Miami Pedestrian Bridge, Part II 55

[hokie66]

That's what they thought. It wasn't a cable stayed bridge, it was a truss, unfortunately made of the wrong material.

 

[dik]

The loads are additive

Thanks for the clarification... I seemed to recall from Lin's book that the precompression was 'absorbed' until it was exceeded by the axial load, was also thinking of preloaded A325 bolts not increasing in tensile stress until the preload was attained... never done work with prestressed compression members.

Dik

 

[BAretired]

No dik, that is not correct. If a member is prestressed, applying external compression will relieve some of the prestress (because of the change in strain) but not all of it. If a member is compressed by externally applied load and then it is prestressed, the two effects will be additive.

BA

 

[dik]

Thanks BART... I'll have to look into it... it's been 40+ years since I read Lin's book... read it starting Friday till Monday night, day and night and had about 1" of notes... can't do that now.

When the load is equal to the prestressed load, what is the stress in the column?

Dik

 

[BAretired]

When the load is equal to the prestressed load, what is the stress in the column?

If the load is applied first, then the prestressing, the stress is 2P/A where P is the applied load.

If the prestressing is applied first, then the load, the stress is less than 2P/A but close to it.

BA

 

[Tomfh]

If the prestressing is applied first, then the load, the stress is less than 2P/A but close to it.

...and then greater when they repull the rod.

 

[Ingenuity]

So there were two bars in #11? Not a central bar?

Correct, as per the attached photo:

 

[BAretired]

It seems clear that the bottom chord was not attached to the pier, so it relied on attachment to the end diagonal and would have a tension equal to the horizontal component of the end diagonal (#11) excluding any prestressing force. The collapse is consistent with the bottom chord pulling away from Member #11.

BA

 

[racookpe1978]

This from the commercial press:
Munilla Construction Management (MCM), the South Miami-based firm that designed the FIU foot bridge, has been sued multiple times for unsafe practices in the past.

In early March MCM was sued by a construction worker who was severely injured when MCM’s “makeshift bridge” at Miami’s International Airport collapsed.

MCM is a Cuban-American, family-owned Miami company founded in 1983 that employs more than 1,000 people in several states. The company is a federal military contractor for the U.S. Army and Navy.

MCM was awarded the $14.2 million minority contract to design and build the cable-stayed bridge. The company is well-connected in Miami politics and it promotes inclusion and diversity in the workforce.

From their linked in reference
Employees at MCM | Munilla Construction Management

Melanie Rowan, P.E.
Senior Project Manager

Nelson Gomez Jr
Project Engineer

Maira Suarez

Alex Suarez
Senior Project Manager

Nelson Nunez
Fleet Manager

https://www.linkedin.com/company/mcm_3

 

[BAretired]

This from the commercial press:
Munilla Construction Management (MCM), the South Miami-based firm that designed the FIU foot bridge, has been sued multiple times for unsafe practices in the past.

I thought it was FIGG who designed the bridge.

BA

 

[MJB315]

If a member is prestressed, applying external compression will relieve some of the prestress (because of the change in strain) but not all of it. If a member is compressed by externally applied load and then it is prestressed, the two effects will be additive.

Is this not the likely culprit?

I know it's too soon to know, but:

[ul]
[li]Say #11 was prestressed to resist the tension caused by the temporary transport load (the end of the truss was basically being hung by #11). No numbers here -- just call that Tension Force A.[/li]
[li]The bridge gets placed on the abutment. Load reversal city as the temporary supports get pulled away.[/li]
[li]Now say #11 is carrying 50% of the bridge self-weight in compression. And, at that steep angle. Call this Compressive Load B. [/li]
[li]Compressive Load B >> Tension Load A. [/li]
[/ul]

Wouldn't those PT rods go "slack" in that case?

If so, wouldn't re-tightening them result in Compressive Load B + Tension Load A to occur at that node?

That'd far exceed any other conceivable design load demand there, right?








"We shape our buildings, thereafter they shape us." -WSC

 

[racookpe1978]

I know one early NTSB member was quoted with the claim that the cable stays were "decorative only" - but is he correct? They are after all, traffic. Not bridge/truss design.

The entire "truss" design was intended to transmit the visual and real loads from each cable stay down through the angled truss members to the lower bridge walkway. They could not be built and tensioned without assuming the load.

 

[racookpe1978]

I thought it was FIGG who designed the bridge.

Munilla Construction Management could be a sub-contractor to FIGG as prime, or the reverse: Put MCM as the "bidder" to the government/political people to get the minority preference (visible) contract, with the real work being done by the FIGG people. Or, the story I found could be dead wrong.

 

[Tomfh]

Wouldn't those PT rods go "slack" in that case?

No, because the rods are far more highly strained (i.e. stretched) than the amount the column shortens under the self weight loading. PT rods/strands stretch a lot.

That'd far exceed any other conceivable design load demand there, right?

You'd include all the loads in the design.

 

[gte447f]

It was design/build. MCM is the prime contractor and Figg is an engineering subconsultant. I think.

 

[epoxybot]

Here is a video showing the bridge move from the pylon side of the bridge: Link

 

[human909]

Is this not the likely culprit?

I know it's too soon to know, but:

Say #11 was prestressed to resist the tension caused by the temporary transport load (the end of the truss was basically being hung by #11). No numbers here -- just call that Tension Force A.
The bridge gets placed on the abutment. Load reversal city as the temporary supports get pulled away.
Now say #11 is carrying 50% of the bridge self-weight in compression. And, at that steep angle. Call this Compressive Load B.
Compressive Load B >> Tension Load A.
Wouldn't those PT rods go "slack" in that case?

If so, wouldn't re-tightening them result in Compressive Load B + Tension Load A to occur at that node?

That'd far exceed any other conceivable design load demand there, right?

This seems somewhat plausible. It is a rookie mistake all round but entirely plausible.

 

[Ingenuity]

First lawsuit to be filed Monday.

http://www.wesh.com/article/orlando-based-attorney...

...and that, along all the many more to follow, will have FIGG's attorneys and insurers issuing FIGG with a immediate cautionary notice (demand?) to talk to no one!

“We are stunned by the tragic collapse of a pedestrian bridge that was under construction over Southwest Eighth Street in Miami. Our deepest sympathies are with all those affected by this accident. We will fully cooperate with every appropriate authority in reviewing what happened and why. In our 40-year history, nothing like this has ever happened before. Our entire team mourns the loss of life and injuries associated with this devastating tragedy, and our prayers go out to all involved.”

Source: Link

Speaking from past experience (a past project of remarkable similarity: bridge, post-tensioning, collapse, deaths...and its following lawsuits) these things get real ugly. The technical part of you wishes to assist the investigators, but you get 'muzzled' by the powers-that-be: i.e. lawyers/insurers/corporate/management.

 

[rapt]

The relatively small amount of load that would have been applied by the stressing of one prestress bar in a diagonal should not be sufficient to cause the collapse if the overall design was ok. With normal factors of safety etc, the amount of force we are talking about compared to the overstrength required to be built into the design is insignificant.

There has to be some other cause.

 

[Ingenuity]

Given the uniqueness of this bridge configuration, I wonder if the design engineers [FIGG] gave any thought to embedding some structural monitoring equipment (e.g. strain or vibrating wire gauges to rebar, concrete, strand, PT bar etc)?

A couple of 16 channel datalogger, a multiplexer, some software, and gauges etc, are readily affordable today, with very good reliability and ease-of-use.

Damn, it would have been a good research project for a select few FIU engineering students.