Types of Loads

A load is any force or action that acts on a structure. Structural engineers must identify every load that could act on a building during its lifetime, determine the magnitude of each load, and design the structure to safely resist all combinations of these loads. Understanding load types is the starting point for all structural analysis and design — and it is essential knowledge for construction professionals who must ensure that the building is constructed to carry the loads it was designed for.

Loads on buildings are categorized by their nature, direction, and duration. The primary reference for determining design loads in the United States is ASCE 7: Minimum Design Loads and Associated Criteria for Buildings and Other Structures, published by the American Society of Civil Engineers.

Dead loads are the permanent, constant loads from the weight of the building itself — its structure, finishes, and all permanently installed components. Dead loads do not change over the building's lifetime (unless the building is renovated). They include:

• Structural elements: Beams, columns, walls, floors, roof structure, foundations
• Permanent finishes: Drywall, plaster, tile, flooring, ceiling systems
• Fixed equipment: HVAC units, plumbing piping, electrical conduit
• Roofing materials: Built-up roofing, single-ply membrane, shingles, insulation
• Cladding: Brick veneer, stone, curtain wall, siding

Dead loads are calculated by multiplying the unit weight of each material by its quantity (volume, area, or length). Common unit weights:

Example calculation: A 6-inch concrete slab weighs 150 pcf × 0.5 ft = 75 psf (pounds per square foot). This 75 psf is a dead load that every beam, column, wall, and foundation element below this slab must support.

Dead loads are the most predictable loads because the materials are known and their weights are well-established. They act continuously — 24 hours a day, 7 days a week — for the entire life of the structure.

Live loads are temporary, moveable loads from occupancy, use, and moveable equipment. They include:

• People: Occupants, visitors, workers
• Furniture: Desks, chairs, cabinets, shelving
• Moveable equipment: Computers, copiers, appliances
• Stored materials: Warehouse inventory, library books, filing cabinets
• Vehicles: In parking garages, fire truck access roads

Because it is impossible to predict exactly how much load occupants and their belongings will impose, building codes specify minimum design live loads based on the occupancy type:

Roof live loads account for workers, materials, and equipment during construction and maintenance. Typical minimum roof live load is 20 psf, reduced based on tributary area and roof slope.

Live load reduction: For large floor areas, it is statistically improbable that the entire area will be loaded to the full design live load simultaneously. ASCE 7 allows live load reduction for members supporting large tributary areas (generally greater than 400 square feet), up to specified limits. This reduction can significantly affect the sizes of columns and foundations in large buildings.

Snow loads are the weight of accumulated snow on roofs. They depend on:

• Ground snow load (pg): The weight of snow on the ground, specified by ASCE 7 maps based on geographic location. Ranges from 0 psf in southern states to 100+ psf in mountainous areas.
• Roof slope: Steeper roofs shed snow more readily, reducing the design snow load.
• Exposure: Roofs in wind-exposed areas have lower snow loads (wind blows snow off); roofs in sheltered areas have higher loads.
• Thermal factor: Heated buildings melt snow from below, reducing accumulation.
• Importance factor: Critical facilities (hospitals, fire stations) are designed for higher snow loads.

The basic roof snow load is calculated as:

pf = 0.7 × Ce × Ct × Is × pg

Where Ce = exposure factor, Ct = thermal factor, Is = importance factor.

Drifting snow is a critical consideration. Snow drifting onto lower roofs adjacent to taller portions of the same building (or adjacent buildings) can create concentrated loads much higher than the flat roof snow load. These drift loads can be the controlling design condition for many roof members.

Rain-on-snow: In many regions, a surcharge of 5 psf is added to account for rainwater absorbed by snow, which can significantly increase the effective snow load.

Wind loads are lateral (horizontal) forces imposed on buildings by wind pressure and suction. Wind is the most significant lateral load for most low-rise and mid-rise buildings (seismic loads may govern for certain building types and locations).

Wind creates pressure on the windward side of a building and suction on the leeward side, sides, and roof. The combined effect is a net lateral force that the building must resist. Wind loads depend on:

• Basic wind speed (V): The 3-second gust speed specified by ASCE 7 maps, based on geographic location and risk category. Ranges from 95 mph in interior regions to 180+ mph in hurricane-prone coastal areas.
• Exposure category: Describes the terrain roughness surrounding the building:Exposure B: Urban/suburban areas with closely spaced obstructions (most common)Exposure C: Open terrain with scattered obstructions (flat, open country)Exposure D: Flat, unobstructed coastal areas
• Building height: Wind speed and pressure increase with height above ground.
• Building shape and size: Pressure coefficients account for how wind flows around different building shapes.
• Internal pressure: Buildings with openings (doors, windows, louvers) experience internal pressure that adds to the structural loading. If a windward window breaks during a hurricane, internal pressure increases dramatically — the building effectively becomes a balloon.

Wind pressure is calculated using:

p = qGCp - qi(GCpi)

Where q = velocity pressure (proportional to wind speed squared), G = gust factor, Cp = external pressure coefficient, qi = internal velocity pressure, GCpi = internal pressure coefficient.

For construction professionals, the key concepts are:

• Wind loads are lateral forces that try to push the building sideways, overturn it, and uplift the roof.
• Wind loads increase with building height and decrease with terrain roughness.
• Hurricane-prone regions require significantly higher wind load design.
• Openings in the building envelope during construction or storm damage dramatically increase wind loading.
• Components and cladding (individual wall panels, windows, roofing) experience higher local wind pressures than the building as a whole, especially at corners and edges.

Seismic loads are forces generated by earthquake ground motion. When the ground shakes, the building's mass resists the motion due to inertia — the result is lateral forces throughout the structure. Seismic design is critically important in seismically active regions (California, Pacific Northwest, central US along the New Madrid fault, Alaska, Hawaii).

Seismic loads depend on:

• Spectral response accelerations (Ss and S1): Ground motion parameters from USGS seismic hazard maps, based on geographic location. Higher values indicate more seismically active areas.
• Site class (A through F): The soil type at the site, which affects how ground motion is amplified. Soft soils (Site Class E and F) amplify earthquake shaking significantly compared to rock (Site Class A and B).
• Building weight (seismic weight): Heavier buildings experience larger seismic forces (F = ma). This is why lightweight construction (wood, light steel) often performs better in earthquakes than heavy construction (concrete, masonry).
• Building period: The natural frequency of the building. Short, stiff buildings experience different seismic forces than tall, flexible buildings.
• Response modification factor (R): Accounts for the ductility and energy-absorbing capacity of the structural system. Ductile systems (steel moment frames, R = 8) can sustain significant deformation without collapse and are designed for lower forces than brittle systems (unreinforced masonry, R = 1.5).
• Importance factor (Ie): Higher for essential facilities (hospitals, fire stations, schools).

The total seismic base shear (the total horizontal force at the base of the building) is distributed to each floor level in proportion to the floor weight and height, producing larger forces at higher levels.

Key seismic concepts for construction:

• Earthquakes produce lateral forces that must be resisted by the structure.
• Ductile structures (that can deform without collapsing) perform much better than brittle structures.
• Connections are critical — most seismic failures are connection failures.
• Soft soils amplify earthquake shaking.
• Irregularities (soft stories, torsional irregularity, vertical geometric irregularity) make buildings more vulnerable to seismic damage.

• Rain loads (R): The weight of rainwater on flat or low-slope roofs. If drainage is blocked (clogged drains, ice dams), water accumulates and the increasing weight causes the roof to deflect further, collecting even more water — a progressive failure mechanism called ponding. Ponding is a significant cause of flat roof collapses.
• Flood loads: Hydrostatic pressure, hydrodynamic pressure, and impact from waterborne debris in flood zones. Governed by ASCE 7 and FEMA regulations.
• Soil lateral loads (H): Lateral pressure from soil against basement walls and retaining walls.
• Thermal loads (T): Forces caused by thermal expansion and contraction of building materials. Significant in long buildings and parking structures.
• Impact loads: Dynamic forces from moving or falling objects — vehicles hitting bollards, crane operations, elevator machinery. Impact loads are typically accounted for by applying an impact factor to the static load.
• Construction loads: Temporary loads during construction — stored materials, equipment, concrete placement, worker access. The structure must be able to support these loads at each stage of construction.

Structures must be designed for the most critical combination of simultaneously occurring loads. It is unlikely that maximum live load, maximum wind load, and maximum snow load will all occur at the same time, so building codes specify load combinations — factored combinations of loads that represent realistic worst-case scenarios.

ASCE 7 specifies load combinations for both LRFD (Load and Resistance Factor Design) and ASD (Allowable Stress Design):

LRFD combinations (examples):

• 1.4D
• 1.2D + 1.6L + 0.5(Lr or S or R)
• 1.2D + 1.6(Lr or S or R) + (L or 0.5W)
• 1.2D + 1.0W + L + 0.5(Lr or S or R)
• 1.2D + 1.0E + L + 0.2S
• 0.9D + 1.0W (checks for uplift and overturning)
• 0.9D + 1.0E (checks for uplift and overturning)

The load factors (1.4, 1.2, 1.6, etc.) account for the variability and uncertainty of each load type. Dead load has a lower factor (1.2) because it is well-known; live load has a higher factor (1.6) because it is less predictable.

The engineer evaluates all applicable combinations and designs each structural member for the most critical combination — the one that produces the highest demand on that member.