Continuum MDOF model for seismic analysis of wall-frame structures

Building design codes of practice in Hong Kong and many other low-to-moderate seismic region do not generally include any provision for consideration of seismic resistance. However, earthquake risks do exist. Hong Kong has recently been rated as a region of moderate seismicity with a peak ground acceleration of 0.15g and a seismic design intensity of VII in Chinese Code GB50011-2001. Considering the high density of population and the extensive built-up infrastructure in the urban area, a moderate earthquake would cause a great loss of life and economic hardship in Hong Kong. Thus there is an urgent need to develop an seismic assessment procedure so that the corresponding retrofit designs can be conducted. This thesis proposes a simplified structural model for seismic analysis of tall wall-frame buildings designed by wind load.The applicability of the existing seismic evaluation procedure, namely Pushover Analysis, for medium-rise to high-rise shear wall buildings is investigated. The displacements and internal forces of shear wall structures are first determined by nonlinear response history analysis (RHA) of detailed structure models, where shear walls are considered as multi-degree-of-freedom (MDOF) systems and modelled by fibre elements. The results of RHA are then compared with those from the pushover procedure. It is found that the pushover analysis underestimates inter-storey drift in the upper storeys and is poor in predicting the overturning moments, shear forces in the shear walls, due to neglecting higher vibration modes.To overcome the deficiency of pushover analysis, a simplified MDOF model, namely continuum MDOF model, is developed for seismic analysis of tall wall-frame structures designed to wind loads. By employing the continuum technique, the structure is treated as a combination of a flexural cantilever and a shear cantilever, of which the nonlinear rigidity property is modelled by bilinear hysteretic model. The lateral storey displacement, inter-storey drift and the internal force of the shear walls can then be estimated from nonlinear RHA based on the continuum MDOF model. The accuracy of the proposed continuum MDOF model in predicting seismic performance of wall-frame structures is verified by investigating three tall wall-frame buildings. Results from the RHA using the continuum MDOF models are compared to those from detailed structural models with member-by-member representations. It can be seen that the continuum MDOF model produce very good estimates of response quantities especially for inter-storey drift and shear forces of shear walls, as compared to the estimate results of general pushover analysis.To develop complete procedure for seismic evaluation of wall-frame structures, the continuum MDOF mode is implemented into the IDA framework. Not only the computational demand but also the data management effort for IDA is then dramatically reduced compared to the detailed structural model. The proposed continuum MDOF model is shown to provide a simple and efficient, yet accurate, method of analysis for estimate of seismic performance of tall wall-frame structures.