That's how ours has been done....pour footing w/re-bar sticking up, then pour slab later.

Here's photos of a 42' bin we put up in '09. Footer, then slab. The actual footer is hard to see since it was back-filled w/ag lime.

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dsc03156 (2).jpg (47KB - 43 downloads)

dsc03179 (2).jpg (53KB - 40 downloads)