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.