What Is Post-Tensioned Slab Design?
Post-tensioned (PT) slabs embed high-strength steel tendons in the concrete, tensioned after the concrete cures — the active compression balancing the gravity loads. The engineer designs the tendon profiles (the draped parabolas), the prestress forces, and the mild reinforcement for the long, thin floors: 30-40 foot spans at 8-10 inch depths. The high-rise residential and the parking structure's floor system.
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How PT slabs work
The tendons (typically 1/2-inch 270 ksi strand in sheathed ducts) drape in the parabolic profile — high at the supports, low at midspan; tensioning (to ~33 kips per strand) compresses the concrete, the upward tendon forces balancing the downward loads (the load-balancing method — the classic PT design approach). The result: the thinner slab spanning farther, with the controlled cracking (the precompression keeping the concrete in compression under service loads).
Design considerations
The tendon layout (the banded-distributed or the distributed-distributed — the two-way systems' tendon arrangements), the stress checks (the concrete stresses at transfer and at service — the ACI 318 limits), the deflection (the long-term — the creep and the shrinkage; the PT slab's deflection controlled but not zero), the punching shear (the column-slab connection — the stud rails or the drop panels where the shear governs), and the restraint (the walls and the columns restraining the shortening — the restraint stresses designed).
Construction and durability
The tendon installation (the chairs and the supports — the profile's accuracy; the concrete placement — the consolidation around the ducts), the stressing (the calibrated jacks — the elongations verified against the calculations; the multi-strand sequence), and the durability (the encapsulated systems — the grease-filled sheathing protecting the strand; the grouted vs. unbonded — the corrosion protection philosophy). The PT slab's Achilles' heel (the future coring — the as-built tendon locations documented; the x-ray before the coring — the severed tendon is the catastrophe). ACI 318 Chapter 8 and the PTI guides govern.
The bottom line
PT slabs (draped 270 ksi tendons, load-balancing design) achieve 30-40 ft spans at thin depths per ACI 318/PTI — control punching shear, restraint stresses, and document tendon locations against future coring.
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