Live Load Reduction (ASCE 7 Ch. 4)
The reduction a big tributary area earns, which sizing to the full tabulated live load misses: ASCE 7 §4.7 L = L0 (0.25 + 15/sqrt(KLL x AT)), where L0 is the unreduced live load, AT the tributary area, and KLL the element factor from Table 4.7-1 (interior/exterior columns 4, beams and cantilever-slab edge columns 2-3, other members 1). It applies only where KLL x AT >= 400 ft^2 and cannot drop below 0.50 L0 (one floor) or 0.40 L0 (two or more). A 50 psf office load on an interior column (KLL 4) with 400 ft^2 tributary reduces 37.5% to 31.25 psf; a 1,000 ft^2 area hits the 0.50 floor at 25 psf. Loads over 100 psf, garages, and assembly spaces are generally not reducible. A design aid; the adopted code and the engineer of record govern.
Formula and source
L = L0 (0.25 + 15/sqrt(KLL x AT)) where KLL x AT >= 400 ft^2; L >= 0.50 L0 (one floor) or 0.40 L0 (two or more); otherwise L = L0.
The ASCE 7 §4.7 reduction in uniform live loads, L = L0 (0.25 + 15/sqrt(KLL x AT)), with KLL from Table 4.7-1, by name; the equation is unchanged across ASCE 7-05 through 7-22.
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