Thesis (M.Eng.) - Asian Institute of Technology, 1999

A thesis submitted in partial fulfillment of the requirements for the degree of Master of Engineering, School of Civil Engineering

The effects of higher vibration modes on nonlinear seismic responses in wall-type buildings are investigated. Wall-type buildings with different numbers of stories and different design inelastic levels are modeled as isolated cantilever walls with bilinear hysteretic behavior. The structures are subjected to a single earthquake ground motion record and their nonlinear seismic responses are determined by performing nonlinear dynamic analyses through DRAIN-2DX Program. Seismic responses of interest are story shears, story moments, and story displacements. In the first step, the higher mode effects on these responses are identified in a typical way, that is by using the ratio of maximum response of a MDOF system to that of the SDOF counterpart. The results confirm that the amplification of seismic responses due to higher mode effects increases significantly with the structure period and the design inelastic level. The importance of higher modes is noticed in this step. A more effective technique for investigating higher mode effects on nonlinear seismic responses, so-called digital filtering, is introduced. By employing this technique, modal responses can be decomposed from the total one. Response histories in the first three modes are obtained. The variation of mode shapes and force distribution patterns, maximum modal responses, and contribution of each mode to maximum total responses are consequently evaluated. These results qualitatively reflect mechanism of higher mode vibrations of inelastic systems. It is evidenced that the validity of application of digital filtering to decompose responses in higher modes is satisfactory in cases of shear and moment responses. Filtered displacement responses in higher modes are found to be inaccurate.