Evaluation of modal combination methods in response spectrum analysis of structures with closely spaced modes

The square-root-of-the-sum-of-squares (SRSS), double algebraic sum (DAS), and double absolute sum (DABS) modal combination methods are evaluated. These methods are used to compute dynamic responses of a large complex building with many closely spaced coupled and uncoupled modes. These responses are then compared to the average response of three time-history modal superposition (T-H) analyses. Deviation of responses using different modal combinations from the average time-history response are compared statistically. It is shown that for structures with closely spaced modes the SRSS method estimates responses which are poorly correlated to those calculated by the T-H method, the DABS significantly overestimates the respones, while the DAS estimates responses reasonably close to those from T-H method. (orig.)