Dynamic response of multi-degree of freedom structure with sliding isolation system and uplift
Vemuru, Venkata Srivishnu Mohan
Master of Science thesis
In this study the two dimensional dynamic response of a Multi Degree of Freedom (MDOF) sliding isolated structure permitted to uplift is studied. The MDOF superstructure is supported by two sets of sliding bearings situated at both the ends of the base mat. The foundation supporting the sliding bearings is assumed to be flexible and modeled using two sets of viscoelastic spring-dashpot systems attached below the sliding isolation bearings. The nonlinear stick-slip behavior of the friction based sliding isolation system is modeled using a hysteretic Bouc-Wen model. A nonlinear analytical model is formulated which takes into account the linear equations of motion of the superstructure and the base mat with stick-slip behavior of sliding isolators and in addition the following effects: uplift and loss of contact at the sliding bearings, and variation of coefficient of friction with velocity and bearing pressure. For comparison purposes fixed base structure with uplift is also modeled. The analytical model is solved using a two-step solution methodology and Pseudo-force method is employed for calculating the nonlinear forces acting on the structure at the interface of sliding isolation system in both the horizontal (stick-slip) direction as well as the vertical (uplift-contact direction). The analytical model developed is validated using experimental results of a 1:4 scaled model. Parametric study is performed with the analytical model (at full scale) by varying the periods of the superstructure, time period of the sliding isolation system, slenderness ratio of superstructure, vertical stiffness of foundation and pulse excitation parameters. The dynamical response of the MDOF structure with sliding isolation and uplift permitted is compared with sliding isolation with uplift prevented and fixed base structure with uplift respectively. The behavior of the MDOF system is studied for both near-source earthquake ground motion and their corresponding cycloidal approximations. The results of the study highlight the advantages of the combined mechanism of sliding isolation and vertical uplift for reduction of structural response of a MDOF system, as compared to the case without uplift and the case without sliding.