Abstract
In many large-span facilities, a structural type in which a large steel roof is installed on the lower RC frame, has been widely used. The conventional roof bearings connecting the roof to the lower RC frame, have often similar detail to that of an exposed column base. However, such roof bearings have frequently suffered severe damages, such as fracture or pull-out of anchor bolts and crash of base mortar, in great earthquake disasters. In the present study, two improvements from the conventional bearings were devised to avoid these damages and their effectiveness was examined through full-scale tests. One improvement was to insert a rubber plate between the base plate and base mortar. The rubber plate was intended to carry the horizontal shear stress due to earthquake response so that the shear and flexural deformation of anchor bolts could be avoided. Another was to introduce disc springs into the anchor bolts. This was to expand the elastic range of the anchor bolt and to avoid the loss of tension due to the bearing rotation as far as possible.
In the tests, the bearing models were loaded by constant vertical and cyclic horizontal loads, together with introducing the initial tension to the four anchor bolts. Three tests (identified by the signs R29-s-d, R87-m-d and F87-m-d) were carried out where the presence of the rubber plate, base mortar and the loose hole for the anchor bolt was changed. A friction chemical coating was used to transmit shear stress between the base and rubber plates and between the rubber plate and base mortar.
In the R29-s-d test, the rubber was inserted but the anchor-bolt hole was not loose so that the anchor bolts were directly subjected to the horizontal force. Moreover the base mortar was replaced to a thick steel plate. In the test, large rotation accompanying yield of the anchor bolts due to tension was observed.
In the R87-m-d test, the anchor bolt holes were loose to avoid contact of the anchor bolts to the base and rubber plates. The horizontal force was carried by shear of the rubber plate so that the anchor bolts were subjected to only axial tension due to rotation of the bearing. This design intent was successful. Significant shear or flexural deformation of the anchor bolts and crash of base mortars were not observable.
In the F87-m-d test, the rubber was replaced to a steel plate but the anchor bolt holes were loose. Without the rubber plate, the shear force was not absorbed by the shear deformation of the rubber so that the steel plate slid on the mortar. The mortar was also crushed by rotation of the steel plate.
In the three tests, the rotations and horizontal loads at the axial yield of anchor bolt were almost same level. However, the presence of the rubber plate influenced on the horizontal behavior. In the R87-m-d test, where the shear force was resisted mainly by the rubber, the horizontal displacement was quite large.
On the elastic shear stiffness, calculation by the theory for the rubber plate itself agreed well with the test results. On the rotational stiffness, a theoretical formulation was carried out considering the axial stiffness of the disk spring and the rotational stiffness of rubber plate. Good agreement was obtained between the calculation applying the derived equations and the results of the R87-m-d test.
