Revisiting Gust Averaging Time and Gust Effect Factor in ASCE 7

Abstract

A peak gust–based wind map was first introduced in ASCE 7-95 to replace the fastest mile wind speed used in earlier versions of the ASCE 7 standard based on wind records using the National Weather Service (NWS) F420C cup anemometer/chart recording system. The peak gust in ASCE 7-95 was considered to be of 2–3 s duration and was referred to as a 3-s gust in the standard. This may imply that an instantaneous gust speed is recorded during the referenced time period, and thus it is not equivalent to the commonly used moving average time. The formulation of the gust-effect factor in ASCE 7 has implicitly utilized a 3-s moving average assumption, which may introduce an inconsistency in gust averaging time between the basic wind speed and the gust-effect factor. This calls for a careful examination in this context of the relationship between gust duration and moving average time to seek consistency and transparency in the formulation of the ASCE 7 standard. In an effort to clarify the gust averaging time, this study firstly investigates the response characteristics of the NWS F420C anemometer/recording system and then identifies an equivalent moving average time. Since the gust effect factor formulation is closely related to the gust averaging time, which is also tied to the gust velocity factor associated with the relationship between the peak and mean wind velocities, the formulation is revisited to evaluate the effect of average time. A modified gust effect factor corresponding to the established equivalent moving average time is established. In addition, the potential impact on the wind profile corresponding to the equivalent averaging time is discussed. Finally, the effects of the gust averaging time on the alongwind response of tall buildings are examined from survivability and serviceability design perspectives.