[ckissick]

Here's my guess as to how it failed.

I'm assuming the floors were post-tensioned slabs. During construction, you lay out the rebar, then use hydraulic rams to pull them tight, like guitar strings. Then you pour the concrete and let it set. Once set, you release the tension on the rebar. It tries to relax, but transfers the stress to the concrete, making it tight and rigid.

Now, let corrosion begin. If the rebar rusts, it loses its grip on the concrete and the whole thing loosens and weakens. The slab is no longer able to hold the load and fails.