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Miami Pedestrian Bridge, Part XIII
36

Miami Pedestrian Bridge, Part XIII

Miami Pedestrian Bridge, Part XIII

(OP)

RE: Miami Pedestrian Bridge, Part XIII

(OP)
Per MikeW7 in the previous thread (Part XII):
https://www.ntsb.gov/news/press-releases/Pages/NR2...

Quote (MikeW7)

The National Transportation Safety Board announced Thursday its intention to hold a public board meeting Oct. 22, 2019, 9:30 a.m., to determine the probable cause of the March 15, 2018, FIU pedestrian bridge collapse.

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RE: Miami Pedestrian Bridge, Part XIII

That's weird. How do you determine the cause of the collapse in a board meeting? Who is on the board? Who is invited to participate? I think we were all expecting a comprehensive report, not a meeting.

RE: Miami Pedestrian Bridge, Part XIII

Quote (NTSB public board meeting Oct. 22, 2019, 9:30 a.m)

I think it is all about PR. A chance to have the spotlight for a moment.
How have they done this in the past? I can imagine a board meeting to review a proposed report and approve or modify it as board members might think proper. But in public?
That must indicate this is the final thing.
Or maybe they want some public input? That could get out of hand fast.

RE: Miami Pedestrian Bridge, Part XIII

It is likely because there were a number of contributors and the meeting is to discuss ranking them.

At some point the draft will be circulated to all parties for more detailed comments, support, and objections. This is typical in major aircraft accidents where recommendations are being made; there is no point in the NTSB making an impossible recommendation and they recognize they can't know everything.

This is part of why it seems to take so long - the goal is to test assumptions as to what the underlying factors are and that proposed fixes will actually make for better outcomes.

RE: Miami Pedestrian Bridge, Part XIII

October 22. There will be a webcast. Should be interesting.

RE: Miami Pedestrian Bridge, Part XIII

Hokie66 if you watch some of the other 'board meetings' accessible form the link posted you'll get a feel for how they are run. I watched one, and it seemed like it was simply a presentation of the facts from the investigators into the accident, with the board members of NTSB able to ask questions and probe particular things a bit further.

RE: Miami Pedestrian Bridge, Part XIII

Actually, rather than speculate with incomplete data, I was just going to wait for the report to come out.

RE: Miami Pedestrian Bridge, Part XIII

Is that satire?

Edit - the post was removed/deleted

RE: Miami Pedestrian Bridge, Part XIII

Quote:

Your turn. Impress me.

The people who have actually studied the available failure info and have written reasons why the lower PT rod in member 11 could not have broken impress me. You ramblings don't so much, meaning I have no want to impress you back.

RE: Miami Pedestrian Bridge, Part XIII

Back again with my collapse theory (25 Jul 19 02:18). I think it also explains the detension/retension conundrum. For this, ignore the upper PT rod, only the lower PT rod is relevant. 11 and 12 are a cohesive unit free of the slab and connected to the diaphragm only through the base of 12 (rebar between the slab or diaphragm that run through 11/12 only act as dowels which provide nothing in the way of constraint jrs_87 (Mechanical) 10 Aug 19 09:29)

When the structure was on the form work and the lower PT rod was tensioned, it served to hold the flaky joinery tight. Although the strain was already pulling at the joinery and causing the cracks to show when the formwork was removed, the full magnitude of the problem had yet to be revealed. The structure was then moved and set in place with the lower PT rod still hiding the issue. When the PT rod was detensioned, the structure settled with the slab sagging and the top of the diaphragm pulled toward the center while 12 bowed out carrying the load transmitted down 11. The structure held in this degraded form until the lower PT rod was retensioned (to the max). This activity had no leverage to pull the structure back into shape. The significant force vector from the retensioning of the lower PT rod is the horizontal component that is pulling the slab (by way of the tab) toward the center of the structure, amplifying the tearing of 11. The structure sags further building the load on 11 etc.






RE: Miami Pedestrian Bridge, Part XIII

You might have something there alright. Add in the fact that those vertical rods adjacent to the no 12 vertical column were not connected and the slippage of the 11/12 node before they tried to apparently tighten it up and it could easily simply wrench the 11/12 node away from the deck.

your top view doesn't show the 2 or 3 tubes set into the deck which appear in some drawings and not others and have a high suspicion factor about removing the interface between 11/12 and the deck. IMHO.



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

RE: Miami Pedestrian Bridge, Part XIII

The chevron surfaces in my model, though not fully detached are easily detached, a.k.a. tear on the dotted line. That's why I try to envision the failure as though they were detached.

RE: Miami Pedestrian Bridge, Part XIII

Quote (Sym P. le (Mechanical)

I would like to expand the discussion a bit - with NO intention of being critical - we both end up at the same place.
First - you present a good description of your thoughts. Second - the graphics are very helpful. They were seen as such when first presented. I will use interrupted quotes as a means to assist my comments.

Quote (When the PT rod was detensioned, the structure settled with the slab sagging and the top of the diaphragm pulled toward the center)

I would view this using the deck as the constant - it is the largest single element in the structure. It is 31 feet wide and 174 feet long and weighs about 1150 kips vs 16 kips kips for member 11. So I would describe the action as member 11 being forced across the deck and out the end of the deck. I am not sure there was a lot of sagging before node 11/12 went into full failure mode.

Quote (The significant force vector from the retensioning of the lower PT rod is the horizontal component)

Full agreement here.

Quote (that is pulling the slab (by way of the tab) toward the center of the structure,)

Since the lower PT rod remained anchored in the deck, and that anchor did not move (within our ability to see at this time) it seems more appropriate to view member 11 as having an unresolved force at node 10/11 from the PT rod tension, with the lower PT rod pulling node 10/11 to the north with the horizontal component of the 280 kips of rod tension. We have seen a picture of a longitudinal crack in the west side of 11 about 6 " above the bottom and maybe within a foot of the fillet between 11 and 12. I read this as the beginning of the splitting of 11 which resulted in the upper part of 11 sliding north and off the end of the deck, with the bottom of 12, while leaving the lower PT rod anchored in the deck.

Quote (amplifying the tearing of 11. The structure sags further building the load on 11 etc.)

Fully agree. The failure of node 11/12 to remain connected to the deck left two load paths to deliver load to the pylon - the Deck and the canopy. With both acting in bending only they failed immediately.
Thank you.

RE: Miami Pedestrian Bridge, Part XIII

Vance, thanks for your discussion.

Quote (LittleInch)

... it could easily simply wrench the 11/12 node away from the deck.
It's ridiculous how basic the explanation is once all the clutter is removed.

Quote (FortyYearsExperience (Structural) 19 May 19 16:32)

There is, in reality, zero amount of steel provided to tie diagonal 11 to the deck.
I would add that 12 also required being tied longitudinally into the deck as it would be a robust backstop for 11.

RE: Miami Pedestrian Bridge, Part XIII

Interesting that the ENR report says the final report will be issued on 22 October, while the NTSB has scheduled a board meeting and webcast that day to "determine the probable cause". Just bureaucratic lingo, I suppose.

RE: Miami Pedestrian Bridge, Part XIII

Quote (SFCharlie (Computer)8 Oct 19 20:49 FHWA Turner-Fairbank Highway Research Center Factual Report for Concrete Interface Under Members 11 and 12 (show members after collapse...))


So the deck surface delaminated about an inch below the surface in areas larger than the contact zone for 11 and 12.
I hope they at least hammer tapped the surface to establish the extent of that delamination thru sounding. Ultrasonic would be better.
I have to wonder if there was a problem in screeding the deck surface and more concrete was added at this area to make grade. That could have created a lack of aggregate across this zone, and created an unintentional pour joint. Bummer location.

Plenty to ponder here. Quite a release of docs. I see FIGG found the SF to be 1.25 for shear friction under 11 and 12, thus proving it should not fail. The overall SF for horizontal shear friction between webs and deck was more than 11. Good head fake there.
Much to digest.
Thanks for the link.

RE: Miami Pedestrian Bridge, Part XIII

Still plowing through it. But one important aspect is that this new tranche of documents corrects a misconception delivered by an incorrect callout on one of the photos in the preliminary report.

In that prelim report, a callout refers to the lower PT rod in #11 as "sheared," implying that it had failed in shear during the collapse. However, the evidence of other photos and diagrams is that the rod was intact after the collapse, and had been cut after the collapse in the process of relocating key portions of debris for later use in the investigation.

New reports available in this docket correct that misconception by omitting references to the "sheared" PT rod, and including photos depicting it as intact.

RE: Miami Pedestrian Bridge, Part XIII

The "sheared" comment was from OSHA. More concerned with people than flawless structural analysis.

RE: Miami Pedestrian Bridge, Part XIII

Lessons will be learned from this tragedy. Hopefully, the victims' families will gain some comfort from that, along with at least some degree of monetary compensation.

I believe that two things are obvious: 1) Concrete truss design is complex, and should not be attempted without exhaustive research to establish the appropriate parameters, and 2) Shear friction as a concept needs a lot of work.

RE: Miami Pedestrian Bridge, Part XIII

There are many photos I have not seen before.
I find pics # 71,88,89,90,92,and 93, plus #102 very revealing. Scary, actually.
My take is member 11 was in bad trouble and the base of 11 and 12 was outbound.
Also it appears the first hoop of #7 shear -friction reinforcing across the plane at the top of the deck was intact after the collapse. That reduces the contribution of the reinforcing. Apparently the south most hoop was in the fillet under #11 and did not share much shear load because the cracking in 11 was north of that hoop.

RE: Miami Pedestrian Bridge, Part XIII

After seeing the collection of photos documenting the slow progression of collapse, it's hard to believe that of the many engineers involved, nobody realized the magnitude of what was happening. I believe I would have declined to even go on the bridge to take pictures.

Brad Waybright

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

RE: Miami Pedestrian Bridge, Part XIII

2

Quote (thebard3)

..declined to go on the bridge to take pictures.

Me too.

After reading some of the docket, it appears parties involved were right and correct, yet wrong. (edited - don't want to go there)

I think the bridge was so massive, they did not feel it could collapse from little cracks. I have severe fear of heights, yet I can look out airplane window with no worries, yet I sweat profusely on a 16 foot ladder.


I feel following link is most interesting. The illustrations inspire me.

Bridge Factors Attachment 73 – FHWA Assessment of Bridge Design and Performance
https://dms.ntsb.gov/pubdms/search/document.cfm?do...

RE: Miami Pedestrian Bridge, Part XIII

Ok, here's something that has puzzled me for a long time on this bridge.

The picture below shows, to me at least, a large discrepancy in resisting forces on the top and bottom chords.

I.e. the top chord has a lot more force going right and the bottom a lot more going left. In a classic truss design these forces are roughly equal.

But here we have large discrepancies (about 2000 kips) which I don't see being taken into consideration anywhere. The rather puny columns at the end aren't going to resist anything much so where does all this out of balance force go?

I know this is a simplification and that the truss was supposed to be rigid, but this really doesn't look to me that these out of balance forces are being accounted for. But then I'm not a civil design engineer, but I would like to know what is happening here.

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

RE: Miami Pedestrian Bridge, Part XIII

Quote (jrs_87 (Mechanical)9 Oct 19 12:35 I feel following link is most interesting. The illustrations inspire me. Bridge Factors Attachment 73 – FHWA Assessment of Bridge Design and Performance https://dms.ntsb.gov/pubdms/search/document.cfm?do..)

Thank you for prompting me - I immediately went there and I find it interesting also.
I am struck by the clarity of the presentation in this document. Compliments to the authors. It is easily understood.

I find this part interesting - and damning to those at the March 15 meeting. From page 66 of 90.


Many items are eerily similar to points and illustrations previously presented in this forum. Kudos to all.

RE: Miami Pedestrian Bridge, Part XIII

Quote (LittleInch (Petroleum)9 Oct 19 17:29 Ok, here's something that has puzzled me for a long time on this bridge.)


I see what you are seeing.
Where does that diagram originate? I think I have seen it but do not recall.
I can't believe that everything from the south end adds to a value at node 10/11. The arrows seem to show the horizontal shear from the diagonals but I am not sure.
Are they superimposing the PI forces?
Thanks,

RE: Miami Pedestrian Bridge, Part XIII

Quote (hokie66 (Structural)9 Oct 19 07:42 Lessons will be learned from this tragedy. , and 2) Shear friction as a concept needs a lot of work.)

I agree with your comment about victims and families.
My thoughts as to shear friction - it seems that 1) there was no preparation of the construction joint area at the deck surface which would have required more steel across the joint; 2) the cracking of Diaphragm 2 from vertical load severely reduced the capacity of any steel in member 12 to contribute and that reinforcing should not have been considered as contributing; 3) that same cracking of the diaphragm reduced the punch out capacity severely; 4)as discussed here before and by FHWA, the clamping force from transverse PT was overestimated; 5)leaving the need to provide enough reinforcing across the construction joint to resist all loads, which was not done (woefully inadequate amount provided); 6) the FHWA report discusses cohesion, which FIGG properly disregards, and was lost before the bridge was lifted by the transporters - the first cracking is evidence of loss of cohesion.
My takeaway is that with another half yard of concrete and half ton or ton of reinforcing at node 11/12 and node 1/2, and with proper joint preparation, the bridge would be standing today. There may be other problems yet undetected, but it would not have failed on March 15, 2018.
And there is much yet to be learned.
Thanks for the opportunity to comment.

RE: Miami Pedestrian Bridge, Part XIII

The diagram came from the link in the post above mine - Attachment 73.

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

RE: Miami Pedestrian Bridge, Part XIII

Quote (LittleInch (Petroleum)9 Oct 19 19:51 The diagram came from the link in the post above mine - Attachment 73.)

Thank you. I do not think that is correct. I do not see how node 8/9 can have so little shear transmitted to the canopy. Must be something to do with elastic changes in dimensions - they found similar numbers 4 times, though the last one was a bit more reasonable. Could this be the effect of elastic shortening from PT in the deck?
The simple truss analysis of years ago assumed things were rigid members with pinned joints. Reality can be different.
The killer "Finding" ----------


I do not comprehend the difference between FIGG calcs and the NTSB results - - anyone???
Surely FIGG will have a comment.
In a previous post I had said "There may be other problems yet undetected" - this may be one of them. If the NTSB report is correct March 15 may have been the most fortunate date available.

RE: Miami Pedestrian Bridge, Part XIII

I haven't read it all, but did anyone find mention of bending/frame action, and also axial shortening in the top and bottom flanges due to both shrinkage and PT?

RE: Miami Pedestrian Bridge, Part XIII

Quote:

hokie66 (Structural)9 Oct 19 21:51
I haven't read it all, but did anyone find mention of bending/frame action, and also axial shortening in the top and bottom flanges due to both shrinkage and PT?
The solid modeling software should address fixities at joints, but I have not read any conditions of that nature.
The Feds ran 4 runs and got different answers each time - by maybe 10% - maybe more.
Strikes confidence in me.

RE: Miami Pedestrian Bridge, Part XIII

Quote (LittleInch (Petroleum)9 Oct 19 17:29 Ok, here's something that has puzzled me for a long time on this bridge.)

Have done some work outside - had time to think.
End vertical reactions are pretty much equal. Member 2 is much flatter angle than 11, so there is a greater horizontal component thrusting into the canopy and pushing on the deck than at member 11.
So it appears that to balance shear loads to the canopy we take node 2/3 at 2182 kips AND ONLY 464 kips northbound at node 4/5 for a total of 2656 kips and that will balance all the remaining shears pointing southward. So the arrows are backwards on several nodes.
Am I not corect?
Same idea for the deck. And arrows are pointed the wrong way there too.
I am basing this on a free body of the canopy - horizontal forces into the canopy must balance to zero or the canopy will wind up in Sweetwater.



RE: Miami Pedestrian Bridge, Part XIII

Quote of the week - or year - -
"I don't know".

Interview of EOR March 20, 2018.

RE: Miami Pedestrian Bridge, Part XIII

One of the supporting documents submitted by Figg is this Party Submission:

https://dms.ntsb.gov/public/62500-62999/62821/6285...

...which includes Figg's own "probable cause":

Quote (Figg)

The FIU University City Prosperity Pedestrian Bridge construction accident occurred because the construction joint at the north end of the main span between the truss members and the bridge deck was not roughened as required by the Florida Department of Transportation (FDOT) Standard Specifications for Road and Bridge Construction. This failure to meet the construction specification requirements was not noticed by either the contractor’s quality control personnel or by the construction inspectors under contract to FIU.

Um, yeah, good luck with that.

RE: Miami Pedestrian Bridge, Part XIII

2
Figg's submission is actually very well done considering. ( https://dms.ntsb.gov/public/62500-62999/62821/6285... ) They make several good points, I will not cite any here, check yourself please. However, I reject their concept of redundancy. And Figg seems prone to wing-it excessively, even though they possess advanced skills.

RE: Miami Pedestrian Bridge, Part XIII

Quote (Vance Wiley (Structural))

The Feds ran 4 runs and got different answers each time - by maybe 10% - maybe more.
Strikes confidence in me.
I got the impression that the different runs were for different conditions of support; un-tensioned, tensioned, transported, etc.

SF Charlie
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RE: Miami Pedestrian Bridge, Part XIII

How is a forensic report as massive as the Figg "submission" not signed and the author(s) identified? It's almost like the FL engineering rules mean nothing.

RE: Miami Pedestrian Bridge, Part XIII

Quote (charliealphabravo )

How is a forensic report as massive as the Figg "submission" not signed and the author(s) identified?

Maybe when the language used in the report, is the result of a negotiation between the forensic investigators & the client.
The report puts a lot of focus on the roughening of the joint between the deck and #11 and rightly so but the in the few instances where surface prep was called out, I do not believe they ever added (TYP.). It would have made a world of difference.

Concrete finishers love their hand tool. I wonder if a small hand cultivator would be an acceptable means of roughening the concrete before it completely hardens.

RE: Miami Pedestrian Bridge, Part XIII

My favorite words are Typ. U.N.O.

RE: Miami Pedestrian Bridge, Part XIII

4

Quote (epoxybot)

...Maybe when the language used in the report, is the result of a negotiation between the forensic investigators & the client.
The report puts a lot of focus on the roughening of the joint between the deck and #11 and rightly so but the in the few instances where surface prep was called out, I do not believe they ever added (TYP.). It would have made a world of difference...

I can't argue that the joints didn't get the prep they ought to have. But I think that if you're in the realm where the absence of 1/4" surface roughening in a construction joint causes your bridge to fall down under its own weight, your design embodies way too little margin of safety.

RE: Miami Pedestrian Bridge, Part XIII

Nodal Shears FHWA analysis of FIGG design - Bridge Factor Attachment 73 - pg 77 and 78 of 90

Bridge Factors Attachment 73 – FHWA Assessment of Bridge Design and Performance
https://dms.ntsb.gov/pubdms/search/document.cfm?do...





FHWA finds FIGG underestimated nodal shear demand at 11/12 and deck. FHWA represents FIGG calculations as showing shear at 11/12 to deck to be twice as great (approx) under full two span condition than when under single main span with full fixity at the pylon/pier (north end of main span), and the greater value should have been used for design and was not. Both conditions are noted to be Stage 4.
Note the Fixed Pylon (Stage 4) is an intermediate construction stage subject to construction loads only. Failure prevented this condition from being encountered.
Note also that the Longitudinal Model (Stage 4) is also an intermediate condition subject to construction loads only.
I do not see how it is possible for moments at the north end of the main span to be greater than when full fixity is assumed at that end. Unless one condition is unfactored DL only and one is full factored DL and LL.
First, DL is about 11 kips/ft and LL is 90#x30 feet = 2.7 kips/ft. Factored that is 11k X 1.25 = 16.75 kips/ft DL and 2.7 k X 1.75 = 4.37 kips/foot factored LL for a total of 18.5 kips/ft factored load. The factored TL load is 1.67 times unfactored DL. So the 2X increase was not from considering unfactored DL vs factored TL. Factors are FHWA representation of those used by FIGG.
The only way I can see the shear demand in node 11/12 to deck increasing beyond that from full fixity at the pier is if enough negative moment is created over the pylon by the continuity PT in the canopy to draw even more load from the main span to the pier. But that condition is Stage 5, after the north span has cured.
So I am confused as to how FIGG could have had such a discrepancy between shear demand at node 11/12 to deck under Fixed Pylon (Stage 4) vs Longitudinal Model (Stage 4) conditions.
As I recall prior discussion here the unfactored DL shear at node 11/12 to deck was in the order of 1300 kips. Or was that the axial load in 11? Adding LL and factoring the loads by the 1.67 from above (DL to Factored TL ratio) yields 2170 kips - pretty much what the graph by FHWA shows for 11/12.
Perhaps FHWA is misinterpreting something also?
Whatever the answer, this points out the complexity of this design and the various support conditions which existed - full form support, full span at casting yard, intermediate span during transport and reversal of loads in many members, and final support in place. All happening in a brittle structure.
And if the non-redundant factor is only 1.05, that would not have saved this structure at this phase.






RE: Miami Pedestrian Bridge, Part XIII

Quote (SFCharlie (Computer)10 Oct 19 15:46 Quote (Vance Wiley (Structural)) The Feds ran 4 runs and got different answers each time - by maybe 10% - maybe more. Strikes confidence in me. I got the impression that the different runs were for different conditions of support; un-tensioned, tensioned, transported, etc.)

Table 2 Pg 81 of Attachment 73 has 4 runs - you are correct, two are with and without PT rods in 11 and 2 and there is a 2D and 3D run to compare.
I should have been more specific. Thanks.

RE: Miami Pedestrian Bridge, Part XIII

Quote (epoxybot (Structural)10 Oct 19 19:00 Quote Concrete finishers love their hand tool.)

Easy there, guys. This is a family hour forum. :)
The tool you recommend cannot be approved. First, it requires a seat, a safety belt, and special gloves.
Seriously - I think special provisions for joint preparation should be made at joints as critical as node 11/12 to deck. The demand and unit stresses and performance required are so much greater than in any construction joint in a pier or wall or at the top of a girder to a composite slab. So I think special forming is required. I suggest formed indentations in a 45 degree shape with amplitudes greater than the largest aggregate size.
Forget cohesion - the slightest slip has destroyed that. Forget raking it - heck, I have seen bridge decks with amplitudes greater than 1/4 inch. So often raking results in dingleberries which are easily dislodged by structural loads but are not loose so washing does not remove them. And yet raking is one of the predominant method of preparation for a joint.
The sawtooth configuration will develop the tension in reinforcing across the interface while the aggregate can move into the "teeth" and those areas will act like concrete instead of paste, dingleberries, and ball bearings. If reinforcing creates access problems great - make the shear plane bigger. If this project demonstrates anything it is "get enough".

RE: Miami Pedestrian Bridge, Part XIII

Vance Wiley - Yes a more "keyed" approach would not have been difficult to form. It is sad that FIGG was able to recognize in the design phase that the area between and north of the plastic pipes penetrations were not a load path but when inspected in the field, failed to appreciate that the loss of bond at the deck/node interface & the loss of the southern most #7 in the chamfer, was so disastrous.

RE: Miami Pedestrian Bridge, Part XIII

I doubt that shear friction theory was ever intended by its inventors to be used in a situation like this. Hopefully, it never will be again.

Shear friction is not mentioned in the Australian concrete standard AS3600, but I know that some Australian engineers use it at times under pressure of contractors who don't want to provide decent bearing at joints.

RE: Miami Pedestrian Bridge, Part XIII

Quote (Vance Wiley (Structural)10 Oct 19 19:46 Nodal Shears FHWA analysis of FIGG design - Bridge Factor Attachment 73 - pg 77 and 78 of 90)

Quote (I do not see how it is possible for moments at the north end of the main span to be greater than when full fixity is assumed at that end.)

I am quoting myself because i may have an answer to my own question.
I have not ran the numbers - that took FHWA 18 months - but in concept the increase in load at node 11/12 to deck may be due to the details of staging the construction.
Calculating for fixity at the pylon end (north) of the main span under dead load is unrealistic - that could only happen if the span were again supported or lifted along its length and then fixed. So calculating fixity at the north end of the main span should only apply to added loads after it is connected to the pylon and north span.
Thus shears at node 11/12 to deck would be from full DL of the main span as simply supported PLUS added load on the main span under finished condition PLUS load drawn from the main span due to negative moment produced over the pylon from removing shoring under the north span and from the continuity PT force added in the canopy under Phase 5.
In concept it appears the loads on node 11/12 to deck could increase. I do not see how they could double - but I have not ran the numbers.
So I get the following approximate added load (vertical) to contribute thru whatever angle to cause shear in 11/12 to deck. All calcs by "back of envelope" method and to that accuracy. Remember - this is the increase at the end reaction of the main span and must be adjusted to show actual shear in the node. So compare only the loads causing end shear.
First, LL from the main span will add about 300 kips shear in the main span at the pylon and is the major thing that will influence the node shear. But this does not allow us to compare the effect of releasing the north span. I propose to do LL and released DL on the north span to see maximum influence on node 11/12 to deck from that action.
Distributing the released DL from the north span to both spans with spans fixed to pylon adds 14 kips shear to end reaction of main span at pylon. Loading LL onto north span only increases shear at the north end of main span by 4 kips. General concept is Unbalanced Moment X stiffness factor of main span / stiffness factor of both spans and pylon.
The pylon, if fixed to superstructure trusses is rather stiff becaus it is short. Pinning the top of the pylon increases the draw of shear to north end of main span to a total of 34 kips. At 32 degrees, that is about -what - 60 kips in a connection that has 1300 or more? Insignificant. Remember - all numbers unfactored. The fixed end moments of DL and LL from the short span remain mostly in the short span because the long span is less stiff, and the shear draw is M/L.
Would appreciate someone checking me out - for a pinned top of pylon the distribution is simply L/(L1+L2) and with same bridge section on both spans it is rather easy.
I find it interesting today because if I do it right it keeps me fresh. If I screw up, I gotta make adjustments to my thinking. So feedback is welcome.
Anyway, I still cannot see the shear at node 11/12 to deck doubling from the release of the north span.

RE: Miami Pedestrian Bridge, Part XIII

2
The interface area was extremely congested. It would have been difficult to put a hand inside that mess of steel, let alone get a tool in there and maneuver it around to roughen the surface of the cold joint. In the Figg rebuttal there is a photograph of of this area showing forms and steel in place before the deck concrete was placed. There are electric conduits going through this area as well, further reducing the strength of the members.

At the risk of being accused of Monday Morning Quarterbacking, I put together a quick sketch showing an alternate way to create the interface area. This is based on Figure 56, on Page 60 of 90 in the Bridge Factors Factual Report Attachment 73.

The sketch shows a buttress above the deck surface that would be placed with the floor, making it monolithic with the floor. The shear plane would not go through a cold joint. The Interface bars would be angled, The resulting cold joint would be in pure compression.

Thoughts?

RE: Miami Pedestrian Bridge, Part XIII

I understand that the requirement to roughen the surface was not explicitly stated on the Drawings. Rather, this requirement is said to be included in a referenced standard: A Florida DOT publication.

Since this is such a critical requirement, would it make sense to restate this requirement on the Drawings so it's right there in front of the Contractor? Or does that create a slippery slope that would end with repeating the entire referenced document?

RE: Miami Pedestrian Bridge, Part XIII

Quote (PA PE (Civil/Environmental))

a quick sketch Thoughts?
I think it is a step in the right direction. The numbers will likely show you will need a greater length and/or width to get enough reinforcing across the plane at the top of the deck. The monolithic casting will improve the friction factor from 0.7 (smooth) to 1.3 (monolithic (if I remember). The sloping joint has been criticized previously. Your illustration is a great representation of the joint I would support. (oops-no pun intended). The fixity of the monolithic nodes (or those with proper joints) has created some moments in the web members with corresponding non critical shears so a formed key would be a good idea in the square joint face.
And to transfer the tear out force to a point where the PT in the deck can handle the load it will need longitudinal reinforcing well anchored under the joint and extending far enough south to transfer the load. Had these ideas been at work on March 15 the structure would be standing today, IMO.

Quote (would it make sense to restate this requirement on the Drawings)

Absolutely. My office issued many many drawings over 40+ years and in every case in which construction joints were needed we included a note in the general form of "Construction joints shall have surfaces removed to expose aggregate solidly embedded and remove all laitance ---" etc. And that was for tops of foundations under walls, joints in walls, - - not in something with the demand and critical nature of node 11/12 and the deck.
Which brings me to a comment about aggregates. In many photos of the damaged structure I see what appears to be fractured aggregates embedded in the cement matrix - and though the photo is a 2D image it apppears the aggregates fractured in the same plane as the matrix. In the use of light weight aggregates there is a requirement for "splitting tensile strength". I wonder if the aggregates used are appropriate for 8500 psi concrete? It seems to me if you want to make 8500 psi concrete you need 8500 psi aggregates with suitable shear strength. I am aware that FHWA found the concrete to be suitable for the design requirements by FIGG.

RE: Miami Pedestrian Bridge, Part XIII

Referenced standards are often seen as boilerplate and for the purpose of CYA. I like to put plenty of notes on the drawings. Referenced standards are supposed to be available on site. How often does that happen, who takes the time to read them, and who enforces that requirement? At the very least, critical requirements should be highlighted during the preconstruction conference, or at a pre-installation meeting.

Figg made a valid point that their involvement during construction was limited. My project managers don't like to send me to the sites either because of my billing rate, and it becomes very annoying when the inspector calls in with questions that should have been addressed before they become a problem in the field. The person who does the design is in the best position to see things before they become a problem, and even allow minor deviations that accommodate a contractor's concerns without compromising the work. I wonder if Figg was tempted to take the position that if they can't be on site then it's not their problem. This could point to an issue in the construction process and the contractual arrangements between the various entities.

I wonder who is paying for all the forensic work?

RE: Miami Pedestrian Bridge, Part XIII

Quote (PA PE (Civil/Environmental))

I wonder who is paying for all the forensic work?
Which has cost probably 10 times the design fee.
The design process can involve maybe hundreds of decisions each day. The litigation process can spend maybe hundreds of hours on just one of those decisions. It does seem that, for the most part, after a year and more of posturing, most of the parties in this event have decided to avoid litigation and settle.
I applaud that.

RE: Miami Pedestrian Bridge, Part XIII

Quote (Vance Wiley (Structural))

It seems to me if you want to make 8500 psi concrete you need 8500 psi aggregates with suitable shear strength. I am aware that FHWA found the concrete to be suitable for the design requirements by FIGG.
I think you're on the right track. The concrete for the new locks in the panama canal was failing specs because the basalt aggregate wasn't strong enough.

SF Charlie
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RE: Miami Pedestrian Bridge, Part XIII

PA PE

Your concept is theoretically acceptable but is more difficult to build/form. There are also other joints that were an issue which would also have to be altered. There is actually more shear at the #10/#11 joint than the #11/#12 joint but it was not an issue since the PT from #10 came through the canopy and up into the blister to form a shear plan on the upper side of the canopy (the PT also clamps the joint). Although the shear stress across the pour joint was still very high.

The steel was actually not that congested (at the slip plan directly below #11). I would increase the base as you have shown, increased the steel (as you have shown) and roughened the surface. For ease of construction, I would keep the pour joint at the top of the deck but make sure my stresses were low enough to be adequate. The steel should all be ties for good anchorage for shear friction.

I would also say, under similar circumstances, I have had hassles from rebar suppliers. They don't like to make each tie a different height. It is more labour and difficult for them to organize. Although I would say that the extra complication is worth in this case.

The other option would be to organize a continuous pour of the deck and diagonals. You could form the a square end at the top of the diagonals and then pour the canopy. You don't want to pour the canopy with the diagonals because of the plastic creep issues. You can get a gap between the diagonal and the canopy.

RE: Miami Pedestrian Bridge, Part XIII

Quote (Earth314159 (Structural)

The other option would be to organize a continuous pour of the deck and diagonals
This seems difficult on first glance. Could you explain how they would prevent the pressures from concrete in the webs from 'boiling out' at the deck surface? I like the idea of reconsolidating the concrete in the deck and diagonal joint to create a monolithic joint. That increases the coeff of friction from 0.7 or 1.0 to 1.3 {or is it 1.4?).
As for the canopy, I have thought that a continuous concrete beam should have been cast full length under the canopy, providing rotational resistance to those joints and plenty of area for shear transfer - something like a bridge deck on a bridge girder. That might leave the plastic creep issues you mention, however. What are your thoughts about creep and elastic shortening of the canopy from PT forces relative to its effect on the already cast diagonals?
It is interesting how many opportunities there are for 'improvements' to this structure. Points out the complexity of the idea.

RE: Miami Pedestrian Bridge, Part XIII

Vance Wiley

Doing a continuous pour takes some experience. We sometimes do it for footings and walls/columns. You have to go back and forth on the pour. This also does not work with high slump concrete but it is done. In my area, it is actually common for house strip footings and foundation walls.

I don't have my code at home but I think it is 1.4 (I can check later). It may also be slightly different in the US. A continuous pour makes the failure plan no different than a failure plane in a typical column. If you look at Figg's report from the NTSB docket, they actually made columns will deliberate cold joints to simulate the failure plane of the bridge.

I though of a continuous beam on the canopy as well but that would have to be poured with the diagonals as well to get an advantage. You might as well pour the canopy with the diagonals.

If you have a true determinant truss, the PT does not affect the member forces in the diagonals or end verticals regardless of the creep. However, you do not have a true truss (nor do you ever have a true truss in the practical world). The bending stiffness of the canopy, diagonals, verticals are affected to some degree by the PT. You can argue that if you have enough ductility demand, it really doesn't mater in terms of safety. Elements like #1 would have double flexure due to the PT. The PT would cause visible damage but not critical damage as the axial stresses due to gravity load on #1 are low. The only way to know for certain what the effect of the PT would be is to model it. Even a simple 2D stick frame could be used to do this. To cause high shear forces at the joints with the PT would require the deck and/or canopy have a relatively high bending stiffness compared to the truss as a whole. In general, creep reduces the level PT force as the structure essentially softens with response to long term loads. Any deleterious effect from the PT would also likely diminish with creep. When I have had a complex PT structure, I model concrete both long term and short term. I reduce the E for concrete but keep the same temperature difference on the PT to determine the relaxation due to creep.

RE: Miami Pedestrian Bridge, Part XIII

*The debate above is focused on the supposedly "correct" angle that the dry joint should have followed.
*Referring to the drawings below, the debate seems to be that the dry joint should have followed the orange plane, and not the blue plane.
*This debate is irrelevant because the failure did not occur along the blue plane; and, the orange plane would have made no difference.
*The joint failed along the two red planes identified in the Side elevation and the Top View.
*Specifically, the surfaces that failed under sheer load are the two surfaces identified as having dimensions b x c
*If you calculate how much steel is required to hold 1500 kips in strut #11 from moving northwards, you will find it needs about 25 square inches of steel.
*If you study the drawings, you will find perhaps 5 square inches of steel crossing the red planes (b x c) that might act as an anchor against strut #11 moving northwards. The concrete itself adds no resistance, because across these red planes, there is no clamping force.
QED





RE: Miami Pedestrian Bridge, Part XIII

For the folks with a lot of concrete experience - what do you think of a shear key in such a critical cold joint?

I have read though some of the reports online and would like to learn something from this to improve my own designs. So far the main takeaway I have is that where critical constriction processes are needed (such as roughing the surface) to call it out instead of referencing standards.
The second takeaway I have is using shear keys any time I have a critical cold joint.

RE: Miami Pedestrian Bridge, Part XIII

Forty years of experience,

I am not quite sure what you mean. The failure plane was the blue plane. Figg actually did a calc on the pink planes in their presentation just before the collapse. It was not a critical link in the chain. They missed the blue plane which was the weak link in the load path.

There is a clamping force from the transverse PT that rests shear friction through the pink planes. The is also a tonne of bars in the deck and diaphragm. The shear stress is substantially less since there are two shear planes. The mu value is also much higher since is part of a monolithic pour.

RE: Miami Pedestrian Bridge, Part XIII

Ideem,

It is good practice in such a situation to use keyways. Our code does not have a means of calculating the shear friction at a keyway. It is part cold joint and part monolithic. It adds capacity but calculating the shear resistance is not necessarily easy.

Beware, using keyways can cause issues as well. The contractor can accidentally leave the wood blocks in place which makes the joint worse off. I don't like keyways where they aren't a significant benefit. I use to see them at strip footing and wall joints but these joints have low shear and it isn't necessary. That practice has tended to fall to the wayside.

RE: Miami Pedestrian Bridge, Part XIII

Quote (Earth314159)

Figg actually did a calc on the pink planes in their presentation just before the collapse. It was not a critical link in the chain.

Figg's calcs are worth nothing. That is why there was a collapse killing six innocents in the first place.

If you look at the cracking that was observed (see 1st photo below), you see that it passes through the two pink planes at 45 degrees (see 2nd drawing below).

There are NO photographs of the cracks passing through the blue plane - obviously, because none were observed.

Also, my point is that there is missing an amount of about 25 square inches of steel to TIE back strut #11 to the deck. Strut #11, with 15000 kips (15 million pounds) is simply not connected to the deck for any tensile forces. You can almost see strut #11 moving northwards in real time for want of being tied to the deck. This is proof positive.

There was not "a tonne of bars" crossing the pink plane. If you look at the relevant drawings, there were maximmum five (5) square inches of steel crossing the pink plane, and all of those were too short to develop any tensile load in those bars.




RE: Miami Pedestrian Bridge, Part XIII

Forty Years of Experience,

The calculations done by Figg on the pink failure plane were not incorrect. It just wasn't the critical failure plane. They calculated the strength of the wrong link in the chain. It is the pour joint that is critical. The area is less, the steel through the joint is less and the mu value is less. The north end of the failure plane near #12 did punch but that was because there was a concentration of vertical steel in #12. The primary failure plane was the blue plane. The vertical steel in #12 was what held it together until the PT in #11 was re-tightened.

There is tonnes of capacity in the pink plane. It was never an issue. The #11 diagonal itself would fail in shear before the pink plane.

RE: Miami Pedestrian Bridge, Part XIII

Earth,

Yep. Shear friction is upper bound, yet people treat it as lower bound.

You can’t just consider one failure surface.

Spoiler:

You have to consider all of them.

RE: Miami Pedestrian Bridge, Part XIII

Earth, you are 100% correct. It's amazing that we're still arguing failure planes at this point in the thread.

My question w/ the FHWA 73, on page 84 in Table 3, they list Avf available for shear capacity. At all nodes they consider the bent #6 & #7 bars as engaged and fully developed. I would question this finding and argue against incorporating these bars into any Avf shear capacity. Or maybe if you were to use them, use them only at some fraction of their whole. IMO, they are not properly developed on either side of the shear failure plane, as required by code.

RE: Miami Pedestrian Bridge, Part XIII

Quote (Earth314579)

It just wasn't the critical failure plane.

If the pink plane was not the critical failure plane, then:

1. How come the photographs show catastrophic failure across that plane?

2. How come there are no photographs of catastrophic failure across the blue plane?

RE: Miami Pedestrian Bridge, Part XIII

Quote (Tomfh)

You can’t just consider one failure surface.

You are correct. I wouldn't such otherwise. However, there are literally an infinite number of possible failure planes. As an engineer you have to rationalize the failure planes that need to be checked. I am also not suggesting you shouldn't or don't need to check the pink planes of failure.

RE: Miami Pedestrian Bridge, Part XIII

Quote (Forthyearsexperience)

If the pink plane was not the critical failure plane, then:

1. How come the photographs show catastrophic failure across that plane?

2. How come there are no photographs of catastrophic failure across the blue plane?

1) I already answered this earlier. #12 has a concentration of vertical steel which helps transfer loads across the blue plane but this is just below #12 and not the rest of the shear plane. That is why you get localized punching at #12.

2) There are photographs of catastrophic failure across the blue plane.

RE: Miami Pedestrian Bridge, Part XIII

Quote (Earth314159)

2) There are photographs of catastrophic failure across the blue plane.

Really? Can you pass those on? I have studied all the photographs carefully. They are not in the record.

RE: Miami Pedestrian Bridge, Part XIII

Forty - what do you consider as "the blue plane"?

RE: Miami Pedestrian Bridge, Part XIII

Quote (Earth314159 (Structural))

Regarding your comment about using keys - would not an equally balanced key design with half area in one member and equal area in the other member leave only half the area for shear? Of course, with one part as a finite area and the other as a deck with relatively unlimited area, the key could be all of the smaller member and be a formed socket the size of the node. The benefit then would seem to be the contribution of the bearing at the loaded side of the socket.
My point would be to check the shear in whatever section remains to resist shears.
Thank you.



RE: Miami Pedestrian Bridge, Part XIII

HI Vance Wiley,

I am not sure a fully understand. However, when I do a key design I will use 2x4s spaced at 7" centres (or 2x6 spaced at 11" centres) oriented perpendicular to the shear force. That way, the shear is forced to go through about the same amount of monolithic concrete from the first and second pour. I am not sure if that what you means by an equally balanced key design. If you have just one big key, it is just pushing out one end and you are back to the same/similar problem. If that you what you are saying, then I agree.

RE: Miami Pedestrian Bridge, Part XIII

Quote (Earth314159)

Check out page 96 of the OSHA report. This photos is other places as well.

Earth314, the photographs at page 96 show that the failure plane was on a combination of the blue plane and the pink plane. Close to the green plane shown below.

However, the fact is that we have a horizontal force in strut #11 of 1,500 kips going north. For equilibrium, we must have enough resistance to hold 1,500 kips onto the deck going south. Therefore, we must find 25 square inches of steel configured to pull in a southerly direction into the plane of the deck. Those 25 square inches must somehow CONNECT to the load in strut #11. The vertical bars in strut #12 have no southerly component and so don't count towards this requirement.

Is it being suggested that by roughing up the surface of the concrete at the blue plane cold joint before pouring strut #11, this would be sufficient to eliminate the need for 25 square inches of steel pulling south?

RE: Miami Pedestrian Bridge, Part XIII

Shear Friction issues.
This was previously posted and deleted so I could clarify items. Here it is again.

The subject of clamping force on the sides of the assumed shear plane where 11 and 12 punch thru and out the end of the deck is interesting to me. The conditions surrounding this failure provide an opportunity to examine the concept of shear friction. I would like to discuss, hopefully not alone, the shear friction concept as it could apply to a "clean" version of the FIU bridge, north end of main span.
The conditions at hand are certainly not 'clean' as this was a multi phased failure with prior cracking at the deck surface for one zone, failure plane defined by the void created by the lower PT duct in member 11, the presence of 4 - 4" dia pvc ducts, proximity to the end of the structure, and presence of already cracked end diaphragm 2, to list a few.
If this were to have occured in an area of deck fully surrounding the zone, it would have been much like a column in a flat slab. Failure would be mostly in diagonal tension and be reinforced for that. Portions of this failure developed into diagonal tension as it neared the edge or end of the deck, and that zone was already cracked by the load in the diaphragm. The diaphragm cracking may have loosened things up for the blow out - or maybe the other way around.

For for discussion and clarity, I would like to just discuss the shear friction issue. My purpose is not to declare some concept as the final thoughts but rather to elicit discussion and consideration of conditions unique to a simplified modeling of this failure. There are many present here with experience, understanding, and inquisitive natures with much to contribute. I hope something in this will interest you.



If reinforcing steel across the shear friction plane can provide shear capacity because it goes into tension and that is measured by 0.9 times yield or 57,000 psi lets see how much stretch that requires to develop 57000 psi (or whatever - the principle remains). Using a #7 bar and development of maybe 24D = 1.12"X24 =27 inches. If tension in the steel is zero at the embedded tip and 57,000 psi at the interface, the average is 28,500 psi and stretch is 28500X27/29,000,000=0.026 inches on one side. Then if the same embedment stretch develops in the other side of the interface there is a total of 2X0.026=0.052 inches gap in the interface at full factored resistance.
Of course if the surfaces have separated by 52/1000 of an inch we can now disregard any cohesion contribution.
I suspect the actual development length is less than that required by code, so lets say it actually develops fully in 12 diameters, so the stretch is half that just calculated, or total gap = 0.026 inches.
For the specific but simplified condition at hand, consider the deck as a monolithic flat member that is 31 feet wide, thickness varying from 9.5 inches at edge to 24" at center with a notch cut out for members 11 and 12. That notch is about 21" wide and 60 inches in length measured from the end of the deck and goes all way through. And lets consider that 25 total sq inches of reinforcing should cross the two interface planes if we cast the notch then pour members 11 and 12 into that notch and try to push them thru. We will adjust for monolithic vs intentionally roughened vs as cast conditions after the concept is defined, which I hope is about now.
So to support aggregate interlock in the two planes each side of the 21 x 60 notch, with those planes being 24" x 60" each, we can provide 12.5 sq in of reinforcing crossing thru each side. That reinforcing can allow - in a two part isolated shear plane condition - a gap of 0.026 inches and still function as a shear friction joint. (according to codes).
But lets disregard any steel and just let the shear capacity of the deck on each side of the notch provide resistance to the movement which would have stretched the steel across the assumed shear planes by 0.026 inches.
12.5 sq in of steel at 57 ksi is 713 kips each side. Average thickness of the deck each side of the notch is (24+9.5)/2 and 14 feet long from notch to edge of deck and that is 16.75" avg X 14'X12= 2800 sq inches. The shear stress in that section from the equivalent 713 kips is 713000/2800 =255 psi. It varies from 0 at the edge to 255 psi at the end of the notch, 60 " from the edge. Shear deflection at the edge will be tne average stress X length of loaded section (60") /Ev of 0.4X5,300,000 or 255/2X60"/(.4X5300000) = 0.0036 inches. That is about one seventh (0.14) of the stretch in the reinforcing which would cross the plane and still meet shear friction design concepts.
There is no active clamping force in this zone because the transverse PT begins just south of the cut out notch zone we have defined. And the shear deformation in the protruding flanges is zero at the end of the notch where the deck is 31 feet wide without a notch. So the average deformation from this spreading shear is far less than the stretch required to produce tension in the cross plane reinforcing. The longitudinal deck PT crosses the plane in the deck at maximum "spread shear" at 60 " from the end so bending moment in the protruding flange is negligible and the shear deformation is the major factor. That longitudinal PT provides an active clamping force for the shear in the deck extensions.

So the questions that arise include:
1) If passive reinforcing across a defined or assumed shear plane can provide shear resistance by developing or retaining a condition of restrained contact and thereby contribute to shear capacity of that plane, can other conditions which create similar restraint contribute also? And if not, why is the reinforcing allowed to do so?
2) Should bar sizes across a defined plane of shear friction be limited in size? Smaller bars develop full capacity in shorter lengths and therefore the stretch is less.
3) If developing the tension in the reinforcing across an actual created joint causes stretch in that reinforcing (I see no other way) with some corresponding separation of mating surfaces, why is cohesion even considered to act in conjunction with the reinforcing?
4) Is the shear friction design based more on formulas which attempt to define what was learned in test results and less on engineering logic? (Dare I challenge the gods?)
5) Is something bad wrong with my logic or calculations? This should be a gimmie - hint: Very probably.

Thank you. I would really appreciate comments.

RE: Miami Pedestrian Bridge, Part XIII

Quote (Earth314159 (Structural))

You describe conditions detailed on a standard detail we used many many times. The shear is resisted by the later placed mating pour of concrete which fills the formed key. That amount of concrete in shear should have the capacity to resist the shear.
Combining the keys with formed surface coeff of friction and reinforcing across the joint would create a shear friction design. I do not know of anyone combining keys and coeff of friction. I wonder what AASHTO would think?
Thank you.

RE: Miami Pedestrian Bridge, Part XIII

Quote (TheGreenLama (Structural)13 Oct 19 14:25 My question w/ the FHWA 73, on page 84 in Table 3, they list Avf available for shear capacity. At all nodes they consider the bent #6 & #7 bars as engaged and fully developed. I would question this finding and argue against incorporating these bars into any Avf shear capacity. Or maybe if you were to use them, use them only at some fraction of their whole. IMO, they are not properly developed on either side of the shear failure plane, as required by code)

You are correct - they do not appear to meet development requirements - but those really involved in node 11/12 appear to have failed at the deck surface so they developed everything available. One question is why is node 11/12 the one with the least number of those #7 bars across the joint?
If they were given a proportionate value as you suggest, there might have been more of them and that would have helped.

RE: Miami Pedestrian Bridge, Part XIII

Quote (Vance)

There is no active clamping force in this zone because the transverse PT begins just south of the cut out notch zone we have defined.

If I understand your example correctly, this statement is in error. Gwideman and I were debating this a while back. I was unable to make my point clearly back then, so here's another tack.

Thought experiment: You have a bridge that is only 2'-8" long w/ one transverse PT tendon. The transverse PT force per unit length would be = 1 PT / 2'-8" . Now imagine we keep increasing the bridge length, and for every 2'-8" we add another PT tendon. Question--at what length of bridge does the PT force at the ends begin to decrease? Answer--it never does. Now our bridge is not exactly like this, but it's close.

Regarding theory, I would look at some of the references cited in ACI 318 for shear friction. Back when I was reviewing the OSHA report I seem to recall that the references cited were in turn searchable (although I may be mistaken on this). Sorry to punt on the meat of your question, but this is all I got right now.

RE: Miami Pedestrian Bridge, Part XIII

Quote (Forty year experience)

Is it being suggested that by roughing up the surface of the concrete at the blue plane cold joint before pouring strut #11, this would be sufficient to eliminate the need for 25 square inches of steel pulling south?

You need to increase the shear friction area, increase the amount of vertical steel and roughen up the joint (and/or add keyways), and then you can start to make the joint work. The longitudinal PT takes the tension component (or F1 decreases the pre-compression in the deck) into the deck. There is also longitudinal mild steel which assists.

You can also have steel going through the joint opposing the force directly rather than through shear friction but it is not a necessity. You can also have part shear friction and part direct transfer.

As I stated before, there was some punching directly below #12 since there is a concentration of vertical steel that locally increased the shear friction (and some doweling action) capacity of the blue plane. From a design perspective, it is better to ignore this localized concentration of the steel and just design for the shear friction through the blue joint so that the loads can more easily be transferred to the PT and other longitudinal steel in the deck.

RE: Miami Pedestrian Bridge, Part XIII

Vance, I'm not sure about 'wrong' but maybe incomplete. I'm thinking that the failure process is along the lines of this:

Friction under 11 initially carried a significant part of the load, but this cannot be uniformly distributed due to compressibility of the concrete. The steel cannot carry much, if any, load out of 11 initially because it doesn't deform enough to see a load, until the friction is mostly lost, at which point 11 is bearing against the steel. 8500 psi compression vs 60ksi steel suggests that the concrete must have been crushed, allowing an S-shape to the steel until the majority of the load is transferred out of 11 to being resisted by the reinforcement in 12.

Once 11 moved more than a fraction of an inch there would be powdered cement and larger asperities would no longer function, just low amounts of friction. Perhaps turning the reinforcement into an S-shape helped enhance the friction by making a small increase in the clamp load?

RE: Miami Pedestrian Bridge, Part XIII

The GreenLama,

I agree with you. The transverse PT does add a clamping force. If I put my hand between two 2x4s that are being bolted together, it will still get squeezed even if it doesn't line up with the bolts. By the time you are on the centre line of the deck, the clamping force from the PT is more or less a uniform distributed line load along the entire length of the bridge. The clamping force will only increase as the rough surface on a shear block tries to pulled out from the deck.

RE: Miami Pedestrian Bridge, Part XIII

Hey, I'm just a silly computer engineer, but since members one and two held even after it was dropped, wouldn't that same adjustment to 11 that was made to two be a first approximation to a fix?

SF Charlie
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RE: Miami Pedestrian Bridge, Part XIII

SFCharlie,

If you build #11 and #12 like #1 and #2, the joint would have been better but still not acceptable. #1 and #2 happened not to fail but there are a lot of structures out there that are inadequate but are not (have not yet) failing. It is also like a wish bone, one side or the other will fail fist. Once one side fails, the other side isn't going to fail unless you can add a higher load.

The fact of the matter is that there is a lot of experience in dealing with these kind of force transfers and the failure should have never happened.

RE: Miami Pedestrian Bridge, Part XIII

Quote (TheGreenLama (Structural)13 Oct 19 23:34 Quote (Vance) There is no active clamping force)

I recall your discussions. And I also think some transverse PT does clamp the zone I am addressing. If the zone of influence fans out at 45 degrees each side that covers the 60" notch 14 feet away. The first 4 transverse tendons probably contribute to a clamping force. It may be somewhat less than if tendons had continued at 2'-8" spacing but it is there.
What I am getting at is cannot the projecting deck at the sides of the failure zone provide as much shear friction as the reinforcing one might design for the blue area defined as blue in recent posts?
There may be indications of this being at work in the failre experienced. There is no failure under the southmost contact area of 11/12. Is that because the surface of the deck failed first because the area of the deck being "clamped" did not fail? I think we can see that the surface failure was the weak link in this south zone. Then as the failure moves farther north it begins to overcome the side friction and dive deeper below the deck surface. I think that is in part because the deck surface had already failed and offered little resistance after slipping, increasing the demand on the remaining part of the joint.
The PT tendons D1 then provided a weak plane and limited the failure laterally until the break out at the edge began to fail in diagonal tension.
Thank you for the reference to ACI background, and for your response.

RE: Miami Pedestrian Bridge, Part XIII

@SFCharlie at 14 Oct 19 00:09

One thing mentioned in the Part 73 analysis (section 6.3.4, third paragraph on page 87) was that not only was truss 2 36" deep (vs 24" in 11), but it also had a much larger diaphragm, 42" versus 24". It also lacked the vertical pipes used on the north end. Figure 68 on the next page implies that the south end was at nearly the same risk as the north end. Page 9 of that report mentioned that node 1-2 was showing similar, but lesser evidence of distress as 11-12.

RE: Miami Pedestrian Bridge, Part XIII

Earth314159 (Structural) RCPete (Electrical)
Thank you both for your responses. I meant no disrespect for the experience on this forum. Yes, you're both right about members one and two being on the edge of a similar failure as the NTSB crack photos point out.

SF Charlie
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RE: Miami Pedestrian Bridge, Part XIII

SF Charlie,

No disrespect was taken. You ask good questions for a computer guy :)

I never got past programming beyond Fortran 77 on my XT IBM PC.

RE: Miami Pedestrian Bridge, Part XIII

Quote (Great post! Report 3DDave (Aerospace)13 Oct 19 23:45 Vance, I'm not sure about 'wrong' but maybe incomplete. I'm thinking that the failure process is along the lines of this:)

Dave - My idea was not to solve this particular failure but to explore the shear-friction issues of an idealized situation like this without the complications.
I am in agreement with your comments - I may hold a bit on your description of the steel and its S curves, but in general I am with you.
The steel that is bent into an S has likely yielded. Whether that increased clamping force is dependent on the configuration, and if yielded the elongation curve has flattened and is relatively constant for some significant elongations. But reinforcing steel is less ductile.
Thank you for responding.

RE: Miami Pedestrian Bridge, Part XIII

SF Charlie, EarthPi

I've been lurking since April 2018, and have found this fascinating. I had a chance to go through the full Part 73 report (recuperating from a bit of foot surgery; 3 months into a 2 month recovery [wince]), so I've had plenty of time to read.

One take I got from the report, was that there were a lot of chances for somebody to throw up a red flag before the bridge collapsed. I'd been on some projects that were going nowhere or down in flames, and I've seen the attitude of "we can make it work, we put too much into it to stop this approach. Let's just try $WHATEVER". However, in my career, catastrophic failure might cost a few tens of thousands of dollars, not lives.

I have this feeling that somebody thought "It's just a foot bridge. What can go wrong?" Not sure they changed their mind before March 15th, 2018.

RE: Miami Pedestrian Bridge, Part XIII

Quote (RCPete (Electrical))

I have this feeling that somebody thought "It's just a foot bridge. What can go wrong?" Not sure they changed their mind before March 15th, 2018.
The FIU Pedestrian Bridge was a "Signature Project" - to be constructed as a towering example of the brilliance of the Accelerated Bridge Construction procedure which is a banner department of the FIU educational program.
The Prime Directive was to maintain traffic on the street at all times.
Not sure but this was likely the heaviest bridge structure ever moved into place on land. The 950 ton monster was moved into place in one weekend - when commuters went home Friday it was not there and when they went to work Monday it was in place and traffic was flowing without a hitch. The accolades were mounting. Dignitaries gathered. Speeches were made. Photos were taken.
Everyone involved understood the importance of this project to the ABC program at FIU. A monument visible for miles telling the world "This is what we do. This is who we are!"
Many men with stellar careers in engineering and construction gathered at a meeting at 9:00 AM on March 15, 2018.
And no one dared to fart at the President's tea.

RE: Miami Pedestrian Bridge, Part XIII

Quote (Earth314159)

From a design perspective, it is better to ignore this localized concentration of the steel and just design for the shear friction through the blue joint . . .

Earth314159. Thanks for your comment above. Two questions arise from this statement:

1. Is it being suggested that there was in fact any actual design for shear through the blue joint ? If you look at the images on page 96, it is pretty clear there was none?

2. Is it being suggested that the collapse arose from a failure by the construction team to roughen up the concrete of the deck surface of the blue joint -- either by mechanical roughening of wet concrete, or more deliberately by cast keyways?

Thanks for your helpful comments.



RE: Miami Pedestrian Bridge, Part XIII

Vance Wiley (Structural)
I'm sure you're 100% absolutely right. ...just a note, it is the largest bridge move in the US by weight to date, but not the world by a long shot...

SF Charlie
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RE: Miami Pedestrian Bridge, Part XIII

FortyYearsExperience (Structural) Thanks for reminding us of this photo. Since the NTSB report came out, you are one of the members that has reminded us of the greater extent of this failure. That, together with the cracks noted at members 1,2, show that something was pervasively wrong with the understanding of the design. Thanks.

SF Charlie
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RE: Miami Pedestrian Bridge, Part XIII

Vance:

As a signature project, it now kind of makes sense. If the priority was to keep traffic flowing, minor details like the cracks spreading from marginally bad to horrific might just possibly be overlooked. Shudder.

Question: Are criminal negligence charges likely to hit anybody in this?

40 Years:

From the official report, (part 73), they mention the lack of roughening as a contributing factor, but the key takeaway from their findings was that the shear demands were underestimated, and that the capacity of the structure was overestimated. I've not seen any argument to the effect that roughening the joints per the specification would have compensated for all the other deficiencies.

RE: Miami Pedestrian Bridge, Part XIII

Quote (RCPete (Electrical)14 Oct 19 16:56 Question: Are criminal negligence charges likely to hit anybody in this?)

No charges have been filed to my knowledge.
And I hope they will not be. Too err is human. Thankfully professionals do not err too often. We would lose thousands of doctors each year if mistakes resulted in criminal charges.
But to not recognize an impending failure or at least a structure in serious peril is beyond the pale.
My son-in-law is not an engineer but watches the documentaries about disasters and says most of the disasters have one thing in common. They are a convergence of several factors. Mistakes - sometimes multiple, combined with conditions which either conceal the impending crisis or cause people to ignore the warning signs. This project seems to fit.
The design firm says they were not informed of the rate of increase of the cracking. One look at those photos of the cracking and knowledge of its location in a truss structure clearly says it will not be long now, regardless of the speed of the progression or the time it has been developing.
I watched cable news from 3000 miles away and once I recognized it was a truss and saw the remains I suspected the truss had lost its heel joint at the north end. That happens in trusses. But it does not have to happen.
Thank you.
Hope that foot heals soon. Do not ignore a warning sign. Don't let your doctor ignore one either.

RE: Miami Pedestrian Bridge, Part XIII

It's possible that among many of those doctors tens of thousands of patients would not have been harmed or died.

It seems like, given all the evidence that was accumulating over the few days before the collapse, that negligence rises to criminal levels. It's not as if hidden features were failing that could not be observed, like a fatigue initiator buried in a turbine fan disk that expands over thousands of hours. This was massive chunks of concrete fracturing off over a matter of days. On an old bridge this should cause concern; how those involved accepted it in a new one is, to my mind, negligence. And that killed people.

There needs to be additional focus that can only be brought by deciding between prison and fudging a project and those unwilling to clearly chose need to face that as a possible outcome.

RE: Miami Pedestrian Bridge, Part XIII

Vance:

Thanks; bunion surgery with bone realignment, complete with (reinforcing pins (temporary #1 stainless "rebar"). I wasn't fully healed at the 8 week point, and the doctor had vacation (no backup, we're rural), so I had an extra month in the comfy chair. Gave me time to read the reports.

I see him tomorrow, and I'm pretty sure the bone is healed enough.

RE: Miami Pedestrian Bridge, Part XIII

Quote (Report Edit Delete 3DDave (Aerospace)14 Oct 19 18:00 This was massive chunks of concrete fracturing off over a matter of days.)

W--e-e-e-e-l-l-l-lllll ------- there is that.
Now that the FHWA/NTSB report is out, criminal negligence charges may develop. You can bet the attorneys have read the reports more carefully than we have.
I can't believe several experienced people observed for real what I saw in a photo and did not pull the plug. I cannot defend that.
I have to hope I would have reacted differently and called for shoring. But that would have been dangerous if began on March 15. It would have taken days to deploy the SMTPs - the outcome was defined before the meeting of March 15 began.

RE: Miami Pedestrian Bridge, Part XIII

The problem is the node blow out was clearly visible, the cracks had opened half inch, and FIGG were on record prior to collapse as knowing they needed to “capture the node”. So it’s past the point where they can say oops.

RE: Miami Pedestrian Bridge, Part XIII

2
Reading thru the FIGG report, I went huh when I hit the part in recommendations where they felt the need to insert some of their own "propaganda" in a technical report. Then I got to the part where they recommended better hard hats may have reduced/prevented injuries and fatalities. This is probably the most disrespectful thing to the injured and killed I have ever seen in a technical report. Just my thoughts. Inappropriate to post what I really think about that, but my recommendation to FIGG is an "ounce of prevention is worth a pound of cure."

RE: Miami Pedestrian Bridge, Part XIII

Reviewing FIGG report, Party Submission (last item in NTSB docket), Appendix A, WJE report, full scale tests of member of 11. A few observations come to mind.

It would seem to have been better to have used the Florida aggregates in the in the full scale tests rather than create an adjustment factor using small slant cylinder samples.

The time between casting the upper and lower halves of the full scale samples was only one week, where as the time between casting bridge deck and diagonals was closer to three weeks. The bridge was also cast outside and exposed to sunlight on one side for some period of time, formwork appears to have blocked three sides. It is may understanding that the titanium dioxide reacts with UV the create the white appearance. I have not been able to find any literature on what effect the titanium dioxide has on coefficient of friction. It would seem to me that it reduces the friction given that one of benefits stated is that the concrete becomes self cleaning. Maybe it is somewhere in one of the docket items, but all I have been able to find is general guidance on compressive strengths and dosage rates.

The tests do indicate how much additional strength can gained from adhesion as indicated by the significantly higher test values of the uncracked samples comparted to the cracked samples, and that bond between the surfaces is great, until it isn't. The tests also indicate the importance of roughening the surface, but, this was already known.

The tests also indicate that the longitudinal reinforcement bars buckled, this appears to be the case also for member 11 as indicated by the spalls in the surfaces. Wide tie spacing and lack of cross ties for the middle bars likely contributed to the bars buckling.

In big round numbers 21"x24"x8.5ksi = 4,284kips, not accounting for additional strength from the steel reinforcement, average roughened surface test result 2,594kips, unroughened, "as placed", 1,455kips. Makes me wonder why anyone would have a construction joint at such an angle to the axial load.

RE: Miami Pedestrian Bridge, Part XIII

Quote (hpl575)

...Then I got to the part where they recommended better hard hats may have reduced/prevented injuries and fatalities...

I'd like to see the hard hat that would protect a motorist from the force of a 950-ton bridge falling on their car.

The NTSB report points out that the structure had adequate ductility that it exhibited clear signs of distress before collapsing. Figg as a company and Pate as an individual held responsibility for reading those signs, understanding them, and responding appropriately, the which they did not do.

Here in the US, there is a type of special airworthiness certificate granted to amateur-built aircraft, that allows their operation in public airspace over houses and towns. The presumption is that self-interest motivates operators of such craft to build and operate them prudently, since they will be the first on the scene of any mishap. And it mostly works; accident rates for such aircraft are only slightly greater than their store-bought counterparts. On that basis I hereby propose that for every bridge, the EOR or their official representative be stationed beneath the structure during every maintenance or repair operation undertaken when that space is open to the public.

They can wear whatever kind of hard hat they want.

--Bob K.

RE: Miami Pedestrian Bridge, Part XIII

The middle bars were still straight following the collapse. The surface did not have containment so as the beam compressed and shifted along the deck, the shell had nothing to enforce that motion. I'd say less than 0.06 inches of displacement was required to pop that face off. In a 6 foot span only 0.06 inches of displacement will produce a 1.5 inch rise. The actual amount seemed to be about 0.25 inches, in accordance with 0.002 inches of displacement.

RE: Miami Pedestrian Bridge, Part XIII

Quote (hpl575 (Structural)14 Oct 19 22:41 Reviewing FIGG report, Party Submission (last item in NTSB docket), Appendix A, WJE report, full scale tests of member of 11. A few observations come to mind. It would seem to have been better to have used the Florida aggregates in the in the full scale tests rather than create an adjustment factor using small slant cylinder samples.)

Say What?? They tested with a different aggregate? What is that supposed to prove? Jeez!

Quote (I have not been able to find any literature on what effect the titanium dioxide has on coefficient of friction. It would seem to me that it reduces the friction given that one of benefits stated is that the concrete becomes self cleaning.)

That is a very good question.

Confinement Steel!
Now to a point made in the WJE report, pages 7 and 8.



This would seem to require further discussion. So WJE says 11 and 12 slipped on the deck then 11 split and failed at its lower end, triggering the collapse.
So how did it "root" out the section below the top of the deck? Did the top of 12 get pulled south as node 10/11 dropped and torque out the deep sections over the 8" sleeve? Was it knocked out in an upward direction as the protrusion of 12 past the end of the deck hung up while going over the edge of the pylon?
Certainly points out the folly in restoring the PT in 11 and increasing the compression load in 11 when it was already failing.
Thanks, hpl575

RE: Miami Pedestrian Bridge, Part XIII

Quote (hpaircraft (Aeronautics)14 Oct 19 23:40 Quote (hpl575) ...Then I got to the part where they recommended better hard hats may have reduced/prevented injuries and fatalities...)

So are they planning on issuing helmets before you drive under the bridge and collect them if you make it out the other side? That could actually work - - when motorists get fed up with the hassle they will take a different route and be far safer.

Quote (I'd like to see the hard hat that would protect a motorist from the force of a 950-ton bridge falling on their car.)

Maybe the turret from an M1A1 Abrams Battle Tank? It would be a little hard on the neck - - -

RE: Miami Pedestrian Bridge, Part XIII

I paused when I read the part about better hard hats too, but as best I know one of the workers on top of the bridge was killed when it fell. I don't think we know any of the details of that, but if it's related to his hard hat flying away when the bottom dropped out from under him and he died of head trauma, then it's a salient point. I think it's more an issue for OSHA to include in their report instead of FIGG, but I guess FIGG thinks that the more trees there are, the harder it is to see the woods.

Brad Waybright

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

RE: Miami Pedestrian Bridge, Part XIII

I was particularly struck by the transcript of the interview of the FIGG stress analyst. He repeatedly distances himself from the bridge design work by asserting that he is an analyst and not a designer, downplays his responsibility as QA manager, and produces this exchange:

"Q. ...when you learned of the cracking, did you ever look at your, the modeling and determine that those cracks were predictable after looking at the modeling?

A. That's a little bit subjective. But there were different stress distribution, so I couldn't say one way or the other."

I don't know about everyone else, but I use FEA quite extensively, and if behavior occurs which was not predicted, we go back to the model and see where the problem is and if we can recreate the behavior that actually happened.

Later on, and i will jump around:

"Q. ...How were the force effects generated from this LUSAS model?

A. ...LUSAS is a stress model. It doesn't report forces easily. You can get reactions at supports pretty easily, but internal forces out of a volume is a little bit more challenging, to say the least..."

Q. Teah, well, I noticed - I'm familiar with the process, so - well, I noticed that it did use the LUSAS model to generate the force effects.

A. Yeah.

Q. So the force effects were calculated by someone. Was that someone you or was somebody else --

A. It was me. Yes, sir."

Wow. Confidence-inspiring.

RE: Miami Pedestrian Bridge, Part XIII

Vance,

The WJE report that you noted (p. 151, https://dms.ntsb.gov/public/62500-62999/62821/6285...) seems to be coming at the collapse from several different angles all at once. But the last bullet point seems most relevant:

"Although the collapse was triggered by sudden failure at the base of Member 11, the underlying cause was northward movement of Members 11 and 12 relative to the deck. As described above, the northward movement started with a shear-friction failure below Member 11 that, in turn, led to breakout failure of the north-end diaphragm below Member 12."

RE: Miami Pedestrian Bridge, Part XIII

Thoughts on WJE Report
The WJE report is a pretty good document. It presents tests of sloping construction joints under shear friction.
But they used Chicago Aggregates and not Florida aggregates, a different supplier of Portland cements and Slag cements, and a different Portland cement to Slag cement ratio. Why would you do that? This is a problem with consequences beyond $50 million - how much does it cost to truck a few tons of cement and aggregate to Chicago?
Think of FIGG being able to stand up in court and say we tested the design under full scale tests using EXACTLY the same aggregates, EXACTLY the same cements, and EXACTLY the same quantities of those EXACT components in our testing so these test results directly prove our design is correct.
Now they have to convince the jury their test is even applicable. They will need to say the results are adjusted by some factor so they can apply to the FIU case - which factor will also have to be explained and sold to the jury. Heck, why not just apply an "Its Not My Fault" factor and be done with it?
I did learn something about shear friction action. The concept that reinforcing across the shear plane contributes to the capacity of the joint seems to be validated. The action seems to correlate well with the 45 degree idea of inducing tension in that reinforcing. It was previously presented here that #7 bars at 12D development lengths could result in a stretch of 0.026 inches at 57,000 psi (0.90X yield) and that would be 0.030 inches at yield. The slip of the tested joints was noted to be 0.02 inches at maximum load, suggesting the #7 cross plane reinforcing developed yield at development lengths closer to 8 diameters. I can buy that. That could explain how the 3 pairs of bars in the #7 hoops in node 10/11 developed enough tension to fail. The WJE analysis looks at strut action with bearing to add the contribution of the bend to the anchorage.
I also see that ANY laitance must be removed - and any paste only matrix. The particles in the paste matrix seem far too small to effectively contribute to a perpendicular movement of 0.020 inches and thereby mobilize the cross plane reinforcing.
The test results seem to support FIGG's position that the failure to prepare the surface of the deck under node 11/12 is why the structure collapsed.
Then the Structural Analysis by WJE finds that the capacity of the joint as NOT intentionally roughened was about equal to the demand at the time of collapse. Add to that prior cracking and movements from moving and the results are now clear. Hindsight is 20/20. The WJE analysis seems to be much different than the analysis by FHWA/NTSB which, as I read it, suggests the FIGG calcs are off by almost 100%.
The WJE analysis suggests that correct preparation of the deck surface at node 11/12 would have resulted in a condition that was completely adequate for this phase of the project.
The following is from the WJE report, Page 127.

9 SUMMARY OF FINDINGS AND CONCLUSION
The following summary of findings and conclusion follow from the research and analysis described in
Sections 2 through 8 of this report. The section that relates to each conclusion is indicated.
Failure Pattern (Section 2). A debonding and sliding failure at the construction joint below Member 11
led to breakout failure of the north-end diaphragm and ultimately collapse, triggered by sudden crushing of
Member 11 near its base.
Construction Joint Conditions (Section 3).
Despite FIGG’s confirmation to MCM that the FDOT Standard Specifications requiring roughening of the hardened concrete must be followed, the construction joint surface below Members 11 and 12 appeared to have been left in an as-placed (non-roughened), relatively smooth condition.
Interface Shear Transfer Testing (Section 4).
The primary finding from the experimental program is that intentional roughening of the construction joint following FDOT Standard Specifications improved the shear capacity of the cracked interface by a factor of 1.78. This factor reduces by 5 to 13 percent if adjustment for Florida aggregate is made based on slant shear tests. This finding is consistent with relative difference according to the AASHTO Code: the maximum allowable shear stress for a roughened surface (1.5 ksi) is 1.88 times that for a non-roughened surface (0.8 ksi).
Comparison of observed axial strengths of the as-placed (non-roughened) specimens to the calculated force
in Member 11 after the shoring was removed suggests that the construction joint was weakened or at least
partially debonded when the shoring was removed.
More significantly, the axial capacities of the roughened specimens, before or after adjustment for Florida
aggregate, are substantially greater than the calculated axial force in Member 11 at the time of the collapse.
As such, if the construction joint were roughened as required by the FDOT specifications, the collapse
would not have occurred. This conclusion is valid for hardened concrete surfaces intentionally roughened
in accordance with FDOT Standard Specifications even if the surface roughness is considered to be less
than the 1/4 inch amplitude referenced in the AASHTO Code. Also note that this conclusion neglects the
additional capacity from breakout resistance of the north end diaphragm, which if included would provide
additional capacity to the connection.
Structural Analyses (Section 5).
A finite-element model of the main span was developed to determine truss member forces and bending moments during construction.
AASHTO LRFD Design Compliance.
The Member 11/12 deck connection was evaluated in accordance with the AASHTO Code, assuming resistance by shear-friction across the entire construction joint.
Although inconsistent with the actual failure mode, resistance by shear-friction across the entire
construction joint is the likely design assumption. Based on WJE test results, the AASHTO friction
coefficient for a roughened surface (which calls for 1/4-inch roughness amplitude) was assumed. However,
AASHTO does not provide specifics on preparation of the joint (including intentional roughening of
hardened concrete) or how roughness is measured. The FDOT Standard Specifications, as proven by
laboratory testing, achieves the requirements of AASHTO Code. The capacity-to-demand ratio was found
to be 1.09 if AASHTO load modifiers for ductility and redundancy are excluded, and 0.99 if they are
included, indicating compliance with AASHTO design requirements.

The following is WJW Report page 128.

Capacity Analysis for Observed Failure Pattern.
The Member 11/12 deck connection was also evaluated
based on results of the interface shear transfer testing in combination with breakout resistance consistent
with the actual failure pattern, ACI 318 design equations, and related research. The results indicate that the
combined shear-friction and breakout resistance is consistent with the calculated horizontal force in the
Member 11/12 deck connection at the time of the failure. This explains the failure due to the unroughened
construction joint surface.
Evaluation of Peer Review (Section 6).
Berger’s peer review fell far short of their contractual obligations.
In particular, by their own admission, Berger did not even attempt to assess the conditions at the
construction stage shown in the plans that was being built at the time of the collapse, which was required
by their contract. Furthermore, the Berger finite element model could not have been used to reasonably
estimate the forces in the concrete truss members during construction or in the structure’s final
configuration because it did not address the construction phasing.
Evaluation of Twist Exceedances during the Main Span Transport (Section 7).
Cracks in the region of the connection of Members 11 and 12 to the deck increased dramatically after the move from the casting yard to the final location, as evidenced by photographs taken before and after the move. The deformations associated with exceeding the established twist limits caused high stresses in the region. Along with other factors, this stress may have been a contributing factor to damage in the region and ultimately to the
collapse.
Re-Stressing of Member 11 (Section 8).
Contrary to FIGG’s instructions, no one closely monitored cracks in the north-end diaphragm during re-stressing of Member 11, even though both MCM and Structural/VSL were aware of the instruction. Also, Structural/VSL’s shop drawings state that stressing operations should stop if existing cracks widen or new cracks are observed. Evidence shows the construction joint was not roughened, so the existing cracks would have widened during re-stressing, and the widening could have been readily detected by several means. In accordance with FIGG’s instruction and Structural/VSL’s awareness of crack monitoring per their shop drawings, widening of the cracks would have required
stopping the re-stressing, thereby preventing the collapse.
Conclusion.
In conclusion, most significant finding from WJE’s research and analysis is that full-scale tests show that if the construction joint below Members 11 and 12 were roughened as required by the FDOT Standard Specifications, the collapse would not have occurred. It is also highly significant that, for the observed failure pattern and relatively smooth as-built condition of the construction joint, the combined shear-friction and breakout resistance determined from testing and analysis is consistent with the calculated horizontal force in the deck connection at the time the failure.

The WJE Report was prepared to support FIGG and seems to present that position pretty well.





RE: Miami Pedestrian Bridge, Part XIII

Quote (TheGreenLama (Structu)

Thanks for responsing. I am trying to wrap my mind around the various stages of failure and how they fit together.
I see the top of deck failure first - maybe beginning in the casting yard.
I have previously thought that member 11 gave it up early because of the severe splitting we could see earlier and supported by the recent photos.
We learn from testing by WJE that it takes only about 0.020 inches slip to develop maximum resistance in shear friction. And that at 1/2 inch slip it is over. So at a half inch slip - maybe less - all shear planes had been developed and had failed.
Somewhere between 0.020 inch and 1/2 inch of slip would be a good time for the moments to change in member 11 and the splitting to increase while the joint still has residual capacity to induce axial load in 11, resulting in the loss of capacity in 11, and triggering the failure.
The last two stages were probably a hand off - but it seems that when node 11/12 has failed there would have been a loss of axial force in 11 - except for the upper PT rod.
I am going in a circle here. More thought required.
EDIT ADD:
Or just disregard WJE conclusion and consider damage to 11 as collateral damage from failure of 11/12

RE: Miami Pedestrian Bridge, Part XIII

Zeroing in on this: "Re-Stressing of Member 11 (Section 8). Contrary to FIGG’s instructions, no one closely monitored cracks in the north-end diaphragm during re-stressing of Member 11, even though both MCM and Structural/VSL were aware of the instruction."

This is such an outrageous statement to make! It makes my blood boil just reading it! How many more people would've died if this had action had been followed? What are FIGG and WJE thinking? Sorry for the rant but, IMO, this is beyond the pale.

RE: Miami Pedestrian Bridge, Part XIII

Quote (TheGreenLama (Structural)15 Oct 19 20:10 Zeroing in on this:)

The die was cast and the future foretold ( if someone had read it) before the Friday 15 meeting began. The danger was too great for anyone to be near the thing.
What everyone failed to comprehend is that the cracking was past the 1/2 inch "I'm gonna fail NOW" mode and as Kenny Rogers said it was time to "Know when to Run!"

One more point about the condition of member 11 - with the failure underway and member 11 already splitting, the idea by the EOR to "capture" node 11/12 would not have repaired member 11. It was all over.

RE: Miami Pedestrian Bridge, Part XIII



Here is the photo of the joint area before any concrete was placed. It would be virtually impossible to effectively roughen the surface due to the congestion in that area. And what are those loopy black things? Anything in this area that is not steel or concrete weakens the thing.

Two other observations:
1. The ties in #11 are uniformly spaced above the joint, but at the joint, the spacing increases, and there is no tie at the very bottom where it connects to #12. There could have been one, maybe two more ties in #11 near the bottom, and it would have helped if they would have gone all the way into the deck instead of going around the bottom steel in #11.

2. The top and bottom bars in #11 are lapped immediately above the cold joint. All these bars plus the PT conduits created planes of weakness, that correspond to the cracks that appeared before the final failure.

RE: Miami Pedestrian Bridge, Part XIII

Temporaty Shoring
Too late now, but has anyone seen inflatable bags of a scale that would have held the north end up? A 20 ft square bag would need 17 psi air - is that within the realm of possibility?
Remobilizing the SMPT was days away - an airbag with a long hose seeems like a safe way to support this thing. Tie a rope on the uninflated bag, with a rock on the end, and throw it under. Run around the end and pull the bag in place. Air up thru a hose with connection outside the danger zone.
Of course 3 hours is no enough time - but last Wednesday maybe?

RE: Miami Pedestrian Bridge, Part XIII

Mr. Vance Wyley:
In conclusion, most significant finding from WJE’s research and analysis is that full-scale tests show that if the construction joint below Members 11 and 12 were roughened as required by the FDOT Standard Specifications,
Sorry, but you have been "fooled" by the WJE report.

The "non- 1/4in-roughenning" done in the tests is not the 'roughenning" called for in the FDOT specs when cleaning a joint. The intent of the spec is to leave the joint clean of loose material. Not to chip the joint with a chisel as they did for their so called "roughenning according to FDOT specs". And Florida engineers, designers and contractors know that.

We have to remmember that you have to think like a "Contractor". Chipping the joints requires alot of time and effort that no contractor is going to pay for. So, the engineer (designer) must assume that the joint is smooth (but clean)and apply the corresponding factors in the shear-friction analysis" "non intentionaly roughened" which means no 1/4" magnitude. This is why MCM asked FGG if the joints needed the application of concrete adhesive. And Figg said no: use the FDDOT spec.

FIGG arguments are being laugh at among structural engineers in Florida. But they may convince a jury or those not experienced in Florida bridge construction.

The WJE report is interesting but does not represent the actual situation at the bridge.

What happen is that FIGG forgot to add the note in their erection draawings to roughenn to 1/4" to match the numbers used in the calcs. That is the initial cause of the failure. Still, the structure told everybody it was in trouble and nobody had the "balls" to face FIGG in the meeting, put the foot down and close the roadway: "It was not their responsability"

RE: Miami Pedestrian Bridge, Part XIII

The green Lama

Zeroing in on this: "Re-Stressing of Member 11 (Section 8). Contrary to FIGG’s instructions, no one closely monitored cracks in the north-end diaphragm during re-stressing of Member 11, even though both MCM and Structural/VSL were aware of the instruction."

This is such an outrageous statement to make! It makes my blood boil just reading it! How many more people would've died if this had action had been followed? What are FIGG and WJE thinking? Sorry for the rant but, IMO, this is beyond the pale.


There was a guy next to the strut #11 taking pics and monitoring the cracks. Lucky him he did not die.

Yes, they were closely monitoring the cracks. But the failure was explosive.

RE: Miami Pedestrian Bridge, Part XIII

Quote (Vance Wiley)

One more point about the condition of member 11 - with the failure underway and member 11 already splitting, the idea by the EOR to "capture" node 11/12 would not have repaired member 11. It was all over.

I'm not so sure of that. I think that if Figg had gotten their collective heads out of their butts and actually looked closely at what the cracks were telling them, they might still have snatched victory from the jaws of defeat.

Just as a "what if," consider the possibility that they could construct a heavy steel appliance that attaches to the exposed threads at the ends of the two D1 longitudinal PT tendons, and buttresses against the north face of member 12. Given a sense of urgency and a bunch of petty cash, I bet they could have put that in place in two or three days time. Once installed, it would get encapsulated inside the backspan, and the external profile of the bridge would be unchanged from the original plan. If they had to, they could probably do the same thing at the 1/2 end as well, and add a decorative fillet to conceal it.

RE: Miami Pedestrian Bridge, Part XIII

Note to all:

The pics show cracks in vertical column 12 which means that there is bending there. Also the #11 must have benting moment on top of comprssion forces.

The reports do not incorporate these elements into their review (not FIGG, MCM, or NTSB). These secondary or "terciary" forces may be part of the straw that "broke the camel's back"

Also, the "restressing procedure" going back and forth from the north bar to the south bar in increments may also be part of the picture in a system that is in the process of colapsing. Think about how you can break a steel wire.

Live long and prosper

RE: Miami Pedestrian Bridge, Part XIII

Quote (PA PE (Civil/Environmental))

It would be virtually impossible to effectively roughen the surface due to the congestion in that area.
True. So if it is not possible within reason, those constructing it should ask for a change to detailing.
One question - if they can't build it, why did they bid it? Not to be harsh here but these are things to be addressed during bidding. Not when penalty days are ahead.
Supposedly there were reviews addressing constructability before approvals were granted.
A closer look while still in design could have traded more steel for less roughening.

And for similar conditions - does anyone remember the cast grid plates with spikes which were used in wood pile construction to connect beams to round wood piers? Could something like that be designed - with bearing plates above and below the shear plane and and holes to pass concrete and torch outs for the rebar and conduits to pass?
Any entreprenuers here? It would be a specialty market - there is not going to be many of these built.

RE: Miami Pedestrian Bridge, Part XIII

Quote (HP)

I'm not so sure of that. I think that if Figg had gotten their collective heads out of their butts and actually looked closely at what the cracks were telling them, they might still have snatched victory from the jaws of defeat.

They were busily designing metal straps to "capture the node" so I suspect they knew.

RE: Miami Pedestrian Bridge, Part XIII

Quote (Vance Wiley (Structural))

if they can't build it, why did they bid it?
It was a design build contract, so it was not designed when they bid it. FIGG worked for MCM.

SF Charlie
Eng-Tips.com Forum Policies

RE: Miami Pedestrian Bridge, Part XIII

Quote (The Mad Spaniard (Structural)15 Oct 19 21:39 Mr. Vance Wyley: In conclusion, most significant finding from WJE’s research and analysis is that full-scale tests show that if the construction joint below Members 11 and 12 were roughened as required by the FDOT Standard Specifications, Sorry, but you have been "fooled" by the WJE report.)

I may have been fooled - the old bait and switch trick. To be clear, the quote is from the WJE report which was requested by FIGG and presented in defense of FIGG.
So what coefficient of friction should be used with a joint prepared as FDOT requires? What coeff would you recommend for one prepared as WJE did?
The WJE tests indicate full/maximum resistance of a shear friction joint is reached at a slip of 0.020 inches. I note FDOT does not specify an amplitude - perhaps because they also think small amplitudes are sufficient. WJE 5.3.4 In the case of the interface shear tests, peak shear-friction resistance occurs at a deformation of approximately 0.02 inches. Deformations at breakout failure are about twice this amount, 0.04 inches.
So FDOT does not specify an amplitude - and apparently Florida Contractors and Engineers know that the 1/4 inch amplitude is not required. I am not sure that excuses the contractor from following FDOT requirements, particularly after the issue arose and directions were given to do so.
If the preparation by chisel by WJE improves the joint and a higher coefficient of friction can be assigned, the industry has learned something. There may be places the increase can be helpful and worth the effort.
We can add the joint preparation by WJE to the list of things FIGG will have to explain to the jury.
Thank you.

RE: Miami Pedestrian Bridge, Part XIII

Quote (SFCharlie (Computer))

Hi, Charlie. As I recall, MCM bid out the forming, concrete, reinforcing, and PT to sub-contractors. It is those contractors who were the focus of my statement.
Thank you.

RE: Miami Pedestrian Bridge, Part XIII

Surely a major lesson to be learned here is that friction is not reliable. Another is that if you build a post-tensioned concrete truss/frame, the design should be based on research specific to that form of structure.

RE: Miami Pedestrian Bridge, Part XIII

Vance Wiley (Structural)16 Oct 19 00:46

To answer your questions:

1 If the connection is designed assuming that with "No intentional roughening" based on AASHTO LRFD (no 1/4" roughening) is enough to transfer the shear friction, then the FDOT spec based cleaning is good enough. If you notice, the value of the coefficient related to the steel capacity is 0.6. This is for practical purposes the coefficient associated with the shear (just typical shear) of a steel section : the steel bars are acting like dowels.

We have to remmember that the equations (with 0.6 coef) are based on tests performed on ( I guess) smooth surfaces. To verify that we may hace to go to the original research. The other case (1/4' roughening) equations (1.0 coeff) for the steel rebar component was based (probably) on tests that involved this level of roughening. If that level is not requested clearly in the plans you would not know if the capacity will be obtained in the field.

Anything in between, you have no clue of the capacity that will be obtained. But we have discover (thanks to FIGG tests) that even something less than 1/4" rougghening could provide you a good capacity. They were just lucky to get those results with what they did. But we can not rely on that for other designs until more tests are done to satisfy AASHTO or ACI bosses.

So, to accommodate typical construction practices, the FDOT specs must say something about cleaning the joint and make sure that no loose matter is there. The way it can be read at this moment , any contractor can use sand paper to "roughen" and clean the joint and still meet the FDOT spec. (and they will !!! and there is nothing FDOT can do about it) If the FDOT wanted to roughen the joint the way FIGG and WJE call "FDOT roughening way", they would explicitly mention the procedure and the result in a very detailed way with measurement of the roughness in the spec. So, to make contractors life easy (and FDOT too) , the spect just make sure that the joint is clean. But for that reason, engineers must design for only "dowel action".

It is not difficult to see a 1/4" roughness in the field. But anything less would require the sophisticated methods that were used in the FIGG test to quantify roughness. And that makes stuctures more expensive and can create arguments in the field if the parties do not agree on the numbers obtained. We are not building "Swiss watches" here.


Note to all: SHEAR FRICTION HAS BEEN USED IN MANY STRUCTURES FOR MANY YEARS. IT IS RELIABLE. BUT YOU HAVE TO KNOW WHAT YOU ARE DOING AND MAKE SURE THAT THE REQUIRED "1/4" LEVEL IS CLEARLY NOTED IN THE PLANS IF THE DESIGNER HAS USED THE 1.0 COEFF IN THE CALCS TO REDUCE THE VOLUME OF REBAR IN THE JOINT. REMEMBER THAT THE PEOPLE IN THE FIELD DO NOT SEE THE CALCS.


Some months ago, when I obtained the calcs from FIU and saw the analysis of the joints and the fact that no note about 1/4" was in the plans, I deducted what was the potential suspect : 0.6 vs. 1.0

Best regards



RE: Miami Pedestrian Bridge, Part XIII

Quote (The Mad Spaniard (Structural)16 Oct 19 02:23 Vance Wiley (Structural)16 Oct 19 00:46)

Thank you for the explanation.
In reading the PARTY SUBMISSION by FIGG to the NTSB, section 6.2.1
The design of the member 11/12 nodal connection was based on AASHTO LRFD
design specifications for shear friction (Section 5.8.4). The calculations used a
friction coefficient of 1.0 which is specified in the design code for “normal weight
concrete placed against a clean concrete surface, free of laitance, with surface
intentionally roughened to an amplitude of 0.25 in.”


It seems FIGG is admitting a special requirement for construction joints beyond the requirements of the FDOT description but did not indicate that special requirement on their plans. And further, FIGG instructed the job to follow FDOT.
FIGG is a firm with a large Florida presence - they did not know the difference between FDOT and AASHTO on this issue?

The next paragraph says this:
Since the bridge was constructed using the RFC plans, the nodal applied loads
and capacities are actually represented by the RFC plan details as opposed to the
design calculations. Section 7.3 of this report provides an analysis of the design for
the member 11/12 nodal connection as shown in the RFC plans.

Uhhh - lets see. Since the thing was constructed from the RFC plans, the loads and resistances are there and NOT in the calcs ?? Did they do the calcs, draw the plans, and ne'er the twain did meet?
It seems the WJE tests followed the intent of the calcs, not the drawings.
But why would FIGG direct or confirm joint preparation to FDOT requirements? Then the contractor did nothing - left it "as poured", because it was too much trouble, according to the testimony of a workman.
And no one at the job noticed the difference.
Another of those converging mistakes. Two, actually. No - this counts for 3.

RE: Miami Pedestrian Bridge, Part XIII

The Mad Spaniard,

No matter how red you make your comments, friction is not reliable in resisting forces of this magnitude. Shear friction theory is relatively new, and not universally accepted. Where I have practiced in Australia, it is not mentioned in our code, as it is considered to be not well supported by long term practice.

RE: Miami Pedestrian Bridge, Part XIII

hokie66

I am curious as to how you handle shear in a cold joint? I have used shear friction method several times (always with 0.6) and find it in my concrete references back to early 1990s (oldest codes I have which are from my undergrad days).

RE: Miami Pedestrian Bridge, Part XIII

Would I not be correct to define shear friction as the ability for 2 separate parts to resist movement between? For load calculation purposes this should be a single monolithic casting of concrete. Once it became two then it has for all intents and purposes failed.

Brad Waybright

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

RE: Miami Pedestrian Bridge, Part XIII

Hokie66,

Would the fact that Australia and NZ do not incorporate shear friction be because that region is more seismically active than say Florida?

RE: Miami Pedestrian Bridge, Part XIII

Quote (thebard3 (Computer)16 Oct 19 15:30 Would I not be correct to define shear friction as the ability for 2 separate parts to resist movement between? For load calculation purposes this should be a single monolithic casting of concrete. Once it became two then it has for all intents and purposes failed. Brad Waybright)

That is basically the case in plain or un-reinforced concrete.
The problem is concrete cracks - for many reasons and almost universally. So we cannot afford to replace every concrete structure because it cracked, and reinforcing is added. And as a double benefit, reinforcing adds strength or load capacity. Plain concrete is relatively weak in tension and sometimes cracks for no apparent reason so as a structure it is not reliable. Reinforcing provides that reliability. As a gravity dam, plain concrete works quite well.
Mild or plain deformed reinforcing can resist the tension forces from loads that are supported, maintain contact across a crack and therefore maintain some shear capacity - and gives engineers jobs. Prestressing is active reinforcing and compresses the concrete, thereby closing the cracks or preventing them from developing. This structure has (had) both.
Most structures are too big and too complicated to be cast monolithically and "cold joints" are required. Incorporating those joints and getting them to perform as if the concrete were monolithic is the purpose of reinforcing and, in this case, the primary critical force acted in shear.
The proper application and manipulation of concrete and reinforcing provides jobs for engineers. Improper applications can result in structural failures and termination of those jobs.
You are totally correct - plain concrete will never be stronger than when monolithically placed and cured and uncracked. Properly reinforced concrete will be much stronger, can be engineered to serve different conditions, and can still serve well after it cracks.

RE: Miami Pedestrian Bridge, Part XIII

What is being discussed here, is why we need to see the "RFI" communications. To see if MCM, Structural Technologies (Steel, Forming & Placing), FIGG & BP, (just who) and were they even discussing or to what extent, the constructability difficulties at the 11/12 Node.

RE: Miami Pedestrian Bridge, Part XIII

Quote (epoxybot (Structural)16 Oct 19 17:16 What is being discussed here, is why we need to see the "RFI" communications.)


FIGG represents the emails during construction as this:

E-MAILS REGARDING CONSTRUCTION JOINT REQUIREMENTS
The construction specification requirements for roughening construction joints
were confirmed and emphasized in an e-mail exchange between MCM (the contractor), BPA (the independent construction quality inspector) and FIGG
(designer) starting on June 10, 2017 and continuing through June 13, 2017.
June 10 at 9:20 a.m. – MCM’s Superintendent to BPA:
“We are scheduled to pour the first bottom 3.5 ft. of the South column on
Foundation Type 3 this coming Monday morning – June 12, 2017 at 8:00 a.m.
… should you have any questions or concerns feel free to contact me.”
June 10 at 10:44 a.m. – BPA’s Project Administrator to MCM’s Superintendent and
others at BPA and MCM:
“Every cold joint generated on structural elements will require a treatment with
a APL [Approved Products List] list product. Please ask FIGG their opinion and
suggestion.”
June 10 at 10:48 a.m. – MCM’s Superintendent to BPA’s Project Administrator and
others at BPA and MCM:
“The question will be asked.”
June 12 at 9:18 a.m. - BPA’s Project Administrator to MCM’s Superintendent and
others at BPA and MCM:
“…any response from Figg regarding potential cold joints? Please advise.”
June 12 at 10:06 a.m. – MCM’s Project Engineer to BPA’s Project Administrator and
others at BPA and MCM:
“Please clarify if you are referring to construction joints or cold joints. For
construction joints we will roughen the surface of the hardened concrete and
remove loose particles prior to placing new concrete.”
June 12 at 10:10 a.m. - BPA’s Project Administrator to MCM’s Project Engineer and
others at BPA and MCM:
“Yes, I am referring to construction cold joints on structural elements, please
get an answer from FIGG of the appropriate treatment.”
June 12 at 10:15 a.m. – MCM’s Project Engineer to BPA’s Project Administrator,
FIGG’s Project Manager and others at BPA and MCM:
“I spoke with FIGG and they advised us to follow FDOT specs which is as
follows:
6-15
400-9.3 Preparations of Surfaces: Before depositing new concrete on
or against concrete which has hardened, re-tighten the forms, roughen the
surface of the hardened concrete in a manner that will not leave loosened
particles, aggregate, or damaged concrete at the surface. Thoroughly clean
the surface of foreign matter and laitance, and saturate it with water.
The plan notes do not mention the use of a bonding agent so it is not required.”
June 12 at 10:37 a.m. - BPA’s Senior Project Engineer to MCM’s Project Engineer,
FIGG’s Project Manager and others at BPA and MCM:
“Lets make sure we keep FIGG informed about the location of the all future
construction joints and represent them accurately in the final as-built.”
The e-mail included an image from the bridge plans showing the south landing
bent (also referred to as the south abutment) with an arrow pointing to a
construction joint at the top of one of the columns.
June 13 at 7:48 a.m. - BPA’s Project Administrator to FIGG’s Project Manager and
MCM’s Project manager with copy to BPA’s Senior Project Engineer:
“Please make sure we have FIGG blessing for the construction cold joints
treatment, my personal experience is that a bonding agent will be a reliable
way to good because proposed method is not easy to do (column with steel)
and achieve good results.
400-9.3 Preparations of Surfaces: Before depositing new concrete on or
against concrete which has hardened, re-tighten the forms. Roughen the
surface of the hardened concrete in a manner that will not leave loosened
particles, aggregate, or damaged concrete at the surface. Thoroughly clean
the surface of foreign matter and laitance, and saturate it with water.”
June 13 at 7:56 a.m. – FIGG’s Project Manager to BPA’s Project Administrator and
MCM’s Project Manager with copies to BPA’s Sr. Project Engineer and others at
FIGG:
“We have had previous communications with MCM regarding this topic and
the FDOT specification referenced below was to be followed. Let us know if
you have any further questions.”
June 13 at 8:04 a.m. - BPA’s Project Administrator to FIGG’s Project Manager:
“Thank you.”
6-16
The NTSB Factual Report in Section 20 incorrectly
states that the above e-mail chain was strictly
limited to a discussion of construction joints at the
columns on the south abutment (south landing
bent). A review of the e-mail text shows that this
is clearly not the case and that the discussion
was regarding the treatment of all concrete
construction joints. MCM’s June 12 e-mail at 10:15
a.m. transmitting FIGG’s instructions to follow FDOT
Specification 400-9.3 was prior to BPA’s e-mail
at 10:37 a.m. that included a sketch of the south
landing bent and did not mention the south landing
bent in the text.

RE: Miami Pedestrian Bridge, Part XIII

[cough] !!! TORSION !!! [/cough]

An ominous foreboding from VVIETCIVIL.





This relates directly to the structural collapse as follows:








As it turns out, the structure is primed for torque induced stress between 11/12 and deck/diaphragm:






With regards to the weak surfaces forming the pocket, also note the blue conduit (previous image) that occupy significant surface area on both sides of 12. Also the shear plane skips laterally from the white tubes to the coil rebar around the PT duct. In short, an abundance of inclusions have created a nightmare scenario.





The rotational forces also put to rest any idea of shear friction as that would require a reliable base which is not provided by way of the slab or by way of 11 as they are both inclined to move opposite and apart from each other. Even a breath of movement eliminates this as a structural design element.

The #6 and #7 rebar ties between diagonal 11 and the slab provide drag and distract from the true nature of the design shortcoming. Unfortunately all of the forces carried by 11 are funneled into 12, and I don't believe that was intended.


Edit: my previous posts (amoungst others) on this page, 23 Sep 19 00:09, and on page XII, 25 Jul 19 02:18, provide more development on this theory, including the lower PT rod as a control mechanism.

RE: Miami Pedestrian Bridge, Part XIII

Vance Wiley (Structural) - Email exchange. Thanks for that, I missed it somehow but why isn't Structural Technologies included? What was the distribution list? A more standardized RFI procedure would have notified all parties working on the project.

RE: Miami Pedestrian Bridge, Part XIII

Quote (Sym P. le (Mechanical))

As it turns out, the structure is primed for torque induced stress between 11/12 and deck/diaphragm:
I think you are forwarding a post from somewhere else.
The comparison of the end of the structure with diagonal cracking was addressed by the EOR as caused by vertical load. He provided calculations that substantiated that and predicted the cracking. Vertical load seems plausible, as the direction of cracking is reversed on the two sides. Note the pictures below the OSHA image are same pic reversed.
The bearing pads were checked for level before the thing was lowered the last inch to ensure level bearing.
I need some help here - I do not see what would cause a lateral torsion (east-west).

RE: Miami Pedestrian Bridge, Part XIII

Vance Wiley, thank you for the thank you.

The Mad Spaniard, thank you, I was hoping someone familiar to the FDOT Specifications and general interpretation would comment.

Both, thanks for the discussion on the FDOT specification topic.

To me, FDOT 440-4.3, is very arbitrary. There is no mention to the degree the joints are to be roughened, it would seem that the degree of roughness should still be defined in Construction Documents. An email telling the Contractor to roughen the joints per FDOT Specifications, in my experience would not have been very effective. My experience, Specifications are not much used after submittals. Sketches of all conditions and all plan groups; substructure, superstructure, etc., issued in a formal RFI to all parties would been much more effective. Many Contractors do still keep a working set of Construction Drawings on the job site and such sketches are taped and pasted into the working set of drawings. At least then it had a chance of being caught.

I have always preferred to spec the surface be roughened while the concrete is still plastic as opposed to mechanically roughening after it is hardened. I can see why some additional treatment after might be desirable, especially for bridge structures where good adhesion is desired for environmental resistance reasons, methods exist that are much easier to execute than chiseling, if the desired roughness profile already exists.

RE: Miami Pedestrian Bridge, Part XIII

Quote (Vance Wiley)

I do not see what would cause a lateral torsion (east-west)

I'm not indicating lateral torsion, it is longitudinal (the axis of rotation is lateral through the lower diaphragm, centered on the 8 heavy rebar). Consider 12 extended to the base of the diaphragm with 11 tied into it. Both are free of the deck. You can push/pull the top of 12 longitudinally and torque the diaphragm. This is the freedom of movement when the weak surfaces let go.

I'm saying the the EOR erred in his design process and in his evaluation of the message that the cracks were giving. I have not expected it to be popular but eventually people might wake up.

Regarding the reversed images, I took the images from the posted and linked slide and applied them to this situation. The torque from 12 will impart opposite shear patterns to the left and right connections with the diaphragm. These are mistaken as punch out, shear stress, vertical load etc. but I believe my presentation is compelling. All things considered, this structure was very tough to bring down.

This is not to disregard the plethora of stress loads that the node and surrounds experienced but the signature torque failure pattern has not been addressed.

My previous posts (amoungst others) on this page, 23 Sep 19 00:09, and on page XII, 25 Jul 19 02:18, provide more development on this theory, including the lower PT rod as a control mechanism.



RE: Miami Pedestrian Bridge, Part XIII

Retensioning the rods, where do I begin, several threads ago I posted a comment to similar to, "I hope FIGG's decision to retention the PT rods in Member 11 went beyond, 'Seems like it was performing better before the rods were detensioned, lets put it back on'". Reading the FIGG employees' post incident NTSB depositions, it would appear not.

Pate, "..., it seemed prudent to try to get back to that state of stress that we had when things were good."

First, the extent of the cracks had changed significantly since the bridge was setting on piers with rods tensioned.

Second, the state of stress changed significantly after the rods where detensioned, restoring the state stress would seem a long shot at best.

Third, observation of the cracks at the end the deck would indicate that the top bar, or north bar in some reports, could not be reliably restored, pulling on the opposite side of the cracks would only further fragment the concrete.

Last, at the angle of Member 11, more shear is created than friction resistance, even at shear friction coefficient of 1 for roughened concrete.

RE: Miami Pedestrian Bridge, Part XIII

Quote (hpl575 (Structural))


Last, at the angle of Member 11, more shear is created than friction resistance, even at shear friction coefficient of 1 for roughened concrete.

Great point. Not only does the angle result in more sliding than normal force from tightening the PT rods, the normal force is only 60% effective or 1/0.6=1.66 times worse than the angle of 31.8 degrees. Using the tan of 31.8 degrees as 0.620 and multiply by 0.6 = 0.37 vert effective (to use coeff of 1.0)with a horizontal component of 1.0 horiz sliding created. That looks like a 20.3 degree angle. So one PT rod at 280 kips X cos 20.3 = 262 kips effective sliding. Two PT rods - wow.

Of course at the time the EOR thought he had a coeff of friction of 1.0 to work with.
Great post.

RE: Miami Pedestrian Bridge, Part XIII

The diaphragm really isn't a torsional member. What you are seeing there is a punching shear. However, what it comes down to is compression struts and tension loads on the rebar. There are only compression and tension stresses. Shear stresses are tensor components of the principle compression and tension. Torsional is a compression strut twisting around the outside of the torsional member and rebar resist the tension components (I am taking some liberties and simplifying here to a certain degree).

You can argue that there is a bit of an eccentricity between the tension load of the cables/rebar and the compression strut of #11 but this doesn't amount to a torsional member as we think of them. You can resolve the forces into compression struts and there is no need for torsion. A cracked torsional member in concrete tends to be far more flexible than the assumed elastic member. Once you have a torsional crack and there are other stiffer load paths, all the torsion is relieved. It is like wood-armer moments in a slab. You can ignore the torsion disappears and redistribute the torsion bending into bending moments.

Also, if this was a torsional moment, you need a resisting torsional force at the other end of the torsional member.

RE: Miami Pedestrian Bridge, Part XIII

I don't think it is a good thing for interface shear (AKA shear friction) to be ignored in the Australian code. Interface shear is a real think in any concrete structure. You can't wish away. Even in a monolithic pour there is interface shear (the mu values and cohesion just have higher values). I don't think it is correct to say the Australian code won't allow it. It doesn't account for it which is worse. Other countries don't ignore it. It is a mandatory check and there are decades of experience using the code equations.

RE: Miami Pedestrian Bridge, Part XIII

Quote (Sym P. le (Mechanical)16 Oct 19 21:31 Quote (Vance Wiley) I do not see what would cause a lateral torsion (east-west) I'm not indicating lateral torsion, it is longitudinal (the axis of rotation is lateral through the lower diaphragm, centered on the 8 heavy rebar).)



I see. And to an extent I agree. Here is a quote from earlier:

Quote (Vance Wiley (Structural)15 Oct 19 03:24
So how did it "root" out the section below the top of the deck? Did the top of 12 get pulled south as node 10/11 dropped and torque out the deep sections over the 8" sleeve?)

That reads a lot like your thoughts.
I posted a spreadsheet study of the shortening of the canopy due to dropping at node 10/11. The top of 12 definitely gets pulled south as the collapse proceeds.

RE: Miami Pedestrian Bridge, Part XIII

We have had many discussions about shear friction theory on this site. The early proponents of the theory acknowledged that the values obtained in tests were a combination of shear and dowel action, but were not able to separate the contributions of each. Shear friction is said to result from passive clamping forces, and that is where the logic falls short. For a clamping force to develop, there must be elongation in the reinforcement normal to the joint, and that elongation must be accompanied by opening of the joint.

An anomaly is that the clamping force is based, I think, on all the developed steel crossing a joint at right angles, but surely only the steel in tension can contribute.

Australia has its share of building issues, but I don’t believe any have been caused by the intentional omission of shear friction as an accepted method.

RE: Miami Pedestrian Bridge, Part XIII

hokie66

ACI accounts for rebar at an angle to the shear plane by resolving for the perpendicular force. Which brings up an interesting question as to how many different shear planes should be considered in design. Typically it would just be the cold joint which is easy to locate. However in something like this truss where it appears the failure plane was partly in the cold joint and partly through the monolithic pour the critical shear plane is much less obvious. When you ad in the drain pipe, conduits and all the other pieces and parts in the joint the complexity jumps up to whole new level. Looking at the picture earlier in this tread showing the joint with all the rebar, conduits, etc. in the forms makes we wonder if in design they ever had a good sense of how crowded this area would be. I recall the engineer considered the 4" conduits and drain pipe but I wonder if all the other conduits and obstructions were known at the time of the design.

Hindsight is 20-20 but looking at the picture of the actual joint I wonder if anyone on the engineering team thought if all that congestion was appropriately accounted for.

RE: Miami Pedestrian Bridge, Part XIII

2
I've been following this since the start, though I'm no structural designer.

My thoughts are that when you get all this level of detail it can be easy to miss the big picture.

I haven't read all the recent docs in great detail, but the key issues which seem to be in danger of getting lost.

This bridge was a one off strange design, but seems to have been designed using "standard" methods and analysis.
It has different sized top and bottom chords.
It's a concrete truss.
The truss design is asymmetrical and appears to lead to out of balance forces which no one other than me seems to care about. I think they might add to the compressive force seen by member 11 and possibly also add to a bending moment on members 1 and 12.
The design of the temporary construction mode does not seem to have received the full level of attention that it should have done.

How the bridge was transported gave rise to potential damage - Never clear how exactly they worked out 0.5 degrees twist was ok but 0.55 wasn't. This was a large rigid, brittle beast of a thing to move across an undulating surface.

The presence and movement during the design of a whole bunch of tubes, large and small into the node area may or may not have been included in the final calculations and analyses - some earlier drawings have the two larger vertical tubes some distance away from member 12, but ended up right next to it. Whatever they appear to be a key factor in reducing strength, increasing congestion and complexity and their impact should be highlighted.

Not withstanding any of that and the construction joint issue, the bridge survived for three days slowly failing. For the PE and the design firm to witness this and not be man enough to say something's wrong, we need to stop is nothing short of criminal (IMHO). The other parties will look to the designer for guidance and re assurance. To provide that reassurance on what seems to be a wing and a prayer about "well why don't we try re-tensioning it and see what happens" seems to be being swept under the table and I'm not surprised it doesn't seem to figure much in the Figg document. It should.

There are many many lessons to be learnt from this collapse but it can be lost if some of the parties are trying to focus in on one particular aspect ( this construction joint / friction factor) and not the big picture. It may well have been a contributing factor, but as other have pointed out - this was a very congested area and as a designer you always need to consider exactly how you're going the build the bloody thing safely and effectively. Then when you see it not working in practice, stop and re consider, don't just keep going.

The NTSB final report awaits.

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

RE: Miami Pedestrian Bridge, Part XIII

Quote (LittleInch (Petroleum)17 Oct 19 14:33 I've been following this since the start, though I'm no structural designer. My thoughts are that when you get all this level of detail it can be easy to miss the big picture. This bridge was a one off strange design, but seems to have been designed using "standard" methods and analysis. It has different sized top and bottom chords. It's a concrete truss. The truss design is asymmetrical and appears to lead to out of balance forces which no one other than me seems to care about. I think they might add to the compressive force seen by member 11 and possibly also add to a bending moment on members 1 and 12. The design of the temporary construction mode does not seem to have received the full level of attention that it should have done.)

Your entire post is quite good. So many points are so easy to agree with.
While the truss is asymmetrical and that causes internal loads not immediately evident, there do not appear to be any issues there that are beyond current engineering knowledge as far as analysis goes. The flatter angle of member 2 compared to member 11 causes thrust into the canopy and tension in the deck that overwhelms or strongly influences expected connection loads and member loads over the southmost 3/4 of the span. The reverse angle of member 3 causes unusual loads in the members and joints in its zone of influence. But these can be predicted and supposedly dealt with. As you correctly note, this is a one-off design, and these geometry issues complicate the design, as does the choice of materials used, the need to correctly deal with connections in concrete having sharp angles between web members, the mixture of PT and normal reinforcing in adjacent members, and strain compatibility. These combine to become major challenges in this structure.
It should be noted the truss geometry is dictated by the visual design intent.
FIGG has proposed some visibly beautiful towers to support a proposed cable stayed skyway structure over congested city streets in another Florida city. While beautiful, with twists and partial curls and tapers, the towers appear at a glance to create unnecessary engineering challenges. This pedestrian bridge also created unnecessary engineering challenges. Apparently many of those were not recognized.

RE: Miami Pedestrian Bridge, Part XIII

There are three important questions.
  1. Why did the bridge collapse?
  2. Why were workers killed in the collapse?
  3. Why were uninvolved people killed in the collapse?
Question 3 is (in my mind) the most important. Why wasn't the road closed? Why were there cars under a clearly failing bridge? This is the question that should lead to criminal charges (for negligence at least AFAICT).

Question 2 is a matter for OSHA. What training was missed for the workers not clipped in? What oversight by the foremen was skipped? Etc, etc. Also possible criminal charges here, but less importantly. It's a dangerous occupation, the PPE was provided, some of the fault is the workers'. (Some of the workers injured were in situations where the PPE wouldn't help, that's a separate issue.)

Question 1 is the least important, but the most interesting. Why did the design fail? Could the architectural design have been implemented safely? Etc, etc. Most of the discussion here focuses on this question, which is appropriate since this is an engineering forum and it's the engineering question.

RE: Miami Pedestrian Bridge, Part XIII

The simple answer to #2 & #3 appears to be because FIGG didn't believe the bridge would collapse.

RE: Miami Pedestrian Bridge, Part XIII

4

Quote (RVAmeche)

The simple answer to #2 & #3 appears to be because FIGG didn't believe the bridge would collapse.

I would phrase it differently: Instead of "FIGG didn't believe the bridge would collapse," I'd say they did believe that the bridge would not collapse. There's a subtle but important semantic difference; the former features a passive lack of belief and the latter has the active presence of belief. A belief that was not founded in an accurate assessment of the status of their structure.

RE: Miami Pedestrian Bridge, Part XIII

hpaircraft (Aeronautics) I agree, there seems to have been a profound ignorance as to what the cracks were telling those who actually examined them on site. Even the photos display a lack of experience and little in the way of increased skill or thought given to better documenting the events.

RE: Miami Pedestrian Bridge, Part XIII

That's a really interesting tweak. I imagine lawyers would prefer that phrasing

RE: Miami Pedestrian Bridge, Part XIII

Quote (epoxybot)

there seems to have been a profound ignorance as to what the cracks were telling those who actually examined them on site. Even the photos display a lack of experience and little in the way of increased skill or thought given to better documenting the events.

This is a profoundly insightful comment. And of course, it is precisely what Denney Pate expressly stated in the voicemail he left behind for the FDOT.

RE: Miami Pedestrian Bridge, Part XIII

Thank-you Vance and Earth for your replies.

Quote (Earth314159 (Structural) 17 Oct 19 03:32)


Earth, I see your response as a blinding example of tunnel vision. In a conventional situation, exactly as you say, but in this deconstruction, my lack of academic rigor may leave me more room to explore.

Quote (Earth314159)

... need a resisting torsional force at the other end of the torsional member

The deck/diaphragm duo compete with the 11/12 duo. Once the 11/12 node is free of the pocket, the structure becomes indeterminate and there is a tremendous amount of torsion as 11 pushes 12 north while the slab rotates down.

Quote (Vance Wiley (Structural) 15 Oct 19 03:24)

... how did it "root" out the section below the top of the deck?

I've questioned this also but I now believe that the 11/12 node became detached as a whole thus rendering this point moot.

Quote (Vance Wiley (Structural) 17 Oct 19 04:08)

The top of 12 definitely gets pulled south as the collapse proceeds.

FWIW, the top of 12 has to move north approx. 4" at the initiation of the collapse (Part XII Sym P. le (Mechanical) 21 Jul 19 02:54)

So to bring my thinking around, 11/12 node is cracked out of the pocket by the lower PT rod, torsion takes over and blows apart the whole bottom end of 11 and 12 ???? 11 slides down the PT rods in a controlled fashion while 12 drops vertically.

RE: Miami Pedestrian Bridge, Part XIII

Quote (Sym P.Le)

The deck/diaphragm duo compete with the 11/12 duo. Once the 11/12 node is free of the pocket, the structure becomes indeterminate and there is a tremendous amount of torsion as 11 pushes 12 north while the slab rotates down.

A structure does not become indeterminant or more redundant as a load path is lost. It becomes less redundant and closer to a determinant structure.

The torsion is not large. There is an equal and opposite force from the deck and PT cables in the same line that resist the horizontal component of the strut force. Once the #11 and #12 lest loose from the pocket, you have no more load paths. It is a failure. To create torsion, you need a couple and there is no couple.

Quote (Sym P. le.)

Earth, I see your response as a blinding example of tunnel vision. In a conventional situation, exactly as you say, but in this deconstruction, my lack of academic rigor may leave me more room to explore.

Sorry, I guess I let facts and evidence get in the way of my analysis.

RE: Miami Pedestrian Bridge, Part XIII

Reinforcing in Member 11
We see photos of pre-collapse cracking in member 11. WJE reports member 11 triggered the collapse by failing near node 11/12 after node 11/12 slipped on the deck.
On sheet B-39 of FIGG's RFC drawings Detail A-A shows a 21 X 24 section with 10 - #7 longitudinal bars and a note "Members with no PT bars". So the interior web members which are in compression have 10 - #7 longitudinal bars. Interior members in tension have PT rods to resist the tension and nominal reinforcing to anchor and retain the concrete.
On sheet B-40 we see Detail B-B as a detail of a section 21 X 24 with 8 - #7 longitudinal reinforcing bars and what I assume are PT rods in ducts, and the note "Typ all members with PT bars".
So when PT bars were added in member 11 for transport, how did the reinforcing detailers interpret the drawings for member 11? Member 11 has PT rods - which is it? Detail A-A or B-B?
It may not make much difference in capacity, but I find it interesting that the 21 X 24 section with the greatest load by far has 8 bars and the far less loaded compression diagonals in the interior have 10 bars of equal size.
The fact is FIGG drawings, ELEVATION on B-40 clearly notes for member 11 " 7S11 (E F)" and there are two arrowheads so I guess that means 2 bars each face.
That would be a total of 8 bars - as Detail B-B requires. So FIGG also fell into their own trap - the highest loaded compression member of 21 X 24 dimension has less reinforcing than all others because PT bars were placed there to resist forces from transporting, and the PT was not really PT because it was intended to be released by detensioning.
In looking at member 4 on the ELEVATION on Drawing B-39 we see Detail A-A cut and the note "7s04 (E F)" and two arrowheads and 4 lines which indicate reinforcing in other locations on the drawings. Are there supposed to be 3 bars in the 24" side as shown on A-A or 2 bars if we count the arrowheads or 4 bars if we count the lines?
The coordination and review exhibited in the drawings is lacking in this case and it seems to have contributed heavily to the failure of member 11.





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