In the Beginning/The Origins of the One-Third Stress Increase

摘要

Ellifritt wrote that the preponderance of literature on the subject supports answer no. 1-the one-third stress increase is allowed because of the "transient nature" of wind; because wind loads "are intermittent and seldom reach their maximum;" because a very high wind load is "a comparatively rare occurrence" and "may never actually occur;" and because wind forces are of "short duration." There was no support for answer no. 2 (rate of loading), and only two references to support answer no. 3 (simultaneous occurrence). [Author's note: It is interesting to find that ASCE 7-02 Commentary Section C2.4.1 statements about the 0.75 load factor indicate that it is essentially the same concept as the one-third stress increase and that it is primarily intended to address simultaneous occurrence.] Ellifritt continued that the one-third stress increase is rooted deeply in our engineering tradition, during a time when wind loads were not so well understood; that modern methods of applying wind loads account for some of the factors used to rationalize the stress increase, such as "short duration" and "rare occurrence." Modern wind codes are based on better meteorological information and wind tunnels, which accurately model the boundary layer. A "rare occurrence" in wind velocity is now programmed into the design pressure selection process in the form of a Mean Recurrence Interval Map. The "short duration" aspect is now accounted for by gust factors.