Florida International University pedestrian bridge collapse

[Anomalocaris]

The Milau Viaduct is a beautiful bridge, I'll grant that.  Videos of it being constructed are some of the most scary, vertiginous scenes I've ever encountered.

As for the pedestrian overpass, a 950 ton, 174 foot long structure is pretty remarkable.  A local truss bridge in this area is probably over 100 years old, spans 185 feet, and couldn't weigh more than 50 tons . . . . .

This is suppose to be a cable stayed bridge.   I see no evidence of the central pylon and cables in the wreckage.  So the bridge was incomplete and seem to lack its main structural support.

Without the suspension cables that put the entire bridge in compression, the bridge surface and the roof would act like a truss beam, with lower bridge surface under tension and upper roof under compression.    Concrete does not so well under tension.   Concrete structure meant to withstand tension needs to be put under pre-compression using tension cables.   This bridge probably doesn’t have tension cables  in its lower surface since the suspension cables, when installed, would play that role.

Since this is a pedestrian bridge, it is probably not designed to withstand any great live load.  Most of the bridge’s designed load would be its own dead weight.  So even before the bridge is finished, the incomplete span’s own weight probably already loaded it to a large percentage of the completed structure’s total designed capacity.

So I bet the bridge was collapsed because it was never designed to sit like a beam between its supports without suspension cables in place to help carry the weight and also put bridge surface under compression. So it probsbly always required the suspension cables to resist its own dead weight.