Counteracting Structural Loads: Treatment in ASCE Standard 7-05

Abstract

The load combinations in ASCE Standard 7 for load and resistance factor design (LRFD) and for allowable stress design (ASD) differ in their treatment of counteracting loads, where the permanent gravity load (dead load) stabilizes the structural system against the effect of uplift or lateral loads. In LRFD, the loads are factored prior to the analysis, and the load factor on dead load is less than unity. In ASD, one factor of safety (the allowable stress factor) had been applied to the difference between the nominal load effects through the 1995 edition of ASCE Standard 7. Beginning with the 1998 edition, the load factor on nominal dead load in the ASD combination was increased to 0.6 to achieve consistency between ASD and LRFD. This particular ASD load requirement has been controversial and has drawn criticism from certain segments of the structural engineering community. This technical note examines the treatment of counteracting loads in ASCE Standard 7 from a reliability viewpoint, and shows that failure to reduce the gravity load for design may lead to inadequate safety.