횡하중을 받는 플랫 플레이트 시스템을 위한 수정된 등가골조법

Abstract

This study is conducted to develop (1) a modified equivalent frame method and (2) a stiffness reduction factor for reinforced concrete (RC) and post-tensioned (PT) flat plate structures under lateral loads. The Equivalent Frame Method (EFM) was developed for two-way slabs as a simple approximate method. Current design codes (ACI 318-05, Eurocode 2, and BS 8110) permit the EFM for the analysis of two-way slab systems under gravity loads, as well as lateral loads such as seismic loads. The EFM, however, was developed based on a semi-empirical formula, derived from the behavior of two-way slab systems under gravity loads. This study shows that the EFM is not appropriate in accurately predicting the response of two-way slab systems under lateral loads. To improve the EFM under lateral loads, this study proposes a Modified Equivalent Frame Method (MEFM). To verify the MEFM, an idealized two story flat plate frame is considered and analyzed by both the MEFM and elastic finite element analysis for estimating slab moments and lateral drifts. Analysis results obtained from the conventional EFM and Effective Beam Width Method (EBWM) are also compared. The lateral stiffness, based on the proposed MEFM, was also compared with experimental data. As the slab moment due to lateral loads increases, slab stiffness decreases due to crack formation. The accuracy of the analysis method (MEFM or EBWM) strongly depends on how the reduced slab stiffness is estimated. For this purpose, this study developed equations for calculating slab stiffness reduction factor (β) for RC and PT flat plate structures by conducting nonlinear regression analysis using stiffness reduction factors estimated from collected test results. Slab stiffness reduction factor (β) is defined as a ratio of reduced slab stiffness due to crack formation to the stiffness of the uncracked slab section. For verifying the proposed equation, the lateral stiffness of RC and PT slab column connection specimens tested in this study was compared with the lateral stiffness calculated using the MEFM with the proposed stiffness reduction factor. Furthermore, flat plate frame specimens having two continuous spans were also considered for verifying the proposed equations for β.