All Structural Design Starts With Understanding the Loads
Before any beam, column, or wall can be sized, the structural engineer - and the architect who must communicate effectively with that engineer - must understand what loads the structure must resist. Loads are forces and effects that act on a structure. The ARE PPD division tests structural load concepts because architects must understand what types of loads exist, how they are defined, and how they combine to determine the maximum force a structure must withstand. This knowledge is prerequisite to understanding structural systems, connections, and failure modes.
Gravity Loads
Dead Load (D)
Dead load is the permanent, static weight of the structure itself and all permanently attached components. This includes the structural frame (beams, columns, slabs), floor and roof finishes, partitions (when permanently fixed), mechanical and electrical systems attached to the structure, and cladding. Dead load is constant throughout the building's life. For structural analysis, dead load is typically estimated as a weight per square foot - structural concrete is approximately 150 psf per foot of thickness.
Live Load (L)
Live load is the variable, occupancy-based load - the weight of people, furniture, movable equipment, and similar transient loads. Live loads are specified by ASCE 7 (Minimum Design Loads for Buildings and Other Structures) based on occupancy type. Office occupancy: 50 psf. Assembly (fixed seats): 60 psf. Assembly (movable seating): 100 psf. Residential: 40 psf. Storage (heavy): 250 psf. Live loads are reduced for large tributary areas through live load reduction provisions.
Snow Load (S)
Snow load is the weight of accumulated snow and ice on roofs. Ground snow load maps in ASCE 7 establish the design ground snow load by geography. Roof snow load is determined from the ground snow load using conversion factors that account for roof slope, thermal conditions, and roof exposure. Sloped roofs with angles over about 30 degrees receive significantly reduced snow loads.
Lateral Loads
Wind Load (W)
Wind exerts pressure on building surfaces - positive pressure on windward faces and negative pressure (suction) on leeward and side faces. Wind loads depend on basic wind speed (from ASCE 7 maps), building height (wind speed increases with height), exposure category (terrain roughness around the site), building shape, and the building's natural period (for dynamic effects). Wind is a reversible load - it can act in any direction.
Seismic Load (E)
Seismic loads arise from ground acceleration during earthquakes. Buildings are not pushed by seismic forces - they are shaken at the base, and the inertia of the building mass generates dynamic forces. Seismic design category (A through F) depends on the site's seismic hazard and the building's occupancy category. Seismic loads are defined in ASCE 7 Chapter 12 and depend on the building's mass, stiffness, and the spectral acceleration at the site.
Load Combinations
Structures must be designed for the critical combination of loads that could occur simultaneously. ASCE 7 specifies factored load combinations for strength design (LRFD) and service load combinations for allowable stress design (ASD). The most common governing combinations for typical buildings include: 1.2D + 1.6L (gravity), 1.2D + 1.0W + L (gravity plus wind), and 0.9D + 1.0E (seismic uplift case).
Key Exam Points
- Dead load: permanent, constant. Live load: variable, occupancy-based.
- ASCE 7 is the reference standard for minimum design loads.
- Wind and seismic are lateral loads - resisted by shear walls, braced frames, or moment frames.
- Load combinations govern design - structures must resist critical combined effects.
- Seismic loads depend on building mass and ground acceleration, not just geographic zone alone.
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