Seismic Risk Assessment of Unreinforced Brick Masonry Buildings System of Northern Pakistan

Abstract

This research work was aimed at assessing the seismic risk of unreinforced brick masonry buildings’ system of Northern Pakistan, constructed in stone dust mortar. To accomplish this, four series of unreinforced brick masonry piers constructed in stone dust mortar were tested in the in-plane direction using quasi-static method of testing. Each pier series comprised of three piers with identical properties and thus a total of twelve piers were tested. Aspect ratio and pre-compression were kept as the main variables. Various properties such as displacement ductility factors, ultimate drift ratios, coefficient of equivalent viscous damping, stiffness degradation and modulus of rigidity were determined using the experimental data from quasi-static cyclic tests on the piers. The effect of pre-compression on the coefficient of equivalent viscous damping and stiffness degradation were studied. Similarly, the effect of drift ratio on the coefficient of equivalent viscous damping was also studied. Various performance levels for unreinforced brick masonry piers, in relation to drift ratios, were also recommended. Based on the results of experimental work, a methodology was proposed for lateral strength assessment of unreinforced brick masonry buildings. The developed methodology produced satisfactory results when compared with the results of full-scale unreinforced masonry (URM) buildings tested at University of Pavia, Italy [MKC 95] and Georgia Institute of Technology, USA [ Yi 04]. Although the proposed methodology was used for brick masonry buildings, it can be applied to other types of masonry (such as stone and concrete block masonry constructed in cement: sand mortar, lime mortar, etc.) if the properties required to quantify the seismic performance (e.g., displacement ductility factors and ultimate drift ratios of masonry piers, etc.) are experimentally known. Various performance levels for unreinforced brick masonry buildings were also recommended in relation to drift ratios. Finally, seismic capacities of thirty-one buildings were evaluated. The buildings’ stock consisted of seventeen single-story and fourteen double-story buildings. The buildings were selected keeping in view their common typological use in urban areas of Northern Pakistan. Fragility curves, showing the probabilities of reaching or exceeding various performance levels at various levels of ground shaking, were drawn for various performance limit states. It was found by studying the fragility curves that the probability of occurrence for various performance levels of single- and double- story buildings do not differ significantly. It was also concluded that unreinforced brick masonry, if properly constructed, can be safely used in localities placed in seismic zone 2b [BCP 07] and below