Estimates of extreme wind effects and wind load factors: influence of knowledge uncertainties

We propose a probabilistic methodology for developing improved load factors in standard provisions for wind loads, and use it to examine: the cause of the discrepancy noted in the 1980s between estimates of safety indices for wind and gravity loads; the relative magnitude of load factors for hurricane and non-hurricane regions; and the effect of the length of wind tunnel pressure records on the estimation of peak wind effects. According to our calculations, (1) the discrepancy between estimates of safety indices for gravity and wind loads is an artifact that can be removed by using current knowledge on probability distributions of extreme wind speeds; (2) the disregard in the ASCE 7-98 Standard of (a) knowledge uncertainties, and (b) errors inherent in the limited number of climatological data on which hurricane wind speed simulations are based, leads to incorrect wind load factor estimates; and (3) increasing beyond 30 min or even 20 min the length of pressure records used for the estimation of fluctuation peaks appears to have a relatively small effect on estimates of overall wind effects. We outline future research on wind directionality, sampling errors in the estimation of peak wind effects, and the use of probabilistic descriptions of wind effects and structural capacity to estimate probabilities of occurrence of nonlinear limit states, including structural collapse.