EXPERIMENTAL STUDY ON STRESSES OCCURRING IN CONNECTION OF RUBBER BEARING UNDER VERTICAL LOAD

Abstract

 Bending moments due to superstructure-derived axial and shear force act on flange plate and fastening bolts, which are used to join rubber bearing to seismic building frame and foundation. Previous studies had confirmed the tensile force of bolt and the von Mises stresses on flange by the loading tests of rubber bearing with diameters of 800 mm and less. Additionally, these studies determined the pressure distributions of the rubber interior using finite element methods, as well as derived formulas for calculating the tensile force generated in bolt in cases where prying actions are formed due to the tensile axial forces of building. However, the phenomenon in which the area decreases in the vertically overlapping rubber along with the shearing deformation results in the concentration of vertical load on a single portion of the flange, and the differences between previous calculation formula for flange bending stress by using the tensile force of bolt and real-world scenarios, had been pointed out but not investigated quantitatively.

 Therefore, we performed biaxial loading tests on two types of natural rubber bearing with diameters of 800 mm and 1300 mm. By the loading conditions ranged from only tensile deformation to compressive stress of at least 20 MPa, we investigated the effects of vertical and horizontal load on flange stresses and tensile force of bolt. The flange stress was evaluated by attaching the strain gauges on the flange diameter in the shear deformation direction and determined by using the maximum principal stress.

 The results show that the tensile stresses of bolt in rubber bearing 800mm where there are significant decreases in the vertical overlapping area accompanying horizontal deformation, are high during high compressive stress and large shear strain. Large prying forces generate in the rubber bearing were confirmed, similar to predictions made by one of previous calculation methods, with the calculated and measured values being close to each another. The results also demonstrate that even the maximum tensile force of bolt is within a factor of approximately 1.5 of the allowable stress. However, significant variations are often observed for the tensile force of bolt introduced by tightening, even when the torque wrench is used. Therefore, the design that incorporate reserve strength are necessary by factoring the prying action of fasten bolt for seismic building and foundation.

 The flange stresses locally increase in the domain of the decreasing overlapping rubber area with the shearing deformation in each rubber bearings. Some of these trends can be predicted when the flange is modeled as a beam of infinite length horizontally laid across an elastic foundation. When this prediction method is used for calculations in other rubber bearing, then the flange stresses are predicted to be decided not only rubber diameter, but also stiffness of steel plates in rubber bearing, the basic coefficient as foundation, and reduction rate of overlapping rubber area. Therefore, rubber bearing flange should be designed in consideration of vertical load from the superstructure and stiffness of foundation. In the future, it will be necessary to verify various effects and improve this prediction method of flange stress.