Seismic performance evaluation of different retrofitting schemes using pushover analysis

Abstract

The traditional approach to seismic design is a force-based design where there is no measure of the deformation capability of a member or of a building. According to Bangladesh national building code,2006 the buildings are designed with equivalent static force method, response spectrum method and time history analysis. However, the actual performance of a structure can hardly be found by these methods. Structural failures in recent earthquake have exposed the weakness of current design procedures and leads to the development of Performance Based Earthquake Engineering (PBEE).

As a relatively new development, Pushover-based seismic evaluation and design methods offered a great opportunity to engineers. Applied Technology Council (ATC-40),1996 and Federal Emergency Management Agency-(FEMA),2000 proposed a simplified nonlinear static analysis (Pushover Analysis) procedure for PBEE which provides a better understanding about the actual behavior of the structures during earthquake. There are established numerical tools like ETABS v 9.7.4 developed by Computers and Structures Inc.. 1995 which can perform the pushover analysis.

The present study investigates and compares the seismic performance of two existing buildings as per as built structural drawings by pushover analysis. Among the two buildings one is a irregular shaped government office building and another one is a regular shaped government residential building which are located at different locations of Dhaka city. The buildings are of 6 storied and constructed 20 years ago. Different infill conditions (i.e. bare frame, full infilled and soft ground storey) along with different earthquake conditions (i.e. serviceability earthquake, design basis earthquake and maximum earthquake) were considered during analysis. For different conditions mainly a structure is analyzed with the help of capacity curve, capacity spectrum, deflection, drift and seismic performance level. Effect of infill is modelled using equivalent strut width theory.

It is found that performance of full infilled frame condition is better than that of bare frame condition. Capacity curve of the both structures meets the demand curve at lower displacement value. Lateral drift ratios are less than that of bare frame structure. Investigation of building with soft storey condition shows that it contains seismic deficiencies and need some remedial measures or retrofitting.

The performances of the structures with different remedial measures (i.e. insertion of shear wall, buttresswall, column jacketing) have been studied both individually and combindly. Among the considered retrofitting measures "insertion of shear wall" shows better performance over "column jacketing and buttress wall" in terms of lateral inelastic drift ratio and number of hinges formed.