Behavior and design of top flange-rotated self-centering steel connections equipped with SMA ring spring dampers

Existing self-centering steel connections developed based on either posttensioned (PT) tendons or shape memory alloy (SMA) elements are usually symmetric ones, in which case the connections rotate via a gap-opening mechanism over the beam-to-column interface and as a result causes unwanted “frame-expansion” (also called beam growth) effect and severe slab damage. This paper presents a novel type of asymmetric self-centering steel connection, which rotates around the top flange of the beam. The bending moment capacity and restoring force are provided by a couple of SMA dampers attached beneath the bottom flange of the beam. The working principles of the damper devices and the beam-to-column connections are discussed in detail, and a comprehensive experimental study on individual SMA rings, individual SMA damper devices, as well as two proof-of-concept beam-to-column steel connections, is conducted. The test results show that this type of connection generally exhibits typical flag-shaped hysteretic behavior with satisfactory energy dissipation and excellent self-centering capability. An extra ‘training’ process with an adequate pre-deformation to the SMA ring spring system can be adopted to effectively mitigate the degradation effect of the connections under repeated loading. Based on the test observations, a preliminary design recommendation is also proposed.