Live load is the changeable part of a structure’s burden: occupants, furniture, and everything else that can arrive, move, and leave. Because a variable load is less predictable than the structure’s own weight, the codes hit it with the biggest multipliers — 1.5Q as the leading variable action in Eurocode 0’s ultimate combinations, 1.6L in the basic ASCE 7 LRFD strength combination. Serviceability checks, by contrast, use near-unity factors, since deflection and comfort are judged at realistic service levels rather than amplified ones.
Residential code tables fix the live load in advance so the lookup stays simple. The 2021 IRC deck joist span table, R507.6, is computed for a 40 psf live load alongside 10 psf dead, with an L/360 deflection limit, No. 2 grade lumber, and wet service — and it explicitly stops applying where ground snow or sustained loads exceed 40 psf, or where concentrated loads like hot tubs land on the deck. Exceed the assumed live load and the whole prescriptive table is off the table: that situation calls for an engineered design.