A factored load is a load multiplied by a load factor, so a structure designed to resist it carries a margin of safety against uncertainty in both the loads and the materials.
Factored loads are the basis of strength design, also called load and resistance factor design (LRFD). Engineers estimate the service loads a structure will actually see, multiply each by a factor that reflects how uncertain it is, combine them, and size members so their reduced strength exceeds the result.
Load combinations
ASCE 7 sets out the combinations to check. Two of the most familiar are 1.4D, for dead load alone, and 1.2D + 1.6L + 0.5(Lr or S or R), where D is dead load, L is live load, and the last term covers roof live load, snow, or rain. Live load gets a higher factor than dead load because it is less predictable.
Factored versus service loads
Strength is checked with factored loads. Deflection, vibration, and cracking are checked with unfactored service loads, because they describe how the structure behaves in everyday use rather than how it fails.
In parking structures
Passenger vehicle garages are designed for a uniform live load of 40 pounds per square foot under the 2024 IBC and ASCE 7-22. In the 1.2D + 1.6L combination, that live load enters design at 64 pounds per square foot. The arrival of heavier electric vehicles raised the question of whether 40 is still enough. Research funded by the Charles Pankow Foundation found that equivalent uniform loads stayed at or below 40 pounds per square foot in nearly all modeled scenarios, including clustered EV charging, and proposed no increase.
Related terms: Service Load, Concentrated Load
Further reading: EV Charging coverage on Parkonomics