A roof system in Hilo experiences 180 mph wind speeds. The roof area is 1,800 sq ft with a shape factor of 1.3. If the base uplift pressure is 50 psf, what is the total design uplift force?
Correct Answer
B) 117,000 lbs
Total uplift = Area × Base pressure × Shape factor = 1,800 × 50 × 1.3 = 117,000 lbs total design uplift force.
Why This Is the Correct Answer
Total uplift force = Area × Base uplift pressure × Shape factor = 1,800 sq ft × 50 psf × 1.3 = 117,000 lbs. The shape factor adjusts the base pressure to account for the roof geometry's aerodynamic effect. This is a straightforward application of the wind uplift formula.
Why the Other Options Are Wrong
Option A: 135,000 lbs
135,000 lbs results from using the wrong shape factor — likely 1.5 instead of 1.3 — or from an arithmetic error in the multiplication sequence (e.g., 1,800 × 50 = 90,000 × 1.5 = 135,000).
Option C: 125,000 lbs
125,000 lbs does not correspond to any combination of the given values and likely reflects a guess or rounding error in the calculation.
Option D: 145,000 lbs
145,000 lbs exceeds the correct answer and may come from using a shape factor greater than 1.3 (e.g., 1.6) or adding an incorrect safety factor on top of the shape factor.
Memory Technique
Uplift formula: A × P × SF = Total Force. Area times Pressure times Shape Factor. 'APS' — like the backup power system keeping your calculations running.
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