Nonlinear Dynamic Analysis of Steel Frames by Modal Superposition

When analyzing steel frames subjected to dynamic loads, several geometrical and material nonlinearities can be encountered in the early stages of loading. These include second-order effects, and semirigid connection and panel zone nonlinearities, respectively. The well-known modal superposition procedure, which is a very popular method for linear dynamic analysis, can be easily modified to perform nonlinear analysis while retaining many of the advantages of the method in the linear range. These modifications are developed in this paper. Element models are derived for panel zone elements and for two types of connection moment-rotation curves. Several approximations of panel zone behavior are also examined and a sample frame is analyzed. Extensive numerical studies are presented. The methodology presented was found to be both a useful and relatively fast method for nonlinear dynamic analysis of steel frames. It was also found that connection and panel zone behavior affect frame behavior and, hence, must be considered in the dynamic analysis. Connection and panel zone behavior are particularly important when frame drift is the controlling criterion in the design, as tends to be the case for taller frames.