## 1 Introduction  
Progressive building collapse, which is defined as \"the spread of an initial local failure from element to element, resulting eventually in the collapse of an entire structure or a disproportionately large part of it\" in ASCE 7-16 [1], can be triggered by man-made and natural hazards, such as blast, vehicle and aircraft impact, fire, and major earthquakes. Although the probability of such catastrophic events is low, the impact is extreme when they do occur. There are a number of examples in history in which structural failures and collapse of buildings resulted in huge economic damage and extensive loss of human lives, such as the partial collapse of [PERSON] federal building in Oklahoma City in 1995 and the collapse of the twin towers of the World Trade Center in New York City in the terrorist attack on September 11 th, 2001. Extensive experimental and numerical studies have been performed to investigate the collapse response of civil structures. Experimental studies on progressive collapse used to be quite rare in the literature because of the expense and difficulty of experimentation. This trend has been reversed thanks to the boomed experimental tests conducted to investigate progressive collapse behavior in recent years, including [PERSON] and [PERSON] [2], [PERSON] and [PERSON] [3], [PERSON] et al. [4], [PERSON] et al. [5], [PERSON] et al. [6], [PERSON] et al. [7], [PERSON] et al. [8], [PERSON] et al. [9], [PERSON] et al. [10], [PERSON] and [PERSON] [11], [PERSON] et al. [12], [PERSON] et al. [13], [PERSON] et al. [14], [PERSON] and [PERSON] [15], [PERSON] et al. [16], [PERSON] et al. [17], [PERSON] et al. [18], [PERSON] et al. [19], [PERSON] and [PERSON] [20], [PERSON] et al. [21], and [PERSON] et al. [22]. These tests provide a great number of high-quality experimental results that can be used for computational model validation, which in turn promotes the development of computational models for progressive collapse. Research efforts pertaining to numerical analysis of progressive collapse in recent years can be found in [PERSON] and [PERSON] [23], [PERSON] et al. [24], [PERSON] and [PERSON] [25], [PERSON] and [PERSON] [26], [PERSON] and [PERSON] [27], [PERSON] et al. [28], [PERSON] et al. [29], [PERSON] and [PERSON] [30], [PERSON] et al. [31], [PERSON] et al. [32], [PERSON] and [PERSON] [33], [PERSON] et al. [34], [PERSON] et al. [35], [PERSON] et al. [36], and [PERSON] et al. [37]. The ultimate goal of all the studies related to progressive collapse is to protect building structures from collapsing under abnormal loading conditions and thus avoiding massive casualties and economic loss. [PERSON] et al. [38] presented retrofit methods suitable for mitigating progressive collapse for both steel frame and reinforced concrete buildings. Approaches that can increase the collapse resistance of concrete structures were proposed by [PERSON] et al. [39], [PERSON] and [PERSON] [40], [PERSON] and [PERSON] [41], [PERSON] and [PERSON] [42], [PERSON] et al. [43], [PERSON] et al. [44], [PERSON] et al. [45], [PERSON] and [PERSON] [46], and [PERSON] et al. [47]. However, quite a few research efforts have been dedicated to the development of strategies which are capable of improving the robustness of both newly designed steel buildings and existing steel buildings, examples of which can be found in [PERSON] [48], [PERSON] et al. [49], [PERSON] et al. [50], and [PERSON] and [PERSON] [51]. As one of the most common steel connections, simple connections allow relative rotation of the connected members and thus can only transfer shear forces. Therefore, they are designed to resist vertical gravity loads and are used in the steel frame structures of non-seismic areas and in the gravity frames of seismically designed frames in the US and Europe. Comparing with moment connections and semi-rigid connections, such as those designed using the method proposed by [PERSON] and [PERSON] [52], the simple connections have weaker strengths.