Seismic Evaluation and Strengthening of Existing RC Buildings

Abstract

Moderate to high levels of seismic hazard are present through out the Pakistan and numerous collapsed by Oct. 8, 2005 Earthquake verified the seismic vulnerability of building type present in the country. Seismic retrofitting of existing structures is one of the most effective methods of reducing this risk. However, the seismic performance of the structure may not be improved by retrofitting or rehabilitation unless the engineer selects an appropriate intervention technique based on seismic evaluation of the structure. Current codes (BCP 2007) do not address the evaluation of seismic resistance of existing building stock, not designed in accordance with the philosophies of current seismic provisions. The primary purpose of this work is to carry seismic evaluation of buildings in Islamabad and propose guidelines for Building Code of Pakistan. Owing to importance of the subject, various organizations in the earthquake threatened countries have come up with documents, which serve as guidelines for the assessment of the strength, expected performance and safety of existing buildings. In this study review of various documents on seismic evaluation of existing buildings from different countries is carried out to identify the most essential components of such a procedure for use in Pakistan and other developing countries, which is not only robust, reliable but also easy to use with available resources. Among these documents, ASCE 31-03 guidelines are found to be most suitable for use in our country. In this study seismic evaluation of buildings is carried out based on ASCE 31-03 provisions in order to understand the procedure in insight. These guidelines are applied on a Case study building which is an actual building located in Islamabad. Building is analyzed for Life Safety performance level and for moderate seismicity. A general checklist is used for Tier 1 analysis. Building was found to be deficient and Tier 2 Evaluation is carried using linear analysis procedures. DCRs for some of the columns exceed the limit at Tier 2 phase so a Tier 3 Evaluation is vii conducted. Pushover analysis is conducted for a typical 12 story RC building. ATC 40 procedure using BCP (2007) parameters was found to be conservative. Retrofitting was proposed for the deficiencies identified in case study building and formation of plastic hinges showed that retrofitting is adequate.