What Is Deflection in Structural Engineering?
Deflection is how much a structural member bends under load. Controlling deflection is critical — excessive sag causes cracked finishes, bouncy floors, and doors that won't close. Code sets strict deflection limits for different applications.
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Why deflection matters
A beam that's strong enough but too flexible will sag visibly, crack drywall and tile above, cause floor vibrations that occupants feel, and create drainage problems on roofs (ponding). Strength prevents collapse; stiffness (deflection control) ensures the building functions properly.
Code limits
IBC Table 1604.3 sets deflection limits: floor beams supporting plaster ceilings are limited to span/360 under live load. Roof beams: span/240. A 20-foot beam at span/360 can deflect only 0.67 inches under full live load. These limits often control member sizing more than strength does.
Long-term deflection
Concrete and wood creep over time — a slab that meets deflection criteria on day one may sag excessively after five years. Engineers account for creep and shrinkage in long-term deflection calculations, especially for concrete. Post-tensioning is often used specifically to control long-term deflection in slabs.
The bottom line
Deflection limits (typically span/360 for floors) often control member sizing more than strength. Long-term creep must be considered.
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