Seismic Retrofitting of Existing RC Structure, A Case Study

Abstract

Islamabad is situated in a region of medium to high seismicity. It is imperative that most of the buildings existing and constructed in Islamabad be able to withstand the loads produced on them by earthquakes. According to recent local research, there is little work being carried out on the seismic evaluation of existing buildings in regions of moderate to high seismic activity in Pakistan. This can be attributed to a lack of expertise in areas of seismic strengthening and earthquake engineering among the relevant segments of Pakistan’s engineering workforce. The main objective of this project was to learn the fundamental concepts behind seismic strengthening or retrofitting of structures and applying the attained knowledge to seismically evaluate and then, if applicable, retrofit an existing building in Islamabad. The building chosen to be evaluated was State Life Building, located in Blue Area, Islamabad. The building had a total of 18 stories and a height of 285’. ASCE 31-03 was used for seismic evaluation of the building. Preliminary evaluation and Initial screening of the building revealed that it had some structural deficiencies as it failed the soft story check, the axial stress check and the shear stress check. Although the building had lateral force resisting systems such as shear walls and bracing beams in place, it was suspected that the building might be deficient in terms of seismic strength. A non-linear static seismic analysis (pushover analysis) of the building was performed and the building was discovered to be globally fit to undergo large drifts ( approximately 3 % of story height) before any global failure would occur as the story shear at failure of the structure was less than the expected maximum shear. An evaluation of the existing 12” thick shear walls revealed that the shear walls, while effective in enhancing the structures lateral stiffness, were uneconomical to use. 9” shear walls placed at a different location compared to the location of the existing shear walls would have sufficed. Local deficiencies in the structure were also identified at the basement level and recommendations have been with respect to their removal.