Statistical Significance of Less Common Load Combinations

The purpose of this work is to provide a simple, rational basis for combining bridge loads in the extreme event limit state. Classic methods are used to evaluate the probabilities of traffic, seismic, and storm-related bridge loads occurring simultaneously and in various intensities. The loads are modeled using a Poisson distribution, which circumvents problems encountered when using normal- or log-normal distributions. The hazard level is evaluated directly using a negative exponential function in the time domain. An acceptable hazard level for combined events is subjectively based on that deemed acceptable for the strength limit state and extreme events occurring individually, as well as fiscal prudence. It is shown that (1) application of seismic loads to a structure already subjected to the combined effects of degradation, local pier scour, and contraction of the waterway is not justifiable; (2) live loads reduced from the anticipated 75-year maximum to a 2-week maximum are appropriate when designing a bridge in its 100-year “scoured-out” storm configuration; and (3) vehicular live loads are likely to be on a bridge during a seismic event, but other issues need to be considered.