How Are Houston Post-Tensioned Slabs Designed?

The Post-Tensioning Institute's method for slabs on expansive soil is the industry standard the engineer follows, and it is admirably direct: it starts with how much the soil will move and ends with a slab stiff enough to tolerate it. The geotechnical engineer provides the predicted edge lift and center lift with their associated moisture variation distances; the structural engineer uses the PTI design charts and equations to select rib depth, rib spacing, slab thickness, and prestress force. The method checks both bending and shear in the ribs under the distorted soil profiles, and it sets minimum prestress levels so the slab stays in compression under service loads. The rib layout is where the design becomes physical. Perimeter ribs run deeper than the interior — the slab edge is where moisture changes hit first and hardest — and interior ribs form a grid that breaks the slab into stiff panels. Tendons run in both directions through the ribs and the slab, banded in one direction and uniform in the other in the typical layout. The engineer details the tendon profiles, the anchorage zones at the slab edges where the jacking force enters the concrete, and the pocket formers that are grouted after stressing. Every element on the drawing traces back to the soil movement number the design started with.

A post-tensioned slab on gumbo clay is a system that includes the ground around it, and the engineer designs that ground. Positive grading carries surface water away from the slab edge; gutters discharge well clear of the foundation; a moisture barrier under the slab slows vapor drive; and the plans note tree setbacks or root barriers where large trees could desiccate one side of the soil. These details are drawn and specified with the same seriousness as the tendon layout, because differential moisture is the load the whole design is resisting — uncontrolled water makes the engineer's movement assumptions wrong. Field inspection closes the loop between design and performance. Before the pour, the inspector verifies tendon placement, profile, and support against the drawings; after the concrete reaches strength, the stressing operation is observed and the measured tendon elongations are checked against the calculated values — elongation is the field proof that the designed force actually went into the steel. Anchorages are grouted and protected after stressing. The engineer reviews these records because a post-tensioned slab whose tendons were never properly tensioned is just an expensive conventional slab sitting on gumbo clay, and the owner deserves the foundation the drawings promised.