CYCLIC BEHAVIOR OF AN INNOVATIVE FRICTION DEVICE FOR RC COLUMN-HSTCB CONNECTION

Abstract

Seismic events result in the loss of human lives due to the collapse of structures. Engineers are trying to figure out an efficient way to ensure dissipative behavior of structures with the primary objective to protect human lives and as secondary to minimize economic loss and to ensure the swift return of users. Beam to column joints play a significant role in the overall seismic performance of structures. For precast concrete and steel moment resisting frames (MRFs) various solutions have been proposed for beam-to-column connections while fewer have been proposed for cast-in-situ reinforced concrete (RC) structures. In this paper, the experimental behavior of a friction-based beam-to-column connection (BCC) for moment resisting frame having hybrid steel trussed concrete beams (HSTCBs) is investigated. In order to mitigate the reduction of strength, stiffness and effect of shear force acting on beam to column joint, the BCC is equipped with a friction device. Thermal-sprayed aluminum was used as a friction material on steel angles based on its high friction coefficient (0.6), and satisfactory performance confirmed by the experimental approach. The specimen tested was constituted by a beam-column joint equipped with a friction device with T-stub on the top chord and central plate on the beam side and steel angles on the column side, a 1.55 m long Hybrid Steel Trussed Concrete Beam (HSTCB), and a 3.3m long column. The connection between the different components was established by bolts. In order to simulate the effect of dead load an axial load of 500kN was applied to the column. Displacement-controlled tests were carried out by applying on the beam at a distance of l= 1.31m from the center of rotation and displacement history defined according to ACI 374.2R-13 (2013). In total, 5 tests were performed with three amplitudes of maximum displacement of ± 21.5 mm, ± 38 mm, ± 71 mm. From the forcedisplacement curves, the initial results showed that the beam-to-column joint provides full stable hysteresis loops, and no damage or cracking was observed on the tested specimen. The test confirmed the ability of disc springs in limiting the loss of bolt preload. An interesting phenomenon was observed in some test, due the inclusion of new bolts. A sudden variation of bolt preload at the start cycles was detected, since friction surface and bolts need to adjust by themselves at the start of the cyclic load. Bolt preload remains constant in the subsequent cycles. It was also observed that the applied sliding force in not symmetric as expected, namely in the case of the hogging (positive) moment, there is a lower sliding force while in the case of the sagging (negative) moment there is higher sliding force of about 15/20% at entire sliding branch of ± 71mm. This is due to two phenomena as with increase in contact pressure due to bugging of plates and due to the variation of lever arm of the external force, due to large displacement effect related to the test setup. As the applied displacement increases there is found a reduction of the lever arm which increases the external force able to activate the sliding of the devices.