Q: How do you calculate overturning moment from wind pressure?

At each height z, the design wind pressure p(z) = q(z) * G * Cf gives a distributed load per unit height of f(z) = p(z) * B (kN/m). Base shear is V = integral of f(z) dz from 0 to H. Overturning moment about the base is M = integral of f(z) * z dz. Using trapezoidal integration over discrete height intervals is the standard practical approach.

Question 1

How is velocity pressure q calculated?

Accepted Answer

Velocity pressure is calculated as q = 0.5 * rho * V^2, where rho = 1.225 kg/m3 (standard air density) and V is the design wind speed (m/s). At height z, the velocity pressure is modified by the exposure coefficient Kz = (z/zg)^(2/alpha), giving qz = q * Kz. In ASCE 7 the formula is expressed as qz = 0.613 Kz Kzt Kd Ke V^2 (N/m2) when V is in m/s.

Question 2

What is the difference between ASCE 7 Exposure Categories B, C, and D?

Accepted Answer

Exposure Category B applies to urban and suburban areas with terrain features, surface irregularities, or closely spaced obstructions (power law exponent alpha ~ 7). Category C is for open terrain with scattered obstructions (alpha ~ 9.5) and is often the default for wind design. Category D is for flat, unobstructed areas and water surfaces (alpha ~ 11.5), producing the highest wind pressures. Sites near large water bodies typically use Exposure D.

Question 3

What does the gust factor G represent?

Accepted Answer

The gust factor G accounts for the dynamic amplification of wind loads due to turbulence and building response. For rigid structures, ASCE 7 allows G = 0.85 as a simplified value. For flexible or dynamically sensitive structures (natural frequency < 1 Hz), a dynamic gust factor Gf must be computed, which can exceed 0.85 significantly. Gf depends on the building natural frequency, damping ratio, and turbulence intensity.

Question 4

How do you calculate overturning moment from wind pressure?

Accepted Answer

At each height z, the design wind pressure p(z) = q(z) * G * Cf gives a distributed load per unit height of f(z) = p(z) * B (kN/m). Base shear is V = integral of f(z) dz from 0 to H. Overturning moment about the base is M = integral of f(z) * z dz. Using trapezoidal integration over discrete height intervals is the standard practical approach.

Wind Load Calculator ASCE 7 / Building Code

Theory