Evaluation of Modal Truncation on Dynamic Responses

Modal contribution indicator which represents the percent contribution of each mode to the total response was used to assess the convergence of the predicted critical structural member loads using Rayleigh-Ritz approach. Two important parameters in the dynamic analysis, dominant mode and truncation frequency, may be effectively identified by the modal contribution indicator. The percent modal contribution of a mode may vary if a different truncation frequency was used in the calculation of the structural member loads. In general, a mode with a relatively larger modal contribution indicator value is considered to be a dominant mode and is significant and essential for predicting accurate loads. An appropriate truncation frequency to ensure the convergence of the load can be determined by examining the ratios of the modal contribution indicators of each mode for various selected truncation frequencies. The loads of a structural member are converged if the ratio of the modal contribution indicator of each mode approaches unity.