13.2 Lateral Force-Resisting Systems, Wind Loads & Seismic Design

Lateral Force-Resisting Systems (LFRS)

Every building must possess a continuous load path that transfers horizontal lateral loads—generated by wind or seismic ground acceleration—from the building envelope, through the floor and roof diaphragms, down through the vertical lateral force-resisting elements, and into the foundations and supporting earth.

Lateral Loads (Wind / Seismic)
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Building Envelope (Curtain Wall, Roof Cladding)
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Horizontal Diaphragms (Concrete Slab, Metal Deck, Wood Plywood / OSB)
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Collectors & Drag Struts (Girders, Rebar Ties, Steel Plates)
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Vertical LFRS (Shear Walls, Braced Frames, Moment Frames)
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Foundations (Spread Footings, Grade Beams, Drilled Piers, Piles)
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Supporting Earth / Bedrock

Diaphragms, Chords & Collectors (Drag Struts)

• Diaphragm: A horizontal structural assembly (such as a cast-in-place concrete slab, metal deck with concrete topping, or plywood/OSB subfloor) acting as a deep horizontal beam. The floor/roof web resists horizontal shear, while boundary elements (chords) resist flexural tension and compression.
• Collectors (Drag Struts): Structural members (beams, girders, or embedded steel ties) that "collect" lateral shear forces from wide open diaphragm areas and "drag" or deliver those concentrated loads into localized vertical lateral elements (such as a shear wall or braced frame core).

Vertical LFRS Typologies Compared

Environmental Lateral Loads: Wind vs. Seismic Dynamics

Designing for lateral forces requires understanding the fundamental differences in how wind and seismic loads are generated and applied to a structure.

Wind Loads (ASCE 7-16 / ASCE 7-22)

Wind is an aerodynamic, external fluid force acting directly upon the exterior surfaces of the building envelope. Wind pressures are not uniform; they produce positive inward pressure on windward facades, negative outward suction on leeward and side walls, and strong uplift suctions on roofs and building corners.

• Height & Aerodynamics: Wind velocity and velocity pressure ($q_z$) increase exponentially with height above ground level ($z$): $q_z = 0.00256 \times K_z \times K_{zt} \times K_d \times K_e \times V^2$ Where $V$ is basic wind speed (mph), $K_z$ is the velocity pressure exposure coefficient, $K_{zt}$ is the topographic factor (accounting for wind acceleration over ridges and escarpments), $K_d$ is the wind directionality factor, and $K_e$ is ground elevation factor.
• ASCE 7 Exposure Categories:
  • Exposure B: Urban and suburban areas, wooded terrain, or closely spaced obstructions having the size of single-family dwellings or larger. Produces maximum ground surface friction, resulting in the lowest wind pressures.
  • Exposure C: Open terrain with scattered obstructions, flat open country, grasslands, and shorelines in hurricane-prone regions. The baseline standard exposure.
  • Exposure D: Flat, unobstructed areas exposed to wind blowing over open water for a distance of at least 1 mile (coastal shorelines, ocean fronts, large inland lakes). Produces minimum surface friction and maximum design wind pressures.

Seismic Loads (ASCE 7 / IBC)

Unlike wind, earthquakes do not apply external pressure to a building. An earthquake causes rapid, multi-directional ground accelerations beneath the building. Because the building possesses mass, its physical inertia resists this sudden acceleration ($F = m \times a$). Inertial forces develop internally at every floor level proportional to the mass of that floor.

• Equivalent Lateral Force (ELF) Procedure: The total horizontal design force at the base of the structure is defined as the Seismic Base Shear ($V$): $V = C_s \times W$ Where:
  • $W$ = Total effective seismic dead load of the structure (including permanent self-weight, fixed MEP equipment, 25% of storage live loads, partition allowances of minimum 10 psf, and a percentage of roof snow load).
  • $C_s$ = Seismic response coefficient: $C_s = \frac{S_{DS}}{\left(\frac{R}{I_e}\right)}$
  • $S_{DS}$ = Design spectral response acceleration parameter at short periods, calculated from mapped United States Geological Survey (USGS) ground motions ($S_s$) adjusted for site soil classification (Soil Classes A through F).
  • $I_e$ = Seismic Importance Factor, reflecting the building's Risk Category under IBC Chapter 16 ($1.0$ for standard commercial/residential [Category II]; $1.25$ for schools, public assemblies >300 [Category III]; $1.5$ for essential life-safety facilities like hospitals, police stations, and emergency operations centers [Category IV]).
  • $R$ = Response Modification Factor, reflecting the ductility, damping, and energy-dissipation capacity of the specific LFRS. Highly ductile systems that deform plastically without collapsing have high $R$ values, significantly reducing design base shear forces (e.g., Special Moment Frames: $R = 8$; Special Concentrically Braced Frames: $R = 6$). Brittle, non-ductile structural systems have low $R$ values and must be designed for much higher elastic forces (e.g., Unreinforced Masonry: $R = 1.5$; Ordinary Reinforced Concrete Shear Walls: $R = 4$ to $5$).
• Seismic Design Categories (SDC): Buildings are classified into Seismic Design Categories A through F based on their mapped spectral accelerations, geotechnical soil site class, and occupancy Risk Category. Structures in SDC D, E, and F face rigorous architectural constraints: limitations on structural height, strict prohibitions against horizontal and vertical structural irregularities (such as soft stories, torsional offsets, or re-entrant corners), and mandatory ductile connection detailing.