Precompression vs. Balanced Load in Post-Tension Concrete

My Post–Tension elevator speech is pretty well rehearsed at this point: “We drape steel cables — tendons — in the concrete forms, and then place the concrete.  Once it cures, we pull the ends of the tendons, which try to straighten out.  That does three things: the tendons lift in the low spots, push down in the high spots, and squeeze the slab together.  The net effect is a slab that’s about 20% thinner than a similar conventional slab.”

That’s a great introduction for the barber or cab driver, but at functions with other engineers, there’s a slightly higher technical bar.  We start talking about two terms — balanced load and precompression.  Both of these are critical to the way that post-tensioning works.

Balanced Load is the technical term for “the tendons try to straighten out.”  Every slab that has draped tendon is taking advantage of this — using the post-tension tendons to lift in the spots far away from the supports.  Unfortunately, nothing comes free, so pushing up on the middle of the slab has to push down on the supports.  What engineers have to ask is “How much is too much?” — as I noted in my post about balancing post-tension, too much lift can actually cause deflection problems in adjacent bays, or curling in cantilevers.  Additionally, the load had to go somewhere . . . and punching shear is a common problem, since everybody loves thinner slabs.  Given these issues — as well as constructability concerns and final performance — most engineers try to start around 75% of the self-weight of the slab.

Precompression is the technical term of “squeezing the slab together.”  With all the wonderful things that balanced load does for a slab, it’s easy to think that once we’ve balanced 75% of the dead load, the design is complete.  Unfortunately, this isn’t the case.  As you’ll recall, elastic losses are directly proportional to the amount of compression that we’ve introduced using the post-tension tendons.  This makes precompression an excellent “sanity check” as we design the slab — once a slab passes a 300 psi “squeeze”, it’s time to ask yourself if you’ve correctly modeled the geometry and loading, or if it’s time to increase your slab thickness.  The ACI also caps the amount of compression that you can throw at any given spot on a member — once you pass that limit, it simply isn’t safe to continue squeezing the slab.

The other side of the design tightrope is that a certain amount of compression is required.  One of the things that makes post-tension slabs so economical is that we can use the gross concrete section — the entire area of the member — for many of our calculations, as opposed to about half, with conventionally reinforced members.  The first step, of course, is to prove that this is reasonable — that the concrete will not crack in service.  This is handled by a check of the largest tensile force in the member, and comparing it to a code-mandated maximum value (which varies by application and jurisdiction).  Since this check is for the live-loaded member, it’s entirely possible that a designer will have to reduce balanced load in order to keep the maximum tension stresses in line without causing problems.  Additionally, since post-tension slabs are placed without expansion joints, there has to be at least enough compression to mitigate shrinkage cracks.  The current version of the ACI 318 states that in no circumstances may the precompression in a slab drop below 125 psi.

At EVstudio, our team of experienced engineers and designers are extremely familiar with the balancing act required to wring the most value out of a post-tension slab — and we’re happy to put that familiarity to work for you.