[steve86]

Even a child building a bridge from popsickle sticks would be uncomfortable with the idea of no support underneath or overhead, whether a crane dropped on it or not.

[ProtectOurFreedom]

You wrote " uncomfortable with the idea of no support underneath or overhead". But that is the very essence of any bridge, isn't it? You have to have an open, unsupported section, otherwise it would be useless as a bridge. The concrete structure that collapsed is a self-supporting truss bridge. For some reason (esthetics? greater load carrying capacity?), a suspension design was to be added to the structure over the next months. But the truss section was obviously designed to support itself and it failed due to bad design, materials problems, or bad construction practices.

Wiki --> A truss bridge is a bridge whose load-bearing superstructure is composed of a truss, a structure of connected elements usually forming triangular units. The connected elements (typically straight) may be stressed from tension, compression, or sometimes both in response to dynamic loads. Truss bridges are one of the oldest types of modern bridges. A truss bridge is economical to construct because it uses materials efficiently.

As long as the concrete is in compression (which can be accomplished via pretensioning), the material can be used successfully in truss bridges.

There was a good FR article last month about the first railroad bridge to cross the Mississippi in 1856. See Bridging the Mississippi: The Railroads and Steamboats Clash at the Rock Island Bridge.