Live load, also known as imposed load or variable load, refers to temporary and movable loads that a structure must support during its service life. Unlike dead loads which are permanent, live loads can change in magnitude and position over time. These include the weight of people, furniture, equipment, vehicles, stored materials, and movable partitions. Live loads also encompass environmental factors such as wind pressure, snow accumulation, and seismic forces in some design codes.

The magnitude of live loads varies significantly depending on the building's intended use and occupancy. Residential buildings typically have lower live loads (1.5-2.0 kN/m² or 30-40 lb/ft²) compared to commercial offices (2.5-4.0 kN/m² or 50-80 lb/ft²) or industrial facilities (5.0-7.5 kN/m² or 100-150 lb/ft²). Building codes specify minimum live load values for different occupancy types to ensure structural safety under normal and anticipated extreme loading conditions.

Live load calculation starts with determining the area over which the load will be applied and the appropriate unit load for the building's use type. For example, an office space of 100 m² with a standard live load of 2.5 kN/m² and a safety factor of 1.5 would have a total live load of: 100 m² × 2.5 kN/m² × 1.5 = 375 kN. This represents the total variable load the floor structure must be designed to support.

For multi-story buildings, live loads are often considered floor by floor, though some codes allow for live load reduction in columns and foundations supporting multiple floors, as it's statistically unlikely that all floors will experience maximum loading simultaneously. The calculated live loads, combined with dead loads and other factors, determine the required strength and size of structural elements including beams, slabs, columns, and foundations.