It is/was the CRANDIC (ex MILW) 105 y.o. thru truss that paralled 8th street. It was ballasted with a string of rock filled hoppers but at that depth and current.............
A view of satelite imagery may explain why 3 spans failed and 1 held.
Here's a video of the bridge collapse from the Weather Channel.
http://link.brightcove.com/services/link/bcpid823425597/bclid877032950/bctid1604978369
John