Seismic vulnerability of some historical masonry structures in Bangladesh

Abstract

Bangladesh is located in a moderately seismic zone. Although no major earthquakes occur in this country in the last few decades, during the past large earthquakes the masonry structures of this country behaved poorly. Ancient masonry structures are particularly vulnerable to dynamic actions, especially seismic actions. In this thesis, 28 historical masonry structures; mainly mosques have been selected which are located in different districts of Bangladesh, built between 13th to 18th centuries. The aim of this thesis is to assess the seismic vulnerability of Masonry building by using simplified method and Microtremor measurements. This research deals with an investigation regarding the possibility of using simplified methods of analysis and simple indices as indicators for fast screening and decision to prioritize deeper studies of historical masonry buildings and to assess vulnerability to seismic loading. These indices, both in-plane and out-of-plane, are established mostly on the in-plan dimensions and height of the buildings. The approach used in the simplified method analysis aims at a simple, fast, and low cost procedure suggested by Lourenco and Oliveira based on a simplified geometric approach for immediate screening of the large number of monuments mainly mosques at risk. Lourenco and Oliveira developed threshold values for analyzing historical masonry churches by this method (Lourenco and Oliveira, 2004). From this threshold one may identify whether the structures are vulnerable or not. In this thesis these threshold values have been used to identify the vulnerable historical structures. The proposed geometrical indices of monuments located in different seismic areas are compared with the respective seismic hazard, expressed by the peak ground acceleration (PGA). The usage of simplified methods was made, taking into consideration the in plan area of the building, its height and seismicity, with the simultaneous verification of two indexes, one related to ratio of in plan area (γ1) and weight (γ2), and another related to the maximum base shear force (γ3). Indices (γ1) and (γ2) do not present a clear trend with respect to seismicity; however, a slight trend observed associates the increase of (γ1) and (γ2) with PGA growth. The analysis of the out-of-plane indices shows that a logical common trend can be established. For low and moderate seismicity, indexes do not exhibit a dependency on seismicity. Microtremor measurements were intended to collect preliminary information on the dynamic properties of the buildings and conducted in the vicinity of the buildings to determine the period of the site and asses the potential effects of soil-structure interaction, which could have a significant effect on the seismic performance of the buildings during a severe earthquake. In terms of average values of six indices, almost all mosques are safe except eight (28.58%) mosques of the samples. For the high seismicity, these eight mosques are vulnerable in the short direction only and required careful attention and deeper investigation at risk. The results of the Microtremor tests were successful at obtaining the site periods for the two locations, which were 0.26 second for Musa khan mosque and 0.13 second for Aambour Shah Shahi mosque. The dynamic response of the older masonry structures during a severe earthquake can be significantly affected by soil-structure interaction effects, and should be considered for possible retrofit design.