Posted on 03/16/2018 12:25:26 PM PDT by C19fan
A steel bridge would have been safer, easier to construct and met the need very nicely.
I don't think the design/construction process involving a precast bridge section is questionable. What baffles me is why anyone would have assumed the newly-installed section could hold up under its own weight WITHOUT any additional support -- either a temporary mid-span column or the final tower/cable arrangement.
Normally, as the concrete is poured, they take sample cylinders and run them over to the lab for testing. Who was in charge of that?
IF it works like other projects I’ve been on the tension is applied with a hydraulic jacking system. House movers used a system that ran jacks from one central location. Each jack still had to be manually operated. I’m not sure how the action would have been balanced on the post tension cables for the concrete bridge. Applying unequal tension on the cables would have skewed the load on the concrete. Any gauges used should have been calibrated. Whether that happened or not is not public. The source of the post tension cable is not known. Given the recent debacle over Japanese steel certifications, the cables may have been faulty. There are lots of factors that could have caused the bottom part of the concrete to be subject to a tensile load which would have led to quick failure. The forensic analysis will eventually provide the answers.
“the decision to use what the bridge builders called an ‘innovative installation’ was risky, especially because the bridge spanned a heavily traveled thoroughfare.”
Vibration from traffic: live loads.
thats a big surprise to me
Actually it is less like that than implied.
In the fully designed project that goes (1) Design, (2) Bid, (3) Build -- the Plans and Specifications show what is to be constructed and specify the exact materials, processes, equipment and quality standards with which the finished product shall be constructed. The classic term-of-art for what is left to the contractor is "means and methods".
On this project, the ABC (Accelerated Bridge Construction) segments and sequence might have been shown with the "means and methods" for the installation rolling tower left to the contractor to implement and also engineered within any limits (like wheel load on the street) listed in the design documents.
The specifications would detail exactly the design mix on the concrete, the mix design history, approve a submission of the mix design, a slump range that the mix shall be placed within and how the testing of test cylinders shall be accomplished.
The contractor can choose means and methods of how to place it in the forms, within certain limits. He can use a crane and bucket, a placement pump or direct chute placement if he can access the area in all those ways.
The big question mark here is what justification is there that the ABC method could be utilized with traffic flow underway with the full structural complement of the parts not yet in place?
You’re are 1000% correct. It’s too early for that BS.
I am not a licensed engineer either. However, from what I remember from Statics and Mechanics, concrete has little to no tensile strength. It is the steel rebar and steel I beams inside the concrete that keep it from collapsing under it own weight.
I would think that the engineering firm would have been onsite when this was set into its final resting place. Without the center pillar and steel cables the concrete must have literally collapsed under its own weight.
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A concrete bridge that is “curing” usually has “falsework” below to support it.
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I don't have it backwards. The dead load of the bridge (950 tons) dwarfs the weight of the pedestrians (1,000 pedestrians -- an unlikely load -- weighing 200 pounds apiece would be 100 tons).
In this context, "dwarfs" means "is very large in comparison to."
Oh, that started yesterday.
What pedestrians? The bridge wasn’t scheduled to be completed until next year.
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>> “what justification is there that the ABC method could be utilized with traffic flow underway with the full structural complement of the parts not yet in place?” <<
Are you ready?
“INCOMPETENCE”
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This was a design build project. The contracting agency basically puts out a call for proposals with the final parameters of the finished project. Zfirms will select various team members who have relevant experience that meet the qualifications and then the team’s will propose their concepts to the agency. The team chosen is awarded the proposal and begins design and construction after the plans are approved.
FIU - Florida International University- has long been a publicity hound university. The wanted a very pretty bridge that would give them more bragging rights. A utilitarian, lightweight, non collapsible steel based bridge could have been put there at less cost. But would not have looked nearly as fantastic.
FIU exploited its political connections to get this FREE bridge, paid for by all US taxpayers. The contractors were happy too as more money was spent for this “architectural statement” kind of bridge. What great publicity for FIU since so many will drive by it each day and see it.
This is another reason this collapsed span was hastily put into place. So that FIU’s lovely new ornament could be seen and admired by motorists as soon as possible. Of course no pedestrians and bicyclists could use the bridge for at least another year because the walkways to the bridge on each end had not been built. Work on them has not even started from I can see.`
That's the idea behind tensioned concrete (I had to look it up). There are cables run through the concrete, and when it is partially cured, the cables are tensioned against holding plates. The stretched cables (33000 psi tension is standard) act like springs, compressing the concrete and holding it together against loads with a tensile or bending component.
The idea is that the concrete is always under some sort of compression, regardless of gravitational orientation or other loading.
1. Flawed construction process (most likely)
2. Poor materials (somewhat likely)
3. Flawed design (not very likely)
I say #3 is the least likely scenario because I've been working in the industry for years and I know that it's the one phase of a project that has the most eyes on it.
Of course, there's also the possibility that a combination of these factors came into play.
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They were actually construction workers, and they didn’t belong there!
The failure began right where they were working! (Watch the video)
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Would a conventional steel bridge withstand the force of a Category 5 hurricane? That was apparently one of the design parameters here.
You don’t that to erect a post tensioned structure. The smaller concrete beams are transported on the highway with the beam being supported only at the ends. If you support it anywhere else, the beam will crack. During transport the beams become the trailer with a connection only to the semi at one end and to a set of axles at the other end.
This type of construction has been around at least since the 60’s when I was in school.
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