Probabilistic Assessment of the Importance Factor

A probabilistic assessment of the effect of the importance factor in ANSI A58.1-1982 on the reliability of structures is performed. It is found that the change in reliability for structures within the same ANSI category may vary widely depending on which load combination controls the design. An alternate format for the importance factor, whereby the importance factor is applied to the entire load combination as opposed to just one single load, is also examined. This format, however, is unable to provide uniform changes in the reliability. Importance factors, including importance factors for live load, that would give uniform changes in reliability are proposed.