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Q.When making structural changes that involve adding a point load to a house built on a post-tensioned concrete slab, is it necessary to add a footing for a structural post? I'm concerned that cutting through the slab to pour a pad footing could sever one of the cables, but I don't want to assume that the slab can support a point load anywhere.

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 5,000 pounds for a typical 5-inch-thick residential slab.

If more bearing is needed, it's possible to cut through the slab and pour a pad footing. But in doing so, it's important to avoid cutting through any tendons, since that will weaken the foundation. Because the tendons are under thousands of pounds of tension, cutting through one is also potentially hazardous. Tendon locations can be pinpointed with a high-strength metal detector or digital scanner. The tensioning force in the cables is transferred to the slab by way of steel fittings - which are ordinarily cast into the slab's exterior edges - so any retrofitted openings should be located several feet away. It's a good idea to consult a structural engineer if you'll be working anywhere near an edge.

As long as they are not cut or damaged, the tendons themselves can safely be exposed within an opening. They're typically enclosed in a plastic sleeve for protection and to keep the concrete from bonding to them. Once the opening for the new footing is complete, prepare the soil/subgrade as usual, install the rebar, and pour the concrete. Adding epoxy dowels and roughening the surface of the existing concrete will make for a stronger bond between the original slab and the new pour.