Performance Based Seismic Design of Reinforced Concrete Building

Abstract

Observing last few decades, seismic design of structure is gaining more attraction among researchers and engineers community. Performance based seismic design is one of the alternate methodologies to carry out seismic design. It is to be done to achieve predefined performance objectives which can be selected with proper understanding of risk to life, downtime cost, repair work, and other economical losses. It is extension of limit state of serviceability provided in current design standards. It incorporates nonlinear characteristics of structural elements for finding out the response of the structure. While in most of major earthquakes the structures are pushed in to inelastic zone so it will be more realistic to predict the response and collapse mechanisms based on nonlinear analysis procedures. But there is requirement of more research and experimental works to make it a general methodology which can be used for seismic design of all type of structures. In addition a damage index to quantify the possibility of damage in future seismic events is studied with Performance Based Seismic Design.

Seismic design according to predefined performance objectives is widely accepted in practice. Performance Based Seismic Design methodology provided in such standards are studied in this project. The idea to incorporate multiple performance objectives as a basic criteria and generalized steps to carry out design has been proposed by SEAOC VISION 2000 document. Further with evaluation of methodologies, many standards have started to include it as a seismic design methodology of existing as well as new structures. Four such standards are (FEMA 273, ATC 40, ASCE 41 and FEMA 440) studied in this project.

Seismic analysis can be divided in to four major types which are linear static, linear dynamic, nonlinear static and nonlinear dynamic procedure. Obviously nonlinear procedures are more realistic and accurate methods in comparison of linear procedures. But nonlinear procedures are more time consuming and complex in nature. The analysis can be done with the help of nonlinear software for accuracy of the results and to reduce the chances of errors without wasting time. Nonlinear Static Procedures are considered in this study. Two such Nonlinear Static Procedures namely, Displacement Coefficient Method and Capacity Spectrum Method are used for seismic analysis.

Four Nonlinear Static Procedures are studied in detail. First one is Displacement Coefficient Method suggested by FEMA 273 standard. It is a simple method aimed to find out the maximum possible response during design level earthquake with the help of different modification factors. This maximum possible response is called as a Target Displacement which is the key element of the procedure. The second one is Improved Displacement Coefficient Method recommended by ASCE 41. Which was proposed as an improvement to previous method. The third procedure is Capacity Spectrum Method presented by ATC 40. Which is a graphical representation method aimed to find out performance point which represents the interaction of capacity of the structure and demand from design seismic level. And the fourth one is Improved Capacity Spectrum Method by FEMA 440 standard. This method remains almost same excepting the approximation of the nonlinear parameters required for analysis. It is also known as Improved Equivalent Linearization Procedure.

A medium rise reinforce concrete building is taken to carry out seismic design following Performance Based Design philosophy. The performance assessment is then carried out according to four above described procedures with the help of ETABS 15.0 and SAP 2000 software. Further park ang damage index for building is found out with the help of simple damage index model.

This project is aimed to study and present the different Nonlinear Static Procedures and damage index which can be incorporated in Performance Based Seismic Design. Capacity Spectrum Method according to ATC 40 gives maximum value of displacement as 38 mm and Displacement Coefficient Method according to ASCE 41 gives minimum value of displacement as 26.4 mm. The damage index found out is 0.371 which matches the results obtained from the analysis. According to study the designed building is in immediate occupancy level and the design is safe for considered earthquake level.