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Concrete weighs ~ 150 pounds per cubic foot.
174 feet long x 15? feet wide x 5? feet thick x 150 lb = 19.575.000 pounds = 978 tons.
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I mean it’s about 75 square feet of concrete in cross section. All to hold maybe 2 or 3 tons of humans at any one time across the entire span.
The 5 foot think part of that is what has me wondering.
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And itself. That’s where maths and engineering become fun friends.
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You might be right, we don’t have a cross sectional blueprint to know for sure. The canopy and supports are probably steel and could account for somewhat less concrete.
Bingo. Static load is not to be trifled with.
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‘I felt exactly how you would feel if you were getting ready to launch and knew you were sitting on top of 2 million parts — all built by the lowest bidder on a government contract.’
Attributed to John Glenn
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Good catch 
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A while back I checked the database below the dozens of bridges I drive over and more importantly, under, every day. The data wouldn’t have foretold the Miami collapse, because that bridge was brand new. But checking can ease, or disturb, your mind depending on the results.
Years ago, I had a partner that was severely gephyrophobic, which at the time I thought was absolutely hilarious. Per divine justice, I am developing my own case now. In my commute there is a stoplight in which I have to sit under an 10 or 12 lane overpass for a few minutes every day. The wait terrifies me. The spot is only a couple hundred yards from the portion of I-85 in Atlanta that burned and collapsed last year. Oh, and that repair job was “accelerated” too. USA infrastructure is horrible.
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I cringe to think that government will draw exactly the wrong lesson from this. They may well say to themselves, “this is what we get for trying to keep pedestrians safe from automobiles”, and just give up on the whole idea.
It used to be roads were for everyone to use, and cars were at the bottom of the priority list. Would that this were possible again.
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I had to look that up; didn’t know it was a thing.
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Yes, it is. My ex used to drive out of her way to avoid even small bridges, or I would have to drive while she closed her eyes. The 7 mile bridge in the Keys and I-10 over the Atchafalaya in Louisiana were a hellish torture for her.
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Sounds terrible.
Is it related to or accompanied by a fear of tunnels?
No - that bridge with the pylon and cables looks like a completely different design. The sketch was probably from early on in the process, and that version was no doubt determined to be more expensive than the budget. (edit - I actually disagree with myself on this now - see my comments in reply below. I think this bridge was to have the pylon and cables.)
The bridge that was installed appears to rely on a simple truss spanning the road. You will note the diagonal members were heavier and thicker than that rendering image. The construction appears to have been reinforced concrete.
So there are two failure modes for reinforced concrete. Either the concrete fails - by crushing, or the steel fails by snapping. Conceptually concrete structures are designed to put the concrete component in compression, and the reinforcing steel in tension. Because the failure of concrete in compression is sudden and without warning, and the failure of steel in tension involves stretching and deformation before breaking, reinforced concrete structures are typically designed with a bias towards the steel being slightly weaker than the concrete so that a failure would progress by the stretching of the steel before breaking, giving warning.
So we can guess here that the concrete failed first, which would be contrary to conventional design approach, which could mean two things - there was too much reinforcing steel, or the concrete was under design strength. Since the construction process involves multiple reviews and observation of the placement of steel reinforcing, its much more likely that the issue here was the concrete mix did not provide the design strength in a critical area and failed. Concrete mixes are tested and evaluated, samples taken during construction and evaluated at labs to assure the strength is met. But there is room for error and/or corruption in that process. Or the engineering design could have been wrong, leading to the incorrect balance of reinforcing and concrete strength. I would still bet on the concrete mix being at fault, because even if the design called for too much steel, that would tend to make the bridge stronger even though it would make it fail without warning. In this case there was a catastrophic failure without warning, without any apparent over-load or unexpected stress. To me that indicates that some element of the construction did not behave as expected by the design, and given the failure mode it looks like the concrete.
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I’ve seen such thing once with a large steel bridge that was built on the river shore and then slid on the previously built concrete supports. During the move parts of the bridge buckled under load and had to be removed and welded again.
Thanks for explaining this. I did measurements (digital image correlation) during bending tests of steel and BFRP reinforced concrete beams and they almost always failed in the way you describe. I didn’t know that it was by design.
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On structures such as this bridge, dynamic loading also must be considered. (Now my engineering background can step in 
) There’s loading from wind (natural frequency to consider; ex: Tacoma Narrows Bridge Failure) and excitation from bridge traffic. There’s also the effect of thermal expansion/contraction, not to mention excitation from street traffic.
Stepping back, @kpkpkp great post links to an article that mentions that the construction of this bridge involved a method never attempted before on one that size. Lots of stuff to think about.
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Ugh. Do they still make that show Engineering Disasters? They did a show on this local tragedy awhile back.
We should have this stuff nailed down.
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That’s a basic steel truss box frame. It’s way more simple to design, build safely for long spans, and pre-build off site than a concrete bridge, which begs the question why wasn’t a basic steel truss box frame used.
The answer will always be money, but I’m just curious what other excuses will come out of the investigation.
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