Live loads are the temporary, movable loads a structure must carry, such as vehicles, people, furniture and equipment, as distinct from dead loads, the permanent weight of the structure itself.
In the United States, minimum live loads are set by ASCE 7 as adopted by the building code. For passenger vehicle garages, the standard requires a uniform live load of 40 pounds per square foot and, separately, a concentrated load of 3,000 pounds on a 4.5 by 4.5 inch area, placed wherever it produces the greatest effect. Garages for trucks and buses are designed for heavier loads. Roof levels must also carry snow where it applies.
Design points
- Load reduction. Unlike many occupancies, live loads in passenger vehicle garages generally may not be reduced for large tributary areas, except limited reductions for members supporting two or more floors.
- Concentrated load. The wheel load often governs thin slabs, double-tee flanges and joist systems. See concentrated load.
- Factored design. Engineers combine live and dead loads with load factors for strength design. See factored load and service load.
- Heavier vehicles. Electric SUVs and pickups weigh more than the cars older garages were designed around. The uniform load is spread over the floor and is generally considered adequate, but owners of older or deteriorated structures should have an engineer review capacity and condition.
The Parkonomics view
The live load is a design promise about what the deck will carry. When the vehicles change, or the structure deteriorates, the promise should be checked, not assumed.
Related terms: Concentrated Load, Factored Load, Service Load, Ultimate Load, SUV