EARTHQUAKE RESPONSE ANALYSIS OF STEEL ROOF GYMNASIUMS CONSIDERING NONLINEAR RESTORING FORCE CHARACTERISTICS OF LOWER STRUCTURE AND ROOF BEARINGS

Abstract

 Roof bearings of many gymnasiums were destroyed by the 2011 off the Tacific coast of Tohoku Earthquake. Typical damages were crush of base mortar, elongation or break of anchor bolt. Because of the damage, the gymnasiums could not be used continuously as a shelter after the earthquake.

 After the quake, seismic response of steel roof gymnasiums has been energetically studied. However, there are few studies considering the detailed non-linear restoring force characteristics of the conventional steel roof bearings. In this study, we firstly propose a nonlinear model of the restoring force characteristics of the conventional roof bearings based on the results of cyclic shear loading tests of bearings in Reference 6, 7).

 The skeleton curve between the shear bearing force and the displacement of the base plate can be expressed in 4 stages. In the stage 1, the shear force increases with very high stiffness until the maximum static friction force. In the stage 2, the base plate slides with almost zero rigidity within the movable range of the anchor bolt hole. After the base plate contacts the anchor bolts, the stage transits into the 3rd where the elastic bending deformation of the anchor bolts occurs. After the shear force reaches the bending yield strength, the stage transits into the 4th. In the stage 4, the shear force is resisted by the axial force of the anchor bolts in consideration of geometrical nonlinearity. The axial forces are calculated using the law of plastic flow. The proposed skeleton curve model of the bearing well approximated the test results 6, 7). Furthermore, the hysteresis curve model was developed, that was able to approximate the test results well by using the slip type restoring force characteristics.

 We modeled a gymnasium damaged by the 2011 off the Pacific coast of Tohoku Earthquake, incorporating the nonlinear model of the bearing to conduct an earthquake response analysis. From the seismic response analysis, it was confirmed that the bending crack by the out-of-plane response occurred in the RC cantilever column as the actual damage. In addition, the bearing shear stress was greater in the in-plane direction of lower frame than in the out-of-plane direction. It is guessed that the bearings resisted the inertial force of the wall plus the steel roof in the in-plane direction of lower frame.

 It was confirmed that the shear stress in the pin bearing can be well evaluated by equation in the Reference 2, 3). In this study, it was found that, when the natural period of lower structure was greater than 0.25 seconds, the bearing shear stress was greater than the inertial force of RC cantilevered column when slide bearings were used. That is considered to be the effect of impact force when the base plate collides to the anchor bolts. It was found that when the range of motion of the slide bearing increased, the bearing stress became 1.4~1.5 times as large as that of the pin bearing.