pedestrian bridge

[ol'stonebreaker]

Don't bring politics into this!! What's your take on cause or causes of this?
Mike

 

[Ronsii]

Very sad what happened..

I saw a dashcam video of it appearing to fail about 30 feet in from the end right at one of the web supports and with the news about the cables being 'retensioned' at the time I'm guessing either the cables failed or concrete in the lower flange had a weak area... also saw where someone mentioned the specific mix was using a high amount of baked clay and flyash... lots of theory's floating around right now but I'm sure there is plenty of info the general public won't see for a while.

 

[Junkyard]

Having read several articles most of which were written by a person with ZERO engineering knowledge it was kind of hard to sift through and gain some insight on what happened. I've heard cracks mentioned, I've heard load/stress testing etc. They darn sure wouldn't have tested it with traffic on the road. I can see a cable tension issue causing it, maybe too much on one or another.

The biggest thing that chaps my a$$ is all the talk of "accelerated" construction. That method implies bridge built nearby and moved into place as a unit. Not that they rushed through without oversight etc. I've seen way bigger bridges and overpasses done that way with no adverse issues. Also, the same company that built this one had another bridge drop some concrete into a roadway and was sued, settled out of court. I know they used a fancy mix to make the concrete appear a certain way. Perhaps that was another mitigating factor.

Bad deal all around. I'll be interested to see what the outcome is. Wasn't as cool to watch as the Tacoma Narrows!

 

[old-iron-habit]

I would speculate that they were pulling the top cords and added stress to the bottom cords. I wonder if the bottom cords were properly tightened first. Pure speculation, I have seen nothing nor heard any rumors about this, just a hunch.

 

[hvy 1ton]

They were tugging one of the post tension rods when it failed. AvE has a interesting video. He's a bit of a goof, but i think that's just to keep the clipboard crowd away.

I forgot to mention Figg Bridge was fined for having a section of precast bridge fall on a job in Virginia a couple years ago.

 

[AzIron]

In my experience around post tension concrete you tension things before you move it from were it was cast and usually to whatever it's supposed to be stretched to

So I am curious why they were messing with the tension once they set it in place you can't tension loaded cables that I know about

 

[Theweldor]

I have to agree with AzIron. I have been around some of this stuff. It is pretensioned before it is poured. Trying to pull on the more after the fact seems rediculouse.

 

[hvy 1ton]

Pretension uses cables and can't be changed after the concrete is poured. Post tension uses rods and can be adjusted after the pour. From what I've been able to find the bottom of the bridge was pre-tensioned and the truss members were post-tensioned. My current failure mode is one of the post-tension rods was damaged during transport/install and when they went back to try to bring the tension up to spec the rod was already in plastic deformation and let go causing the brittle failure of the structure.

 

[Ronsii]

^^ Just read they were tensioning one of the webbing/truss members when it collapsed, not sure if this was in the area of the cracking or not...

 

[AzIron]

I am still surprised someone thought it was a good idea to be doing tension work once the structure was set in place cause the cables/rods are under working load at that point the only way to release the load would be to shore up the bridge along the length of it to hold the weight and then adjust the tension

 

[td25c]

What's your take on cause or causes of this?

Construction methods .. It's 2018 and everyone wants it to happen real fast .. And it did !

The accelerated method was also supposed to help out on worker safety issues .

My God ! This is a pedestrian bridge ....... Build it out of wood on the next round if they can find the funding .

Wood will hold up a train .

To much education & not enough practical experience is my take on the whole situation .

 

[Junkyard]

Not to mention wood is cheaper! Why overthink and over engineer simple stuff. 950 ton pedestrian bridge? Wtf ladies and gentlemen. All because a student was struck by a car trying to cross that street. Darwin would say that particular person wasn't smart enough to walk this earth. Thinning the herd a bit.

Who the heck knows.

 

[Raildudes dad]

If you look at the concept bridge drawings, this was the southern span of a 2 span cable stayed bridge. The northern span, the tower and the cable stays were yet to be built. There had to be significant tensioning to keep it up while the rest of the bridge was built. IMO, that's the non-standard part of the construction. I suspect that's where they will find the cause.
I don't like the term "accelerated" construction. The only part that goes faster is putting the new structure in place. The construction is the same, just not on site.
This was more than a pedestrian bridge, it was designed to be a plaza over the road.

Edit:
I just watched that video above. I think he has it figured out. The road definitely should have been closed Always err on the side of caution in this business.

 

[td25c]

Not to mention wood is cheaper! Why overthink and over engineer simple stuff. 950 ton pedestrian bridge? Wtf ladies and gentlemen. All because a student was struck by a car trying to cross that street. Darwin would say that particular person wasn't smart enough to walk this earth. Thinning the herd a bit.

Who the heck knows.

Preach it brother !

We have lost allot of old school engineers & this topic is a classic result .

Think about it every time we pull in to a job with the 1969 Grove crane ....

Had one kid on a job ask what century the rig was from ??

" It came off the assembly line the same time Neil Armstrong was stepping out on the moon "
https://en.wikipedia.org/wiki/Apollo_11#Lunar_surface_operations
And had the same quality engineers

 

[Birken Vogt]

If you look at the concept bridge drawings, this was the southern span of a 2 span cable stayed bridge. The northern span, the tower and the cable stays were yet to be built. There had to be significant tensioning to keep it up while the rest of the bridge was built. IMO, that's the non-standard part of the construction. I suspect that's where they will find the cause.

It looks like it was not a cable stayed bridge. The "cables" were really to be pipes just bolted to the top of the bridge with 4 bolts. Does not sound structural to me. Tower and pipes just decorative. With dollars taxed from us.

 

[Raildudes dad]

The announcement says it's cable stayed.

 

[Junkyard]

It would be an interesting procedure to tension the cable stays. You'd almost have to support or preload the center to get things supported correctly. Kinda like shimming a big lowboy. Can't do it once it's loaded. I wonder how much that thing deflected under its own weight. Also, what did they have for foundations on each end. When the Golden Gate was built the ends were massive to anchor the giant cables.

So I guess my current question is this (well questions)....did improper tensioning procedure yield a failure? Did concrete structure fail at point of securement somewhere along span? Did foundation on one end or the other fail at a cable anchor point thus unevenly preloading concrete structure to point of failure? Did some guy go "spec says 10,000 psi is called for so 11,000 psi is better?" Was it an "supplier" issue in regard to concrete quality or cable quality? I dont recall which bridge it was but they'd bring the good stuff for inspection and then swap it out for cheap stuff upon assembly.

It's not hard to get a bad batch of concrete even when it's a normal mix. We drilled and poured some tower crane piers awhile back. We packed up to head to another job and they stopped us. Mix in those piers failed at 500 psi!!!! History of company building it isn't that good....

 

[ol'stonebreaker]

My initial opinion is there was a whole lot of stupid from top to bottom and I don't think it reached all the way to bottom. If the pole and pipes were just for aesthetics, then that in itself was stupid.
Mike

 

[AzIron]

there are literally thousands of variables to account for in all this between material and workmanship a few big things stand out

they prefabbed the bridge to to make it modular when they put it in place so it had a smaller impact on road closures

there was no false work to support the bridge to help with road closure during construction

the design i have seen is supposed to be cable stayed or (suspension Bridge) they were not in place yet



i have seen tilt panels get scraped because a tension cable broke you cant fix that in most cases very easily when tension cables break it damages the concrete around it because of the massive compression release most of the time not catastrophically because the concrete itself is still in the position it was to pour it so it is not bearing load. now once one cable breaks the rest have to bear the load the shock force from a cable breaking in a structure under load is immense as things settle
thats why you dont tension cables in a structure that is under load the bridge was set in place so it was under its own load when one leg in the truss fails the whole bridge does because there was nothing else in place yet to support it

 

[hvy 1ton]

Being pretty was definitely a driving factor in the design of this thing. They added titanium dioxide to the concrete to make/keep it white. The tower and cables for the cable stay had not yet been installed when it failed. It was supposed to hold it's own weight during installation.