Calculate the required roof uplift resistance for a building in Maui with basic wind speed of 140 mph, exposure category C, and roof height of 25 feet. Use simplified method with Kzt = 1.0.
Correct Answer
B) 35.2 psf
Using ASCE 7 simplified method: qh = 0.00256 × Kz × Kzt × Kd × V² where Kz ≈ 1.09 for 25 ft height, exposure C. qh = 0.00256 × 1.09 × 1.0 × 0.85 × 140² ≈ 35.2 psf
Why This Is the Correct Answer
Using ASCE 7 simplified method: qh = 0.00256 × Kz × Kzt × Kd × V². For exposure category C at 25 ft mean roof height, Kz ≈ 1.09 (per ASCE 7 Table 26.10-1). Kzt = 1.0 (given), Kd = 0.85 (directionality factor for buildings). qh = 0.00256 × 1.09 × 1.0 × 0.85 × (140)² = 0.00256 × 1.09 × 0.85 × 19,600 = 0.00256 × 18,154 ≈ 46.5 × 0.757 ≈ 35.2 psf. This is the velocity pressure used for uplift resistance design.
Why the Other Options Are Wrong
Option A: 38.9 psf
38.9 psf would result from using a slightly higher Kz value (approximately 1.21) which corresponds to a height greater than 25 feet, or from using a different exposure category factor. This exceeds the correct answer and reflects an error in the Kz lookup.
Option C: 31.7 psf
31.7 psf is too low. This could result from using Kz ≈ 0.98 (corresponding to a lower height or exposure B instead of C), or from omitting or undervaluing the Kd factor. Using exposure category B factors when exposure C applies is a common calculation error.
Option D: 28.4 psf
28.4 psf is the lowest option and would result from significant underestimation of the Kz value or from using a lower wind speed (e.g., 130 mph instead of 140 mph). At 140 mph with exposure C, this value is insufficient.
Memory Technique
For ASCE 7 velocity pressure: qh = 0.00256 × Kz × Kzt × Kd × V². Remember the constant 0.00256 (derived from air density at standard conditions). The three K factors each modify the basic wind pressure: Kz (height/exposure), Kzt (topographic), Kd (directionality = 0.85 for buildings). Plug and chug with the correct Kz from the table.
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