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.