Miami Bridge Collapse

There is a "high early strength" concrete, but I don't know the specs and restrictions. I also don't know if they used it.

I have read that the cracking was observed visibly, and that it was observed audibly. Which is true (possibly both?) I don't know. Another detail to await in some future explanation.

do we know what activity the audible cracks corresponded to ?

I've only heard/read a bare mention of audible cracking--no details.

Back in my early days as a materials tech, we would take 6" diam. concrete test cylinders for the measurments of 3-day, 7-day, and 28-day strengths, to confirm or deny that the concrete batch mixes were performing as intended...

Since the entire structure is ''lost'', I would be very surprised if additional test cylinders were not cored to measure the actual concrete strength at the time of collapse, as part of the forensic portion, of the (accident) investigation...

There should also be test specimens of the tension cables, at least, as well...

Numerous posts refer to cracks in the concrete, but although there should not be any, does this weaken the structure to the point of collapses.

To me a crack is indicative of a separation of components, a gap consistent with a shear or stretching force, against which concrete is known to have little resistance.

The stretch strength is provided by the steel reinforcing anchored under tension. The concrete provides compression strength and stiffness.

When in place, and in working order the structure might have been prone to sagging or bending - but certainly not collapsing.

Since it did collapse, assuming the steel had adequate tensile strength and was anchored and under tension, I guess it was concrete crumbling under the compressive force - maybe due to not being left long enough to cure.

Something that can be blamed on the site crew.

Does there have to be a price tag on everything (rhetorical question - NNTR)? Much is made of the largely-irrelevant <...$8.3 million...> price tag for this bridge in the media.

Of much more relevance is the loss of life, the injuries, and the devastation to the families and friends of those involved in this tragedy.

One presumes the price tag doesn't include the investigation, the subsequent legal maelstrom, the clean-up, the redesign, rebuild and the compensation costs from this tragedy (rhetorical question - NNTR)?

Which is why it is irrelevant.

_{"Angry" of Dodman's Lane.}

Short and sweet, but post #8 accounts for lives lost and $ yet to be spent.

This will be part of the curriculum of a Structural Engineering course, just like The Hyatt in Kansas City and Tacoma Narrows Bridge.

GA from me, with the reservation that decision-makers such as politicians, business persons, bureaucrats, etc., don't generally take a single structural engineering class...

Can someone please explain to me why, in this early PT bar drawing, strut 2 is pre-tensioned, but strut 11 (implicated in the collapse) is not? I realise the structure is not exactly symmetrical, but surely both members are heavily in compression? (at least before any other section of the bridge is in place)? My back-of envelope suggests 11 would be under about 940 tons compression.

Taking all that you say at face value as being correct, and since you have focused on a couple of design weak points you attribute to the collapse, and assuming the technology you have have used for your analyses was available to the original designers, would these points have been spotted as 'weaknesses' and if so what could have been their decision logic to allow them. 'Cheap or pretty' as opposed to entirely safe.

Or being cynical, was the construction based on a buck-passing protocol that ensured that whatever choice was made the 'management' could not be blamed for failure if it occurred.

"Taking all that you say at face value as being correct, and since you have focused on a couple of design weak points you attribute to the collapse, and assuming the technology you have have used for your analyses was available to the original designers, would these points have been spotted as 'weaknesses' and if so what could have been their decision logic to allow them."

I doubt that those concerned spotted these weaknesses because presumably they would not have allowed them if they had.

However, they obviously didn't allow for weaknesses which they didn't spot - by insisting on rigorous testing - to destruction - of models and full-size new components such as the joints in their innovative I-beam - because any such rigorous testing would have highlighted those weaknesses or, if the design had been good, would have proved the performance limits of the design.

If we neglect to test for any weaknesses in an innovative design then we probably won't spot them.

That's the logic of rigorous testing and there is no logic in not testing. Not testing is seriously negligent and possibly criminally so.

" 'Cheap or pretty' as opposed to entirely safe."

The "cheapest" thing to do would have been not to build any bridge at all but rather to enhance the safety of the pedestrian crossing only 25 yards away - with better lighting, good quality CCTV and automated reporting and fines of owners of vehicles which commit traffic law violations at the crossroads - speeding, jumping lights etc.

https://s25.postimg.org/58dres9bz/FIU-bridge-collapse-crossroads.jpg

I believe that one of the reasons for the FIU building the bridge was to use it as a signature "pretty" landmark to advertise the federal funding which the FIU's professor of bridge engineering had been awarded as an official "Champion of change in transportation" - originally by the Obama administration in 2015.

US Department of Transport - 2015 Champions of Change in Transportation

So the bridge was to help boost their profile, to help to recruit more students and get still more funding, to climb up their ivory tower and claim to have all the answers, for a price.

"Or being cynical, was the construction based on a buck-passing protocol that ensured that whatever choice was made the 'management' could not be blamed for failure if it occurred."

Well the management can be blamed, if we want to blame them and I, for one, do.

1. Mark B. Rosenberg, FIU President

2. Kenneth Jessell, FIU Vice President and Chief Financial Officer

3. Atorod Azizinamini - Chair of FIU’s Civil & Environmental Engineering Department and director of FIU’s Accelerated Bridge Construction University Transportation Center

Project contractor organisation chart.

https://s25.postimg.org/up8qilwfz/FIU_bridge-collapse-organistion-chart.jpg

Project contractor company presidents

4. Linda Figg, FIGG President

5. Jorge Munilla, MCM President

Project contractor managers

6. Dwight Dempsey, FIGG, Project Design Manager

7. Rodrigo Isaza, MCM, Project Construction Manager

8. Eddie Martinez, MCM, Project Safety Manager

National Society of Professional Engineers

9. W. Denny Pate, FIGG, Lead Technical Designer and Engineer of Record

10. Robert Murphy, MCM, Project Director

* Dwight Dempsey, FIGG, Project Design Manager

11. Kenneth Heil, FIGG, Design Quality Manager

12. Manuel Feliciano, FIGG, Bridge Engineer

13. Kristian Navarro, MCM, Project Engineer

US Army Corps of Engineers

* Eddie Martinez, MCM, Project Safety Manager

* Kristian Navarro, MCM, Project Engineer

14. Carlos Hernandez, MCM, Quality Control Manager

More ...

I'm including the authorities in the "management" of this bridge collapse. How much you want to blame them probably depends on your politics.

Elected Representatives of the People

15. Mario Diaz-Balart, U.S. Representative and Chairman of the House Appropriations Subcommittee on Transportation, Housing and Urban Development.

16. Orlando Lopez, Mayor of the City of Sweetwater

17. Anthony Foxx, Obama's Transport Secretary

(Mug-shot posted above)

State and Federal Authorities

18. Rick Scott, Governor of Florida

19. Donald Trump, President of the United States

....the answers to your 'what' questions of the collapse may soon be forthcoming - but it will be a lot harder to get answers to the 'who' questions.

Current (Standard Practice) prescribes that, within metropolitan areas as big as Miami, all construction (assembly) is ''inspected'' by, and signed-off by, City Inspectors, or their (duly designated representatives), and that (building components) fabricated off-site are inspected by said (city inspection agents) and/or by the fabricator's own inspectors, both of which are positions requiring special licensing and/or certification(s) via State Licensing Board(s) and/or applicable (certifying authorities)...

In an up-to-date (City Public Works Department), (best-business and/or best-management practices) prescribe that construction photos should be on-file, and one, or more, videos may be on-file...

Mass (finger-pointing) is certain to follow...

"Mass (finger-pointing) is certain to follow..."

And $$$$.

Neither the Governor of Florida nor POTUS oversee the construction. Nor would any Congressman or Senator. The field engineers..... ooops, there were not any..... so likely the laps to lay this upon are with the construction firm & Florida State Dept of Transportation. From all accounts, the changes they made to locations of the support "trucks" was NOT approved by either the non-existent field engineers, nor did they seek prior approval to this MAJOR engineering field change. Whoever approved lanes of traffic flow for convenience over safety needs some prison time, but that is just my pinion.

Irony....... look up the specialty of the very University that this bridge was connecting to opposite side of roadway.

SCORE: CYA !

But cracks were found in the deck before moving it and _ignored_

the question i still want answered is , did the transport “trucks” have compensation to adjust for different gradients in the road surface during the move , or did the different gradients twist the bridge during transit causing more structural failures ?

Why are you insinuating yourself into American politics?

It’s just to attract rednecks for the 2015 redneck census

yes we know it’s 2020 now but they are a bit slow

This FIU news web-page identifies the main management culprits

FIU NEWS - First-of-its-kind pedestrian bridge “swings” into place

with assumption being made if this is correct,... then it falls to engineering.

I’ll wait for the investigations.

''GA'' for Post #109 from me for a Goooooooooode Answer (and texts, and pics, and especially links.)

(It was at least theoretically possible that (FIGG, et al) had started to post-tension the diagonals in preparation for the installation of the stays above the canopy...)

(Even so, it was waaaaay too soon to start that step of the structure (assembly) before the entire bridge/truss and tower were fully in-place, and deemed safe enough to continue with installing the (super-structure) stays...)

(So, that even that excuse is even inexcuseable...)

Good ''pinned'' connection to resist tension in the horizontal direction, but how is compression conveyed from a rebar-and-stirrup ''cage'' to the ''H-beam(?)'' diagonal portion of the linkage?...

The tensioning cable doesn’t serve as a compressive element , the concrete truss with rebar accepts compression loads ,but the tensioning cable adds compression to the truss which increases the strength of the concrete.

"Good ''pinned'' connection to resist tension in the horizontal direction,"

Thanks!

"but how is compression conveyed from a rebar-and-stirrup ''cage'' to the ''H-beam(?)'' diagonal portion of the linkage?..."

Before I consider "how?" I should ask "why?" because I don't agree that engineers should ever use rebar-and-stirrup "cage" concrete columns for a bridge truss, like FIGG have, I think, mistakenly designed for the collapsed FIU pedestrian bridge.

Instead, I'd recommend metal-only for a bridge truss's structural elements.

The obvious way to go to complete a revised design of truss member #11 (M11) starting from my joint design would be to use H / I steel beam for the full length of M11 and for all truss members which are in compression.

For a design which specifies a concrete surface for the truss members then a superficial, non-structural concrete facade could be added around the truss members easily enough.

First of all, thank you for understanding what I actually said.

Second of all, rebar-stirrup-cage and pre/post-tensioning are standard ways to construct beams and columns, etc., but the application of mixing methods can clearly cause unanticipated problems to occur, as unfortunately demonstrated recently...

I don't know of any American Companies that do (linkage-type) constructions any more, but maybe some will have to re-visit that option now...

I think that even if it had been completed as designed, it would still not have withstood the first major hurricane...

I think that having pre/post-tensioning in multiple directions at single joints creates additional complexity, and then, sufficiently detailed modeling and testing is called-for, but did not happen... (ie: the ''innovative'' design was not thoroughly ''proofed'' in the first place...) (ie: overlapping bousinesque-type shredding...)

It appears from the structural details provided that the central tower was to be off-set enough to be out side of the bridge/truss, which woulld have imparted unbalanced twisting loads in high winds... (another intrinsic weakness designed-in ?...)

The more that I see and read about this failure, the more that I'm convinced that the support column and suspension stays were part of the original design, and for reasons of cost, time or other factor(s), was not included without any significant changes to the span that failed.

Even so I doubt that the structure would have been able withstand a Category 5 hurricane as noted in early news reports.

As someone I used to work with would say, the job was done "cheap and dirty".

I think the fake super structure would have placed an additional load on the bridge and weakened it even more. The pipes rather than cables would have created a lot of windage which in turn would have caused a lot of vibration even in moderate winds. This bridge is "a fail" in every respect. It wasn't even low cost as the payments to the lost and injured victims may exceed the initial cost of the bridge construction. Even the adage "you get what you pay for" doesn't apply as a tremendous sum of money was spent on a concrete rubble pile that had the additional cost of clean up and disposal. Crossing the street is just as problematic as before the commencement of the project.

You're right. Completeing one, relatively simple, coherant, comprehensive, design solution, with competent construction, from start-to-finish, would have been the much better, and more affordable, choice. GA from me.

I think vortex shedding, windage vibration, is worst at low wind speeds. I have seen several galloping phone and electric lines.

In Sheboygan Wisconsin an insurance company has a huge flag for over 30 years....

The "world's tallest tribute to freedom," is a 400-foot tall flagpole waving an American flag that's 7,200 square feet. The pole is about 100 feet taller than the Statue of Liberty, its flag is four stories tall, and it stands in an unlikely location: an insurance company campus in Sheboygan, Wis.Aug 19, 2014

this was after the first attempt at erecting a replacement new pole in 2008, they wanted one out of stainless steel because they were spending $10,000.00’s of dollars a year to paint the flag pole... well when they were installing it in 2008, they didn’t count on vortex shedding, I was a couple of miles away, and what I thought was a the crane working it,... I was thinking, Christ that crane is going too fast,... only to notice it when I got closer, it wasn’t the crane moving, it was the flag pole...

(I couldn’t find any information on it... insurance company probably washed the net of any bad publicity)

WoW!... Any news photos at the time?...

No actual reports of (wind shedding) in Miami, Florida, at the time of the subject accident, though ? ...

That pole was arcing a good 30 degrees back and forth, I think the website was washed of all that bad publicity

That's why (towers) have multiple guy-wires...

As they were originally replacing it,... it was 'boasted' in the papers, that a firm that design the towers for wind generations was doing the design for this flag pole. The client, (Insurance firm in this case) wanted a clean looking pole, so no spiral flites on the pole like they do to stacks to avoid the formation of Kármán vortex.

The pole design from 2008 also doesn't have these, but it is a stout pole.

I also want to reiterate, that a huge flag has been at this location for over 25 years.

Why do we keep being reminded that, at some point, (Mother Nature) will be a very tough (negotiater) ?...

A link with more information was posted on Eng-Tips.

Florida Department of TRANSPORTATION - Denney Pate signed and sealed FIU bridge construction plans - 2016 & 2017

https://cdn2.fdot.gov/fiu/13-Denney-Pate-signed-and-sealed-FIU-bridge-construction-plans.pdf

From this new information ...

https://farm1.staticflickr.com/885/41562757232_e2b98846b2_b.jpg

- which is completely different from the PT bar tensioning requirements suggested in the proposal pdf, but there still doesn't seem to be any obvious method employed in calculating the PT bar tension from the maximum tension force in the truss members that I calculated.

https://farm1.staticflickr.com/940/40892067954_daf903b3c5_b.jpg

My method was somewhat arbitrary - recommend a PT bar force equivalent to about 130% of the maximum tension force that member experiences.

I don't know what the recommended practice for setting PT bar tensions is - or if indeed there is a recommended method - how much in excess of the tension required is the bar tension set to - is but my guess is that neither does W. Denny Pate. Or perhaps he is using a method but he starting from a radically different truss calculation?

Analysis of the newly released FIU bridge plans has revealed that my earlier suspicion that member 11 was dangerously under-reinforced has been confirmed, to such a degree that the collapse of member 11 under the compression load after the bridge was placed on the piers but before destressing was to be expected.

The first point to note from the plans is that the plan's P.T. bar tensioning begins once the concrete reaches a strength of 6,000 psi or more, as this quote shows -

"CONSTRUCTION SEQUENCE - STAGE 2 - SUPERSTRUCTURE PRE-CASTING

2. AFTER CONCRETE COMPRESSIVE STRENGTH HAS REACHED 6000 PSI, STRESS POST-TENSIONING OF THE MAIN SPAN IN THE FOLLOWING SEQUENCE ..."

Next, I have shown in my post on Saturday 04/21/2018 6:47 PM, #140, that the plans recommend a P.T. bar setting for the 2 P.T. bars in member 11 which together total a P.T. bar tension of 560 KIP.

Together with the results of my truss calculations, also reported again in my post of Saturday, the dead weight of the bridge exerts a tensile force of 304 KIP while the bridge is being transported and a compressive force of 1367 KIP when the bridge is placed on the piers.

Now let us consider what all those forces together in the sequence they were applied mean for the compressive force on the reinforced concrete of member 11.

I have considered 5 different stages, A, B, C, D and E. The bridge collapsed either in stage D or as a result of damage sustained in stage D, so the bridge never got to stage E in good order, sadly, but inevitably given the plan followed.

https://farm1.staticflickr.com/975/41680074411_0753edcc7c_b.jpg

Stage A The concrete has hardened to at least 6,000 PSI and so post tensioning is about to begin but at this stage the mainspan is still resting on the ground, so there are no troublesome forces on member 11, no PT bar tension, no bridge dead weight and so the reinforced concrete is not being compressed very much at all except under its own weight and that of the canopy immediately above it, but we will ignore that for now.

Stage B The P.T. bars of member 11 have been tensioned to the recommended amount - a total of 560 KIP and that tension force on the P.T. bars is being provided by an equal and opposite compressive force of 560 KIP on the reinforced concrete. But the mainspan is still on the ground so not much in the way of dead weight of the bridge to worry about yet.

Stage C. The mainspan has been lifted onto the transporters and now the dead weight of the bridge is exerting an external tension force of 304 KIP on member 11. This has the effect of reducing the compressive force on the reinforced concrete of member 11 by 304 KIP down to 256 KIP.

Stage D, "D" for danger and for "Doom". This is when things take a turn for the worse. The bridge gets placed on the piers and now the dead weight of the bridge is applying a compressive force of 1367 KIP on to member 11. So now the reinforced concrete has to take the full 560 KIP from the P.T. bars plus the 1367 KIP dead weight to suffer a whopping 1927 KIP of compressive force, which is more than the member is able to cope with, especially so if the concrete has not reached is full strength of at least 8500 PSI, as is shown in this table from my concrete column calculator and this bar chart.

https://farm1.staticflickr.com/965/40971810834_4d0153b665_b.jpg

https://farm1.staticflickr.com/966/41680080591_0a81ec9d90_b.jpg

The plans only call for 8 number 7 (diameter 7/8" inch, area 0.6 square inches each), axially orientated reinforcing bars, just barely 1% of a reinforcement ratio for that size of concrete member.

https://farm1.staticflickr.com/975/26813198797_460a49671c_b.jpg

As I have noted there, W Denney Pate's drawings don't draw a section through member 11 with only 2 P.T. bars, only section B-B "(TYP. FOR ALL MEMBERS WITH P.T. BARS)" which draws 8 reinforcement bars. Bars marked here "7S11" are 8 size 7 (7/8" diameter) bars confirmed by the table on page 98, SHEET B-98, SUPERSTRUCTURE REINFORCEMENT BAR LIST AND NTSB images.

It's inadequate reinforcement for the compression before destressing the P.T. bars.

So it turns out that this picture was "the smoking gun" after all, but not for the reason I thought at the time, but because it shows one of the far too small and far too few reinforcement bars, marked "<-B->".

https://farm1.staticflickr.com/830/39875129800_cc2cc3d7f2_b.jpg

Stage E. After destressing is complete. We don't know for sure if this stage was actually reached, but even if it was, by that time the damage was done and the member 11 was failing and bridge had begun the process of collapsing and it was just a matter of time.

One small post for a scientist, one giant leap for forensic engineering.

It’s been 9 months, anyone sighted an accurate report into the cause of this collapse yet ?