The AISC ASD specification permits allowable stresses due to wind or earthquake forces, acting alone or in combination with dead and live load, to be increased by one-third (Art. 6.11). The required section computed on this basis, however, may not be less than that required for the design dead, live, and impact loads computed without the one-third stress increase. This stress increase cannot be applied in combination with load-reduction factors in load combinations. Also, the one-third stress increase does not apply to stresses resulting from fatigue loading.
In LRFD design, equivalent allowances are made by the prescribed load factors when appropriate.
