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Engineering
Grade college
Wind Loads on Structures Reference Cheat Sheet
A printable reference covering dynamic pressure, ASCE 7 velocity pressure, exposure factors, pressure coefficients, and design wind pressures for grades 13-16.
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Wind loads on structures connect wind speed, air density, terrain exposure, building shape, and structural response. This cheat sheet helps college engineering students organize the main pressure formulas used in preliminary wind design. It is especially useful when comparing basic fluid mechanics pressure with ASCE 7 velocity pressure and building surface pressures. The goal is to make the calculation path clear before using a full design standard.
Key Facts
- Dynamic wind pressure from fluid mechanics is q = 0.5 rho V^2, where rho is air density and V is wind speed.
- In U.S. customary ASCE 7 form, velocity pressure is qz = 0.00256 Kz Kzt Kd Ke V^2 in psf, with V in mph.
- At mean roof height h, the commonly used pressure is qh = 0.00256 Kh Kzt Kd Ke V^2.
- In SI units, a common velocity pressure form is qz = 0.613 Kz Kzt Kd Ke V^2 in N/m^2, with V in m/s.
- The main wind force resisting system pressure is commonly based on p = q G Cp - qi GCpi, using external and internal pressure effects.
- Positive pressure acts toward a surface, while negative pressure or suction acts away from a surface.
- The design wind force on a surface can be estimated from F = p A, where p is net design pressure and A is tributary area.
- Exposure category affects Kz because open terrain usually produces higher wind pressure at low building heights than dense urban terrain.
Vocabulary
- Basic wind speed
- The mapped wind speed used by the design standard for a specified risk category, location, and return period.
- Velocity pressure
- The pressure associated with wind speed after applying code factors for height, topography, directionality, elevation, and exposure.
- Exposure category
- A classification of surrounding terrain roughness that affects how wind speed changes with height above ground.
- Pressure coefficient
- A dimensionless factor that converts velocity pressure into pressure on a specific building surface or zone.
- Gust effect factor
- A multiplier that accounts for wind turbulence and dynamic response of the building or structural element.
- Internal pressure coefficient
- A factor representing pressure inside a building caused by openings and air flow through the enclosure.
Common Mistakes to Avoid
- Using basic wind speed directly as pressure is wrong because pressure depends on V^2 and must include density or code pressure factors.
- Forgetting exposure and height factors is wrong because wind pressure near the roof can differ greatly from pressure near the ground.
- Mixing mph with SI pressure constants is wrong because qz = 0.00256 uses mph and psf, while qz = 0.613 uses m/s and N/m^2.
- Ignoring pressure signs is wrong because windward pressure and leeward or roof suction can combine differently for uplift, shear, and overturning.
- Using the same coefficient for the whole building envelope is wrong because corners, edges, roofs, walls, and internal pressure zones often have different design coefficients.
Practice Questions
- 1 Compute the dynamic wind pressure q = 0.5 rho V^2 for air density rho = 1.225 kg/m^3 and wind speed V = 35 m/s.
- 2 Using qz = 0.00256 Kz Kzt Kd Ke V^2, find qz in psf for V = 120 mph, Kz = 0.85, Kzt = 1.0, Kd = 0.85, and Ke = 1.0.
- 3 A wall panel has net design pressure p = 32 psf and tributary area A = 48 ft^2. Find the total wind force F on the panel.
- 4 Explain why a roof corner zone can require a larger uplift design pressure than the middle of the same roof.