How Is Precast Double Tee Design Done for Floors and Roofs?

Double tees are designed as pretensioned beams for gravity loads — stem depth, strand pattern, and concrete strength sized for span and loading — while the flange system is designed as a structural diaphragm with grouted or topped joints transferring shear between tees. Connection details at supports handle bearing, lateral stability during erection, and the diaphragm chords and collectors that tie the floor into the lateral system. Think of it as a product plus a system. The precaster engineers the product — the individual tee and its prestressing. The engineer of record engineers the system — how hundreds of tees become a floor diaphragm, how it connects to the frames and walls, and how the whole thing behaves under lateral load.

The gravity design is prestressed concrete at its most standardized: stems sized for the span, strands draped or straight to match the moment diagram, concrete strength matched to the prestress force, and camber calculated so the tee arrives flat under its own weight plus topping. Release, handling, and transportation stresses get checked because a tee is lifted and hauled in conditions nothing like its final supports. The diaphragm design is where the engineering judgment lives. Shear has to flow from tee to tee through flange connections — weld plates with grout, or the composite topping — and accumulate into chords and collectors that deliver it to shear walls or frames. Openings for stairs, ramps, and shafts interrupt the diaphragm and need dedicated detailing. In seismic regions the diaphragm design forces, amplified for the flexibility of the precast system, often control the connection design rather than gravity.

Double tee floors are remarkably forgiving of gravity overload and remarkably sensitive to connection and movement details. The long-term serviceability issues I see — leaking joints, cracked flanges, distressed connections — trace to movement that wasn't accommodated or water that wasn't managed.

• Design the diaphragm, not just the tees: flange connections, chords, and collectors sized for lateral forces
• Accommodate volume change: prestress shortening and thermal movement need somewhere to go
• Detail bearings for rotation and movement: elastomeric pads, not hard bearing that cracks stems
• Manage water in parking structures: joint sealants, drainage, and topping details that keep chlorides out
• Coordinate openings early: every shaft and ramp opening interrupts the diaphragm and needs a detail