Ambient vibrations of unreinforced masonry buildings in Dhaka city

Abstract

The ambient vibration investigations are performed for use in structural health monitoring and in structural control studies within the technical assessment of different kinds of structures. The ambient vibration tests define the linear behavior of structures, since the amplitudes of vibration are small. The actual responsive behaviour of structures under an earthquake is similar to the ambient vibration behaviour. It was shown that the dynamic behaviour of the masonry building, even if not regular and with deformable floors, can be effectively represented. Considering the above fact the main objectives of this research was to identify the pre-dominant or natural periods of the targeted unreinforced masonry (URM) buildings by performing ambient vibration tests, to compare the obtained structural pre-dominant period with the structural period calculated by the formula given in Bangladesh National Building Codes (BNBC) and to develop a relationship of structural period with different structural parameters of the URM buildings. This paper presents evaluation of ambient vibrations and the identification of the pre-dominant frequencies of seventeen low-rise URM buildings by ambient vibration tests. The buildings are of different stories from single storey up to five stories with regular and irregular shape in plans and elevations located in Dhaka, the capital city of Bangladesh. These masonry buildings were built in between 1960 to 1990 without any seismic guidelines. Attempt was made to correlate structural periods with heights and structural periods with total length of walls of structures by curve fittings and regression models. The multivariate linear regression analysis was selected for model development. Total three approaches were followed where structural period was the dependent variable. Approach 1was taken to find out an empirical model with the consideration that the dependent variable i.e. structural period is a cubic function of both the independent variables i.e. height of structures and total length of walls. Again Approach 2 was taken for the same with the consideration that the structural period is a quadratic function of both height of structures and total length of walls. Lastly Approach 3 considered that the structural period is a quadratic function of height of structures and a linear function of total length of walls, separately. Total four methods, namely the enter method, stepwise method, backward method and forward method, for each approach were used one after another as to check which one work better and provide realistic solution. Finally both forward selection and stepwise methods of Approach 1 and Approach 2 provided similar and the best possible statistical result combining height and total length of walls of structures. However analyzing all the findings of the regressions the parabolic curve fitting equation was selected for calculating structural periods of URM buildings. The endeavours of identifying the correlations through regressions were successful. The study suggested an insertion of the finding in the form of a formula in the BNBC in future.