Floor response spectra in isolated structures subjected to earthquakes weaker than the design earthquake-Part I: Isolation with high-damping rubber bearings

The seismic response of secondary oscillators mounted on structures isolated by high-damping rubber bearings (HDRB) and subjected to earthquakes weaker than the design earthquake is analyzed in the first of two joint articles. The shear modulus of rubber can substantially increase at small deformations. Consequently, the initial and post-elastic tangent stiffness of HDRB can be significantly different. Hence, the effective stiffness of HDRB can considerably increase and the effectiveness of the seismic isolation can be substantially reduced when they are subjected to earthquakes weaker than the design earthquake. This can jeopardize the equipment mounted on the structure, since the seismic demand can be even larger for weak earthquakes than for the design earthquake. The effects of the increased effective stiffness of HDRB on the response of the equipment were studied using floor spectra. The main parameters that influence the floor spectra at low-seismic intensities were identified and analyzed. They are the following: the effective stiffness of the bearings, which depends on the intensity of the seismic load and the deformation of the rubber, the periods (stiffnesses) of the isolated and the non-isolated superstructure, the initial stiffness of the bearings, the non-linearity of the bearings, the effective damping of the bearings, the effective damping of the superstructure as well as the effective damping of the equipment, and the higher modes of vibration (the position of the equipment in the building). In the case of weak earthquakes, the floor spectra are quite sensitive to the numerical model of the bearings. Copyright (C) 2010 John Wiley & Sons, Ltd.