**1. Introduction**

Progressive collapse refers to a devastating phenomenon in which failure of one key structural component, due to abnormal events, leads to chain reaction and spreads to other structural members, causing disproportionate or even entire collapse of the structure [1]. Vehicle impact, terrorist attack, and gas explosions are among incidents that can produce progressive collapse in structures. By the growth of tall and complex structures throughout the world and the increasing number of terrorist- or accident-induced catastrophic events, progressive collapse has received extensive attention from scientists and structural engineers in recent decades [2–7]. The vast majority of structural design codes provide general mitigation strategies to deal with the effects of progressive collapse on structural components that may experience a relatively high demand capacity ratio (DRC).

The design of structures to resist against progressive collapse was first introduced by the UK construction regulations [8] in the aftermath of the Ronan Point building collapse in 1968. Many research and scientific efforts were concerned with this phenomenon, especially after the 9/11 terrorist attack on the World Trade Center in the US. Several design codes including general service administration (GSA) [9] and department of defense (DoD) [10] proposed the alternate load path method as an important design measure to mitigate progressive collapse. In this technique, the structural component is allowed to experience local damage when subjected to extreme loading events. However, it seeks to afford alternate paths so the damage will be localized without spreading to the surrounding areas.

During sudden column removal from the frame, high attention should be paid to the load-carrying capacity of the double-span assembly above the removed column since it has an essential function in the progressive collapse prevention. Generally, as the middle column becomes ineffectual, the catenary action develops in connected beams and slabs, leading to large deformation in beam-to-column connections. Since the robustness of connections preserves the integrity in a double-span column removal scenario, there is a necessity to investigate the beam-to-column connection performance under the simultaneous presence of moment, shear, and tension in conjunction with high ductility demand. Such a complex loading protocol negatively affects beam-to-column connection performance and poses the risk of unexpected brittle failure.

In steel structures with fully rigid and semi-rigid beam-to-column connections, the catenary mechanism plays a major role to resist progressive collapse through the axial tension in the connected beams. In fact, the catenary mechanism is the fundamental resistance source of structures to vertical loads in the large deformation stage. Beam-to-column connections with appropriate robustness and reliable axial resistance are compulsory for developing the catenary actions where this stage is the final line of defense against progressive collapse. F Wang et al. investigated the behavior of bolted and welded flange plate connections subjected to progressive collapse [11]. They concluded that the connection with welded flange plates can lead to greater flexural strength than that with bolted angles and the application of welded haunch plates can arrest fracture failure on the welds within the beam-column joint. Hao Wang et al. performed experimental tests of steel frames with different beam-column connections under falling debris impact [12]. They concluded that the majority of the external work applied to the system was absorbed by bending deformation, especially by the plastic rotation at mid-span of the beam. Besides, they concluded that the catenary action was shown to significantly improve the load-carrying capacity and energy absorption in specimens with high levels of rotational ductility. Alrubaidi et al. investigated the behavior of different steel intermediate moment frame connections under a column-loss scenario [13]. Performance of different connections was compared based on their modes of failure and load-displacement response in both flexural and catenary action stages. They concluded that significant axial tensile forces were generated in the beams and the catenary action stage was then fully mobilized, providing an increase in the progressive collapse resistance.

It has been well documented that beam-to-column connections play a vital function in mitigating progressive collapse potentials in steel structures. There is a large volume of published studies investigating the connection performances under sudden column removal, where flexible, semi-rigid [14–18], and rigid [19–22] connections have been studied in detail.

The connected beam's axial forces significantly contribute to the development of the catenary mechanism. In the case that a double-span assembly experiences large deformation, the beam-span-to-depth ratio, *R<sup>i</sup>* , is also a major parameter that has been studied by several researchers [23–25]. The effect of the *R<sup>i</sup>* ratio on the mitigation of progressive collapse in steel moment frames was investigated by Rezvani et al. [26]. They concluded that the vertical resistance of frames increases as the *R<sup>i</sup>* ratio decreases.

A large and growing body of literature has also investigated the robustness effects of steel beam-to-column connections to mitigate the progressive collapse [27–30]. More recently, several attempts have been made to investigate the influence of the seismic design of beam-to-column connections on an anti-progressive collapse mechanism [23,31,32]. The behavior of welded unreinforced flange-bolted web and reduced beam section connections subjected to column removal were investigated by Chen et al. [33]. Using an experimental test, Yang and Tan investigated the performance of flexible and semi-rigid connections including different types of bolted beam-to-column connections [34]. They concluded that maximum tensile resistance of the connection significantly contributes to the development of catenary action after large rotations. Driver et al. [35] reported experimental results of several shear connections including 15 bolted single-angle and 6 double-angle specimens subjected to double-span assembly. They came to the conclusion that rupture or tearing of the cross-section in the

vicinity of the angle heel leads to sudden failure. Qin et al. investigated the progressive collapse behavior of conventional and reinforced welded flange-bolted web connections using numerical simulations validated by experimental tests [36]. Their study confirmed that the reinforced flange-bolted connection possesses higher ductility and robustness compared to the conventional connection, leading to more reliable collapse performance. Many researchers, such as Oosterhof and Drive [37] and Shen and Astaneh [38], have established or implemented several mechanical spring techniques for bolted-angle beam-to-column connections. Stylianidis and Nethercot [39] investigated the progressive collapse performance by using component-based connection models.

Overall, the previous literature is mainly concerned with the anti-progressive collapse behavior of typical beam-to-column connections using experimental tests or numerical simulations. However, the comprehensive comparisons of common practice beam-to-column connection performance subjected to column removal addressing the load transfer mechanisms requires robustness and ductility, and *R<sup>i</sup>* effects are still very limited. Therefore, this research comprehensively investigates the anti-collapse behavior of double-span assemblies with flexible, semi-rigid, and fully rigid beam-to-column connections. This is done with the aid of available test results on steel beam-to-column connections including top-seat angle and welded unreinforced flange-bolted web. Meanwhile, for a reliable comparison of different types of double-span assemblies and to evaluate the anti-collapse performance of steel beam-to-column connections, the vertical pushdown load and equivalent rotation were normalized against connected beams' plastic hinge and plastic rotation, respectively.
