Miami Pedestrian Bridge, Part III 99

[JStephen]

"Site management has requested that we limit the length of the other thread by forming a new one. This subject may require III, IV...."
So here's Part III. Please don't post any more in Parts 1 and 2.

Part I thread815-436595
Part II thread815-436699

 

[Meerkat 007]

New interesting post processed dashcam video at

https://youtu.be/D6pGzgm4zZ4

Compression of member #11, before deck fall, could be pushed out PT rod, as someone suggested already.

 

[Ron]

Were you able to determine if the failure occurred at the ends of the web members or at the panel point?

dik....not sure. In one of the NTSB photos, it shows the anchor "blister" relatively intact but broken through the top chord. This is what led me to suggest perhaps a punching shear of the entire panel point through the top chord. There did not seem to be much, if any, lateral mild steel reinforcement across this interface.

 

[drawoh]

A defense of the design:

To the structurals here, who hasn't, at the behest of an architect, worked their tail off to make something superflous work? Is that not part of our job? Ok, so the "cable stays" really aren't. So?

...

Drawoh's rules of style...
[ol 1]
[li]Style costs more. There is a way to do it that is cheap and functional.[/li]
[li]Keep it simple. We are techies, not artists. [/li]
[li]Bad styling looks a lot worse than un-styled functional.[/li]
[/ol]

Okay, I am mechanical, not civil or structural. If management is determined to make it cool, ask for a budget. Maybe you will get a big one! I perceive rule[ ]2 as an artistic issue, but maybe there is a structural component too.

What if they had designed a simple, well proportioned bridge, and added twenty tons capacity for artists to play with?

--
JHG

 

[WinelandV]

Drawoh,

I agree with your rules of style. That being said:

1) The people who decided to lay out the money for the bridge wanted something snazzy. Without actually looking in to it (I'll save that for epoxybot), I'm willing to guess that the style of the bridge contributed to the DB team getting the project. So yes, the city/state/university most-assuredly paid more for this bridge than a "standard" (whatever that is) long span bridge.

2) Unfortunetely, with bridges, as you adjust how it looks, you're adjusting how it works. You've essentially got a line with 2 or more supports. The load has to get to those somehow and there's just not much play with it. *shrug*

3) The style of this bridge was interesting enough (to me) to bring it out of the "bad styling" category. YMMV

4) A simple well-proportioned bridge with additional capacity just wouldn't have had the WOW factor that (apparently) everyone wanted.

Long story short: municipalities like fancy bridges in high-visibility areas. You'll notice that interstate bridges over no-name river in the middle of nowhere very rarely deviate from girders and a deck slab.

 

[AUCE98]

I worked for VSL as an engineer for two years several years ago, mostly monostrand commercial building design work. My understanding from some former coworkers was that they were in the process of de-stressing one of the threaded bars when it collapsed. So they were attempting to reverse the stresses within the no. 11 diagonal when the collapse occurred.

 

[Meerkat 007]

Okay, I am mechanical, not civil or structural. If management is determined to make it cool, ask for a budget. Maybe you will get a big one! I perceive rule 2 as an artistic issue, but maybe there is a structural component too.

Like this ones:

 

[LittleInch]

Well if they were actually destrssing it maybe the PT rod was the only thing keeping that column straight and once they loosened it it just bent and failed in bending?

I've also been wondering if the bottom slab got damaged/ cracked during the lift due to reverse of anticipated loads / sheer.

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

 

[oldrunner]

How about form follows function?
Due the "simplicity" of the truss, one might be able to run a 2-D analysis on SAP80, which would run on a Radio Shack Model II which would run the CPM system. $600 program at the time. About that time we were moving over from VisaCalc to Lotus 123.

 

[PE2015]

In the attached utube video posted today, there may be a crack, or a construction wire/cable, several feet south of where #9 and #10 connect to the lower deck. The crack or wire appears to past through, or over, the floor drain.

http://www.youtube.com/watch?v=SBoE34-WZoE

 

[epoxybot]

Everything about the FIU TIGER-2013 award is here: Link
It is all part of a bigger scheme to expand into Sweetwater aka University City

 

[TrafficDad]

I've also been wondering if the bottom slab got damaged/ cracked during the lift due to reverse of anticipated loads / sheer.

I also thought about this. Specially since in the transit video it seems like 2 of the 4 support points were between the nodes instead of directly underneath the nodes and there were no load distributing elements between the bridge deck and the supports that moved the bridge into place. However, even if that was a screwup and the concrete on the deck was cracked as a consequence (which may be what that engineer's voicemail message was about), the bottom deck only would have tension forces going through it once the bridge was placed in it's final position, which (I guess) would mean that the concrete would not be taking much stress. The bottom slab is in tension so the steel tendons would be doing all the work. So a crack on the bottom deck should not have been an issue, right?... Unless perhaps, if the crack was not between two nodes but rather quite close to one of the nodes where the diagonal elements meet.

 

[12345abc6ttyui67]

PE2015, I'm sure that's just a cable / wire. There seem to be some wrapped around the chord member, and some further along the bridge too. Seems it connects into some little grey box either side of the deck. Not sure what those might be though...

 

[dik]

There did not seem to be much, if any, lateral mild steel reinforcement across this interface.

Thanks, that's what made me wonder if the failure had occurred at the panel point and not the actual member.

Dik

 

[epoxybot]

PE2015 - That looks to be electrical extension cord but looking at those drains directly at the base of the truss is unnerving.

 

[azcats]

Are those strain gauges on the members shown on the video posted by PE2015 on 20 Mar 18 18:57?

 

[3DDave]

At 720P the lines leading to the drain terminate at little boxes near the edge of the platform. There is a similar configuration farther along the bridge where the guy is standing.

 

[LittleInch]

I know we now know that the bridge failed, but in that video posted by pe2015, does it just feel like the struts are too small and at bad angles?

When you see it like that the width of the bottom flange just feels huge compared to the struts.

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

 

[bones206]

In the safety section of the proposal document, there’s a brief statement that all work on the main span, after being set, would be performed over closed lanes during off peak hours.

My point is: that entire document shows an impressive amount of diligence and forethought put into the safe implementation of this design. Yet, when things apparently didn’t go as planned, those conservative safety principles somehow didn’t carry through and they ended up working over live traffic. Either the risks of this work weren’t appreciated by the team, or there was some sort of decision made based on risk vs cost/schedule/hassle/whatever.

Besides the technical aspects of this disaster, the lesson to be learned concerning conservative decision making when unexpected conditions are encountered is one we should all take to heart and keep in mind when we encounter similar situations our future endeavors. I feel that we have an obligation to get the full story on the decision making process in this case, then learn from it and reinforce that aspect of our profession going forward, just like we learn from the technical aspects of disasters.

Having worked in the field on multi-billion dollar projects for a good part of my career, I’ve taken serious flack for sticking to my guns on safety issues. I remember feeling like I was on an island with very little backup from senior engineers, who were more interested in not rocking the boat and having a smooth sail into retirement. There was always an implicit pressure to be a “team player”. Probably because of those personal experiences, the decision making aspect of this disaster has resonated with me more than anything else. Even if they thought the crack was harmless, the written safety plan was no work over traffic.

 

[replicab]

https://www.cbsnews.com/news/florid...hind-schedule-over-budget-updates-2018-03-20/

"Documents obtained by The Associated Press through a public-records request show that the Florida Department of Transportation in October 2016 ordered Florida International University (FIU) and its contractors to move one of the bridge's main support structures 11 feet north to the edge of a canal, widening the gap between the crossing's end supports and requiring some new structural design."​