See another blog analysis here.
Basically the bridge is a truss design, while the cable stays are both for aesthetics and vibrational dampening only. The cross-section profile is basically an I-Beam.
My amateur questions below …
QUESTIONS:
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Why is the deck of the bridge composed with less support mass than the roofline? Due to self-supporting bottom-heavy mass, PT rods would need higher tensioning along the roofline, with fewer rods embedded in less concrete.
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Why are the truss diagonals staggered in such a way as to shift the static load to one end of the bridge? Typically, sag would occur in the middle of a symmetrical set of diagonal braces. In this case, asymmetry would shift the compressive forces from the middle of the span toward the end near the pilings. This would mean that approximately 1/4 of the diagonal braces would need to support 3/4 of the cumulative load.
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Why was there increasing spacing between braces without reinforcing vertical members? This would increase the compressive forces on the bottom deck with fewer load-spreading members to dissipate stress more evenly.
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Why do the counter-diagonal supports meet at a singular point above the height-line of the cable-stay tower? Although it’s for aesthetics rather than support, the cable-stay tower mimics the evenly-spaced attachment points of a cable-stay bridge versus a typical suspension bridge. This dissipates force along the tower rather than a single point at the top. However, if we imagine the counter-diagonals cable-stayed in the opposite direction, we can see that support would not be dissipated properly. This would suggest that even without a support-tower, the angles of force should be adjusted, especially if compressive tensioning has to therefore vary inside each beam.
DIAGRAM OF THEORETICAL REASON FOR FAILURE (enlarge to read):
VIDEO OF COLLAPSE:
This appears to be a design flaw or design oversight. Here are some video still shots showing the collapse path. The collapse appeared to be partially due to over-tensioning and snapping a PT rod. But why were they overtensioning?
Video stills:
CONJECTURE:
From video pictures you can see there was an abrupt, shattering single point of failure. As the deck collapsed it was mostly intact until hitting the ground. It was reported that the failure-point occurred where sagging had loosened the PT rods.
The compressive forces are roughly analogous to a ladder propped up against a building at an extreme angle, while you jam your foot against the feet of the ladder to act as a brace. Remove your foot or simply shift your body-weight back as you straighten your leg, and the ladder is going to slip and fall. Your center-of-gravity shifts and your leg will have less strength to keep the ladder in place, because your leg is acting as a kind of diagonal cantilever with less to buttress against the length of the ladder.
It’s possible they actually needed to tighten PT rods on the opposite end of the bridge to try and re-distribute the load tensioning. Instead, tightening the sagging rods simply removed the static support on the counter-diagonal members?
