For a low-rise building, what specific exposure category and topographic factor would be applied if the building is situated on a hill within a densely suburban area?

For a low-rise building situated on a hill within a densely suburban area, the specific exposure category applied would be Exposure Category B, and a Topographic Factor (Kzt) would be applied and calculated.

Exposure Category defines the characteristics of the ground surface roughness over which wind flows to a building. These categories account for the effect of obstructions on the wind velocity profile. Exposure Category B is characterized by urban and suburban areas, wooded areas, or other terrain with numerous closely spaced obstructions having the size of single-family dwellings or larger. A “densely suburban area” fits this description because the closely spaced buildings and other structures create significant surface roughness, causing the wind speed to decelerate near the ground. Other common categories include Exposure C for open terrain with scattered obstructions, and Exposure D for flat, unobstructed areas like shorelines.

The Topographic Factor (Kzt) accounts for the increase in wind speed over isolated hills, ridges, and escarpments, known as the speed-up effect. Since the building is situated on a hill, this factor must be applied. The value of Kzt is greater than 1.0 when significant speed-up effects are present and is used to amplify the design wind pressures. The precise value of Kzt is not a fixed number but is *calculatedbased on the specific geometry of the hill and the building's location on it. This calculation typically involves specific parameters from standards like ASCE 7, including the hill's height (H), the horizontal distance from the crest to the point where the elevation is half of the hill's height (Lh), and the horizontal distance from the crest to the building's location (x). If a hill is small or its geometry does not meet certain criteria, Kzt may be considered 1.0, meaning no topographic speed-up effect is considered; however, for a building *ona hill, the factor must be evaluated. The application of Kzt ensures the design adequately accounts for the localized acceleration of wind over the elevated terrain.