Posted on 03/16/2018 12:25:26 PM PDT by C19fan
Engineering experts say investigators looking into the collapsed ‘instant’ bridge in Miami will want to know why a central tower which is usually built to support a suspension bridge was not in place when it collapsed onto Tamiami Trail on Thursday afternoon.
Last week, Florida International University’s official Twitter account posted a rendering of the bridge in its completed form as envisioned by the planners before its opening to foot traffic in early 2019.
The rendering shows a tall central column with cables connecting it to the main span.
Engineers say the design is known as a ‘cable-stayed bridge,’ which is a kind of suspension bridge, according to USA Today.
(Excerpt) Read more at dailymail.co.uk ...
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I think the engineering firm must have been a moslem affirmative action recepient.
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Used both.
NO. The bridge was not open to pedestrians. It was under construction and only one of the two main spans had been moved into place. The ramps and walkways approaching each side of the bridge had not yet been constructed. It is a construction zone, but someone decided that they could allow traffic to resume down below while they still had much work to do on the bridge.
IMO this is a ridiculously elaborate and ostentatious bridge for an overhead walkway. Such things are usually reserved for spans across large bodies of water.
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The dead load of the structure itself created a large tensile element.
That is why there should have been falsework in place until the cables took up the load.
I would guess they were looking at the developing cracks to decide if it was a problem.
Bragging rights for the School's Engineering Dept
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Stil have Log tables and trig tables, and fully expect to be using them again when we run out of batteries!
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>> “ looking at the developing cracks to decide if it was a problem.” <<
SPLAT !!!!
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So, 100 tons dwarfs 950 tons. Ok.
Also, construction live loading would not be anticipated to be the same magnitude as service live loading. Further, I'm not quite sure that the safety factors during construction would need to be of the same magnitude as service loading safety factors for such a structure.
The issue here is most likely the tightening of the post-tensioning tendons. If the tendons had somehow "loosened" (de-stressed) as was reported, with the structure resting in place, that indicates a real problem. If anything, as the bridge rested in place, one would anticipate the tendons to increase in stress. The only thing I can conceive of causing that is that the cable anchoring mechanism somehow became compromised over time. One most suspicious cause would be a crushing of the concrete surrounding and supporting that mechanism. And if the engineer did order tightening of the tendons as was reported, that would only further crush the surrounding concrete.
As a side note, Florida is known for not having access to high volumes of hard aggregate for strong concrete. However, as these units were a specialty design of light weight high strength concrete, I suspect that wasn't an issue here, unless there were issues with curing the precast units before they were post-tensioned.
But all my comments are pure speculation, as I haven't examined any of the design drawings or construction specs or activities.
Another Florida P.E.
That’s where we disagree. The design should have allowed for support of the dead load (no tension below what would be the neutral axis as in a steel beam) and a modest working load to put the bridge on the piers and allow construction of the upper structure.
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How do you think they got to be the low bidder?
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>> “light weight high strength concrete” <<
Something new?
All of the “light weight concrete” I’ve worked with used expanded aggregate, which has little compressive strength.
This is what happens when people no longer accept objective truth — in anything.
I would agree that this is more common in bridge construction in the last 15 years.
There still is a liability issue that is affixed differently between the Design side partner and the Construction side partner due to the difference in (another term of art) "standard of care" as the designer is a licensed professional engineer with a longer and more detailed standard of care required compared to the Construction Manager or the Concrete Contractor.
There was some talk about liability insurance earlier saying one million or two million was all that was there other than the net worth of the parties involved. I don't feel that paints a proper picture.
First of all the Owner is likely to also be sued by the injured and killed parties. I don't know if Florida has participatory negligence where the judge apportions the fault amongst the parties but many states do have that. The Owner has risk of ownership and may have requested aspects of construction that increased risk or dictated design aspects. Assuming the Designer and the CM are a JV under one overall contract, the lead partner probably required the design team be covered by what's called an E & O Policy (Errors and Omissions) and that will come into play behind the designers liability and the C/M General and Comprehensive Liability. Workmen's Comp will come into play for employee's injured and that has state dictated limits separate from the liability. Generally in a project like this the Liability limits are 3 Million minimum with an Umbrella all-risk policy over the whole JV team that can range from ten to fifty million although sometime the actual policy is more that what is called for under the contract executed with the Owner.
Additionally we are hearing the contractor side of this referred to as a Construction Manager. That may be just part of their corporate name and they may not be performing as a C/M. If they were performing as a CM, the trade contractor's contracts for concrete work, slide placement etcetera might be direct with the Owner.
State projects sometimes do that but rarely on bridges to my recollection.
So you have:
Designers Professional Liability 1MM
Consultant Designers' Professional Liability 1MM for each involved
Construction partner's General and Comprehensive Liability 3MM (+/-)
The JV (if that is indeed the case) may have a project specific Gen Liability Policy
The probable Umbrella policy, let's say 30 million as that is fairly common for bigger firms.
Workmens' Compensation Insurance for all workmen killed or injured with statutory limits barring special negligence or circumstances.
Add it the insurance of any party, Owner, testing lab, manufacturer or subcontractor found to have a percentage of fault assigned and you have a pretty big insurance coverage even before you get to the net worth of the parties or "pierce the corporate veil" and go after stock holders and individuals.
Hadn't seen anything else on that.
I got the information that the bridge was open to pedestrians several times yesterday just after it collapsed. CNBC especially reported it several times as well as local affiliates. If its not true, I never heard them recant it. I guess its just fake news.
So maybe it’s self supporting without cable stays - barely. Figg (and Muller) were known for complex designs that were tough to build. (I think there was a precast segment accident during the Sunshine Skyway build by Paschen in the mid 1980’s).
The temporary supports were pulled and the top of the new bridge goes in compression. When cable stays are installed and tightened later the top of the concrete canopy is stretched out again - all is good.
So somebody notices the “loose” tendons on the top of the canopy before cable stays are installed and decides they need to be stressed more - not “Stress tested”. The canopy gets overstressed, buckles and the whole thing comes down. Note a crane and manlift at the point of failure in the blurry video.
Failure of the “Responsible Engineer in Charge” regardless.
I looked at that and kind of thought the same thing. But I was not certain as the video wasn’t particularly good.
If that was the case, the crane operator was totally incompetent or was criminal!
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