EFFECT OF USING PERFORMANCE-BASED APPROACH FOR SEISMIC DESIGN OF TALL BUILDING DIAPHRAGMS Naveed Anwar 1 , Jose A. Sy 2 , Thaung Htut Aung 1 and Mir Shabir Talpur 1 1 AIT Consulting, Asian Institute of Technology, K.M. 58, Moo 9, Poholyothin Highway, Khlong Luang, Pathumthani, 12120, Thailand. 2 Sy^2 + Associates, Inc., Unit 504 Pryce Center, 1179 Chino Roces Ave., Corner Bagtikan St., Makati City, Philippines ABSTRACT This paper presents how performance-based design (PBD) approaches can help to improve the structural performance and cost effectiveness in design of floor diaphragms of tall buildings under earthquakes. In contrast to the prescriptive design approaches, performance-based design provides a systematic methodology for assessing the performance capability of overall building system and its components. The performance-based design explicitly evaluates the response of the building under the potential seismic hazard, considering the probable site-specific seismic demands as well as the uncertainties in the post-yielding response and behaviour of the building under seismic events. Case study of 57-story reinforced concrete residential building with 4 basement levels is presented. The building was designed for Design Basis Earthquake (DBE) level in accordance with traditional code-based design procedures at the preliminary design stage. After preliminary design, the performance of the building was checked explicitly at Service Level Earthquake (SLE) (43-year return period) and Maximum Considered Earthquake (MCE) level (2475-year return period), using linear and nonlinear response history procedures. Diaphragm design forces at podium level and tower levels were explicitly checked at site-specific MCE level event rather than application of code-specified modification factors to estimate the forces and deformation under code-specified earthquake level. Cost effectiveness of the design was evaluated by comparison of the indicative quantities and parameters between the code-based design and the modified design based on PBD. KEYWORDS Performance-based design, diaphragm, tall building, earthquake. INTRODUCTION Performance-based design is a state-of-the-art design approach for the seismic-resistant design, which has been widely used, for seismic evaluation of existing buildings and seismic design of number of new tall buildings. The conventional seismic design codes consider the reduction in design seismic force which implies the structural inelastic behaviour through the application of seismic response modification factor, R, in the simplified elastic analysis methods. While allowing inelastic deformation in the deformation-controlled members detailed for ductility, force-controlled members that are designed to remain elastic would experience the significant higher seismic force demand than reduced design forces by seismic response modification factor. To account for this effect, structural overstrength factor, Ω0, is multiplied to the design seismic forces to predict the maximum forces in members that are to remain elastic, especially in design of diaphragms. The intent of those factors is to simplify the structural design process by application of elastic analysis procedures. Those procedures do not consider the structural performance of component level, the ground motion characteristics and redistribution of seismic demand in the various components of building at the state of inelastic behaviour under strong seismic events. In contrast to the prescriptive design approaches, performance-based design provides a systematic methodology for assessing the performance capability of a building, system or component. The performance-based design explicitly evaluates the response of the building under the potential seismic hazard, considering the different probable site-specific seismic demand levels as well as the uncertainties in the post-yielding response and behaviour of the building. The case study building is a high-rise residential tower, which is 57-story high-rise building, approximately 192 meters from ground level to lower roof deck level. The building is the second tower of two-tower residential development project, sharing a common podium with 4-story below-grade parking. The seismic force-resisting 1323