Structural loads are the forces and effects that a building, bridge, or other structure must safely resist. Engineers classify loads so they can predict how beams, columns, slabs, bracing, and foundations will behave under everyday use and extreme events. Understanding load types matters because an unsafe structure can fail, while an overbuilt structure can waste materials and money.
Good design balances safety, serviceability, durability, and cost.
Key Facts
- Dead load D is the permanent weight of the structure and fixed equipment.
- Live load L is the variable load from people, furniture, vehicles, storage, or movable equipment.
- Total vertical gravity load on an area can be estimated by W = qA, where q is load intensity and A is area.
- Wind pressure can be approximated by p = 0.5 rho V^2, where rho is air density and V is wind speed.
- Thermal strain is epsilon = alpha Delta T, where alpha is the coefficient of thermal expansion.
- A typical strength load combination is 1.2D + 1.6L, but actual design combinations depend on the building code and load case.
Vocabulary
- Dead load
- A dead load is a permanent load caused by the weight of structural members, finishes, fixed equipment, and other parts that stay in place.
- Live load
- A live load is a temporary or movable load caused by occupancy, furniture, vehicles, stored materials, or maintenance activity.
- Lateral load
- A lateral load is a horizontal load, such as wind or earthquake force, that pushes a structure sideways.
- Load combination
- A load combination is a code-specified equation that combines different load types with factors to check structural safety.
- Load path
- A load path is the route forces follow through slabs, beams, columns, bracing, and foundations into the ground.
Common Mistakes to Avoid
- Treating dead load and live load as the same thing is wrong because dead load is permanent while live load changes with use and occupancy.
- Forgetting lateral loads is wrong because wind and seismic forces can control the design of tall buildings, long-span roofs, braced frames, and foundations.
- Adding unfactored loads when a strength check requires factored loads is wrong because design codes use load factors to include uncertainty and safety margins.
- Ignoring the load path is wrong because every load must be transferred continuously through structural members to the foundation and supporting soil.
Practice Questions
- 1 A floor bay has an area of 48 m^2 and carries a dead load of 3.5 kN/m^2 plus a live load of 2.0 kN/m^2. Find the total unfactored gravity load on the bay.
- 2 Using the strength combination 1.2D + 1.6L, calculate the factored load per square meter for a slab with D = 4.0 kN/m^2 and L = 3.0 kN/m^2.
- 3 A building has heavy concrete walls, a roof that can collect snow, and a tall flexible frame in a windy region. Explain which load types are likely to be important and how a continuous load path should carry them to the foundation.