Lateral Forces: Wind, Seismic, and the Lateral Force-Resisting System
Load path and system behavior
Lateral load starts at the cladding or diaphragm, moves through collectors and chords into the vertical lateral force-resisting system (shear walls, braced frames, moment frames), and down to the foundation. Identify the diaphragm as rigid or flexible first: rigid diaphragms distribute lateral force in proportion to element stiffness (V_i = V·k_i/Σk), while flexible diaphragms distribute by tributary length. Most wood and metal-deck diaphragms are treated as flexible; concrete slabs are typically rigid.
Wind versus seismic
Wind pressure scales with height and exposure and is applied as a pressure on the projected area; seismic base shear scales with mass and spectral acceleration, V = C_s W with C_s = S_DS/(R/I_e) for the equivalent lateral force procedure. Learn what each parameter does: R reduces elastic demand for ductility, I_e increases demand for occupancy, and Ω₀ amplifies force only for specific overstrength-design elements and certain irregular structures.
Traps
- Applying wind to a strength combination at service level, or vice versa.
- Neglecting accidental torsion (5 percent offset) even when the plan looks symmetric.
- Assuming a rigid-diaphragm distribution in a wood-frame building.
- Skipping drift checks, or using unreduced/unamplified seismic drift incorrectly.
- Forgetting orthogonal effects (the 30 percent combination rule) and vertical irregularities like soft stories.
Worked rule of thumb
Base shear 400 kip on two shear walls with stiffness ratio 3:1 gives 300 kip to the stiff wall and 100 kip to the flexible one, before torsion. If the center of rigidity is offset from the applied load, add the torsional shear from V·e·d/J, and always include the accidental eccentricity.