A.Bryan Allred, a structural engineer with Seneca Structural Engineers in Laguna Hills, Calif., responds: Buckling is a design issue with slender masonry or concrete tilt-up walls that resist vertical loads while supporting their own weight. But with a post-tensioned slab, the weight of the concrete, the weight of the structure, and the supporting soil all act against a buckling-type response. And even though it's theoretically possible to buckle an improperly post-tensioned slab, it's more likely the concrete at the anchors would crush from the extremely high force that would have to be delivered by the tendons before buckling would occur. In fact, this anchorage zone is typically the weak link in the system. In all the slabs I've designed and observed (both slab-on-grade and elevated), I've never witnessed a horizontal post-tensioned element buckle; however, I have seen several concrete blowouts at the anchors.
By the way, in most residential post-tensioned foundations, only a very small amount of rebar is placed. This rebar is typically located in the footings to resist high, concentrated loads from posts and columns, or under shear walls that resist lateral (seismic and wind) forces. The slab itself will typically have only trim rebar around...
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