Horizontal capacity of base isolation rubber devices under large vertical design stress, valued through full-scale tests

An experimental campaign to investigate the effect of vertical stress on horizontal capacity of rubber bearings is argued in this paper. Results from a test program on full-scale HDRBs are discussed. They concern three different bearings (ϕ = 500–600 – 700 mm), having variable shape factors [S1 = 19,44–22,72] [ S2 2,8 ÷ 3,4]. The experimental data refer mainly to static shear tests, at varying levels of shear strain (up to γ = 250%) combined to increasing compressive stress (from 6 MPa to 20 MPa). Re-creating load conditions similar to the one occurring in situ, the experimental tests want to assess the impact of growing vertical stress on device horizontal deformability. The elaborations of data underline a progressive decrease of horizontal stiffness when even higher pressure acts. This is confirmed by visible softening effects that occur when horizontal displacement exceeds a certain percentage of device diameter, variable with the value of the applied compressive stress. This response is undoubtedly related to variations in axial load carrying capacity of bearings under large deformations: for this reason, evaluations on the critical load have been included in the discussion. Analytical formulations have been compared to experimental results to support the argumentations. The elaborations have been accompanied by the use of dimensionless parameters, in particular the ratio γ/S2, available also for design aims: this type of representation helps to evaluate deformation limits for rubber devices.