Seismic Behavior of Multi Storied RCC Structures Using Different Sets of Stiffness Modification Factors

Abstract

Stiffness Modification Factors (SMFs) are utilized as a part of linear analysis of reinforced concrete structures to represent the impact of concrete cracking. Different sets of SMFs are recommended in different codes. Before availability of finite element softwares and even after the availability, designers have been using gross stiffness of the members for reinforcement design as this practice yields more reinforcement for beams. However, this practice may be against the seismic design philosophy of weak beam-strong column. Therefore, there is a need to verify the seismic performance of these buildings through advanced techniques under framework of performance based seismic design (PBSD). In this study, seismic behavior of a realistic 7-storied building designed with code-based, in-practice (uncracked stiffness for reinf. design) and hypothetical (slab and beams uncracked) sets of SMFs has been investigated. The performance of the building thus determined is compared with code-based, (American Concrete Intitute (ACI)) SMFs as reference for each set of SMFs. Decrease in reduction factor due to inherent ductility “Rµ”, storey displacement and storey drift has been observed while increase in over strength factor “Ω”, storey shears and overturning moment inelastic demand has been observed for in-practice and hypothetical sets of SMFs indicating more capacity requirement i.e. more reinforcement demand. Higher demand may greatly affect the elements/ behavior that intended to be elastic i.e. formation of hinges in columns in upper stories, shear in frame sections, bond and slip failure and foundation pressure. In code-based SMFs case, the load is observed to be transferred from slab to beam approximately equally along the length of the beam and from beam to column. This mechanism is according to the philosophy of strength based design mechanism of load transfer in RC structures. In other SMFs cases, significant load is observed to be directly transferred from slabs to columns deviating from realistic behavior. Plastic hinge formation in code-based SMFs indicates code intended behavior, i.e. reasonable formation of plastic hinges at ends of beams and bottom of bottom storey columns. Whereas for other stiffness modeling cases, formation/ status of hinges is rather low. This may result into formation of hinges ix at undesirable locations such as columns at upper stories. Reinforcement quantity with code-based modifiers is observed to be 20% more economical than the other systems. It is concluded that Code-based SMFs are better and economical than other sets in terms of load transfer mechanism, shear and moment inelastic demand, reinforcement demand and seismic behavior.