Dynamic behaviour of unreinforced masonry building

Abstract

Unreinforced masonry (URM) buildings represent a large portion of the buildings stock around the world and in Bangladesh. During the construction of those buildings very little or no seismic design requirements are considered. Recent earthquakes e.g. Gorkha earthquake in 2015 have shown that many such buildings are seismically vulnerable; therefore, the demand for dynamic analysis and upgrading strategies of these buildings has become increasingly stronger in the last few years. Moreover, no dynamic test on unreinforced masonry building made by Bangladeshi indigenous material is done yet. Therefore, the objective of this study is to understand the dynamic behavior of a typical unreinforced masonry (URM) building of Bangladesh. For this purpose, a half-scale URM room having dimension of 6' x 5' (length x width) with 5' height and 2.5'' wall thickness, were built using M2 type mortar (c:s=1:4) and half-scale brick units. Time history of Imperial Valley Earthquake was chosen as an input motion because pre-dominant frequency (5Hz) of Scaled Imperial Valley earthquake is close to the model's pre-dominent frequency (6Hz). The URM model was tested as reference specimen. Then, the reference specimen was retrofitted on all faces using 18 gauge wire mesh (12mm x 12mm (1/2'' x 1/2'') opening), which is locally available and retested again. The tests were conducted on a uni-directional earthquake simulator. The tests show that wire mesh may be used as a retrofitting material for URM buildings. The retrofitting measure decreased the specimens’ lateral deflection by a factor of 4.3 to 4.8 times compared to the reference (URM) specimen for different intensities of earthquake. In addition, the upgrading enhanced the cracking resistance and the energy dissipation of the upgraded specimen. The upgraded specimen is able to undergo 1.42 times more acceleration than reference specimen. Lateral load of the reference model is compared with the prescribed shear force (capacity) as per Bangladesh National Building Code (BNBC 1993). The reference model failed after reaching the capacity. Therefore, the code is conservative in terms of lateral load resistance capacity.