Displacement-based Design of R.C.C. Buildings

Abstract

The traditional earthquake resistant design is quanti ed by obtaining ductility demands based on calculated force demand-capacity ratio. This methodology completely miss an important parameter, structural displacement, which is a measure of damage in structures subjected to earthquake excitations. A codal based Force-Based Design (FBD) approach has following de ciencies - (1) the initial stiffness is used to estimate the time period of the structure, (2) seismic code use empirical expressions for the time period, T, (3) Response reduction factor (R), is appear to be arbitrary and are difficult to justify. Therefore, recently a design procedure known as Displacement-Based Design (DBD) has been developed with the aim of removing the de ciencies of widely used codal based FBD provisions. This method is based on displacement, since it is basic quantity and can be easily obtained. DBD de fines the structure under consideration by a single-degree-of-freedom (SDOF) representation and consider its performance at peak displacement response, instead of its initial elastic characteristics. This is realized by Substitute-Structure Approach. The Substitute-Structure Approach is a procedure where an inelastic system is modelled as an equivalent elastic system. In the present study, traditional FBD approach given in IS 1893 (Part 1) : 2002 is reviewed and its limitations are discussed. DBD is implemented for single storey building (SDOF) and multi storeyed building (MDOF). The advantages of DBD over FBD are also presented. Single storey R.C.C. building (SDOF) is analyzed by FBD and DBD and the parameters like time period, sti ness and base shear are compared. The influence of percentage reinforcement on curvature and sti ness are also examined by performing parametric study. Three di erent MDOF systems like four, ten and twenty ve storey R.C.C. buildings are analyzed by FBD and DBD and the storey forces and base shear demand are compared. The in uence of beam aspect ratio on yield displacement, displacement ductility and equivalent viscous vidamping are examined through parametric studies. The major focus of the study is to compare DBD with codal based FBD approach prescribed in IS 1893 (Part 1) : 2002, and understands the merits and demerits of DBD. Results pertaining to single storey building (SDOF) reveals that, DBD estimates base shear higher as compared to FBD. Results pertaining to multi storey buildings (MDOF) shows that DBD estimates base shear closer to that obtained from FBD if ductility factor is approximately `2'. The results are very much in line to the ones reported in literature. It is clear from the present study that, FBD is conservative in nature while DBD is quite realistic and simple in implementation.