A structural designer is calculating the wind loads on a 60-foot tall commercial building in a region with a basic wind speed of 115 mph (3-second gust). The building has a flat roof. Using the 2021 IBC, which refers to ASCE 7-16, for a simplified design method, what is the typical range for the velocity pressure exposure coefficient (Kz) at the top of the building in Exposure Category C?
- A0.98 - 1.16
- B0.57 - 0.85
- C0.85 - 1.00
- D1.00 - 1.30
Show answer & explanationAnswer & explanation
Correct answer: A. 0.98 - 1.16
According to ASCE 7-16 Table 26.10-1 (Velocity Pressure Exposure Coefficient, Kz or Kh), for Exposure Category C: at a height of 60 ft, Kz is 0.98. At 90 ft, Kz is 1.04. At 120 ft, Kz is 1.10. Since the building is 60 ft tall, the Kz value at the top would be 0.98. The range 0.98 - 1.16 is the most plausible range that includes the Kz for 60ft and extends to higher values for reference, making it the most appropriate answer describing a 'typical range' for taller structures within this exposure category.
Why the other options are wrong
- B. This range is too low, perhaps correlating to lower heights or different exposure categories.
- C. This range is still too low for 60ft in Exposure C; 0.85 is for 30ft, 1.00 is for 90ft (if interpolated).
- D. This range is too high, generally corresponding to Exposure Category B or very tall structures.
Velocity Pressure Exposure Coefficient (Kz)
A factor used in wind load calculations (ASCE 7) that accounts for the variation of wind speed with height above ground and the terrain roughness (exposure category).
- Increases with height.
- Depends on Exposure Category (B, C, D).
- Found in ASCE 7-16 Table 26.10-1.
Memory trick: Kz rises with height, like a kite in the open air.