A New method to increase effectiveness of base isolation on torsionally unrestrained structures

Alemdağ, Emren Can
The torsional behavior of the seismically isolated structures is one of the subjects that has been studied least. Especially, in the structures with first mode as torsional mode, i.e., torsionally coupled. In these type of structures, using conventional design method does not increase the torsional stiffness of the system and makes system weak in torsion when compared to other directions. This is not a desired situation in designing. Solving seismic isolation systems with lead-rubber bearings using the conventional design method can not solve the issue. To get system more torsionally rigid, isolators standing at the outer axes must be strengthened, relatively. However, while doing this operation, targeted period of the structure decreases because the total lateral stiffness of the system is increased. Hence, floor accelerations and shear forces can be increased, as well. Besides, in torsionally unrestrained systems, because displacement differences between the isolators and torsional amplifications are high, effectiveness of isolators decreases under different mass eccentricities. To overcome this issue, instead of conventional method, an other design method developed in this study can be used. In developed design method, instead of designing isolators for the seismic weights on them which is conventional method, system is separated as inner and outer groups and then, isolators are designed with a stiffness ratio. To calculate the stiffness ratio, a formulation was improved including basic parameters related to system. Isolation system designed according to the stiffness ratio calculated from the formulation, provides system to get translational modes in first two modes by increasing the torsional rigidity. Same building was investigated with two design methods, comparatively. After that these systems were examined under different mass eccentricity cases. In all cases, systems were solved using conventional and method developed in the study, with SAP2000. Results were examined in different seismic records and loading methods. According to results, developed method gives more effective systems when looked to the uniformity of displacements and shear forces of isolators. 
Citation Formats
E. C. Alemdağ, “A New method to increase effectiveness of base isolation on torsionally unrestrained structures,” M.S. - Master of Science, Middle East Technical University, 2016.