be issued. Even if they did not, that is the amount of time almost every final report takes; they do not issue any intermediate reports.

RE: Miami Pedestrian Bridge, Part VIII

3DDave... that report and $2.00 will get you a cup of coffee. Report was useless. Taxpayers' money hard (ly) at work.

Dik

RE: Miami Pedestrian Bridge, Part VIII

The report was exactly what NTSB can release at this point in their investigation, and it is exactly what they are supposed to release. It sets forth the facts that are knowable by inspection (what happened, when, and where, etc) and lays the groundwork for the ensuing investigation. It does not speculate.

RE: Miami Pedestrian Bridge, Part VIII

hpaircraft... it was pretty light for that even. It did not provide any factual information they had. The information has been specifically held back, so, characters like those on this forum could not come up with 'real' information. To reiterate, the report was useless, and, a waste of paper and manhours.

Dik

RE: Miami Pedestrian Bridge, Part VIII

The reason that the NTSB is so backlogged is due to budget cuts. They have only just gotten around to holding the hearings on the train crash outside of Tacoma. They will hold hearings on the FIU bridge and they will leave no stone unturned. Everyone will take their turn being grilled. Be patient.

SF Charlie
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