Section L3/Deflections
AISC 360-22 Section L3 covers serviceability deflection criteria for steel members at service loads with IBC Table 1604.3 limits.
Deflections of structural steel members under service loads must not impair the serviceability of the structure. AISC 360 Section L3 states that deflections shall be checked at service load levels (not factored loads) and compared to limits that are appropriate for the building type, function, and finishes supported. The specific deflection limits are typically taken from IBC Table 1604.3: L/360 for live load deflection of floor members supporting plaster, L/240 for floor members without plaster, and L/180 for roof members. Long-span steel beams and open-web steel joists are particularly susceptible to deflection issues.
Why this section exists
A steel beam can be strong enough to carry the load without yielding but still deflect enough to crack plaster, cause doors to bind, or create ponding on a flat roof. Steel has a relatively low modulus of elasticity compared to its strength, meaning long-span steel members often have deflection as the controlling design criterion rather than strength. Serviceability checks ensure the structure performs acceptably under everyday loading conditions, not just under extreme loads.
What plan reviewers look for
Plan reviewers check the structural calculations for deflection checks at service loads. They verify the deflection is compared to the correct limit from IBC Table 1604.3 based on the member type and supported elements. For long-span beams, they check whether camber is specified to offset dead load deflection. For flat roofs, they check for ponding analysis per IBC Section 1611.