Low-Rise Steel Structures under Directional Winds: Mean Recurrence Interval of Failure

The Commentary to the American Society of Civil Engineers (ASCE) Standard 7-05 states that the nominal mean recurrence interval (MRI) of the wind speed inducing the design strength is about 500 years if the specified load factor is 1.5, as in early versions of ASCE 7, and “somewhat higher than 500 years” if the specified load factor is 1.6, as in ASCE 7-05. However, the Commentary also states, “it is not likely that the 500-year event is the actual speed at which engineered structures are expected to fail, due to resistance factors in materials, due to conservative design procedures that do not always analyze all load capacity, and due to a lack of a precise definition of ‘failure’.” In this paper, we propose a working definition of “failure” for steel structures using nonlinear finite-element analysis, and we present a methodology for estimating the MRI of failure under wind loads that accounts in a detailed and rigorous manner for nonlinear structural behavior and for the directionality of the wind speeds and the aerodynamic effects. The methodology uses databases of wind tunnel pressure (database-assisted design), nonlinear finite-element analysis, and directional wind speeds from the National Institute of Standards and Technology (NIST) hurricane database augmented by statistical techniques. As a case study to illustrate the methodology, we consider a single frame of a steel industrial building. Under the assumption that uncertainties with respect to the parameters that determine the wind loading and to the material behavior are negligible, the minimum MRI of failure for the steel frame being investigated was found to be of the order of 100,000 years, which corresponds to a probability of 1/2,000 that the frame will fail during a 50-year lifetime.