Load Calculation
Working out every force a structure must safely resist.
Quick Answer
Load calculation is the process of determining the forces a structure must resist, including dead loads from its own weight, live loads from occupants and contents, and environmental loads from wind, snow, and earthquakes. Engineers combine these using code-prescribed factors, typically from ASCE 7, to size members and foundations with an adequate margin of safety.
The Full Picture
Before an engineer can size a single beam, they have to know what it will carry. Load calculation answers that question. It identifies each source of force, estimates its magnitude and location, and sets up the conditions the structure must survive over its design life.
The main categories are dead loads, which are the permanent weight of structural and fixed nonstructural elements; live loads, which are variable occupancy and use loads; and environmental loads such as wind, snow, rain, flood, and seismic. Soil and fluid pressures, thermal effects, and construction loads also apply in some cases.
In the United States, minimum design loads are generally taken from ASCE 7, which the International Building Code adopts by reference. Wind and seismic loads depend on location, building height and shape, occupancy category, and site conditions. Loads are then combined, for example gravity plus wind, using factors prescribed in the standard, and members are checked against the governing combination.
Contractors see the results of load calculations on structural drawings as design criteria, floor live-load values, and member schedules. Knowing them matters in the field too: stockpiling materials on a slab, parking a crane, or adding rooftop equipment can exceed the loads the engineer assumed, which is why such changes need engineering review.
Real Examples
Common Misconceptions
People assume: Load calculation just means adding up the weight of the building.
Actually: Weight is only the dead load. Live, wind, snow, and seismic loads, plus how they combine and act laterally, often govern the design.
People assume: A structure that is strong enough today will always be strong enough.
Actually: Added equipment, changed use, deteriorated materials, or a heavier roof assembly can change loads. Alterations require a fresh check by an engineer.
Frequently Asked Questions
What is the difference between dead load and live load?
Dead load is the permanent weight of the structure and fixed components. Live load is the variable load from people, furniture, and movable equipment, which changes with the building's use.
What standard defines design loads in the US?
ASCE 7, Minimum Design Loads and Associated Criteria for Buildings and Other Structures, is the primary reference, and the International Building Code adopts it for most jurisdictions. Local codes may add requirements.
What are load combinations?
Load combinations are code-defined ways of adding different load types together, each with factors that reflect how likely they are to act at the same time, so members are checked against the worst realistic case.
Why do wind and seismic loads matter so much?
They act horizontally and can be very large relative to gravity loads, so they often control the design of the lateral system, connections, and foundations, especially in tall buildings or high-hazard regions.