Placing two-column floats at bottom of page

You specified placement '[htbp]' for your full-width figure or table, but they always get placed at the top of the page... Well, it is what the documentation says: LaTeX, unadorned, only allows full-width floats at the top of a page, or occupying (part of) a float page.

The stfloats package ameliorates the situation somewhat, and makes LaTeX honour '[b]' placement as well.

The FAQ team doesn't know of any package that will make LaTeX honour '[h]' placement of double-column floats, but the midfloat package can be pressed into service to provide something approximating the effect it would have.

stfloats.sty, midfloat.sty
Distributed as part of macros/latex/contrib/supported/sttools.tar.gz