Current methods for estimating design wind speeds in non-hurricane regions of the United States do not distinguish between winds originating from large-scale extratropical pressure systems and small but intense thunderstorms. These two different meteorological phenomena have separate statistical distributions and, based on the Phase I results, should not be handled using a single (mixed) distribution, as is currently the case. From the Phase I study, it appears that there may be many sites in the United States where thunderstorm winds dominate the extreme wind climatology (for the return periods relevant to conventional structural design) and that the traditional approach tends to underestimate the design winds and too narrowly models the wind directionality distribution. Since thunderstorms differ from large- scale extratropical winds in the details of wind structure, storm duration, frequency, and directionality, these differences could have an important influence on the future codification of design winds, the safety of structures, and wind tunnel testing in the United States. The objective of the Phase II research is to evaluate the importance and merits of treating thunderstorm gust front winds as a separate and distinct wind loading event. The integrated research plan includes three basic tasks: (1) develop wind engineering models of the spatial and temporal characteristics of thunderstorm gust fronts; (2) analysis of thunderstorm and non-thunderstorm windspeed and direction statistics at selected sites; and (3) evaluate the effects of thunderstorms on structural response for several categories of structures. The work could have nationwide significance and, if successful, influence future wind load design criteria in the United States and other countries where thunderstorms are important contributors to extreme wind frequencies.
