Proposed new equivalent lateral force design method for low-rise reinforced concrete wall-frame mixed building systems

Abstract

Reinforced concrete wall-frame mixed buildings have been widely used asa seismic force resisting system because walls and frames can play complementary roles in earthquake resistance. In conventional design procedures (e.g., per ASCE 7), the walls and frames in mixed buildings have often been analyzed and designed separately by assigning a specific portion of the design lateral load to each one. These conventional procedures may in part be a product of a bygone era when computerized structural analysis programs were not widely used. However, advanced structural analysis programs are currently available, which can easily simulate the walls and frames simultaneously, and even three-dimensionally. Korean structural engineers have taken advantage of those programs and typically design such mixed buildings without separating the walls and frames or assigning any particular required strength to each one. This new design method is simple and easy to conduct, but it does not exactly follow the conventional design process for a wall-frame interactive system, even though it still uses the same seismic design parameters (from ASCE 7). Therefore, this study has investigated the feasibility of the new design method. Target buildings for the study have reinforced concrete ordinary shear walls in low-to-medium-rise wall-frame mixed buildings, and specifically those with plans having fairly limited bays except for the ones with walls, which makes it difficult to separate the walls and frames. To investigate the performance of buildings designed by the simple new method based on elastic analysis, nonlinear static pushover and nonlinear dynamic analyses have been conducted as part of this study, and engineering demand parameters such as story drift and concrete compressive strain at the wall base have been examined. Results indicate that buildings designed by the new method have good performance even for very conservative failure criteria. However, they exhibited less satisfactory performance than those designed by the conventional procedure. This shortcoming could be compensated for by a slight decrease in the response modification factor, or by specifying a lower bound wall area ratio or an upper bound wall axial load ratio. In order for these options to be applicable, some additional study may be necessary, where a wider variety of prototype buildings are examined utilizing probabilistic approaches.