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Old 15th March 2018, 04:56 PM   #41
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Originally Posted by ddt View Post
I gotta wonder what impact this has on the teaching at the university, whose campus this bridge was designed to connect. They advocated this very method.


The "innovation" part was supposed to be building the bridge section off to one side to keep from blocking traffic except for the few hours required to set it into place.

I'm not sure exactly how innovative that is. We did exactly that with a pedestrian bridge we built connecting the Dental School addition to an already existing pedestrian bridge in Chapel Hill back in the early 90's. The road it spanned was the exit side of a loop which provided access to UNC Hospital, and we had to move bureaucratic mountains just to get permission to close it for four hours very early on a Sunday morning. We built the bridge beside the building we were putting up, and set it into place with two very large mobile cranes.

I concede that it wasn't as big as this, but it was still in the 200 ton range, and the principle was identical.

No one remarked on it being particularly unique at the time. Just a good idea which solved a logistics problem.

Anyhow, I don't think that engineering or materials failure (which are my first suspicions of a culprit here) would invalidate anything that the students were learning as part of the planning process for this project. That would remain the same. It wasn't the concept that was flawed.
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Old 15th March 2018, 05:16 PM   #42
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The problem here may have been that installing a structurally complete bridge in that way, they installed part of one.
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Old 15th March 2018, 05:18 PM   #43
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Originally Posted by quadraginta View Post
Anyhow, I don't think that engineering or materials failure (which are my first suspicions of a culprit here) would invalidate anything that the students were learning as part of the planning process for this project. That would remain the same. It wasn't the concept that was flawed.
All true but academia, public academia especially, is highly susceptible to the power of negative publicity.
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Old 15th March 2018, 05:35 PM   #44
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My former employer was big on "STP" as one of its many management fads of the month.
Situation: Crossing the road is hazardous for students, who may be hit by cars.
Target: Students not being hit by cars.
Proposal: Drop a hundred tons of steel across the road. No cars. Problem solved.
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Old 15th March 2018, 05:36 PM   #45
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Originally Posted by Trebuchet View Post
The problem here may have been that installing a structurally complete bridge in that way, they installed part of one.

If the structural engineers involved in the project did not consider the need for that bridge section to be self-supporting during the completion of the remainder of the support system then what we are looking at is very much a design failure.

It isn't like the design engineers would have been caught by surprise to learn that the plan was to erect the bridge in that fashion. It was all part of a well publicized (and in my opinion over-hyped) project design process.
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Old 15th March 2018, 05:43 PM   #46
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Originally Posted by quadraginta View Post
If the structural engineers involved in the project did not consider the need for that bridge section to be self-supporting during the completion of the remainder of the support system then what we are looking at is very much a design failure.

It isn't like the design engineers would have been caught by surprise to learn that the plan was to erect the bridge in that fashion. It was all part of a well publicized (and in my opinion over-hyped) project design process.
Absolutely correct. The fact that the bridge came down essentially intact suggests the foundations to me. The fact that the pier apparently has some angles designed into it so it looks pretty makes it hard to tell if it's leaning.
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Old 15th March 2018, 05:51 PM   #47
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Is it really weird that this is an American news story that actually does not involve daily senseless guns or Donny being a right twat?
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Old 15th March 2018, 06:40 PM   #48
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Originally Posted by Norman Alexander View Post
Is it really weird that this is an American news story that actually does not involve daily senseless guns or Donny being a right twat?
Not a right twat, though, actually a wrong'un!!!!!! Not to mention **** - breath!!!
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Old 15th March 2018, 06:41 PM   #49
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Though, of course, I felt compelled to to keep it truthful!!!
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Old 15th March 2018, 06:47 PM   #50
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Originally Posted by Norman Alexander View Post
Is it really weird that this is an American news story that actually does not involve daily senseless guns or Donny being a right twat?
Originally Posted by fuelair View Post
Not a right twat, though, actually a wrong'un!!!!!! Not to mention **** - breath!!!
Well, he's certainly not a left one.
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Old 15th March 2018, 06:50 PM   #51
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Originally Posted by Trebuchet View Post
Absolutely correct. The fact that the bridge came down essentially intact suggests the foundations to me. The fact that the pier apparently has some angles designed into it so it looks pretty makes it hard to tell if it's leaning.

That's just it. It doesn't look like it failed essentially intact to me. It looks like it broke some distance from the end across the road from the canal, and the other end, closest to the canal, was dragged off of the pier when that happened.

That's why part of it is resting at an angle from the ground up to its own pier, and the rest is pancaked flat to the ground. The point that is on the ground angling up to the pier is where it failed. That landed on the road, and the rest was dragged down after it.

At least, that's my first guess so far.
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Old 15th March 2018, 07:46 PM   #52
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Originally Posted by Trebuchet View Post
Well, he's certainly not a left one.
Unfortunately people are already using this tragedy to go after the Left, there is one blog who's creator repeatedly rails against the introduction of 'social justice' into engineering/physics courses (Usually by claiming that teachers are telling students that standards and rigor are 'racist'.) and this story provides them with a perfect example.

Quote:
Whether due to a lack of rigor in the design process, the manufacturing process or the construction process… *someone* didn’t run the numbers right. And as a result, people are dead. Honestly: anyone who argues against the value and importance of engineering rigor can go...
http://up-ship.com/blog/?p=37050#disqus_thread

Some of the comments are quite nasty, but I will say and this is purely speculative is that what we have here is a classic example of letting the 'bean counters' dictate safety and that is never a good thing.
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Old 15th March 2018, 11:30 PM   #53
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Originally Posted by Graham2001 View Post
........ what we have here is a classic example of letting the 'bean counters' dictate safety and that is never a good thing.
You haven't got the tiniest piece of evidence to back up that conclusion.

Did you consider and dismiss the fact that there is a lot of steel kicking around which claims to be of a certain specification, but is actually inferior? Did you somehow determine that there was no design failure? Or that there had been some sort of geological issue at the support columns? No, of course you didn't, so speculation like this is unwarranted.

In fact, looking at this bridge, it looks overly large and involved, and nothing like a bridge built with a constrained budget. It looks to me like it was designed to make a big visual statement. A bridge designed or constrained by the "bean counters" could have been a narrow steel-girder bridge with multiple utilitarian supports, and maybe have simply crossed the road, not the canal (there is another bridge in the near background of the image I posted yesterday).
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Old 16th March 2018, 02:50 AM   #54
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Don't be surprised if it turns out a subcontractor was substituting cheap concrete for high performance concrete then charging for the better material. That has been behind some mysterious collapses before.
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Old 16th March 2018, 02:53 AM   #55
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There is a tiny video of the actual collapse, here.

It's in the first few seconds, and appears to show the truss failing, rather than a support giving way. It "breaks" about a quarter of the way across the span.
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Old 16th March 2018, 04:31 AM   #56
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Old 16th March 2018, 05:36 AM   #57
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My initial thoughts are that this structure was under-designed.

After all, the structure only had one truss section to support the entire structure. Whereas, with truss structures the normal procedure is to provided two truss sections; that way the load is distributed over a wider area and if one section were to fail, then there is the other section to help keep it together.

However, since there is only one truss section, then when that one section fails it soon leads to the entire structure failing.

I can think of three reasons why they went with just one truss section:

First, is that one truss section would make the structure cheaper to design and build.

Second, that since the structure was just being used for pedestrian traffic, it was believed that two trusses would be an expensive over-design.

Third, one truss section looks neat.
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Old 16th March 2018, 05:43 AM   #58
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No Crossbow, this is not a truss bridge. It is a suspension bridge. The bit that collapsed was only the initial part of the structure......see my image posted yesterday. There was to be a tower with a system of cables to hold the whole thing up. However, they must also have calculated that it was safe in the part-built situation, and they clearly got that wrong.

As to your point about this being designed as a cheap bridge: it clearly isn't. It looks like a hugely overblown design to me, designed to make a statement, and not just to get pedestrians over a road and canal as cheaply as possible. It would have been perfectly possible, as I said previously, to have a simple steel girder construction, much narrower, and without the tower & suspension stays....maybe 10% of the price. This was a $14 million+ budget.

ETA......here's that image again:

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Old 16th March 2018, 05:45 AM   #59
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Originally Posted by quadraginta View Post
The "innovation" part was supposed to be building the bridge section off to one side to keep from blocking traffic except for the few hours required to set it into place.

I'm not sure exactly how innovative that is.
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I was wondering about that too. How is a (pedestrian) bridge over a road different than a (car) bridge over the busiest river in NW Europe? Here's some pictures of the bridge put in place near my home, Nov 2013. I'm not in the construction business, so I may be overlooking a simple explanation, but the idea to build the bridge off-site and then sail/drive it into place doesn't seem very revolutionary to me.
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Old 16th March 2018, 06:00 AM   #60
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Originally Posted by MikeG View Post
No Crossbow, this is not a truss bridge. It is a suspension bridge. The bit that collapsed was only the initial part of the structure......see my image posted yesterday. There was to be a tower with a system of cables to hold the whole thing up. However, they must also have calculated that it was safe in the part-built situation, and they clearly got that wrong.

As to your point about this being designed as a cheap bridge: it clearly isn't. It looks like a hugely overblown design to me, designed to make a statement, and not just to get pedestrians over a road and canal as cheaply as possible. It would have been perfectly possible, as I said previously, to have a simple steel girder construction, much narrower, and without the tower & suspension stays....maybe 10% of the price. This was a $14 million+ budget.

ETA......here's that image again:

https://i.imgur.com/nszajWv.jpg
Ugh!

You are quite right, my mistake.

Sorry about that. I was basing my ideas on incorrect data and that serves me right for writing hasty Forum posts.

Anyway, this structure is a cable stayed bridge where the cables are tied into the structure via the one truss section; and, unfortunately, the structure collapsed before all of the cables could be installed.

And you are quite correct, they were trying to make a statement with this design.

Cable stayed bridges are a very popular design right now and even though this is a pedestrian bridge, they tried to get on to the cable stayed bandwagon. Furthermore, I am sure that you are also correct in stating that a simple deck girder bridge could have done the job just as well and would have been much less costly.

It just would not have looked so cool.
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Old 16th March 2018, 07:55 AM   #61
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Originally Posted by Crossbow View Post
Ugh!

You are quite right, my mistake.

Sorry about that. I was basing my ideas on incorrect data and that serves me right for writing hasty Forum posts.

Anyway, this structure is a cable stayed bridge where the cables are tied into the structure via the one truss section; and, unfortunately, the structure collapsed before all of the cables could be installed.

And you are quite correct, they were trying to make a statement with this design.

Cable stayed bridges are a very popular design right now and even though this is a pedestrian bridge, they tried to get on to the cable stayed bandwagon. Furthermore, I am sure that you are also correct in stating that a simple deck girder bridge could have done the job just as well and would have been much less costly.

It just would not have looked so cool.

Not going to try to find the link right now, but I read that the purpose of the bridge was not merely to provide pedestrian access across the road, but also to create a social space, for meetings, gatherings, etc.. Like an overhead plaza. This is why the pedestrian deck was nearly 60 ft. wide. So the job being done had more elements than that

As I mentioned upthread, the load characteristics of anything designed for the weight of potential crowds of people are extreme, because a crowd of people is a huge live load. And ever since the walkway collapse back in 1980 at the Hyatt Regency in KC, which killed 116 people and injured nearly twice that, structural engineers have been hyper-vigilant about this consideration in their designs. That was a real wake-up call.

It was definitely intended as a showpiece as well as a show space, and the cable supported design may well have also been the simplest and most cost efficient one to achieve those goals. They wanted to span an over two hundred ft. gap with a structure which anticipated loads far in excess of anything mere foot traffic (or even vehicle traffic, for that matter) would require.

And yes, they wanted it to look real nice and make a statement. Architects do that a lot, and bridges are certainly among the more popular places for them to do it.

Non of this is necessarily or even probably a proximate cause of the collapse.
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Old 16th March 2018, 09:02 AM   #62
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Originally Posted by MikeG View Post
There is a tiny video of the actual collapse, here.

It's in the first few seconds, and appears to show the truss failing, rather than a support giving way. It "breaks" about a quarter of the way across the span.
There was something on the news this morning about some cables which had gone slack and were being tightened. I wonder if they might have inadvertently pulled it right off the pier.
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Old 16th March 2018, 09:04 AM   #63
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CNN says that 6 are known dead (bodies recovered) but there are still more cars beneath and those will contain an unknown number of dead people. Also from CNN...

Originally Posted by CNN
Florida Sen. Marco Rubio, who visited the bridge site Thursday, posted on Twitter: "The cables that suspend the #Miami bridge had loosened & the engineering firm ordered that they be tightened. They were being tightened when it collapsed today."
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Old 16th March 2018, 09:06 AM   #64
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Originally Posted by Trebuchet View Post
There was something on the news this morning about some cables which had gone slack and were being tightened. I wonder if they might have inadvertently pulled it right off the pier.
Extraordinary. I wonder what they are referring to. I can't see any cables, and as there was no tower, I can't see what use cables would have been anyway.

Rubio seems to have completely misunderstood something, if he thinks the bridge was already suspended.
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Old 16th March 2018, 09:09 AM   #65
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Maybe an engineer at the site told Rubio that they were tightening cables because they were tightening cables.
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Old 16th March 2018, 09:13 AM   #66
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Originally Posted by Trebuchet View Post
The problem here may have been that installing a structurally complete bridge in that way, they installed part of one.
This is a strong possibility. My bet is on some change do design that occurred during construction. Mostly because most structural failures that occur during or shortly after construction are due to the unforeseen consequences of such changes.

Originally Posted by quadraginta View Post
If the structural engineers involved in the project did not consider the need for that bridge section to be self-supporting during the completion of the remainder of the support system then what we are looking at is very much a design failure.

It isn't like the design engineers would have been caught by surprise to learn that the plan was to erect the bridge in that fashion. It was all part of a well publicized (and in my opinion over-hyped) project design process.
Most such projects are overhyped. Its generally the builders responsibility to support structures during construction. Most construction contracts clearly state that as well. The design engineer should understand how the project can be built but the typical don't provide that plan to the builder.




Originally Posted by Travis View Post
Don't be surprised if it turns out a subcontractor was substituting cheap concrete for high performance concrete then charging for the better material. That has been behind some mysterious collapses before.
It's pretty difficult for that to go unnoticed during construction. The truck arrives with a ticket that lists the concrete mix design and relevant specified properties. It under goes visual inspections prior to placement and their are regular samples taken for later testing.

Granted, you have to wait 28 days to test those samples.
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Old 16th March 2018, 09:23 AM   #67
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Originally Posted by MikeG View Post
Extraordinary. I wonder what they are referring to. I can't see any cables, and as there was no tower, I can't see what use cables would have been anyway.

Rubio seems to have completely misunderstood something, if he thinks the bridge was already suspended.
Obviously not the suspension cables. May have been some sort of temporary ones associated with the move or intended to stabilize it until completion.
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Old 16th March 2018, 09:32 AM   #68
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Originally Posted by Trebuchet View Post
Obviously not the suspension cables. May have been some sort of temporary ones associated with the move or intended to stabilize it until completion.
I suspect it is far more likely that Rubio got hold of the wrong end of the stick, then spouted something about cables to the press, who without checking have passed it on verbatim, and thus a whole new set of rumours have started.
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Old 16th March 2018, 09:39 AM   #69
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Originally Posted by ahhell View Post
...... The design engineer should understand how the project can be built but the typical don't provide that plan to the builder.......
Really? I'd be amazed if that was the case. In fact, if the design engineer hasn't made supporting details clear to the builder, then I'd say the engineer was liable. I would also expect there to have been a design team meeting prior to moving the bridge, and for the design engineer to have been on site for such an important move. S/he'll at the very least have wanted a photo of her/himself standing in front of it.
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Old 16th March 2018, 09:40 AM   #70
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Semi-professsional opinion.
It was a suspension bridge supporting a truss, the load from the bridge was meant to be supported from the center tower via the suspension cables. It doesn't appear as though the cables were in place yet or they were slack. The load was likely supported entirely by the truss and the end points of the bridge. This condition was not accounted for in design. Either the truss failed because it was meant to be supported at multiple points midspan instead of just the ends or the embankment failed because it wasn't designed to support the bridge at all.

Originally Posted by MikeG View Post
Really? I'd be amazed if that was the case. In fact, if the design engineer hasn't made supporting details clear to the builder, then I'd say the engineer was liable. I would also expect there to have been a design team meeting prior to moving the bridge, and for the design engineer to have been on site for such an important move. S/he'll at the very least have wanted a photo of her/himself standing in front of it.
I am a structural engineer, I rarely account for how parts of an incomplete structure are meant to be supported prior to completion. We design details for the completed structure, not for the intermediate stages. (Typically)

Last edited by ahhell; 16th March 2018 at 09:44 AM.
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Old 16th March 2018, 09:50 AM   #71
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Well I'm an architect and deal with structural engineers almost daily. If there were any circumstances here in which the part-built situation required support in a certain way, the engineers are obliged to make it explicit in their drawings and calculations, or in separate instructions. I had such a conversation with a structural engineer only a couple of hours ago about a project we're working on.

The tower wasn't yet built, so the cables are irrelevant. They couldn't have been attached because there is nothing to attach them to (and you can see from the photos that there are no cables). Which is why I have said all along that the truss must have been designed such that it could span that distance in the part-built state.........or, as you say, it should have had a temporary support designed and in place.
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Old 16th March 2018, 10:09 AM   #72
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Originally Posted by MikeG View Post
They couldn't have been attached because there is nothing to attach them to (and you can see from the photos that there are no cables).
I know virtually nothing about bridge design.

Can there possibly be internal cables which are not visible in photos? Can there possibly be cables along the side which are not visible in photos?
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Old 16th March 2018, 10:10 AM   #73
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Originally Posted by Travis View Post
Don't be surprised if it turns out a subcontractor was substituting cheap concrete for high performance concrete then charging for the better material. That has been behind some mysterious collapses before.
And should result in the death penalty for that sub-contractor and any others involved in it!!!
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Old 16th March 2018, 10:10 AM   #74
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Originally Posted by MikeG View Post
The tower wasn't yet built, so the cables are irrelevant. They couldn't have been attached because there is nothing to attach them to (and you can see from the photos that there are no cables). Which is why I have said all along that the truss must have been designed such that it could span that distance in the part-built state.........or, as you say, it should have had a temporary support designed and in place.
We would typically say, that is the contractor's means and methods.

It should have been planned for, who's responsibility it was to account for the half built state depends on the contract. I'd now bet the Engineer figured it was the contractor's responsibility and the Contractor didn't ask if the engineer had planned for it.

Originally Posted by William Parcher View Post
I know virtually nothing about bridge design.

Can there possibly be internal cables which are not visible in photos? Can there possibly be cables along the side which are not visible in photos?
In high seismic areas(and I'd guess high wind and flood risk areas) you will see bridges resting on a bearing pad with cables from the bridge the abutment to allow movement but not enough for the deck to slide of the foundation. You can only see them on the bottom side of the bridge. That might be what they are talking about.

Could also be a post tensioned concrete structure. There are internal cables that are tensioned to bot the concrete in compression. Concrete being strong in compression and weak in tension. Hard for me to tell just looking at the pictures. The university of Washington does research in accelerated construction of post tension concrete bridges, I can't rule that out.

Last edited by ahhell; 16th March 2018 at 10:15 AM.
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Old 16th March 2018, 10:11 AM   #75
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Originally Posted by ddt View Post
I gotta wonder what impact this has on the teaching at the university, whose campus this bridge was designed to connect. They advocated this very method.
Was the method itself at fault or was it just badly executed?
Moduler construction for bridges is nothing new;dates back to the Bailey bridges still used by the military.
I am not denying somebody messed up bigtime, I just want to get the facts before I condemn an entire method.
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Old 16th March 2018, 10:13 AM   #76
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Originally Posted by William Parcher View Post
I know virtually nothing about bridge design.

Can there possibly be internal cables which are not visible in photos? Can there possibly be cables along the side which are not visible in photos?
Not that would make any great difference to the structure. I say again, I reckon this is Rubio misunderstanding what he's been told on site, blurting out some nonsense to the media, who are now pursuing "cables" as their latest craze. It's a red herring.
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Old 16th March 2018, 10:47 AM   #77
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Originally Posted by MikeG View Post
Extraordinary. I wonder what they are referring to. I can't see any cables, and as there was no tower, I can't see what use cables would have been anyway.

Rubio seems to have completely misunderstood something, if he thinks the bridge was already suspended.

I wonder if the slab itself was reinforced with post-tension cables. That might explain a use of the term "cables" which a lay person might misinterpret.

Post-tension cable reinforcement would be a likely choice in a long-span flat-plate application like that bridge slab, and until they are trimmed and the anchor openings grouted in they could still be "fine-tuned" after the bridge section was put in place.

Although I have installed a lot of post-tension reinforcement I have never done it for a structure which was intended to be moved subsequent to construction and placed in a support configuration different from that which it was originally built. I could see how an adjustment of cable tensioning might be anticipated.

I have seen some relatively impressive failures when tensioning post-tension reinforced flat slabs. Generally as a result of local failures of the concrete, either as a result of materials, mixing, or poor placement technique. Sometimes damage to the design profile of the cables themselves during concrete placement can result in some interesting failures as well.
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Old 16th March 2018, 11:06 AM   #78
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Originally Posted by ahhell View Post
This is a strong possibility. My bet is on some change do design that occurred during construction. Mostly because most structural failures that occur during or shortly after construction are due to the unforeseen consequences of such changes.

Most such projects are overhyped. Its generally the builders responsibility to support structures during construction. Most construction contracts clearly state that as well. The design engineer should understand how the project can be built but the typical don't provide that plan to the builder.

I built poured-in-place concrete buildings for nearly my entire construction career. Shoring plans which detailed the placement, duration, and removal of supports for suspended slabs were a standard part of the contract drawings.

They might not have always been generated by the structural engineer of record, but they were always reviewed and signed off on by their office.

This was SOP.

Quote:



It's pretty difficult for that to go unnoticed during construction. The truck arrives with a ticket that lists the concrete mix design and relevant specified properties. It under goes visual inspections prior to placement and their are regular samples taken for later testing.

Granted, you have to wait 28 days to test those samples.

No. It is also SOP to take multiple samples for use at far shorter intervals, to determine when it is safe to remove the original shoring for recovery for use in later floors. No project retains primary shoring for all floors for the entire structure. Generally there will be three shored or post-shored levels and everything below that will be supporting itself as long as those floors have achieved their design strength. (That 28 day break you mentioned is usually the time specified for the concrete mix to achieve its design strength.)

Test cylinders are generally made at point of delivery (when the concrete comes down the chute of the truck), with enough made to provide breaks at 7, 14, and 28 days. More might be specified by the contractor or by other parties. High-early concrete mixes which achieve design strength more quickly are often used in suspended slab applications to facilitate earlier removal of the primary shoring. 4 and 5 day breaks are standard when a post-tension design is used, so the tensioning of the cables can be done as early as possible, for the same reason.
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Old 16th March 2018, 11:20 AM   #79
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Originally Posted by quadraginta View Post
I built poured-in-place concrete buildings for nearly my entire construction career. Shoring plans which detailed the placement, duration, and removal of supports for suspended slabs were a standard part of the contract drawings.

They might not have always been generated by the structural engineer of record, but they were always reviewed and signed off on by their office.

This was SOP.

I do not disagree, I don't think this contradicts anything I've said.
As the Engineer of Record I've rarely designed any temporary shoring or designed structures to be self supporting in the anything but its complete form. When review shoring plans, we typically included clear disclaimers disavowing responsibility for the shoring. No idea how well that would hold up in a court room.

Quote:
No. It is also SOP to take multiple samples for use at far shorter intervals, to determine when it is safe to remove the original shoring for recovery for use in later floors. No project retains primary shoring for all floors for the entire structure. Generally there will be three shored or post-shored levels and everything below that will be supporting itself as long as those floors have achieved their design strength. (That 28 day break you mentioned is usually the time specified for the concrete mix to achieve its design strength.)

Test cylinders are generally made at point of delivery (when the concrete comes down the chute of the truck), with enough made to provide breaks at 7, 14, and 28 days. More might be specified by the contractor or by other parties. High-early concrete mixes which achieve design strength more quickly are often used in suspended slab applications to facilitate earlier removal of the primary shoring. 4 and 5 day breaks are standard when a post-tension design is used, so the tensioning of the cables can be done as early as possible, for the same reason.
I don't see how this contradicts my statement either. Which was basically, you might have to wait 28 days to find out if the concrete bad. My point being, it usually pretty clear early on that the concrete is bad. If that was the problem, they probably already know it, and probably should have known it a week ago.

Edit, in retrospect, I wasn't clear. I said you have to wait 28 days. Which isn't true. Most concrete is tested at 7 and 28, sometimes 14. Often at different intervals for reasons mentioned by Quad.

The traditional bench mark is 28 days and I believe that is still the most common acceptance criteria.

Last edited by ahhell; 16th March 2018 at 11:26 AM.
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Old 16th March 2018, 11:28 AM   #80
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Originally Posted by EventHorizon View Post
Wow. If that's true that's unbelievable.

Seems this bridge was being constructed using a new Accelerated Bridge Construction method. Now I don't want to jump to conclusions but...

Anyway, here are some articles and a fact sheet about the bridge:

https://news.fiu.edu/2018/03/communi...-street/120395

url]http://communitynewspapers.com/florida-international-university/first-of-its-kind-pedestrian-bridge-swings-into-place/[/url

https://news.fiu.edu/wp-content/uplo...18_DIGITAL.pdf


Originally Posted by quadraginta View Post
I wonder if the slab itself was reinforced with post-tension cables. That might explain a use of the term "cables" which a lay person might misinterpret.

Post-tension cable reinforcement would be a likely choice in a long-span flat-plate application like that bridge slab, and until they are trimmed and the anchor openings grouted in they could still be "fine-tuned" after the bridge section was put in place.

Although I have installed a lot of post-tension reinforcement I have never done it for a structure which was intended to be moved subsequent to construction and placed in a support configuration different from that which it was originally built. I could see how an adjustment of cable tensioning might be anticipated.

I have seen some relatively impressive failures when tensioning post-tension reinforced flat slabs. Generally as a result of local failures of the concrete, either as a result of materials, mixing, or poor placement technique. Sometimes damage to the design profile of the cables themselves during concrete placement can result in some interesting failures as well.
I think you are very much on track here. On the highlighted link above there is a statement that says the slab is post-tensioned in two directions. I suspect that is going to turn out to be the weak link.
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