ACI 318-19 Table 24.2.2 | Maximum Permissible Calculated Deflections
Concrete structures must remain functional and serviceable. Excessive sagging causes cracked drywall, ponding water, and misaligned doors. Table 24.2.2 sets strict span-to-deflection ratios based on what elements the floor or roof supports.
Governing Formula
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Notation & Variables
Worked Example
Determine span and criteria
For a 24-foot floor span (l = 288 in) supporting nonstructural partitions that are likely to be damaged by large deflections.
Identify governing limit
The code mandates that the long-term deflection (immediate live load + creep/shrinkage under sustained load) shall not exceed l/480.
Compare with calculated deflection
Calculate cracked moment of inertia (Icr), effective moment of inertia (Ie), and time-dependent multiplier λΔ to verify that Δtotal ≤ 0.60".
When does this govern?
Deflection limits frequently control the required depth of long-span commercial office floors and cantilever balconies rather than flexural strength.
Common Mistake
Using gross moment of inertia (Ig) instead of effective moment of inertia (Ie per §24.2.3.5), which severely underestimates deflections by a factor of 2 to 3.