Keywords: seismic isolation, seismic gap, pounding, structural impacts, base isolation, earthquake-resistant design.

This paper presents simulations of seismically isolated buildings under strong earthquakes considering the possibility of impact with the moat wall and discusses the consequences of such poundings on the effectiveness of seismic isolation [1,2,3]. In order to seismically isolate a building, flexibility is introduced at the isolation level to shift its fundamental frequency outside the dangerous resonance range of typical earthquakes. Seismic isolation can be used to reduce the shear forces, interstory deflections, and floor accelerations of a building, and prevent damage of its structural and non-structural elements, as well as damage of its contents.

However, a seismic gap should be provided around a seismically isolated building to facilitate the large relative displacements at the isolation level. Since the width of the seismic gap is usually finite due to practical limitations, a reasonable concern is the possibility of poundings of seismically isolated buildings with adjacent structures during strong ground motions. Therefore, it is important to investigate that possibility and understand how potential poundings of seismically isolated buildings with adjacent structures, due to stronger than expected earthquakes, may affect the effectiveness of seismic isolation. This research work aims to address some aspects of this problem using numerical simulations and parametric studies, examining how the maximum floor accelerations, story shear forces and interstory deflections are affected by poundings and relevant parameters.

After a brief description of the simulation approach and the simplifying assumptions that have been used, a dynamic analysis of a typical seismically isolated four-story building under a strong earthquake is used to demonstrate the problem. Next, simulation results and parametric studies are presented for some aspects that have been considered. In particular, the flexibility of the isolation system for a typical four-story building is varied in order to obtain seismically isolated buildings with fundamental periods in the range of 1.0 to 4.0 seconds, which are analyzed for the Northridge earthquake excitation considering different seismic gap sizes. The results indicate that the interstory deflections and, consequently, the corresponding story shear forces, increase due to poundings. However, the major consequence of poundings on a seismically isolated building is the substantial increase of the floor accelerations and inertia forces, which may happen with very wide seismic gaps due to the increased velocity that can be reached prior to impact. The effect of pounding is more pronounced for relatively flexible seismically isolated buildings, especially when their fundamental period corresponds to a local maximum in the displacement response spectrum of the earthquake excitation.

The effect of the impact stiffness is considered by varying its value relatively to the stiffness of the superstructure. According to parametric studies, as the impact stiffness increases, the relative displacements at the isolation level are reduced, approaching the size of the seismic gap. However, at the same time, the maximum floor accelerations and consequently the inertia forces increase substantially due to pounding and may become much higher than what the building experiences without seismic isolation. The maximum interstory deflections and the base shear forces also increase with the impact stiffness, but, in general, remain lower than that which the corresponding fixed-supported structure would experience. The substantial increase of the floor accelerations with the impact stiffness indicates that the latter should not be more than a fraction of the stiffness of the superstructure.

Next, the effect of the flexibility of the superstructure on the response of seismically isolated buildings during impacts is considered, by varying the flexibility of the superstructure. The simulations indicate that as the flexibility of the superstructure increases, the interstory deflections are significantly increased, the total floor accelerations are very slightly increased and then, as the superstructure becomes very flexible, slightly decrease, while the story shear forces are reduced but at a lower rate than the increase of the interstory deflections. If the prevention of damage to structural and nonstructural elements is a major design concern, the stiffness of the superstructure should be increased to limit the interstory deflections.

In order to avoid abrupt stiffness changes due to impacts, collision bumpers and restrainers are suggested as potential practical mitigation measures against poundings, which is demonstrated through a numerical example. Pieces of flexible material with damping properties can be added at certain locations of a seismically isolated building and used as shock absorbers to smooth the stiffness changes during pounding with the moat wall and also toprovide additional damping prior to impact.

1

Kelly, J.M., "Earthquake Resistant Design with Rubber", Springer-Verlag, Second Edition, 1997.

2

Komodromos, P., "Seismic Isolation for Earthquake-Resistant Structures", WIT Press, 2000.

3

Skinner, R.I., Robinson, W.H., McVerry, G.H., "An Introduction to Seismic Isolation", DSIR Physical Sciences, Wellington, New Zealand, John Wiley & Sons, 1993.

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