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Exposure Categories
Exposure Categories from ASCE 26.7.3 are based on corresponding Surface Roughness categories from 26.7.2. This choice affects the calculation of the velocity pressure co-efficient Kz and the gust effect tor G in generating wind pressures.
Exposure B – Roughness B applies for 1500 ft. upwind for structures less than 30 ft in height, and 2600 ft for those greater.
Roughness B – Urban and suburban with numerous closely spaced structures.
Exposure C – Not Exposure B or D.
Roughness C – Open terrain with scattered obstructions, e.g. grasslands.
Exposure D – Structure is within 600 feet of an area where Surface Roughness D applies for 5000 ft or greater
Roughness D – Flat and unobstructed - e.g. water surfaces, ice, mud or salt flats.
The Exposure category selected applies to all wind directions; refer to Load Generation Assumptions for more details.
Minimum Loads
Certain local design codes require a different minimum load, for example New York City requires or at some time required a minimum load 20 psf. Therefore, the program now allows you to adjust the intensity of the minimum load for walls and for roofs, with the defaults being 16 psf and 8 psf respectively, the required loads from ASCE 27.1.5 and 28.3.4.
These are input in the Site Information dialog box, and are recorded in the Site Information output table
Ground Elevation Factor Ke
You enter the altitude above sea level of your structure and the ground elevation Ke factor is calculated using the equation in Note 2 of Table 26.9-1 and applied to the velocity pressure qz in equation 26.10-1.. Note that this is optional; the default sea level altitude can always be used.