Reliability Basis for Counteracting Load Combinations in ASCE Standard 7-05

Abstract

Load combinations in Section 2 of ASCE Standard 7 are provided for both Load and Resistance Factor Design (LRFD) and for Allowable Stress Design (ASD). Included in Section 2 are combinations addressing counteracting load effects, where the permanent gravity (dead) load stabilizes the structural system. Counteracting effects occur most commonly when destabilizing forces occur due to wind uplift or lateral wind or earthquake effects. In the 1998 and subsequent editions of ASCE Standard 7, the load factor on nominal dead load in the corresponding ASD combinations has been 0.6 rather than 1.0 to achieve some measure of consistency between ASD and LRFD. This reduction has been questioned by some engineers. This paper examines the treatment of counteracting loads in ASD from a structural reliability viewpoint similar to that used to develop the ASCE 7-05 load combinations for LRFD, and explains why the reduction in the dead load factor was necessary to achieve consistent reliability between ASD and LRFD.