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The following assumptions were made in applying Figure 27.3-1 to the determination of pressure coefficients Cp.
Height, Length and Width of Building
In determining the ratios L/B and h/L, the height h is the mean roof height of the highest roof in the structure (excepet eave heights are used for angles less than 10 degrees). The length L parallel to the wind loads and width B perpendicular to the load are measured from the extremities of the building as a whole, and not based on individual blocks,
Windward Roof Coefficients
Where ASCE 7 gives positive and negative pressure coefficients, Cp, on windward roof surfaces, the positive coefficients are considered to govern. Given that the windward roof is the only surface that has pressure acting opposite to the wind direction if negative coefficients are used, the positive coefficients will always govern for shear wall design.
It is possible that a negative coefficient may be used for interpolation purposes in some cases.
Horizontal Distance From Windward Edge
Windward loads for angles less than 10 degrees vary according to the distance from the windward edge. The program calculates a weighted average of co-efficients to be used on a surface that has more than one area with different coefficients, and uses the average co-efficient on the entire surface The value of the resulting lateral wind load is not affected.
Area Used for Reduction Factor
For height to width ratios greater than 1.0, and roof slopes less than 10 degrees, a reduction factor is applied based upon roof area. The area used is that of the roof panel for which loads are being generated.
Monoslope Roofs
For the wall portion on the monoslope roof, on the leeward side, a Cp coefficient of -.6, corresponding to roofs, rather than -.2 to -.3, corresponding to walls, is used. The windward co-efficient is the same in both cases, 0.8.