Load Combination Factors for Extreme Events

Abstract

The AASHTO load and resistance factor design (LRFD) specifications were developed with the use of a reliability-based calibration that covered the basic combination of dead load plus live load. The other load combinations were obtained from previous generations of specifications and the experience of bridge engineers and thus may not be consistent with the reliability methodology of the LRFD specifications. Work done as part of a recent NCHRP project to study in a systematic way the reliability of bridges under the combined effects of extreme load events and to develop a rational set of load combination factors consistent with the LRFD philosophy is described. The load events considered include live loads, seismic loads, wind loads, ship collision loads, and short-term scour. The analysis considered structural safety as well as foundation safety, and member safety was compared with system safety. The study recommended revisions to the AASHTO LRFD load combination factors and also calibrated new factors for combinations of events that include the presence of short-term scour. The calibration took into account previous experience with so-called safe bridge structures to provide acceptable and consistent levels of reliability.