Florida pedestrian bridge collapses onto roadway, cars. Was up for 5 days

[realibrad]

they put in the bridge b4 the support?!

so the bridge could support its own weight w/o the suspension.
it just couldn't support any additional weight, like foot traffic?

It was under construction and not taking any additional load. Remember this is FL and we get winds. Bridges are built with the ability to withstand more stress than what is expected under normal use. It needed compression to be complete. It was concrete after all. But no, it was far from done and not taking any traffic.

What it looks like is that the bridge failed from lateral tension forces. It looks like someone had the bright idea of not having supports while it was being built.

 

[Zorba]

https://news.fiu.edu/2018/03/first-of-its-kind-pedestrian-bridge-swings-into-place/120385
Why is there no support post in the middle of the bridge?
$14.2 million and several lives due to someones mistake gone.
Let's see where the pointing the blame on begins.

Links to some pics, drone footage, time lapse and videos during construction and a few days before the collapse.
https://www.dropbox.com/sh/u8qicuu22xmfhg1/AADPXPe1obmF_0JwDpcqLJeMa?dl=0

There is a middle support, on the right side of your image. That is only half of the bridge shown, the other half hadn't been installed yet. Regardless, you don't have to have a mid span pier if the span is built right. 176 feet isn't even that impressive of a span for post-tensioned concrete.

 

[BlueWeasel]

I'm a licensed structural engineer but I've never done bridge design. I'm shocked at the dead load of the 174-foot long bridge section (950 tons = 1,900,000 pounds). That's over 10,000 pounds per foot of span.

Given the span and hurricane load conditions, a bridge made of pre-stressed or post-tensioned concrete makes sense. Pre-stressed/post-tensioned concrete uses high strength tendons that when tensioned, introduce compression stresses into the concrete. Concrete loves compression yet has very little tensile strength. Pre-stressed or post-tensioned concrete allows for much longer spans using less concrete material. But I just don't see enough concrete there to equal 950 tons...the concrete main deck and roof deck don't look thick enough for it to weight that much.

 

[Zorba]

It was supposed to be a suspension bridge.

This is what it was supposed to look like when done.

*Cable Stayed, not suspension. https://www.quora.com/What-is-the-difference-between-the-cable-stayed-bridge-and-a-suspension-bridge

this only costs $14M?!?!

i would have thought $50M+.
guess the new technique does save quite a bit of $.

too bad it didn't work.

$14M is about 10x what a similar concrete or steel bridge would've cost using normal methods, at least in this part of the country.

 

[realibrad]

*Cable Stayed, not suspension. https://www.quora.com/What-is-the-difference-between-the-cable-stayed-bridge-and-a-suspension-bridge


$14M is about 10x what a similar concrete or steel bridge would've cost using normal methods, at least in this part of the country.

You are correct.

 

[SKORPI0]

So the cables that were supposed to support the weight of the bridge were not installed yet when part a section of it was put in place.
Videos showed that vehicles were already passing underneath a few days before the collapse, how safe was that?

schematic of the bridge. Red: collapsed parts, Blue: not installed yet

https://en.wikipedia.org/wiki/Florida_International_University_pedestrian_bridge_collapse

 

[MomentsofSanity]

Apparently engineers found a crack in the bridge two days before the collapse and notified the DOT. They didn’t listen to the voicemail.

http://www.fox5dc.com/news/engineer-reported-crack-in-florida-bridge-2-days-before-collapse

 

[Saylick]

I'm a licensed structural engineer but I've never done bridge design. I'm shocked at the dead load of the 174-foot long bridge section (950 tons = 1,900,000 pounds). That's over 10,000 pounds per foot of span.

Given the span and hurricane load conditions, a bridge made of pre-stressed or post-tensioned concrete makes sense. Pre-stressed/post-tensioned concrete uses high strength tendons that when tensioned, introduce compression stresses into the concrete. Concrete loves compression yet has very little tensile strength. Pre-stressed or post-tensioned concrete allows for much longer spans using less concrete material. But I just don't see enough concrete there to equal 950 tons...the concrete main deck and roof deck don't look thick enough for it to weight that much.

I am pretty confident that the quoted 950 tons is for the whole bridge; only half of the span was installed, and the center pylon looks pretty heavy. I'd imagine the portion of the bridge that collapsed was closer to 300 tons.

*Cable Stayed, not suspension. https://www.quora.com/What-is-the-difference-between-the-cable-stayed-bridge-and-a-suspension-bridge


$14M is about 10x what a similar concrete or steel bridge would've cost using normal methods, at least in this part of the country.

The bridge is NOT a cable-stay bridge in mechanics. It only looks that way for aesthetic reasons. If you look at the detailing of the connection from the cables to the deck, it's held together by a few anchor bolts and is not as robust of a connection than if this were a true cable-stayed bridge. Also, a true cable-stayed bridge would not be self supporting until the cables were taut and properly secured to the deck.

 

[Saylick]

For what it's worth, I posted in one of the Reddit threads to shed some light on what I think happened.

/tldr; the construction workers caused the bridge to fail when they prestressed some of the diagonal members on the bridge. Whether or not the engineer or contractor is liable depends on what instructions where given to prestress the diagonal members.

---

Professional Civil Engineer working in structural engineering here (legally can't call myself a Structural Engineer as I have yet to obtain my license), but figured I clear up some potential confusion and weigh in on a possible cause of failure here:

1) The media seems to have heard the word "stressing" and ran with the notion that the construction crew was "stress testing" the bridge. I personally do not design bridges - I design buildings - but one that you do NOT do is "stress test" a structure while it is open to the public. Any testing of structural components should be performed in a lab or in a closed off area such that if something were to fail, it poses no immediate danger to anyone. In other words, this was not a stress test. The media simply misheard, which is understandable given that they are not engineers. What was really happening is that the construction crew was applying prestress to one of the bridge members. According to Marco Rubio's tweet, some of the prestress tendons' anchorage was loose during construction or transportation and there were orders from the lead engineer to tighten the tendons, presumably to get them back up to their design prestress value. Nothing fishy so far about this.

If you look at the security cam footage of the bridge right before collapse, what you see are the construction crew standing right on top of the bridge with their hydraulic jacking equipment ready to tension the loosened tendon. heck, even one of the dudes in the video quips about how one of the crew got hang time as the bridge collapses.

Normally, for a concrete bridge to fail in flexure, i.e. it fails under bending load, the top chord fails in compression load (i.e. it buckles or crushes) or the bottom chord fails in tension (i.e. the rebar/prestress tendons gave out). Where does the point of maximum flexure occur for a simply-supported bridge like this one? Right at mid-span. The failure did not occur at mid-span for this bridge, which suggests the cause of failure was not due to the prestress tendons giving out along the bottom chord.

If you watch closely, you can see that the failure point of the bridge is right directly under where the crew is standing. Note that their position with respect to the length of the bridge; they are standing at the first intersection of the diagonal brace members of the bridge, not at mid-span. Given that the bridge stood up under its own weight earlier in the day, this seems to suggest that the failure mechanism was related to the work the crew was doing.

Rubio's Tweet: https://twitter.com/marcorubio/status/974486176336957440

2) So what was the construction crew doing exactly? They were prestressing the tendons for the diagonal brace members in the bridge (not the tendons in the top and bottom deck).

Some background info: the role of these diagonal braces (i.e. the truss members) is critical to the structural load carrying capacity of the bridge. To use an analogy, treat this bridge as an I-beam. What makes an I-beam strong and stiff is the fact that its shape allows you to have a deep (i.e. tall) cross section where a lot of the material is at the top most and bottom most part of the section. This results in a section with a high area moment of inertia. Note that intuitively, this only works if you still have some material connecting the top and bottom parts of the beam, resulting in a shape that lends the name "I-beam". Without the strip of material (known as the "web") connecting the top and bottom parts of the beam (called the "chords"), the overall section cannot act as a deeper section and it loses most of its stiffness.

How an I-beam works: https://www.quora.com/Why-is-an-I-b...l-design-come-to-fulfill-its-primary-function

Now, going back to the pedestrian bridge, the diagonal braces are the web and the top and bottom deck are the chords of the "I-beam".

For those who have taken some form of physics or statics class will have some experience designing a spaghetti truss bridge of some sort during their schooling. They know that the diagonal brace that sees the most compression load are the ones at the far extremes of the span. Yes, the exact same one that were below the construction workers. If this brace (or any brace, really) were to fail, the bridge cannot as as an I-beam at the point where the brace failed and it thus loses most of its strength and collapses. Recall that the bridge failed right under the construction workers...

3) Okay, so if the hypothesis is that one of the diagonal braces failed, which one failed? Good question.

It so happens that there's a 2015 version of the design proposal for the bridge floating around the internet: https://facilities.fiu.edu/projects/BT_904/MCM_FIGG_Proposal_for_FIU_Pedestrian_Bridge_9-30-2015.pdf

If you go to page 115, one sees the elevation of the bridge. The span on the left (left of the Pylon) is what collapsed. The span on the right was not put in place yet. So, if we focus just on the left span, the failure occurred right where Member 10 and 11 meet. If you look at the table for PT Bar Requirements (PT = "Pre-tensioning" i.e. prestressing), Member 10 is prestressed while Member 11 is not. This makes sense because you typically prestress concrete when you want it to withstand tension loads. Prestressing a member that is expected to take compression loads will only add more compression load to it.

So which diagonal was the construction crew prestressing? From pictures, it appears that it was Member 11, the one that is supposed to take compression loads. One can spot the hydraulic jack that applies the prestress to the tendon by the teal cylinder. It appears that the jack is oriented in a rather flat orientation (i.e. it's almost horizontal). Between Member 10 and 11, the brace that is more horizontal is Member 11, not Member 10.

Pic: https://res.cloudinary.com/engineering-com/image/upload/v1521183436/tips/MIAMI_3_i7ildl.png

Now, concrete is pretty strong in compression and it takes a lot of compressive force to crush concrete. However, applying compressive force on Member 11 results in Member 10 seeing more tension load, which concrete does not handle well. Recall, Member 10 has prestress tendons for this reason.

Where the mistake could have occurred is anyone's guess but it might boil down to one or more of the following:

• Too much prestress was applied to Member 11, causing Member 11 to fail in compression.
• Too much prestress was applied to Member 11, causing Member 10 and its prestressing tendons to fail in tension.
• The workers accidentally released all prestress in the tendons in Member 11 while prestressing Member 11, causing Member 11 to fail in tension.
• The workers were actually prestressing Member 10 (not 11), and by applying too much prestress, it causes Member 10 to go from resisting tension to resisting compression loads, thus effectively negating it's function as a web member in the "I-beam" and causing the bridge to fail.

4) I'd like to add that the design proposal from 2015 is NOT the final construction documents used to construct the bridge. All information presented in the proposal was subject to change in the final design. Member 11 likely had prestress tendons specified for it in the final construction documents, which means that the construction workers were correct in prestressing Member 11 (assuming they were indeed prestressing Member 11, of course).

Thus, without more information, the crux of the hypothesis remains is as follows: the act of "tightening" the prestressing Member 11 caused either Member 10 or Member 11 to fail, thus taking out the bridge with it.

For more information, you can follow this thread: http://www.eng-tips.com/viewthread.cfm?qid=436595

The engineers there have essentially laid out the arguments I presented above and credit is due to them for fleshing out some of the finer details presented.

 

[Puffnstuff]

Here's the video of the bridge collapsing caught on a truck's dash camera. Traffic was stopped at the moment it happened which is why so many were trapped underneath it.
http://www.miamiherald.com/news/local/community/miami-dade/article205637539.html

This is another camera view from the intersection.

 

[Hans Gruber]

For what it's worth, I posted in one of the Reddit threads to shed some light on what I think happened.

/tldr; the construction workers caused the bridge to fail when they prestressed some of the diagonal members on the bridge. Whether or not the engineer or contractor is liable depends on what instructions where given to prestress the diagonal members.

---

Professional Civil Engineer working in structural engineering here (legally can't call myself a Structural Engineer as I have yet to obtain my license), but figured I clear up some potential confusion and weigh in on a possible cause of failure here:

1) The media seems to have heard the word "stressing" and ran with the notion that the construction crew was "stress testing" the bridge. I personally do not design bridges - I design buildings - but one that you do NOT do is "stress test" a structure while it is open to the public. Any testing of structural components should be performed in a lab or in a closed off area such that if something were to fail, it poses no immediate danger to anyone. In other words, this was not a stress test. The media simply misheard, which is understandable given that they are not engineers. What was really happening is that the construction crew was applying prestress to one of the bridge members. According to Marco Rubio's tweet, some of the prestress tendons' anchorage was loose during construction or transportation and there were orders from the lead engineer to tighten the tendons, presumably to get them back up to their design prestress value. Nothing fishy so far about this.

If you look at the security cam footage of the bridge right before collapse, what you see are the construction crew standing right on top of the bridge with their hydraulic jacking equipment ready to tension the loosened tendon. heck, even one of the dudes in the video quips about how one of the crew got hang time as the bridge collapses.

Normally, for a concrete bridge to fail in flexure, i.e. it fails under bending load, the top chord fails in compression load (i.e. it buckles or crushes) or the bottom chord fails in tension (i.e. the rebar/prestress tendons gave out). Where does the point of maximum flexure occur for a simply-supported bridge like this one? Right at mid-span. The failure did not occur at mid-span for this bridge, which suggests the cause of failure was not due to the prestress tendons giving out along the bottom chord.

If you watch closely, you can see that the failure point of the bridge is right directly under where the crew is standing. Note that their position with respect to the length of the bridge; they are standing at the first intersection of the diagonal brace members of the bridge, not at mid-span. Given that the bridge stood up under its own weight earlier in the day, this seems to suggest that the failure mechanism was related to the work the crew was doing.

Rubio's Tweet: https://twitter.com/marcorubio/status/974486176336957440

2) So what was the construction crew doing exactly? They were prestressing the tendons for the diagonal brace members in the bridge (not the tendons in the top and bottom deck).

Some background info: the role of these diagonal braces (i.e. the truss members) is critical to the structural load carrying capacity of the bridge. To use an analogy, treat this bridge as an I-beam. What makes an I-beam strong and stiff is the fact that its shape allows you to have a deep (i.e. tall) cross section where a lot of the material is at the top most and bottom most part of the section. This results in a section with a high area moment of inertia. Note that intuitively, this only works if you still have some material connecting the top and bottom parts of the beam, resulting in a shape that lends the name "I-beam". Without the strip of material (known as the "web") connecting the top and bottom parts of the beam (called the "chords"), the overall section cannot act as a deeper section and it loses most of its stiffness.

How an I-beam works: https://www.quora.com/Why-is-an-I-b...l-design-come-to-fulfill-its-primary-function

Now, going back to the pedestrian bridge, the diagonal braces are the web and the top and bottom deck are the chords of the "I-beam".

For those who have taken some form of physics or statics class will have some experience designing a spaghetti truss bridge of some sort during their schooling. They know that the diagonal brace that sees the most compression load are the ones at the far extremes of the span. Yes, the exact same one that were below the construction workers. If this brace (or any brace, really) were to fail, the bridge cannot as as an I-beam at the point where the brace failed and it thus loses most of its strength and collapses. Recall that the bridge failed right under the construction workers...

3) Okay, so if the hypothesis is that one of the diagonal braces failed, which one failed? Good question.

It so happens that there's a 2015 version of the design proposal for the bridge floating around the internet: https://facilities.fiu.edu/projects/BT_904/MCM_FIGG_Proposal_for_FIU_Pedestrian_Bridge_9-30-2015.pdf

If you go to page 115, one sees the elevation of the bridge. The span on the left (left of the Pylon) is what collapsed. The span on the right was not put in place yet. So, if we focus just on the left span, the failure occurred right where Member 10 and 11 meet. If you look at the table for PT Bar Requirements (PT = "Pre-tensioning" i.e. prestressing), Member 10 is prestressed while Member 11 is not. This makes sense because you typically prestress concrete when you want it to withstand tension loads. Prestressing a member that is expected to take compression loads will only add more compression load to it.

So which diagonal was the construction crew prestressing? From pictures, it appears that it was Member 11, the one that is supposed to take compression loads. One can spot the hydraulic jack that applies the prestress to the tendon by the teal cylinder. It appears that the jack is oriented in a rather flat orientation (i.e. it's almost horizontal). Between Member 10 and 11, the brace that is more horizontal is Member 11, not Member 10.

Pic: https://res.cloudinary.com/engineering-com/image/upload/v1521183436/tips/MIAMI_3_i7ildl.png

Now, concrete is pretty strong in compression and it takes a lot of compressive force to crush concrete. However, applying compressive force on Member 11 results in Member 10 seeing more tension load, which concrete does not handle well. Recall, Member 10 has prestress tendons for this reason.

Where the mistake could have occurred is anyone's guess but it might boil down to one or more of the following:

• Too much prestress was applied to Member 11, causing Member 11 to fail in compression.
• Too much prestress was applied to Member 11, causing Member 10 and its prestressing tendons to fail in tension.
• The workers accidentally released all prestress in the tendons in Member 11 while prestressing Member 11, causing Member 11 to fail in tension.
• The workers were actually prestressing Member 10 (not 11), and by applying too much prestress, it causes Member 10 to go from resisting tension to resisting compression loads, thus effectively negating it's function as a web member in the "I-beam" and causing the bridge to fail.

4) I'd like to add that the design proposal from 2015 is NOT the final construction documents used to construct the bridge. All information presented in the proposal was subject to change in the final design. Member 11 likely had prestress tendons specified for it in the final construction documents, which means that the construction workers were correct in prestressing Member 11 (assuming they were indeed prestressing Member 11, of course).

Thus, without more information, the crux of the hypothesis remains is as follows: the act of "tightening" the prestressing Member 11 caused either Member 10 or Member 11 to fail, thus taking out the bridge with it.

For more information, you can follow this thread: http://www.eng-tips.com/viewthread.cfm?qid=436595

The engineers there have essentially laid out the arguments I presented above and credit is due to them for fleshing out some of the finer details presented.

Good analysis. Wouldn't it have made sense to close off the center lane and place a hyraulic jack in the middle of the bridge simply to add structural support until the entire support structure was added. Why would they stress test or add tension to the bridge when traffic was moving under the bridge? You would think they would have done tensioning or stress testing during the middle of the night while the road below was closed off to traffic.

 

[Mark R]

Good analysis. Wouldn't it have made sense to close off the center lane and place a hyraulic jack in the middle of the bridge simply to add structural support until the entire support structure was added. Why would they stress test or add tension to the bridge when traffic was moving under the bridge? You would think they would have done tensioning or stress testing during the middle of the night while the road below was closed off to traffic.

There was no other support structure. The "suspension" cables were apparently not intended to be load bearing stays, and instead were a stiffening element for damping of wind induced oscillations. The bridge was intended to be a completely self supporting truss.

The thing that is interesting is that if you consider the design as a simply supported truss, why would member 11 need pre-stressing, surely from its location, it would be purely compressively loaded due to the dead weight of the upper deck? So why did the design change between drawing and construction and lead to pre-stressing of 11?

One possibility is discussed in this video commentary: https://www.youtube.com/watch?v=KtiTm2dKLgU

In this video, the hypothesis is related to a late change in the bridge lift plan. The initial drawings show the bridge span being lifted into place by transporter modules located close to the ends of the span, effectively treating it as a simply supported truss which is the loading the span was intended to see during service. However, pictures of the actual lift, show that the modules at that end of the bridge located a significant distance from the span end due to obstructions at the roadside. In effect, during the lift, that end of the bridge was cantilever loaded, drastically changing the load pattern on the web members. Just by eyeballing this, this may well have required member 11 to support a tension load, and hence this may be the reason why the design was changed to accomodate pre-stressing of 11. Then once, the span was it its final position, the pre-stressing could be adjusted to the service configuration.

 

[Saylick]

Good analysis. Wouldn't it have made sense to close off the center lane and place a hyraulic jack in the middle of the bridge simply to add structural support until the entire support structure was added. Why would they stress test or add tension to the bridge when traffic was moving under the bridge? You would think they would have done tensioning or stress testing during the middle of the night while the road below was closed off to traffic.

They simply decided to perform this work mid-day because they underestimated the risk of a bridge collapse. From their perspective, they were probably thinking, "All we need to do is tension a few PT strands, right? I mean, what's so hard about that?"

I will give the engineering team the benefit of the doubt that had they known the magnitude of the situation, they would have requested that this work be done when no public lives were at risk.

There was no other support structure. The "suspension" cables were apparently not intended to be load bearing stays, and instead were a stiffening element for damping of wind induced oscillations. The bridge was intended to be a completely self supporting truss.

The thing that is interesting is that if you consider the design as a simply supported truss, why would member 11 need pre-stressing, surely from its location, it would be purely compressively loaded due to the dead weight of the upper deck? So why did the design change between drawing and construction and lead to pre-stressing of 11?

One possibility is discussed in this video commentary: https://www.youtube.com/watch?v=KtiTm2dKLgU

In this video, the hypothesis is related to a late change in the bridge lift plan. The initial drawings show the bridge span being lifted into place by transporter modules located close to the ends of the span, effectively treating it as a simply supported truss which is the loading the span was intended to see during service. However, pictures of the actual lift, show that the modules at that end of the bridge located a significant distance from the span end due to obstructions at the roadside. In effect, during the lift, that end of the bridge was cantilever loaded, drastically changing the load pattern on the web members. Just by eyeballing this, this may well have required member 11 to support a tension load, and hence this may be the reason why the design was changed to accomodate pre-stressing of 11. Then once, the span was it its final position, the pre-stressing could be adjusted to the service configuration.

I agree with your assessment in that Member 11 does NOT need pre-stressing (or to use the correct term here, "post-tensioning", since it's done after the concrete has cured) if this was a simply supported truss. Member 10 sees the tension load; Member 11 sees compression.

Ah, good ol' AvE. The guy has a knack for engineering with the vocabulary of a sailor that I find immensely relatable (I mutter "F*ck me..." all the time at work whenever a contractor makes a mistake and I have to deal with it). Looks like he's coming to a very similar conclusion as well.

With respect to having the transporters more inboard of the span, that would indeed flip the loading of the outside truss members, so Member 10 would see compression and Member 11 would see tension. If this is the case, then Member 11 should have been post-tensioned during transportation so that the ends of the bridge don't collapse as it's being moved in place. Once the bridge is set down on it's abutments, the loading reverses and Member 11 sees compression and Member 10 sees tension. Therefore, both Members 10 and 11 need to have some amount of post-tensioning prior to the transporters moving the bridge in place. Okay, no problem there.

Now, going back to how the intent of the workers was to "tightening" up some PT strands, the member whose strands would be loose is indeed Member 11. The reason for this is as follows:

1) Member 11 was in tension during transportation but it effectively is in a state of compression because of the PT strands.
2) By resting the bridge on the abutments, Member 11 now sees a compression load IN ADDITION to the compression applied by the PT strands.
3) Therefore, the PT strands would relax as Member 11 saw the full compression load from the bridge resting on its own weight.

Cue lead engineer asking the workers to tighten back up the PT strands in Member 11. Now, the theory says that Member 11 does not need any post-tensioning anymore at this point because it's only going to see compression loads from the bridge's own self weight, but for some inexplicable reason, they needed the PT tightened back up.

AvE's hypothesis is that in the process of re-post-tensioning the PT strand in Member 11, they over-stressed the PT tendon, causing it to violently shoot out the back end of Member 11 which causes Member 11 to fail. The bridge thus fell down.

If it were up to me, I'd argue that the PT in Member 11 was no longer needed once the bridge was self-supporting and thus a re-tightening was not needed. We will find out in due time why they were post-tensioning that member.

 

[crashtech]

Such slender concrete trusses don't seem to have left much margin for error. From an aesthetic standpoint it's easy to see why they wanted them to be that way, but somehow I doubt it will be done again, not in concrete anyway. Interesting to note that the truss on the opposite end of the span has a much larger cross-section, in retrospect it might have been prudent to use a larger cross-section throughout.

 

[Red Squirrel]

Not sure if posted yet, but AvE did a few videos explaining the possible failure:

https://www.youtube.com/watch?v=ioC61QW7SHQ
https://www.youtube.com/watch?v=KtiTm2dKLgU

 

[Puffnstuff]

AvE does some good stuff on his channel and I've spent many hours watching him pick things apart.

 

[Lanyap]

They should have diverted traffic until the construction was farther along. A local Tampa Bay TV channel showed a video of some official, I think from the engineering department at the school, several days before the failure who said this new bridge building technique was going to revolutionize bridge building. They should also have used steel beams instead of concrete for this project.

 

[OutHouse]

Not sure if posted yet, but AvE did a few videos explaining the possible failure:

https://www.youtube.com/watch?v=ioC61QW7SHQ
https://www.youtube.com/watch?v=KtiTm2dKLgU

wow thats was interesting.

 

[bshole]

They should have diverted traffic until the construction was farther along. A local Tampa Bay TV channel showed a video of some official, I think from the engineering department at the school, several days before the failure who said this new bridge building technique was going to revolutionize bridge building. They should also have used steel beams instead of concrete for this project.

The entire point of this kind of construction technique (ABC) is that you don't have to divert traffic. That is the justification for its higher price tag. If you are going to divert traffic anyways, there is no reason to use the technique at all.

 

[Zorba]

The entire point of this kind of construction technique (ABC) is that you don't have to divert traffic. That is the justification for its higher price tag. If you are going to divert traffic anyways, there is no reason to use the technique at all.

It has also been done many other times. There is more risk, but it isn't like this was the first time ABC has been used.

The problem here isn't even really the ABC, it was the tensioning of the PT with an active roadway underneath, which is done on non-ABC bridges as well.

 

[crashtech]

Like most disasters, it's likely the confluence of several factors. Designing such slender trusses out of concrete may have worked a charm in Autodesk, but software can't always anticipate real-world situations, like the accidental overloading of a PT rod, especially if some light damage was caused by the move.

 

[hal2kilo]

It was under construction and not taking any additional load. Remember this is FL and we get winds. Bridges are built with the ability to withstand more stress than what is expected under normal use. It needed compression to be complete. It was concrete after all. But no, it was far from done and not taking any traffic.

What it looks like is that the bridge failed from lateral tension forces. It looks like someone had the bright idea of not having supports while it was being built.

It would be interesting to see what kind of strong backs they were using. God, we had this 10000 pound van holding an RF test set that had to be lifted onto the back of a submarine. For years we had a couple of padeyes that the manufacturer of the van OK'd. For some reason the entity that would do the annual weight testing did not want to do it anymore. This other outfit says oh my gosh those padeyes are inadequate. Well I can't remember the government lifting specification rule numbers, but the safety margins border on ridiculous. They assume if you have like 4 cable with attachments to lift the item that you have to design it with enough capacity that 2 of the cables can break and still hold the weight. Bottom line is we ended up having to build a strong back using 8 inch rectangular metal tubing. The damn thing weighed probably almost half of what we were lifting. I understand though that the bridge collapsed after it was landed. Not sure if those walkways were supposed to be under some kind of tension as a lot of modern concrete works better if the rebar was installed under tension.

 

[crashtech]

It was all pretensioned, but many of the bridges members shifted from tension to compression and vice versa between moving and placing the span. The member they were working on at the time of the collapse was in tension while being moved, but it held much of the weight of the span in compression once placed.

 

[crashtech]

"A Bridge Too Faux: Florida’s Pedestrian Bridge Would Only Have Looked Safe"

The location of the failure, off to one side, rather than the middle, is making engineers scrutinize that part of the structure. Dashboard video show a crane on the road and worker(s?) on top of the span. Workers may have been adjusting tension to these steel tendons, said engineers reviewing pictures and videos of the disaster.

This was confirmed by National Transportation Safety Board investigators on the scene. “Crews were tightening tensioning cables in a diagonal truss at the north end of the bridge on the day of the collapse,” reported the Miami Herald on Saturday, although that may not be the reason it collapsed—or the only reason—according to the investigators, who may take as long as 18 months to complete their investigation.

 

[theeedude]

https://www.politico.com/states/flo...g-running-role-in-collapsed-fiu-bridge-324278

The Scott administration quickly worked to distance itself from the collapse of a Florida International University pedestrian bridge that left six dead, but documents from the state’s transportation department and the university paint a different picture.

From the selection of the politically powerful firm that led the project to the days leading up to the collapse, the Florida Department of Transportation, overseen by Gov. Rick Scott, had direct involvement in a project whose collapse has rocked South Florida and sparked a federal investigation.
...
MCM has long had local political ties and is also a player on the statewide level. During the time MCM, a Miami-based firm, was seeking the bid, the company gave nearly $50,000 in contributions to Scott or his affiliated political committee, according to state campaign finance records. The decision to hire the firm was made by FIU, but Scott’s transportation agency was involved from the very beginning of that process. It's among $250,000 in state political contributions the company has given in recent election cycles.

Oops.