DIAPHRAGM BEHAVIOUR IN REINFORCED CONCRETE BUILDINGS

Abstract

Floor diaphragms are a vital component of seismic resistant structures, which not only require analysis & design to resist out of plane bending but also require special attention to resist its in-plane bending. Unfortunately, despite their crucial importance, there are insufficient codal provisions & design guidelines related to in-plane bending of the floor diaphragm. Due to diaphragm design deficiencies such as lack of adequate shear & flexural capacity against in-plane bending, non-existence of valid load paths, an improper connection between diaphragm & shear wall, failure of the structure takes place due to separation of diaphragm & vertical lateral load resisting elements. With particular reference to shear wall supported buildings, this study aids in the development of a simplified methodology for determining the diaphragm design force & designing the floor diaphragm & its components to resist & safely transfer those forces to the vertical lateral load resisting elements. Lateral force at each floor diaphragm was determined using four methods: Equivalent Static Analysis (IS 1893-Part I), Response Spectrum, Spectral Acceleration & Diaphragm Design Force (ASCE 7-16). Out of four methods adopted, DDF was mainly governed by the spectral acceleration method & was used to assess & design the floor diaphragm along with its components such as Chord, Collector & Shear Friction Reinforcement. Equivalent Static Analysis Method being a simplified method & used for practical purposes, this study looks into the relationship between Diaphragm Design Force (DDF) & Equivalent Static Method to ease the calculation. To establish a relationship, three residential apartments were categorized into 4 groups (7, 10, 13 & 16 Storey) & were analysed using ETABS. The results of the analysis were used to create an equation that links DDF with Equivalent Static Method. This study discovered that the established equation is primarily influenced by the overall height of the building and the soil conditions on the site, but the equation remains valid when all other seismic parameters are varied. It was concluded that the DDF obtained from the equations developed were comparable to the actual values up to a building height of 10 stories. To achieve more reliable values when increasing the number of stories, dynamic analysis is recommended, as the dynamic concept analyses building attributes such as mass, stiffness, and damping, whereas the static concept only addresses mass.