I thought it was an AISC requirement that steel beams be supported by steel, concrete or masonry. But now I am finding it difficult to locate the specific reference. In Colorado it seems the practice in commercial buildings is not to use wood support and for residencial it is acceptable to use wood to support steel.
Used to be that UBC did not allow wood to be used to support concrete or masonry (with a few exceptions like floor toppings, see UBC 94, 2316.1). Don't remember any code prohibitions regarding timber supporting steel framing. IBC has different provisions for long term loading of timber with masonry or concrete loads, see IBC 2006, 2305.1.
