Wind Directionality Factors for Nonhurricane and Hurricane-Prone Regions

The wind directionality factor, $K_d$, is a nondimensional quantity smaller than unity that reflects the fact that the climatologically and aerodynamically or dynamically most unfavorable wind directions typically do not coincide. The ASCE 7-10 standard specifies for buildings a directionality factor $K_d=0.85$, although no justification appears to have been adduced for this value in the literature. This paper presents a more informed discussion of this topic by the engineering and codification communities. The results of this work show that the directional factor $K_d$ varies as a function of type of wind storm, geographical location within an area with a given type of wind climate, type of wind effect (e.g., local pressures, internal forces in members of wind force resisting systems, global shear or torsion), and position of the wind effect being considered in the structure. These results suggest that, in most if not all cases, the dependence of the directionality factor on mean recurrence interval of wind effects is weak, and they confirm a recent finding that $K_d$ values are larger for hurricane-prone than for nonhurricane regions. For pressures at individual points on building envelopes, the value $K_d=0.85$ is typically found to be adequate for hurricane-prone regions and conservative for nonhurricane regions. For internal forces in wind force resisting systems (e.g., frames), the value $K_d=0.85$ is typically adequate for nonhurricane regions while $K_d=0.90$ is required for hurricane-prone regions, in spite of its conservatism in some situations. Finally, for global effects, such as building base shears and global building torsion, the values $K_d=0.90$ and $K_d=0.95$ are appropriate for nonhurricane and hurricane-prone regions, respectively.