Parasitic Seismic Response of Equipment in Base Isolated Structure Mounted on Sliding Bearings

Abstract

All the implementations of seismic base isolation of structures until today—except for one (1982)—have used laminated rubber bearings inserted at foundation level which transfer simultaneously both horizontal and vertical forces. The double function of these bearings and the buckling problem resulting from it have led designers to restrict the fundamental period of the isolated structure to values less than 1.5–2 s in order to avoid large displacements, and this has caused a limitation of isolation efficiency. On the contrary, when the transfer of vertical forces is uncoupled from the transfer of horizontal forces, i.e., when these two functions are accomplished by independent components (for example, sliding bearings and steel or rubber springs, respectively), the buckling problem is overcome. The present study deals with equipment installed in an isolated structure mounted on such a dual support. It carries out a parameter analysis of the parasitic response of the equipment produced by horizontal friction force transferred to the structure through the sliding bearings. The parameters considered are: 1) the fundamental period of the isolated structure (1.5 to 7 s), 2) the stiffness of the sliding bearings, 3) the friction coefficient μ (2 to 15 percent), 4) the ratio σ of equipment mass to the whole structure mass (σ = 0.125 to 50 percent), 5) the frequency content of the ground seismic motion, 6) its intensity (0.3 to 3.0g GPA), and 7) the fundamental frequency of the equipment (3 to 30 Hz).