Stability issues for elastomeric bearings: analytical formulations compared to experimental results

During earthquake strong motions, anti-seismic devices are subjected to large horizontal deformations combined to vertical stress due to gravity loads. For elastomeric bearings, the most widespread technology used to this aim, it is necessary to value critical load capacity under this combined effect, to ensure their stability in service. In this paper, elaborations of data from experimental campaign on full-scale rubber bearings (HDRBs) are compared to results form a brief theoretical overview, to evaluate their stability limits. To examine the effect of device geometrical characterization on bearing mechanical behavior, to check instability mode and to evaluate the interaction between vertical pressure and shear deformation, three different types of full-scale devices have been considered (ϕ500, ϕ600, ϕ700). Experimental data refers to “soft” natural rubber bearings, having a shear modulus G= 0.4MPa (at γ=100%), an equivalent damping factor ξ = 10÷15, a primary shape factor S1 varying in the range [19,44 – 22,72] and a secondary shape factor S2 varying in the range [2,8÷3,4]. Data derive from static shear tests, at increasing levels of shear strain (up to γ = 250%) combined to growing compressive stress (from 6 MPa to 20 MPa). Their elaboration has been accompanied by the use of dimensionless factors, in particular the ratio γ/S2, that results an effective design parameter to describe bearing performance. On the base of analytical formulations from literature, the experimental results have been matched with theoretical evaluations of the critical load. The experimental occurrence of buckling instability has been read also in light of design guidelines provided for rubber devices.