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.
