Development of the low damage self-centering Sliding Hinge Joint

Abstract

The Sliding Hinge Joint (SHJ) is a low damage beam-column connection that rotates inelastically with minimal damage through sliding in Asymmetric Friction Connections (AFCs). The AFC is a type of slotted bolted connection, which is installed in the bottom web and bottom flange bolt groups. Previous studies showed that under a major earthquake, the SHJ undergoes permanent losses in elastic strength due to bolt tension losses. The frames may also be subject to residual drifts. This thesis presents research undertaken to better understand and improve the SHJ and AFC, and develop a self-centering version of the SHJ (SCSHJ). AFC specimens with shims made of three steel grades were tested to determine the influence of shim hardness on the frictional performance. Abrasion resistant steel, the hardest material tested with a specified hardness of 370 – 430 HB, performed the best with the highest friction, least wear and most stable hysteretic behaviour. Abrasion resistant steel was therefore recommended for use in future construction. Some of the specimens were tested with Belleville Springs installed under the nut, where it was found that the springs increased the sliding shear capacity by reducing the loss in bolt tension that occurs when sliding takes place. Further AFC tests were conducted to establish recommended sliding shear capacity values for use in design and residual joint strengths once subjected to sliding. It was established that the AFC has minimal residual joint strength after an earthquake and the bolts have to be retightened or replaced. The proposed SCSHJ incorporates ring springs installed below or above the beam bottom flange to improve self-centering properties. Ring springs are friction damping springs that exhibit flag-shape hysteretic behaviour. In the SCSHJ, the ring spring will partially develop moment resistance, store and release energy which improves self-centering, and reduce elastic strength losses through pre-stressing. The objective is to improve dynamic self-centering behaviour, taking shake-down of the building into account. The SCSHJ thus does not reflect flag-shape hysteretic curves typical of other self-centering systems. The SCSHJ is designed based on the percentage of ring spring contribution to total flexural capacity (PRS). Prototype 10-storey frames with the SHJ or SCSHJ at PRS levels up to 50% were designed and studied analytically using a suite of 10 ground motions, showing reduced residual drifts with increasing PRS. Frames with PRS of 25% or more had residual drifts under construction tolerance limits for all ground motions scaled to the Design Level Event. The level of PRS investigated was however insufficient to significantly increase the elastic strength of the post-earthquake joint. The benefits of the ring springs were limited under the ground motions scaled to the Maximum Considered Event due to the high drift demands. Experimental tests of the SHJ and SCSHJ were undertaken on a full-scale subassembly representing an internal connection. The joints tested ranged from the standard SHJ to the Ring Spring Joint, where the flexural capacities of the latter were developed only by ring springs. The joint self-centering hysteretic response improved with increasing PRS. The hysteretic models used in previous analytical studies of the joints were modified based on the experimental results. Further analytical studies on the 5 and 10 storey frames showed the viability of using ring spring at selected joints within the frame to reduce overall cost.