PT flat slabs vs banded beam slabs

The two layouts

• PT flat slab (flat plate): a uniform slab with tendons banded along one column line direction and distributed (uniformly spread) in the other. It gives a flat soffit, fast formwork and free service routing.
• Banded beam slab: wide, shallow PT beams on one grid direction, with a thinner PT slab spanning between them. It suits longer spans in one direction, such as car parks and offices.

Load balancing and tendon layout

In both layouts the tendon profile is draped so the upward force balances a chosen share of the dead load. Banding concentrates tendons in the column strips, where they also help punching shear; distributed tendons give the slab its two-way action. The balanced load, precompression level and tendon spacing are tuned together during PT design optimisation.

What drives the choice

• Spans and grid: flat slabs suit regular grids; banded beams suit long spans in one direction.
• Punching shear: heavy column loads may need drop panels, shear studs or a banded layout.
• Deflection and cracking: long-term deflection under creep, and crack control at service, often govern slab thickness.
• Floor-to-floor height and services: a flat soffit maximises headroom and simplifies MEP.
• Programme: flat slabs usually give the fastest floor cycle.

Slab thickness optimisation

For a given grid, the thinnest economical slab balances concrete, strand and reinforcement quantities against deflection and punching limits. Preliminary design support compares layouts and thicknesses before the architecture is frozen; for designs already done, an optimisation study checks whether a thinner slab or fewer strands will still pass ACI 318, Eurocode 2 or SBC 304 as applicable.