# Earthquake-Induced Pounding of Medium-to-High-Rise Base-Isolated Buildings
## 1 Introduction  
Earthquakes may cause bounding between adjacent buildings having limited gap and different dynamic properties [1; 2; 3; 4]. This phenomenon, known as seismic bounding (see [5; 6]), often causes damage and, in some cases, may even lead to the collapse of the buildings. Many pounding-induced damages have been observed in past earthquakes, including the Mexico City earthquake in 1985, the Loma Prieta earthquake in 1989, the Northridge earthquake in 1994, and the Kocaeli (Izmit) earthquake in 1999 (see [7; 8] for example). In order to prevent structural collisions during ground motions, it is required to consider a minimum gap for adjacent buildings according to the seismic codes [9]. However, in the case of the large number of existing buildings, which have not been designed according to the current seismic codes, or in the case when the gap between structures has been filled by inappropriate materials, structural bounding has to be mitigated [10]. In recent years, there has been an increasing demand to minimize structural and non-structural damages so as to avoid functionality disruption and to protect sensitive and expensive equipment in buildings, even under extreme earthquake excitations. One of the possibilities is the applicationof seismic isolation at the base of buildings, which is considered as an effective earthquake resistant design method [11; 12; 13; 14]. The application of this type of protection system leads to the elongation of the natural period of a structure. Additional damping is also often provided so as to increase the amount of energy dissipation during vibrations of a building. As a result, the structural response under seismic excitations is substantially reduced [15; 16]. Generally, base isolators can be divided into two groups, i.e., laminated rubber bearings and sliding bearings. The first type of isolators is usually represented by high damping rubber bearings (consisting of hard rubber and steel plates, constructed in alternating layers) and lead rubber bearings (with the lead core inside) [17]. In turn, the friction pendulum bearing is a popular sliding system in which low frictional interfaces are applied [18]. Different types of base isolators have wide practical applications in many structures all over the world. Their effectiveness has already been confirmed during a number of seismic events (see [19] for example). The base isolation systems introduce flexibility at the isolation level of relatively stiff buildings, which results in avoiding resonance with the predominant frequencies of the ground motion. Floor accelerations are substantially reduced and deformations are observed mainly at the seismic isolators, which are specifically designed to accommodate large relative displacements, withstanding several cycles of deformation. However, a kind of limitation for the implementation of seismic isolation is the seismic gap that must be provided around the building to facilitate the expected large relative displacements. The results of the numerical simulations conducted by [PERSON] et al. [20], for example, indicate that the minimum gap size can be as large as 50 cm in the case of major ground motions. [PERSON] and [PERSON] [21] obtained the value of 43.7 cm required to prevent collisions of the base with the surrounding moat wall in the case of 10-story base-isolated steel building exposed to the Kobe earthquake of 1995. The peak base displacements for different base-isolated reinforced concrete frame structures with masonry infill walls were also investigated by [PERSON] and [PERSON] [22]. The results of the study for the El Centro earthquake of 1940 show that the minimum gap size should be larger than 20 cm. The situation can be somewhat improved by the application of some methods of reduction of excessive displacements at the isolation level. [PERSON] et al. [23], for example, proposed to use the supplemental dampers in order to reduce the structural displacements without significant increase in the forces transferred to the superstructure.