Types of Structural Loads

Why This Matters

Every structure you'll analyze in Statics and Strength of Materials must resist multiple types of loads simultaneously—and understanding why each load behaves differently is the key to solving problems correctly. You're being tested on your ability to classify loads by their behavior (static vs. dynamic, permanent vs. temporary), determine how they create internal stresses, and apply the right analysis method for each situation.

The loads covered here connect directly to equilibrium equations, stress-strain relationships, factor of safety calculations, and combined loading scenarios. When you see an exam problem describing a bridge deck, a retaining wall, or a building frame, your first job is identifying which loads apply and how they act on the structure. Don't just memorize definitions—know whether a load is gravity-driven or pressure-driven, static or time-varying, and distributed or concentrated. That conceptual understanding will carry you through FRQs and design problems.

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Gravity-Driven Loads

These loads act vertically downward due to the weight of objects. The key distinction is whether the load is permanent and predictable or variable and uncertain—this determines your safety factors and load combinations.

Dead Loads

• Permanent self-weight of the structure—includes beams, columns, walls, floors, and any fixed equipment that won't move over the structure's lifetime
• Calculated from material density and geometry using $W = \gamma \cdot V$ where $\gamma$ is the unit weight and $V$ is volume
• Most predictable load type, which is why dead loads typically carry lower safety factors than variable loads in design codes

Live Loads

• Temporary, movable loads from occupants, furniture, vehicles, and stored materials—anything that can change position or magnitude
• Building codes specify minimum values based on occupancy type (offices, assembly halls, warehouses all have different requirements)
• Must consider worst-case positioning for maximum moment and shear calculations, especially on continuous beams

Snow Loads

• Accumulated snow and ice on roofs, varying dramatically with geographic location, elevation, and local climate patterns
• Roof geometry matters—flat roofs accumulate more snow, while steep slopes allow sliding; drift loads can concentrate near parapets and valleys
• Ground snow load converted to roof load using exposure and thermal factors specified in codes like ASCE 7

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Lateral and Environmental Loads

These loads act horizontally or at angles, creating overturning moments, shear forces, and stability challenges that gravity loads alone don't produce. Dynamic behavior often governs the analysis.

Wind Loads

• Pressure from moving air acting perpendicular to surfaces, creating both positive pressure (windward) and negative pressure/suction (leeward)
• Increases with height and exposure—taller buildings and open terrain experience higher wind pressures following a power-law or logarithmic profile
• Causes lateral sway and overturning that must be resisted by bracing, shear walls, or moment frames; calculated using $p = \frac{1}{2} \rho v^2 C_d$

Earthquake Loads

• Inertial forces from ground acceleration during seismic events—the structure's own mass creates horizontal forces when the ground moves
• Requires dynamic analysis because structural response depends on natural frequency, damping, and the frequency content of the ground motion
• Base shear distributed up the height with larger forces at upper floors; ductile design allows controlled yielding to dissipate energy

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Pressure-Induced Loads

These loads result from contact with soil, water, or other materials that exert pressure on structural surfaces. The pressure typically varies with depth, requiring integration to find resultant forces.

Soil Pressure Loads

• Lateral earth pressure on retaining walls and basements, calculated using Rankine or Coulomb theories based on soil properties and wall movement
• Pressure increases linearly with depth as $p = K \gamma z$ where $K$ is the lateral earth pressure coefficient
• Active, passive, and at-rest conditions depend on whether the wall moves away from, into, or remains fixed relative to the soil

Fluid Pressure Loads

• Hydrostatic pressure from liquids acting on dams, tanks, and submerged structures, always perpendicular to the surface
• Pressure varies linearly with depth following $p = \rho g h$, creating a triangular pressure distribution on vertical surfaces
• Resultant force acts at the centroid of the pressure diagram—for triangular distributions, this is at $h/3$ from the base

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Dynamic and Time-Varying Loads

These loads change rapidly with time, requiring consideration of inertial effects, stress concentrations, and fatigue. Static equilibrium equations alone won't capture the full structural response.

Impact Loads

• Sudden, short-duration forces from collisions, falling objects, or rapidly applied weights—think vehicle crashes into barriers or dropped machinery
• Dynamic amplification factor accounts for the fact that sudden loading produces stresses greater than the same load applied gradually
• Impulse-momentum analysis relates the change in momentum to the average force: $F_{avg} \cdot \Delta t = m \cdot \Delta v$

Thermal Loads

• Internal stresses from temperature changes causing expansion or contraction that's restrained by supports or adjacent members
• Thermal strain is $\epsilon_T = \alpha \Delta T$ where $\alpha$ is the coefficient of thermal expansion; if constrained, this produces stress $\sigma = E \alpha \Delta T$
• Expansion joints and flexible connections are design solutions that accommodate movement and prevent thermal stress buildup

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Construction-Phase Loads

These temporary loads exist only during building erection but can be critical for preventing failures before the structure reaches its final, fully-braced configuration.

Construction Loads

• Temporary loads from equipment, materials, and workers during the building process—cranes, concrete forms, material stockpiles, and construction crews
• Partial structures lack full lateral bracing, making them more vulnerable to wind and instability than the completed building
• Sequencing and shoring requirements must be specified to ensure each construction stage can safely support the loads present at that time

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Quick Reference Table

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Self-Check Questions

1. A warehouse stores heavy pallets that are regularly moved by forklifts. Which two load types must be considered for floor design, and why does one require more conservative safety factors than the other?

2. Both wind and earthquake loads act laterally on a building frame. Explain why doubling the building's mass would increase earthquake forces but have no direct effect on wind forces.

3. A retaining wall holds back 3 meters of saturated soil. Compare and contrast how you would calculate the pressure from the soil versus the pressure from groundwater behind the wall.

4. An FRQ describes a steel bridge girder that experiences daily temperature swings of 40°C. If the girder is fixed at both ends, what type of load develops, and what formula relates temperature change to stress?

5. Rank the following loads from most predictable to least predictable, and explain your reasoning: dead load, live load, earthquake load, snow load.