Fig 1: Wind load calculation example structures
ASCE 7’s Applications
Comparing one of ASCE 7’s Applications with the two equations included in the last blog. First was a generic drag force calculation, second was incorporating two other coefficients to account for exposure and the intensity of wind gusts. As an example, the use three objects on top of a 100-foot tall building:
- A 10’ tall, 10’ wide cylinder.
- A cube with the same volume as cylinder A, oriented with a face normal (90°) to the wind direction.
- Another cube with the same volume, but rotated 45° for a slightly more aerodynamic front.
For the wind speed, I chose 110 mph. This may seem high, but is the standard design wind speed for buildings in California as set in ASCE 7. Representing the speed of 3-second intense wind gusts that structures must be designed to withstand.
Wind Load Equations
Numerous methods are used for calculating wind load on a structure or object. ASCE 7 is one of the most detailed, other organizations like the International Code Council and the National Association of Tower Erectors have adapted the equations from ASCE 7 or developed their own, based on their industry’s specific needs. For this brief examination I will use the generic drag calculation (Eqn 1-2), a drag calculation modified with two additional wind-related coefficients (Eqn 3-5), and ASCE 7’s equation for “other structures” (chimneys, tanks, rooftop equipment and similar structures) (Eqn 6-7).
F = wind load (in N or lbf)
P = wind pressure (in Pa or psf)
A = projected area (in m2 or ft2)
Cd = drag coefficient (unitless)
V = wind speed
Kz = exposure coefficient
G = gust response factor
z = height of the midpoint of the object above the ground
h = height of the top of the object above the ground
qz = velocity pressure
G = gust effect factor (as defined in ASCE 7 §26.9)
Cf = force coefficient (as defined in ASCE 7 §29.5)
Kz = velocity pressure coefficient (as defined in ASCE 7 §29.3.1)
Kzt = topographic factor (as defined in ASCE 7 §26.8.2)
Kd = wind directionality factor (as defined in ASCE 7 §26.5)
Wind Load Factors