A.Brian Allred, S.E., of Seneca Structural Engineering in Laguna Hills, Calif., responds: A typical post-tensioned foundation is reinforced with high tensile-strength steel strands instead of conventional rebar or wire mesh. Once the slab is poured, the strands are tensioned to a predetermined level with a hydraulic jack to apply compressive force to the concrete, allowing it to bear heavy loads and resist cracking. In most single-family homes, these strands - or "tendons," as they're called - are spaced on a 3- or 4-foot grid and centered within the 5-inch-thick slab. Integral interior footings, or "ribs," are usually spaced about 12 feet apart in each direction, giving the underside of the slab a waffle-like configuration.
If the new post load can be located over an interior footing - which should be called out on the foundation plan - there's ordinarily no need for any other reinforcement. A typical interior footing should support a 10,000-pound load. Even between footings, a post-tensioned slab can safely bear about 1,000 pounds per inch of thickness, or about...
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