Concrete foundations for metal buildings: what your slab has to do

The slab is structural

A rigid frame pushes down and outward at each column base. The foundation resists that horizontal thrust, either through a hairpin tied into the slab, a tie rod across the building, or a deepened footing. Which one you get comes out of the reactions on your sealed drawings and your soil bearing capacity.

Anchor bolt tolerance is unforgiving

Bolt patterns are set from a template before the pour. Out-of-position bolts mean field drilling, epoxy anchors, and an engineer's approval — an easy week lost. Use the template, brace it, and check it after the pour truck leaves.

Typical specification for a shop or garage

• 5" slab, 4,000 psi, with fiber or #4 rebar on 18" centers
• Thickened edge 12–16" wide, below local frost depth
• 10 mil vapor barrier under any heated or finished space
• Compacted granular base, 4–6" of #57 stone
• Control joints on a spacing of roughly 12–15 feet

Get a geotechnical report on commercial work

Assumed bearing capacity is a guess. On any commercial building, a soils report costs a small fraction of the foundation and frequently pays for itself by letting the engineer design a lighter footing.