In 2011, some roof bearings of gymnasiums suffered severe damage by the pacific coast of Tohoku earthquake. Typical damage was the elongation and break of the anchor bolts or crush of base mortar. Since gymnasiums are used as shelters for the neighboring residents in earthquake disasters, damage prevention of roof bearings are required.
Roof bearings are the connections between the lower RC frame and the steel roof. They are similar to conventional steel exposed column bases. However, the dominant stress and the mechanism to resist the horizontal forces of the roof bearings are different from those of steel exposed column bases. The dominant stress of steel exposed column bases is axial and bending stress while shear stress of the anchor bolts is dominant for roof bearings since the vertical load is extremely lower than that in steel exposed column bases in ordinary buildings.
In present paper, the full-scale cyclic shear loading tests of roof bearings were carried out to investigate the fracture behavior of roof bearings and the restoring force characteristics were also evaluated. Three different height (20mm, 50mm or 100mm) of the base mortar were used to compare the fracture behavior and the restoring force characteristics. Moreover, the friction tests were conducted to investigate the friction coefficient between the base mortar and base plate.
It was found from the results of friction tests that the friction coefficient between the base mortar and the base plate was about 0.5. This result agrees very well with the friction coefficient given in the AIJ recommendation for Design of Connections in Steel Structures.
The fracture behavior and the restoring force characteristics of roof bearings are summarized as follows. The anchor bolts were leaned after the base plate contacted the anchor bolts. After that, the base mortar cracked since the deformed anchor bolts pushed it out. After the tests, the base mortar was crushed especially outside of anchor bolts. When the base mortar height is 100mm, the test body couldn’t support the vertical load since the base mortar located in inside of anchor bolts was crushed. However, the friction coefficient of base mortar was approximately 0.5, and we assumed that the restoring force characteristics of roof bearings is the summation of strength of anchor bolts and the friction force between the base mortar and base plate.
The anchor bolts and the base mortar were already severely damaged before the horizontal load reached the yield shear strength. Meanwhile, we observed that the anchor bolts were bended at two parts, the bottom of the base plate and the slightly below of the surface of the concrete. For the small deformation, we proposed that the restoring force characteristics of roof bearings is evaluated by the yield bending strength considering this bending mechanism. For the large deformation, we proposed that the restoring force characteristics of roof bearings is evaluated by considering the bending and axial stress of anchor bolts. These stress of anchor bolts were calculated by the generalized plastic hinge theory applying kinematic hardening.