High Damping Rubber (HDR) is used in the manufacturing of elastomeric bearing for seismic isolation of building and structures. In service rubber bearings are subjected to a permanent vertical load which may change at the occurrence of the earthquake, and potential shear deformation induced by the seismic ground motion. This study investigates the response of five commercial elastomeric compounds used in the manufacture of HDRB with shear modulus between 0.4MPa and 1.4 MPa, and equivalent viscous damping between 0.06 and 0.16 under combined compression and shear, with compressive stresses ranging from 0 up to 18 MPa. Soft rubbers seem to be more sensitive to the compression load than hard rubbers as far as the damping factor is concerned, whereas the influence of the compression load on the shear modulus seems to be peculiar of every mixture and not apparently related on the hardness, which can explain the inconsistencies found in literature relevant to the studies on full scale rubber bearings
