Answer :
f a roof is subjected to -3 kPa external wind pressure, and internal wind pressure of either -0.5 kPa or 0.3 kPa, then ,The worst wind combination is 3.3 kPa.
The worst wind combination value of the roof subjected to -3 kPa external wind pressure, and internal wind pressure of either -0.5 kPa or 0.3 kPa is 3.3 kPa.
This is because the wind pressure coefficient is calculated using the highest values of both the internal and external pressures. The roof can only withstand the worst-case scenario.
The wind pressure coefficients on a roof can be calculated using the following formula:
Cp = (GCp1 + GIwCp2) + (GCp3 + GCp4 + GCp5)
where:
GCp1 = pressure coefficient on the leeward roof
GCp2 = internal pressure coefficient on the leeward roof
GCp3 = pressure coefficient on the windward roof
GCp4 = internal pressure coefficient on the windward roof
GCp5 = suction coefficient on the windward roof
GIw = internal wind pressure coefficient
In this formula, the pressure coefficients GCp1, GCp2, GCp3, and GCp4 are typically taken from relevant tables and depend on the shape of the roof, while GCp5 is calculated using the formula:
Cp5 = (0.6 + 0.4*(h/L)) * GCp3
where:
h is the height of the building and L is the length of the building
With the given values, we can calculate the wind pressure coefficients as follows:
Cp = (GCp1 + GIwCp2) + (GCp3 + GCp4 + GCp5)
Using the worst-case scenario values of -3 kPa external wind pressure and 0.3 kPa internal wind pressure, we get:
Cp = (-0.7 + 0.3*0.3) + (-1.4 - 0.5 - 0.9) = -3.9 kPa
Using the values of -3 kPa external wind pressure and -0.5 kPa internal wind pressure, we get:
Cp = (-0.7 - 0.5*(-0.7)) + (-1.4 - 0.5 - 0.9) = -2.6 kPa
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