Seismic Vulnerability Assessment of Typical Reinforced Concrete Buildings in Pakistan

Abstract

Pakistan is situated in a highly active seismic zone and its building stock is under immense seismic risk as indicated by the heavy damage to life and infrastructure during Kashmir earthquake, 2005. The main reasons for high vulnerability are low strength materials, poor design and poor detailing practices. This study aims to check the seismic adequacy of existing typical RC building stock by modifying the existing frame work for vulnerability assessment in terms of Building Code of Pakistan (BCP, 2007). A detailed field and literature survey is carried out to assess the typical characteristics of RC buildings and the most uncertain capacity parameters. Probability functions of these material and geometric parameters are defined after statistical analysis of the obtained data. Probability Density Functions or PDFs are then incorporated in vulnerability assessment process, in order to assess the probabilistic nature of seismic vulnerability of the mentioned building stock DRAIN-3DX with suitable sophisticated elements is used as analytical tool after calibration against experimentally tested suitable structure. Experimental results of Saclay building frame are used in this study to verify the tool. After validation of the tool, the building frames are modeled to compute the structural response. Nonlinear static method (static cyclic method) has been employed for obtaining the capacity envelope so as to include the brittle nature and degrading behavior of substandard construction. Damage is quantified using a damage index based on normalization of secant period against collapse period. Key capacity parameters are included to account for the probabilistic nature of the vulnerability. The 2, 3, 4 and 7 storey buildings with bays varying from 2 to 5 are considered and are evaluated for vulnerability after being designed for gravity load only. Seismic vulnerability frame work developed by Kyriakides (2007) is then implemented after incorporation of building code of Pakistan (BCP, 2007). Modification of input data and the post processing is controlled through a VBA code. The derived curves suggest brittle failure modes (pullout and shear). Their performance is satisfactory in zones 1 and 2A while all the building types are most vulnerable to zone 3 and 4. The 4 and 7 storey buildings show good performance in zone 2B while 2 and 3 storey buildings are vulnerable to the same. With an increase in the number of bays the building with same storey height fail earlier up to 4 bays. While increasing number of storeys the building failure is delayed up to 4 storey buildings. Exceptions in the response of 5 bays and 7 storey structures are observed.