Seismic & Wind Load Basics

Quick pick (TL;DR)

Wind force scales with exposed area and the square of wind speed: F = ½·ρ·V²·Cd·A.
Seismic base shear scales with weight: V = Cs·W, where Cs comes from the site seismicity, soil class and the structure's response factor R.
Compute both, design for the larger, and combine with gravity per the code load combinations.
Tall light structures are wind-governed; heavy structures in seismic zones are seismic-governed.

Step 1 — Two different physics

	Wind	Seismic
Driven by	Area & V²	Weight
Worst for	Tall & light	Heavy
Code	ASCE 7 / EN 1991-1-4	ASCE 7 / EN 1998

Step 2 — Worked comparison

Vessel, W 2000 kN, 3×10 m

Wind force F = ½ · ρ · V² · Cd · A ρ = 1.225 , V = 40 m/s , Cd = 0.7 A = 3 × 10 = 30 m² F = 0.5×1.225×40²×0.7×30 / 1000 F = 20.6 kN Seismic base shear V = Cs · W Cs = 0.12 (site D, with R) , W = 2000 kN V = 0.12 × 2000 = 240 kN Which governs? 240 kN ≫ 20.6 kN → seismic governs Design → design lateral = 240 kN + load combos

Heavy vessel → seismic governs; tall light structure → wind governs. Always compute both.

Step 3 — Beyond the numbers

Wind: apply gust/exposure/shape factors.
Seismic: Cs from Ss/S1, site, R; modal if flexible.
Combine per code; check overturning/uplift.

Wind vs seismic calculator

Lateral load — which governs

Wind F = ½ρV²CdA, seismic V = Cs·W — design for the larger.

Weight W (kN)

Area A (m²)

Wind speed (m/s)

Air density ρ

Drag Cd

Seismic Cs

Base values only — apply gust/exposure (wind) & full Cs (seismic); combine with gravity.

Common mistakes

Checking only one of the two.
Base pressure without factors.
Ignoring overturning/uplift.

Requisition & datasheet

Type, weight, height, area, site data.
Wind, seismic, governing, design load.

Frequently asked questions

What should I know about Seismic & Wind Load Basics?

Wind force scales with exposed area and the square of wind speed: F = ½·ρ·V²·Cd·A. Seismic base shear scales with weight: V = Cs·W, where Cs comes from the site seismicity, soil class and the structure's response factor R. Compute both, design for the larger, and combine with gravity per the code load combinations. Tall light structures are wind-governed; heavy structures in seismic zones are seismic-governed.

Which standards or codes apply?

ASCE 7 (wind & seismic), IBC, EN 1991-1-4 (wind), EN 1998 (Eurocode 8 seismic), and the Thai DPT/ มยผ. seismic standards.

Standards — ASCE 7; IBC; EN 1991/1998; มยผ. (DPT).