- Q. Is it possible that a post-tensioned garage slab could literally buckle in half when the tendons are tensioned, or does rebar in the slab prevent that from happening? Also, why are these types of slabs so common in Las Vegas? Is it because of the soil?
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 penetrations, re-entrant corners, or any other odd condition.
Las Vegas is indeed a hotbed for post-tensioned concrete, both elevated and slab-on-grade, but it's not necessarily because of the soil. There is some expansive soil in the northern part of Las Vegas, but the majority of the sites are nonexpansive. Las Vegas developers have used post-tensioned foundations for decades because they provide a proven and economical solution even for nonexpansive sites; they minimize pad footings for post loads and interior footings under bearing walls. As a rule of thumb, a properly designed post-tensioned slab can resist a post load of about 1,000 pounds per inch of slab thickness, so a 5-inch-thick slab can resist a 5,000-pound post load. In addition, a post-tensioned slab can resist most residential bearing-wall loads without the use of a down-turned footing.
