Keywords: poundings, seismic isolation, impact, adjacent structure, moat wall.

Seismically isolated buildings may experience large displacements relative to the ground during strong earthquake excitations, especially when they contain long period impulses. Therefore, a sufficiently wide clearance, which is known as "seismic gap", must be available around the building in order to accommodate such large displacements. Since the width of the provided seismic gap cannot be unlimited due to practical constraints, especially in cases of retrofitting existing structures, and considering the uncertainties regarding the evaluation of the expected design displacement, a reasonable concern is the possibility of structural poundings of a seismically isolated building against the surrounding moat wall or even adjacent buildings during very strong earthquakes.

In order to investigate the consequences of such structural impact on the seismic behaviour of a seismically isolated building, a specialized software application has been developed, which provides the ability to effectively and efficiently perform dynamic analyses and parametric studies. The software developed can be used to simulate multi-degree-of-freedom systems in two dimensions, allowing the consideration of poundings of a seismically isolated building either with the surrounding moat wall, which is assumed to move with the ground, or with an adjacent building. This is achieved by simulating together under the same excitation both the seismically isolated building and the adjacent fixed-supported buildings, while impact is allowed both at the isolation level and the floor levels. The impact forces acting on the colliding structures are calculated using a force-based linear viscoelastic impact model, whenever a contact is detected.

Using the software developed, large numbers of numerical simulations are performed in order to investigate the effects of certain parameters, such as the structural characteristics, the size of the separation gap and the earthquake characteristics on the response of a four-story seismically isolated building during poundings.

The results indicate that even with a small exceedance of the available clearance, significant changes on the response of the seismically isolated building may be observed. In particular, floor accelerations and interstory deflections are amplified significantly due to impact, with regard to the excitation characteristics and the width of the seismic gap. In addition, the parametric studies showed that even if a sufficient gap is assured to avoid poundings with the moat wall at the base, the seismically isolated building may pound against the adjacent buildings at the upper floors for wider gaps, due to the deformation of the superstructures.

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