In the not-so distant past, authorities recommended leaving the windows to a hou

Question

In the not-so distant past, authorities recommended leaving the windows to a house open in a tornado to equalize the pressure and avoid blowing off the roof. We can estimate the validity of the advice.

Assume the pressure inside the house is one atmosphere.

Approximate Densities of roofing materials: Doug Fir 2x6 1 kg./ft.

5/8" plywood 0.85 kg/ft2

gravel roofing 2.5 kg/ft2 For a 40ft x 60ft. area of roof, there are 40x20 ft of 2x6 (1.5 ft spacing between 2x6's) and 2400 ft2 of plywood and gravel roofing. Assume 6 screws per 2x6 with a holding force of 500N each.

Using the maximum pressure difference across the roof and the above data, can the roof blow off because of the pressure drop during a tornado, or is this not a valid concern?

Explanation / Answer

mass of Doug fir = 40x20 x1 kg = 800kg

mass of plywood = 2400 x 0.85 = 2040kg

mass of grvel = 2400 x 2.5 = 6000kg

force due to screws = 6 x500 x 800 =2.4 x 106 N

total mass M= 6000+2040+800= 80840

weight = Mg = 7.92 x 105N

total force required = 3.19 x 106N

assuming the pressure outside to be zero

pressure difference P = 1 atm = 1.1 x 105Pa

area = 40 x60 ft2 = 223 m2

force due to pressure difference = 1.1x105 x 223 = 2.45 x 107N

as the force due to pressure difference is greater than the force keeping the roof intact

it can be blown due to tornado

Related Questions

Navigate

• Browse (All)
• Browse I
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.