Non Linear Dynamic Analysis of High Rise Buildings in Islamabad Pakistan

Abstract

Pakistan is located in one of the seismically active zones of the world. Historic earthquakes like Taxila earthquake of 25 AD, Quetta earthquake of 1935 and Kashmir earthquake of 2005 bear testimony to the seismicity of the country. Islamabad being the capital city holds a large number of high rise buildings and is located in seismic zone 2B according to Building Code of Pakistan (BCP) 2007. Islamabad is surrounded by a number of faults. Traditionally Peak Ground Acceleration (PGA) is used in design calculations for lateral loads on structures. However alone PGA does not amply define the seismic load and modern building codes emphasize on use of spectral acceleration values. But such parameters don’t exist for any city of our country. In order to derive these parameters probabilistic seismic hazard analysis has been carried out for the capital city of Pakistan. Short period spectral acceleration at 0.2 second (Ss) and one second spectral acceleration (S1) were calculated for the entire district of Islamabad and some portions of Rawalpindi, Haripur and Abbotabad districts. Seismic waves while travelling upward from bed rock to surface change its frequency and are generally amplified in soft soil. Using the data of few available bore holes, a soil amplification spectrum was computed for Islamabad using SHAKE 2000 software. Seismic analysis of buildings is generally carried out by applying an equivalent lateral load. However dynamic analysis is required for important buildings or those of irregular geometry. Dynamic load can be defined as a response spectrum function or time history function. Response spectrum function was created for typical soil type D. Results of PSHA were de aggregated and a maximum earthquake scenario was defined for Islamabad. Time history functions of real recorded events matching with the scenario earthquake were selected and scaled to match the design spectrum. A suite of three accelerograms each with two horizontal components were selected for time history analysis. In order to check the adequacy of defined loading, a ten story building with irregular geometry was modeled and analyzed using static, response spectrum and time history load cases, the results were found comparable. In order to improve the response of structure viscous dampers were modeled and non linear dynamic analyses were carried out. An application of dampers was found useful in reducing the base shear and story drifts.