3.0 Load Combinations in Limit States
Because there are many different types of loads, the manner in which the loads are combined to create a limit state has sometimes been unclear in the traditional use of ASD. For instance, it is unlikely that the most extreme values of the live loads, wind load, stream load, and earthquake load will occur at the same time. The AASHTO-LRFD provides a solution to this problem by specifying several load combinations with load factors based on probability of occurrence. In essence, the AASHTO-LRFD approach implements Turkstra’s rule (Turkstra, 1970) which is based on the observation that when one load component reaches an extreme value, the other load components are often acting at their average values. In other words, the probability of two or more load components acting at their extreme values simultaneously is so remote that it is negligible. Turkstra’s rule states that for i load components, the designer should consider i possible combination of the loads to get the maximum value of the total load. The essence of this rule is reflected in the AASHTO-LRFD approach by consideration of several load combinations within each limit state, e.g., Strength I, Service II, etc. The intent of each load combination is to create a maximum load effect. The key is that in the AASHTO-LRFD framework, each combination of the loads within a given limit state has an equal probability of occurrence.
Since each combination of load has an equal probability of occurrence, all possible applicable load combinations in all limit states should be considered in design. Note that not all possible load combinations may be applicable for a given bridge structure, e.g., Strength IV may not govern for low (< 3) dead load to live load ratios as in the case of short-span bridges. For the new user who may not be familiar with such considerations, it may be prudent to check all possible load combinations and then develop a feel for the applicable load combinations for a given bridge structure.