Stability issues and pressure–shear interaction in elastomeric bearings: the primary role of the secondary shape factor

Abstract

Stability of isolation rubber bearings is a topic widely studied and concerns (1) the critical load capacity in the undeformed configuration, under long-term load (gravity) effects; and (2) the stability condition under short-term vertical pressure (due to gravity plus seismic loads) at large lateral deformations. In this paper the problem of elastomeric bearing stability under large lateral displacements is addressed through FEM parametric analysis; rubber bearings typical of current design practice, characterized by S 1 = 20 are considered, with the value of the secondary shape factor S 2 being varied between 1.5 and 6.2 in order to assess the effect of slenderness on the mechanical behavior, failure mode, and interaction vertical pressure-shear deformation. The analysis results show that the sensitivity of the shear response to the applied vertical pressure is directly related to the value of S 2 and that S 2 has an overwhelming effect on the stability behavior and shear response of the bearing, as compared to the effect of the primary shape factor and of the rubber shear modulus. Finally, S 2 is the parameter which governs the failure mode of the bearing in the seismic condition (vertical pressure-shear deformation). On the basis of these results, design implications are discussed.

Developing collapse fragility curves for base‐isolated buildings

“…As far as shear failure is concerned, recent studies have pointed out a lower bound limit for rubber failure ranging between 260% and 380% in terms of shear deformation (γ), regardless the shape factor of the bearing and the applied pressure. Such value seems to be rather conservative if compared with the experimental results reported in Muramatsu and Inoue, where values of γ of the order of 400% to 500% have been found.…”

“…The effect of large horizontal displacements is usually accounted for approximately by means of a correction factor equal to the ratio between the effective bearing area at large displacements and the actual bearing area. In this study, a reduction of the critical buckling load of about 15% has been considered for elastomeric bearings which have first experienced cavitation in tension, based on the experimental observations by Kumar et al 45 As far as shear failure is concerned, recent studies 46 have pointed out a lower bound limit for rubber failure ranging between 260% and 380% in terms of shear deformation (γ), regardless the shape factor of the bearing and the applied pressure. Such value seems to be rather conservative if compared with the experimental results reported in Muramatsu and Inoue, 47 where values of γ of the order of 400% to 500% have been found.…”

Stress softening behaviour of HDNR bearings: modelling and influence on the seismic response of isolated structures

“…At this regard, it is found, to a good approximation, that for a typical design of laminated rubber bearings (e.g. primary shape factor S 1 > 20 and secondary shape factor S 2 > 3, ) and under typical design axial loads, bending moments and shear deflections (far from collapse condition as imposed by Standards), the shear load‐deflection behaviour of a laminated HDNR bearing is the same as that of a block, constrained to be in simple shear, of the same total thickness, cross‐sectional area and material properties as the rubber in the bearing. In fact, under these conditions, the effects of axial load and bending moment on the shear‐load behaviour of such a single rubber layer in the bearing are negligible, and the extra compliance resulting from P‐Δ effects estimated from the appropriate beam‐column theory can be assumed to be small.…”

Rintc Project: Nonlinear Analyses of Italian Code-Conforming Base-Isolated Buildings for Risk of Collapse Assessment

“…The global collapse condition has been conventionally fixed when the 50% of elastomeric devices simultaneously reaches a value of the axial compressive force equal to the critical buckling load. Finally, with regard to the shear failure, recent studies [14] pointed out a lower bound limit for rubber failure in terms of shear deformation (γ) of about 260%, regardless the shape factor value and the applied pressure. Such value seems to be excessively precautionary if compared to the experimental results obtained by Muramatsu et al [15] and Kawamata and Nagai [16], which propose values of the order of 400-500%.…”