Higher mode effects on friction pendulum base isolated asymmetric buildings

Abstract

When a structure's fundamental period lengthens, often shear and floor acceleration increases. The contribution of higher modes in the dynamic response of structural systems with multiple degrees of freedom is significant in both the design of new structures and the evaluation of existing structures.Few research have looked at the impact of higher modes on base isolation systems in structures. To investigate the effects of higher modes on a building with a friction pendulum base isolation, a study was conducted on four different types of buildings: one with an L-shaped plan with two, four, and six floors; another eccentricities of buildings 5%, 10%, 15%, and 20%; and two others with a building with an L-shaped plan, where the second floor is half the mass of the first floor and next S shape plan. Eight layouts of the pendulum base isolation are considered for each building and their first mode, the second mode, and the mass participation rate of the first mode, the second mode, and higher modes are derived. Finally, in a linear time history analysis, the change in the maximum displacement of the base level is compared to the percentage of mass participation on the modes. The findings show that in four models, the percentage of first mode mass participation is decreased if the center of stiffness of the base isolation is aligned to the center of cumulative mass of the building, and the percentage of mass participation is increased in the second or third modes. The responses of the structure are thus determined by the second mode or higher modes. In general, the stiffness center of the base isolation must line up with the center of the base cumulative mass to produce the optimal design. And this pendulum base isolation layout’s displacement of the base level is also reduced.

Keywords

• high modes
• mass participation ratio
• center of stiffness of pendulum
• center of the curvature radius of pendulum