Miami Pedestrian Bridge, Part XII 34

[zeusfaber]

A continuation of our discussion of this failure. Best to read the other threads first to avoid rehashing things already discussed.

Part I
thread815-436595: Miami Pedestrian Bridge, Part I

Part II
thread815-436699: Miami Pedestrian Bridge, Part II

Part III
thread815-436802: Miami Pedestrian Bridge, Part III

Part IV
thread815-436924: Miami Pedestrian Bridge, Part IV

Part V
thread815-437029: Miami Pedestrian Bridge, Part V

Part VI
thread815-438451: Miami Pedestrian Bridge, Part VI

Part VII
thread815-438966: Miami Pedestrian Bridge, Part VII

Part VIII
thread815-440072: Miami Pedestrian Bridge, Part VIII

Part IX
thread815-451175: Miami Pedestrian Bridge, Part IX

Part X
thread815-454618: Miami Pedestrian Bridge, Part X

Part XI
thread815-454998: Miami Pedestrian Bridge, Part XI

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[Vance Wiley]

Ahhhhhh - A voice of reason. Clearly in the Structural discipline.
Thank you.
Someone with their head above deck to watch where the ship is going.
Berger is being reluctant to release detailed information on their involvement but some things are known. Do we know if Berger reviewed the main span as an independent structure, without the referenced future contributions from the back span?

 

[Earth314159]

Isn’t the steel supposed to be perpendicular for shear friction to be valid?

It doesn't have to be perpendicular but you do have to use the perpendicular component (I suspect it is the same in the US code).

 

[SFCharlie]

Do we know if Berger reviewed the main span as an independent structure

I thought we knew that he did not. He was specifically not paid to do that.

SF Charlie
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[Vance Wiley]

Thank you. I missed that or forgot.
If the independent span check was not in the Berger scope of work that should ease the pressure on them. That is probably why they have not participated in a settlement.
Is there a way to search all 13 parts of this issue at one time? I have trouble doing one at a time - jumping to the nexxt sectiun is cumbersome.

 

[Vance Wiley]

That seems counter intuitive to me. If the bars have compression then they are actually "stealing" some of the normal (as in perpendicular) force from the joint and the concrete to concrete force would be less.
Also, if the bar is in compression it must have shortened - however little - and would not this compression shortening need to be overcome before the steel can be mobilized to assist shear friction?

 

[hokie66]

Some good can come from this. Two things...the concept of a post-tensioned concrete truss can be consigned to the dust bin. And hopefully, shear friction theory can be given better scrutiny. It has been seized on some in the industry as a convenient way to deal with construction joints without having to develop the forces in bearing. I remain a skeptic, as expressed many times in the structural forum.

 

[Vance Wiley]

hokie66 - hopefully, shear friction theory can be given better scrutiny.

Reinforcing for shear-friction.
Lets see - the development length of a bar is dependent on its size - the bigger the bar the longer the development length. Conversely, the smaller the diameter, the shorter the development length.
And is not the elastic lengthening dependent on the length of the bar under tension?
So smaller bars should exert greater forces (per unit area) at less elongation than larger bars, because they are fully developed in a shorter distance.
I am not sure the codes address this.
Too small an issue to be concerned? Accounted for in equations? Should it be? Is the dowel action an offset?
Too fine a point? It is a bridge, not a space shuttle.

 

[The Mad Spaniard]

SFCharlie (Computer)11 Aug 19 19:34
Quote (Vance Wiley)
Do we know if Berger reviewed the main span as an independent structure
I thought we knew that he did not. He was specifically not paid to do that.

Greetings to all:

What if the team MCM/FGG (or only FIGG because it was going to be from their part of the "pot") wanted to reduce design costs and ask Berger to only do a strength check of the main members using a FE model and not of the connections? Is it possible that the connections (the element that I am gessing created the problem) may have been designed between 60% and 100% and therefore Berger never saw the final reinforcing plans?

This is why I believe Berger is holding up in the restitution agreement.

I believe Berger had nothing to do with the failure because they were not contracted to evaluate the connections or the final plans and they never reviewed the final plans. What would you do if you were now in their position if my assumptions regarding their involvement are correct?

 

[hokie66]

If Berger accepted a commission like that, they neglected their primary duty to the public.

 

[hokie66]

Vance Wiley,

To many of us, shear friction remains a thought bubble. The Australian concrete code, which has often followed, and sometimes led, the ACI code, still does not recognize shear friction as a legitimate design procedure. I note that you, and many of the contributors to this particular discussion, are relatively new members of Eng-Tips. If you search the Structural Engineering general discussion forum, you will find extensive debate about shear friction.

 

[The Mad Spaniard]

hokie66 (Structural)11 Aug 19 20:35
Some good can come from this. Two things...the concept of a post-tensioned concrete truss can be consigned to the dust bin. And hopefully, shear friction theory can be given better scrutiny. It has been seized on some in the industry as a convenient way to deal with construction joints without having to develop the forces in bearing. I remain a skeptic, as expressed many times in the structural forum.

I disagree. I can not accept to be beaten as an structural engineer by the failures of others. We are suppose to be the masters of loads and strength. We control them. This bridge only needs about a few (about let's say 5) addicional cubic yards on concrete and about an additional 200 pounds of steel to be safe at all times. My recommendation is to rebuild it almost identically so nobody will know the difference except those who have added the needed concrete and rebar in the right places.

Well, maybe eliminate a few stays and struts in the short back span (by about 1/2). It does not look good now.

Heck, I will designed it for free. . It is 99.99% done. Just a couple of pages of additional calcs and slightly changing a couple of drawings. Just the publicity will provide me more additional bussines.

Live long and prosper.

 

[The Mad Spaniard]

hokie66 (Structural)11 Aug 19 22:30
If Berger accepted a commission like that, they neglected their primary duty to the public.

This is a wonderful comment and shows a philosophical item in our bussiness. Because it is a bussiness. My guess it that nobody at the time couuld believed that once you get all the forces thru analysis the structure would not be reinforced properly by any well known firm.

Now that we know more, what?

This case will be teached in engineering school forever.

 

[Earth314159]

That seems counter intuitive to me. If the bars have compression then they are actually "stealing" some of the normal (as in perpendicular) force from the joint and the concrete to concrete force would be less.
Also, if the bar is in compression it must have shortened - however little - and would not this compression shortening need to be overcome before the steel can be mobilized to assist shear friction?

That is why I put compression in quotation marks. The member can be in compression or you could be in the compression zone of a bending element. However, the bar is actually in tension even though the member is in compression. The compression on the concrete interface increases (as the bar begins to stretch) with shear movement since the interface is forced to split apart under the shear load. The plane sections assumption is not valid when there is a large shear at a pour joint (concrete is in a compression strain and the rebar is in a tensile strain). There is a very high strain differential between the bar and the surrounding concrete.

I think you are also correct in assuming that you need more roughness for larger bars since the development length is longer. You would also have an issue with stronger steel for the same reason. I think I have only ever used a 55M a couple of times (pretty rare to get a bar that big) and I don't have my concrete hand book with me but I will assume a development length of 70" as a rough estimate (this is a development length and not a splice length). Assume a linearly increasing strain distribution along the length. 2x6mm/(70x25.4)= 0.0066 which is greater than the yield strain. It is conceivable that a 0.25" or 6mm roughness still works with big bars. A smaller and/or weaker bar would work with less roughness. High strength concrete may not necessarily require as much roughness in theory since the development lengths are less. The actual shear friction parameters are more complex than the code equations imply.

I hope this incident doesn't serve as an pretext to unnecessarily increase the complexity of the shear friction equations.

 

[Earth314159]

I disagree. I can not accept to be beaten as an structural engineer by the failures of others. We are suppose to be the masters of loads and strength. We control them. This bridge only needs about a few (about let's say 5) addicional cubic yards on concrete and about an additional 200 pounds of steel to be safe at all times. My recommendation is to rebuild it almost identically so nobody will know the difference except those who have added the needed concrete and rebar in the right places.

Well, maybe eliminate a few stays and struts in the short back span (by about 1/2). It does not look good now.

Heck, I will designed it for free. smile. It is 99.99% done. Just a couple of pages of additional calcs and slightly changing a couple of drawings. Just the publicity will provide me more additional bussines.smile

I agree. There are advantages to having a concrete truss bridge. It was the design over-sight and not the truss that was a problem. I would say though that there are other issues with the bridge that are difficult to design but feasible.

Personally, I think the tube stays just confuse the aesthetics. I would use fewer stays and replace the tubes with viscous dampers. They would be more effective for dampening vibration and allow the bridge to creep up or down without effecting the loads on the truss. With the design as is, an upward creep from the longitudinal deck PT could overload the truss. This adds a compression load in the cable stays which makes the loading on the bridge worst. If the deck creeps downward, the load path is difficult to predict. Even though the stays were not "structural" they can add significant adverse loading at connections, members etc. if not properly addressed. The stays add redundancy but they also add complexity and reduce predictability.

 

[Tomfh]

I hope this incident doesn't serve as an pretext to unnecessarily increase the complexity of the shear friction equations.

In my opinion there should be more onerous checks and balances if you wish to rely on roughened surface for a critical connection. Construction joint preparation is prone to failure. It's not like concrete strength or steel strength, where 99%+ chance you get what you ask for. For deliberately roughened surfaces the chances are the concrete will be too smooth. It is a major hassle roughening concrete, and contractors won't generally do it, and if they do they often won't do it properly. It is labor intensive.

 

[hokie66]

The Mad Spaniard,

So by your comments, I take it that you believe the rest of the bridge was just fine, and that the only problem was at the joint which failed first. That is a big assumption, but one which seems to be common in the recent discussions here.

 

[Vance Wiley]

Charlie - I cannot find that in a post here -I have searched for "berger" in each section.
Do you have a reference?
Thank you.

 

[hokie66]

I think what Berger was supposed to do, as well as what they actually did, remains unknown.

https://www.enr.com/articles/47312-what-the-fiu-bridge-collapse-says-about-peer-review

 

[SFCharlie]

Charlie - I cannot find that in a post here -I have searched for "berger" in each section.
Do you have a reference?

Here is an example ... there are others.

FIGG did not contract for 30%, 60% & 90% PEER Review as required by FDOT. They originally tried to finagle FDOT to allow one of their OTHER FIGG offices do the PEER Review. They finally finagled FDOT to agree to PEER Review when plans were at 100% Construction ready but they used that instead to contract with Louis Berger for a 100% completed structure PEER Review instead of the still required incremental PEER Review. Louis Berger submitted the PEER Review Certifications to Alfredo Renya the FDOT LAP Coordinator, bypassing FDOT Structural Reviewer: Tom Andres. This seems a bit slippery behavior by FIGG.

SF Charlie
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[Vance Wiley]

RE: BERGER Scope
I have found the scope of Berger work in the OSHA Report pages 36 and 37.

https://www.osha.gov/doc/engineering/pdf/2019_r_03.pdf

It basically says Berger did only a completed structure review at 100% design.
And - thank you, Charlie.