Evalution of structural and economic implications of incorporating base isolator as earthquake protection device in buildings in Dhaka

Abstract

Introduction of flexible elements at the base of a structure and at the same time ensuring enough damping is probably the best option for the seismic isolation technique. The device that is capable to meet such criteria is known as isolator. In this study incorporation of such base isolator in buildings in Dhaka has been in~estigated. A Time History Function as well as response spectrum corresponding to It has been established for the detail dynamic analysis and design along with static analysis. A feasibility study was done considering different buildings with different plan to justify the applicability of base isolation. The characteristic, cost and performance effects of a building with and without base isolotion system were compared. The design of Lead rubber bearing and High damping Rubber Bearing isolator for several multi storied residential buildings, located in Dhaka has been prepared and it's effectiveness in multistoried building is studied. Isolation on building with soft bottom story was also explored. Finally the recently commissioned 10 storied residential Tower Building for Police Personnel established at Saidabad, Dhaka under "Public Works Department (PWD)" has also been investigated here to propose for base isolation. Response spectrum as well as time history dynamic analysis using proposed Dhaka earthquake ground motions of varying seismic intensity were performed for both fixed and isolated base building. Time history was derived from recently occurred Natore earthquake and from this time history response spectra was constructed. The results of lateral load investigation with time period focus on a no. of buildings with different building plan. Analyses were performed using the finite element method hy SAP2000 and ETABS. Isolator design has been done through excel spreadsheet following an iterative procedure. Study shows the applicability of base isolator up to the range 29.3m-44.5m building height as a suitable concern. The installation of isolator in building considerably increases the time period of building, which means it reduces the possibility of resonance of the structure. The amount of lateral load taken by the structure reduces in a range of 20-45 % and therefore member sizes decreases providing the same reinforcement. Provision of Isolator in building often increases the total cost in a range of 2% - 5%. But as reduction of reinforcement requirement and construction material reduces the cost savings in the range of 19% - 25% so the net cost reduces due to the installation of isolator in building up to 5-8%. For Building with soft story, story drift behalves a peculiar characteristics, For first story it shows a larger value and for the immediate story, inter story drift falls 44-50% for LRB and 48-60% for HDRB .. In practical building, analysis shows that isolation reduces the base shear by 40 % and base moment by 45% of conventional value; T=0.5241 sec, earthquake is the governing lateral load. So isolator (Lead Rubber Bearing or High damping rubber Bearing) may be incorporated at the bottom of the building to exploit economic and structurally safe alternative.