# Earthquake-Induced Pounding of Medium-to-High-Rise Base-Isolated Buildings
## 3 Results and Discussion  
### Response for Different Gap Sizes  
The influence of the gap size between buildings on the response of base-isolated medium-to-high-rise structures has been firstly studied. Figures 3-5 show the peak accelerations of the 5 th story of the isolated building with respect to different gap size values for various configurations and ground motion records.  
The results presented in Figures 3-5 clearly indicate that bounding between the isolated and non-isolated buildings may significantly increase the response of the medium-to-high-rise isolated structure in the case of all earthquake records considered. It can also be seen from the figures that the response of colliding base-isolated building depends very much on the gap size between structures. The results indicate that with the increase in the gap size, the response becomes smaller and this trend is observed up to the gap size big enough to prevent pounding. It should be underlined that the size of the gap preventing collisions depends on a number of parameters, including the earthquake intensity, its particular frequency content, the dynamic characteristics of buildings, their configuration, etc.  
Figure 4: Peak accelerations of the 5 th story of the base-isolated building with respect to different gap sizes for the second configuration under various earthquakes.  
Figure 3: Peak accelerations of the 5 th story of the base-isolated building with respect to different gap sizes for the first configuration under various earthquakes.  
For the cases analyzed, for which all earthquake records have been scaled so as to attain PGA = 0.5 g, its maximum value has been found to be as large as 30 cm (see Figure 5).  
Moreover, the results of Figures 3 and 4 indicate that the increase in the peak acceleration due to bounding can be as large as 1140% for interactions between the five-story isolated building and the five-story fixed base structure, while it is equal to 828% when the five-story isolated building collides with the eight-story fixed base structure. It should be underlined, however, that the effects of bounding can be even more pronounced when the height of the base-isolated medium-to-high-rise building is bigger, and the increase in the peak acceleration due to bounding between the eight-story isolated building and the five-story fixed base structure can be as large as 2529% (see Figure 5).  
It can also be seen from Figures 3-5 that, except for the Buena-Park station record of the Northridge earthquake where the stability in the response is observed from 6 to 18 cm gap size, the remaining records result in structural response modifications due to collisions for gaps as wide as 12 to 30 cm. Moreover, the peak acceleration responses are quite similar for each case of two different buildings interacting under seismic excitation. This is an interesting result, given the fact that the earthquake records are very dissimilar, with PGD of 4.53 to 23.04 cm (see Table 2). It should be underlined, however, that scaling the earthquake records to the same PGA value resulted in reduction in differences between PGD values for various scaled ground motions. It should also be added that PGD is not the only factor influencing structural displacements, and the dynamic parameters of the structure itself have a significant influence on its response, especially when the natural structural period falls within the range of the predominant frequencies of excitation. Moreover, the bounding-involved response is much different comparing to the regular response without collisions (see also [5; 35] for example). Therefore, the above reasons caused the situation observed in Figures 3-5 so that different records provided quite similar results.