Post-Tensioned Slab Calculator

Post-tensioning is a method of prestressing concrete after it has hardened, using high-strength steel strands or tendons that are tensioned against the hardened concrete. This technique places the concrete in compression, which counteracts the tensile stresses caused by applied loads. Post-tensioned slabs are thinner, span longer distances, and crack less than conventional reinforced concrete.

In a post-tensioned slab, steel tendons are threaded through ducts or placed in unbonded sheaths before the concrete is poured. After the concrete reaches sufficient strength (typically 75% of design strength), the tendons are stretched using a hydraulic jack and anchored at the edges. The force in the tendons compresses the concrete, creating a precompressed zone that resists service loads.

This calculator helps engineers design post-tensioned slabs by computing tendon forces, precompression levels, balanced loads, friction losses, and concrete quantities. It is a preliminary design tool that provides starting values for detailed structural analysis per PTI and ACI 318 requirements.

The calculator uses several fundamental formulas from post-tensioned concrete design:

Average precompression is the total prestress force divided by the gross concrete area. It represents the uniform compressive stress applied to the slab by the tendons. The target range for post-tensioned slabs is typically 125 to 300 psi.

Precompression below 125 psi may not adequately control cracking and deflection. Above 300 psi, the slab may experience excessive short-term deflection (camber) and the concrete may be overstressed during the jacking phase. The calculator flags whether your design falls within this target range.

The formula is: Precompression = Total Force (lbs) / Slab Area (in²). The slab area is calculated as the plan dimensions converted to square inches.

The load-balancing method is the most intuitive way to understand post-tensioning. The parabolic profile of the tendon creates an upward equivalent load that counteracts the downward gravity loads on the slab. This upward load is called the balanced load.

The balanced load equals the equivalent upward force from the tendon drape divided by the tributary width. The net load on the slab is then the total applied load minus the balanced load. If the balanced load equals the dead load, the deflection under dead load is theoretically zero.

Typically, designers balance 60–100% of the dead load, leaving the remaining dead load plus the full live load as the net load that the slab must resist in bending.

Enter the following parameters to design your post-tensioned slab:

1. Slab Dimensions: Enter the length, width, and thickness in feet and inches.
2. Concrete Strength (f'c): The specified compressive strength in psi (typically 4,000–6,000 psi).
3. Tendon Layout: Select strand size (1/2-inch or 0.6-inch), tendon spacing, and jacking stress percentage.
4. Loading: Enter superimposed dead load and live load in psf.

Results include tendon count, jacking force, precompression level, balanced load, friction losses, and concrete volume. Review the precompression range indicator and adjust tendon spacing if needed.

Post-tensioned slabs are used in high-rise buildings, parking garages, bridges, mat foundations, and residential floor systems. They allow longer spans with thinner slabs, reducing floor-to-floor heights and overall building weight. In residential construction, PT slabs enable open floor plans without intermediate beams or columns.

Commercial buildings use post-tensioning to reduce foundation costs by minimizing the number of columns needed. Parking structures benefit from the reduced slab thickness, which allows more parking levels within a given building height. Bridges use post-tensioning for long-span box girders and segmental construction.