ASSESSMENT OF THE SHEAR PROPERTIES OF HDRBS UNDER DIFFERENT COMPRESSION LEVELS

Despite it is well known that the shear properties of High Damping Rubber Bearings (HDRBs) are affected by the instantaneous compression load developed during the seismic ground motion, only permissible variations of their design properties with frequency, temperature and ageing are prescribed in the standards while the influence of the compression level is usually disregarded. Within this framework, this research addresses this drawback through both experimental and numerical investigations. In the first part of the study, small scale laminated isolators are tested on a custom biaxial machine in order to assess the secant modulus, and damping factor of the elastomeric compound under different compression levels. In the second part, the same phenomenon is investigated through cyclic shear tests on full scale HDRBs under three different levels of axial load. In the last part, a 3D finite element model of the isolator is eventually formulated in Abaqus FEM software. The mechanical response of the elastomer is simulated by means of a hyperelastic strain energy function combined with a relaxation function. The experimental results highlight the substantial influence of the axial load on the damping capacity of the elastomer, while the shear modulus is less affected. The numerical analyses demonstrate that the vertical – horizontal coupled response of HDRBs can be accurately predicted, within the proposed formulation, with constitutive parameters estimated from simple uniaxial tests.