Base-isolated building with asymmetries due to the isolator parameters

The effects of torsional coupling, due to isolator parameters, on the seismic response of base-isolated buildings are presented. The isolated building is modelled as a single-storey structure mounted on different isolation devices such as elastomeric and sliding systems involving non-linear restoring forces. The governing equations of motion for the uncoupled and torsionally coupled system are derived and solved in time domain by Newmark's method of integration to obtain lateral-torsional displacement response. The displacement response of the isolated system with different combinations of structural configurations, isolation systems and the ratio of uncoupled torsional to lateral frequency of the system is investigated. A comparison of the response of the torsionally coupled base-isolated building is made with the corresponding response obtained from torsionally uncoupled base-isolated building. In addition, a parametric study is conducted to observe the effect of superstructure flexibility on the displacements in torsionally coupled base-isolated building. The eccentricities arising due to the asymmetries in the isolation stiffness and/ or yield strength of the isolators are compared with the eccentricity in the system as specified by the Uniform Building Code (UBC 1997). It is observed that the torsional coupling arising due to the dissimilarity in the isolator properties considerably influences the seismic response of the base-isolated building. Effects of superstructure eccentricity are found diminishing when the isolation eccentricities exist. The design bearing displacement suggested by the UBC static formula incorporating accidental torsion is found conservative for the isolation eccentricities arising due to the dissimilarity amongst the isolators

Impact Response of Torsionally Coupled Base-isolated Structures

The seismic response of a single-story asymmetric structure supported on various base isolation systems during impact with adjacent structures is investigated. Eccentricities arising due to elastic forces in the columns at the top-deck and the restoring forces in the isolation systems at the base-raft are considered. The adjacent structures (i.e. retaining walls or entry bridges) surrounding the base-isolated structure on all four sides are modeled in the form of springs and dashpots. The coupled differential equations of motion for the isolated system are derived and solved in incremental form using Newmark's method to obtain the seismic response with and without impact. The variation of superstructure acceleration and isolation level displacement during impact upon the adjacent structures under the action of real earthquakes are computed to study the behavior of torsionally coupled structures and compare the performance of various isolation systems. The torsional impact response of isolated structures is studied under the variation of important system parameters such as the sizes of gaps, the stiffness of adjacent structures, superstructure flexibility and different eccentricities in the base-isolated structure. It is concluded that the lateral-torsional response of the base-isolated structure is adversely affected when impact takes place with the adjacent structures. The superstructure acceleration increases and the base-raft displacement decreases due to impact with the adjacent structures; nevertheless, the isolation remains effective as compared to the nonisolated case. Further, it is also observed that superstructure acceleration increases with increase of the isolation gap distances up to a certain value and then the acceleration decreases with its further increase. The effects of impact are found to be severe for systems with flexible superstructure, stiffer adjacent structures and increased eccentricities