Deflection is the amount a structural member, such as a beam, slab or double tee, bends or sags under load.
All floors deflect a little. Building codes set limits, usually written as a fraction of the span, such as span divided by 360 for floor live load, to keep floors feeling solid and to protect finishes. In a parking structure deflection matters for another reason: water. Garage floors are sloped gently to drain, and too much sag can create low spots where water and salt collect. See clear-span construction.
Why it matters in a garage
- Ponding. Low spots hold water, which freezes, stains and carries salt into the concrete.
- Long spans. Clear spans of around 60 feet make layouts efficient but must be stiff enough to limit sag and bounce.
- Camber. Precast members are built with an upward bow that offsets deflection; it has to be accounted for in drainage slopes. See precast concrete.
- Creep. Concrete keeps deflecting slowly for years under sustained load.
How engineers control it
- Deeper members, prestressing and post-tensioning. See post-tensioned concrete.
- Drainage slopes that account for expected long-term deflection.
- Checking vibration as well as sag on long spans.
The Parkonomics view
Most people never notice deflection until there is a puddle in the same place every time it rains. Good garage design treats drainage and deflection as one problem.
Related terms: Clear-Span Construction, Post-Tensioned Concrete, Precast Concrete, Cantilever, Structural Systems of Parking, Expansion Joint