Performance-Based Seismic Design considering Slab Flexural Stiffness and Arrangement of Vertical Reinforcement in Wall

Abstract

As a risk of earthquakes increases drastically in Korea, seismic performance evaluation and performance-based seismic design (PBSD) projects have been frequently. In the PBSD, slab is designed to resist gravity loads only and usually modeled as a rigid diaphram with no flexural stiffness. Because slab thickness of most residential buildings built in 1990s was 135~150 mm which was not thick enough to consider flexural stiffness of slab, so it has not been used as a lateral resistance component. However, after standard slab thickness of residential buildings was set to be 210 mm due to the tightening of noise regulations in apartment in 2009, there is a need to take into account the flexural stiffness of slab. Considering the flexural stiffness of slab, seismic loads applied to slab of each floor are not same. This makes slab deisgn to be different and increases a workload compared with a typical design method. This also takes more time for modeling and analysis of buildings. Therefore, in this paper, the maximum flexural stiffness of slab that can be considered without changing design is found using FEA and PBSD considering flexural stiffness of slab is conducted. Also before applying the slab stiffness in the PBSD, response spectrum analysis is performed in order to investigate the effect of in-plane stiffness and out-of-plane stiffness of slab on the dynamic behavior of 20-story shear wall buildings. In addition, PBSD is carried out considering an arrangement of vertical reinforcement in walls for more economical deisgn. Generally, the typical vertical reinforcement of walls is equally spaced. In this study, however, nonlinear static analysis and time history analysis are conducted for the shear wall buildings where vertical reinforcement of walls are concentrated at the ends remaining the same amount of reinforcement to evaluate a seismic performance and dynamic characteristics. As an analysis result, the in-plane stiffness of slab is relatively large compared to the lateral stiffness of vertical members in concrete buildings, considering flexural stiffness of slab with rigid diaphram is more efficient than modeling the slab as shell element or plate element. Taking into consideration a flexural stiffness of slab, inter story drift ratio of the building is reduced and lateral loads are redistributed. Also shear force distribution between the upper and lower floors, reaction force distribution and the load difference acting onthe large wall and the small wall become more uniform due to redistributed loads. The maximum flexural stiffness of slab which can be considered without design change is about 10%. When this is reflected in the PBSD, the bending strength of the building is increased more than 1.5 times and also plastic rotation angles of coupling beams is considerably decreased. Furthemore, it can become more economical by reducing the amount of reinforcement in total walls by about 6% compared with the existing design using rigid diaphram only.

When the vertical reinforcmenet of walls are concentrated at the ends, the bending strength of the whole building increases by 5~6% and inter story drift ratio decreases slightly. Also it can be possible to reduce the amount of reinforcement in all walls about 4% by downsizing the increased bending strength to the standard model level. But there is no big difference in dynamic characteristic change in nonlinear time history analysis.

Keywords : Performance-based seismic design, Slab flexural stiffness, Nonlinear analysis, Perform-3D, Wall reinforcement