Reliability evaluation of axially loaded steel members design criteria in AASHTO LRFD bridge design code

Reliability based structural design insures a uniformly designed structure, in terms of safety. By considering an adequate reliability index (or probability of failure) for different parts of a structure, a reasonable balance between cost and safety of the structure can be achieved. In this study, the reliability of steel tension and compression members designed with AASHTO LRFD bridge design specifications (2007) is evaluated. These members are prevalent in different types of truss or arch bridges. Various conditions such as redundancy, ductility and importance of the bridge are taken into account by changing the load modification factor, η. To include the effect of the span length, a variable ratio of dead load to total load is considered. Current load factors in AASHTO LRFD code are accepted due to their verification in a comprehensive study for reliability of girder-type bridges. Furthermore, load and resistance distribution models are chosen based on the latest existing experimental data. The Monte Carlo simulation technique with randomly generated samples is applied in numerical calculations. For tension members, analysis results show relatively high reliability indices in yielding design, while having slightly low reliabilities for the fracture mode. For fracture design of steel tension members, an increase in vehicular dynamic load allowance (IM) from 33% to 75% is suggested to insure a safer behavior. Also, it is shown that the resistance factor for yielding of gross section, φy, can be increased from 0.95 to 1.00 while maintaining enough safety for designed tension members. In addition, obtained reliability curves for steel compression members show a safe behavior of designed compression members with conservative response in some cases. More results and plotted curves are discussed in detail and possible adjustments in code criteria are presented in this paper.