Seismic performance of CHS X-connections under out-of-plane bending Bida Zhao a , Cheng Fang b,c, ⁎, Wei Wang b,c , Yangzheng Cai a , Yuning Zheng a a College of Civil Engineering and Architecture, Zhejiang University of technology, Hangzhou 310023, China b State Key Laboratory of Disaster Reduction in Civil Engineering, Tongji University, Shanghai 200092, China c Department of Structural Engineering, School of Civil Engineering, Tongji University, Shanghai 200092, China abstract article info Article history: Received 21 August 2018 Received in revised form 18 March 2019 Accepted 10 April 2019 Available online xxxx This paper presents a comprehensive investigation of the hysteretic behavior of unstiffened circular hollow sec- tion (CHS) X-connections subjected to out-of-plane bending moment (OPBM). The study commences with tests on three full-scale CHS X-connection specimens with varying geometric parameters. The test results showed that the specimens mainly failed in tearing of the chord wall near the connection zone. Evident energy dissipation was provided when the connections experienced yielding, and this was followed by ductile crack propagation of the chord wall. The strength and ductility of the connections depended significantly on the brace-to-chord diameter ratio and the in-plane brace-to-chord angle. In particular, a decrease in the brace-to-chord angle led to increases in both the strength and connection efficiency (i.e., the ratio of the connection ultimate flexural capacity to nom- inal brace flexural yielding capacity) of the connection specimens. The test results also suggested that the current design code tends to make conservative predictions of the OPBM capacity of the connections, especially when the in-plane brace-to-chord angle is small. Moreover, either decreasing the in-plane brace-to-chord angle or increas- ing the brace-to-chord diameter ratio is found to benefit the ductility ratio and energy dissipation capability. This observation is further confirmed by a ring analytical model proposed in this study. FE models are then built and calibrated through comparisons against the test results, and subsequently, a parametric study was carried out to consider the influences of an extended range of geometric parameters. © 2019 Elsevier Ltd. All rights reserved. Keywords: CHS X-connections Out-of-plane bending moment Seismic Ring analytical model Experimental and FE analysis 1. Introduction Circular hollow section (CHS) tubes are commonly used as primary members in various types of building and offshore structures. In prac- tice, the intersections of the members are often treated by directly welding the CHS braces to the surface of the CHS chords. These unstiffened X-connections are one of the most common connection types employed in single-layer reticulated shells, which have become a popular choice for large-span roof systems because of their light weight, reliable structural performance, and appealing architectural ap- pearance. A typical engineering application of such connections is shown in Fig. 1. In the event of vertical excitations caused by earthquake or wind action, significant out-of-plane bending moment (OPBM) can be induced in the X-connections. The OPBM can become particularly large for roof systems with a large span-to-rise ratio or with a very small curvature. From a seismic design point of view, the performance of the connections under repeated OPBM during earthquakes may sig- nificantly depend on their geometric configurations which affect the en- ergy dissipation and fracture behavior. The focus of the previous studies on tubular connections has been mainly on the ultimate static capacity and appropriate strengthening techniques. Through experimental and finite element (FE) studies, Fung et al. [1,2] investigated the ultimate capacity of completely overlapped CHS connections with the brace member under axial com- pression. It was found that the connection strength increases as the lap-to-through brace diameter ratio increases, and that the design model provided by Kurobane et al. [3] for predicting the ultimate strength of CHS K-connections is reasonable. Iskander et al. [4] per- formed experimental and FE studies on through-bolt strengthened CHS T-connections with their braces under axial compression. It was revealed that the ultimate capacity is increased by 35% when the through-bolt is used. Zhu et al. [5] performed an experimental study looking into the static behavior of both unstiffened CHS X-connections and stiffened ones with external stiffening rings. It was indicated that the stiffening ring can significantly increase both the ultimate capacity and initial stiffness of the connections, and the benefit is particularly ev- ident for the connections with a small brace-to-chord diameter ratio (β). The presence of the stiffening rings also transforms the failure mode from chord yielding (unstiffened connections) to both chord yielding and brace wall buckling (stiffened connections). In addition to the studies on the static response, the seismic behavior of the connections has also received attention, although the focus was Journal of Constructional Steel Research 158 (2019) 591–603 ⁎ Corresponding author at: State Key Laboratory of Disaster Reduction in Civil Engineering, Tongji University, Shanghai 200092, China E-mail address: chengfang@tongji.edu.cn (C. Fang). https://doi.org/10.1016/j.jcsr.2019.04.019 0143-974X/© 2019 Elsevier Ltd. All rights reserved. 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