I'm trying to envision a scenario where a prefabricated concrete bridge deck that weighs 950 tons can support its own weight over its full span length, but needs the tower and cable-stayed support when the entire bridge is completed and is subject to live loads only from pedestrians.
It seems obvious to me that the dead load of the bridge would dwarf the typical live load it is designed to carry.
Alberta's Child, P.E.
It seems obvious to me that the dead load of the bridge would dwarf the typical live load it is designed to carry.
Alberta’s Child, P.E.
I am not a P.E., but do have an M.S. in Mining Engineering.
Was this architect-designed without an engineer? Lawsuits to follow.
Yes, it is obvious. I thought of that and posted on it yesterday.
Although a PE, I’m not that kind of an engineer (electrical/electronic, actually). But, I can think.
Also, if you look at the bridge section, all you see is shattered concrete and the occasional reinforcing or tensioning rods. There is no substantial steel in the base section, which is what was supported at the ends and what broke.
In contrast, the superstructure and roof assembly is massively trussed, as it should be if it was planned to be bearing the load through tensioned cable stays. But, if the cable stays aren’t in place, it’s just so much more dead weight!
I wonder who was in charge of the assembly. Blaming the workers, undocumented or legal, is besides the point. Who gave the order to pin the bridge up by its ends without the central tower even being built, let alone attached by the staying cables? There has to be an assembly plan for this tinker toy!
my uncle was a steel erector, did a number of pro football stadiums and basketball arenas, he said it is easy to be structurally sound when in place, the engineering is to keep it sound all along the way.
Really? For a concrete pedestrian bridge? 950 tons? How many people does it take to weigh 950 tons? I think you must have it backwards.
They were playing the percentages, no doubt, and something went amiss. Situations transpired.
I’m just a little old lady married to a P.E.; I used to type his papers in college (on a manual typewriter, of course) about pre-stressed concrete and water hammer but know nothing about either.
Just from watching my father mixing up concrete by hand 75 years ago, could it have been way too much sand and way too little cement? He explained to me how important the correct ratios were but that’s probably considered old-fashioned nonsense in todays “superior” institutions of higher(?) learning.
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With that long of a span, the bridge’s own weight was likely too much to stand without uniform support.
Adding pedestrians in motion brought in resonances that easily took it down. It is amazing that it didn’t collapse immediately.
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You are right, of course, so the question would be whether placement of the span could be accomplished prior to installation of the cable stay system.
As you know, in the typical arrangement both the design team and the contractor would have agreed on the calculations that demonstrated the structure was capable of supporting itself prior to installation of cable stays. In fact, we can speculate the structure supported itself for some duration after fab was completed while it was yet on the ground.
It seems relevant the span remained in place for some days after placement and prior to the post-tensioning phase. Even if the roof didn't reguire p-t, it is almost certain there was at least one engineer onsite during p-t. Back in the day, p-t was accomplished by a qualfied specialty subcontractor; is it possible there was no enginer present and the work was performed by the regular crew?
IMHO, at this point, the p-t of this structure is likely a significant element and it either failed on its own or in conbination with some unrecognized error by contractor.
I will agree with you, however, that it would have been prudent to install the foundation and column system to the bottom of the span before placing the span.
There is definitely steel in the bridge but I was expecting much more.3x or 4x times what I am seeing.
A rope bridge would have sufficed.
Disclaimer: this is merely a retired carpenter's guesstimate re the maximum total mass of pedestrians on that bridge at a given time.
Gleaned from a couple different sources that the ped span was 175 ft long x 40 ft wide.
Assume an area occupied by the average pedestrian at 2ft x 2ft (4 ft^2), and an average mass of each ped at 150 lbs.
+ 175 bridge length (ft)
* 40 bridge width (ft)
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+ 7,000 bridge area ft^2
/ 4.00 avg area ft^2 each ped
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+ 1,750 max # peds @ 4 ft^2 each
* 150 avg mass per ped (lbs)
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+262,500 total max ped mass (lbs)