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,
3,
4,
5,
6,
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,
15,
16,
17,
18], and rigid [
19,
20,
21,
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,
Ri, is also a major parameter that has been studied by several researchers [
23,
24,
25]. The effect of the
Ri 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
Ri 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,
28,
29,
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 Ri 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.
2. Ri Effects on Progressive Collapse
To evaluate the effects of the
Ri ratio, one set of pushdown analyses was performed in this research using structural analysis program (SAP) 2000 academic version 21. In this case study, three steel double-span assemblies with
Ri ratios of 5, 10, and 15 were considered. All specimens possess fully rigid connections which were designed according to the strong column–weak beam theory in compliance with The American. Institute of Steel Construction (AISC) seismic design [
40].
Figure 1 shows the topology of one studied frame with double-span assemblies.
In
Figure 1, the pushdown force,
F, represents the concentrated load in the pushdown analysis applied at the top of the removed column. This concentrated load is equivalent to the progressive collapse resistance of the double-span assembly.
Table 1 shows the beam and column section properties used in progressive collapse analysis.
The plastic hinge capacity of connected beams,
Fp, of double-span assembly can be calculated from the following equation:
where
is the plastic moment of the connected beam,
is the double-span assembly length,
is the plastic modulus of the connected beam, and
is the yield stress.
To facilitate the comparison of different types of double-span assemblies against each other independently and regardless of connected beam plastic capacity, the pushdown force,
F, should be normalized against the plastic hinge capacity of connected beams,
Fp, as in the following equation:
Besides, to highlight the differences of different beam sections during the development of the catenary action, it is necessary to normalize the chord rotation,
θ, over the plastic rotation,
θp, based on the following equations:
where
Ib is the moment of inertia of a connected beam, and
Ke is the elastic stiffness of a simply supported beam subjected to pushdown force.
Figure 2 shows the plots of the pushdown analysis results of double-span assemblies with different
Ri ratio, in which the vertical and horizontal axes are the normalized force,
, and the normalized rotation,
θ/θp, respectively.
Figure 2 shows that the frame with the larger
Ri has a higher capacity to develop progressive collapse resistance. Also, the double-span assembly with the larger
Ri possesses relatively high initial stiffness.
4. Test Results
Table 3 shows the summary of test results, including maximum vertical loads, displacement, connection rotation, and failure mode.
Since the pushdown force is applied to the middle column, the vertical displacement gradually increases. In simple connections such as top and seat angle where the connection possesses a limited capacity to develop the full plastic moment of connected beams, the specimen rotates at both ends following a major deflection below the removed column. The normalized force versus normalized rotation for all tested simple connections is shown in
Figure 4.
Figure 4 indicates that at the initial pushdown stage, there is no substantial loading resistance. The average
F/Fp is around 0.2 at the normalized rotation of 5, until developing axial tensile force in the connected beam as a result of large deflection, indicating the beginning of catenary action. Besides, the shear fracture of bolts causes several jumps in the curve in which the connections experience major localized bearing deformations in the vicinity of bolt holes. Overall, the previous literature indicated that axial tensile force has a major contribution toward progressive collapse resistance in flexible double-span assemblies in which the catenary mechanism takes place before connection component rupture or shear fracture of bolts. Also, the important feature of progressive collapse performance of flexible connections is that although double-span assemblies experienced large deformation, the connected beam remains in the elastic region and connection components experience large plastic strain.
The normalized force versus normalized rotation for all tested semi-rigid connections is shown in
Figure 5.
Figure 5 indicates that semi-rigid connections have higher initial stiffness as a result of high flexural capacity. Nevertheless, after normalized rotation of 5, the stiffness experiences a decrease in most of the specimens as a result of limited capacity connections’ components, i.e., top and seat angles. Overall, the previous literature indicates that connections in this category mainly fail due to bolt thread stripping and fracture or fracture at the web angles. Besides, connection failure is categorized in two phases, in which, at the first phase, the connection resists vertical pushdown force through flexural action, and after large plastic rotation, the connections go into the catenary mechanism, indicating phase 2.
The normalized force versus normalized rotation for all tested fully rigid connections is shown in
Figure 6. This category possesses the highest stiffness, where in the normalized rotation of around 2, almost all specimens develop the full plastic moment of connected beams. In addition,
Figure 6 shows that by considering unique configuration and large
Ri, SidePlate has the highest stiffness and ultimate strength. Generally, the previous literature indicated that ductility demand for traditional rigid connections in the case of column removal is mainly controlled by the column shear panel zone, while in the SidePlate connection, it is controlled by a connected beam. Also, in fully rigid connections, the fracture at beam flange or shear plate are the main reasons for failure mode without premature weld or connection’s component failure.
Yield mechanisms and failure modes are the factors that control both the resistance and ductility or rotational capacity of the connection. Failure modes and yield mechanisms are related but are inherently different. Failure modes cause fracture, loss of deformation capacity, or significant loss of resistance. Yield mechanisms induce inelastic deformation and result in dissipation of energy and changes in stiffness without inducing fracture or excessive loss of resistance. In fully rigid connections, the ratio of rotational capacity of connection to rotational capacity of beam ( is around 12, where the failure mode is controlled by fracture at shear plate and beam flange. For some specimens in this category, such as I-W, weld fracture causes significantly less ductility, energy dissipation, and plastic rotational capacity. The ratio of is around 16 and 13 for semi-rigid and flexible connections, respectively. The failure modes in semi-rigid connections are governed by local buckling and deterioration caused by the large inelastic deformation web cleats and bolts. So, it can be concluded that semi-rigid connections provide large ductility and a ductile failure mode compared to fully rigid connections.
6. Summary and Conclusions
This paper presented the descriptions and experimental results of available full-scale double-span systems subjected to the middle column loss scenario. Several parameters and features including beam span-to-depth ratio, catenary mechanism, stiffness, and ductility have been investigated for fully rigid, semi-rigid, and flexible connections. The following conclusions can be drawn:
I. After middle column removal at the preliminary phases, the behavior of the beam is controlled by flexural resistance, and the tensile force is almost zero, recognized as a flexure action-dominated phase. With increased downward displacement, the axial tension also increases in the beams, developing a catenary mechanism recognized as a catenary-dominated mechanism phase. The results of this research show that the magnitude of axial force in the flexible connections, i.e., top and seat angle, is significantly small compared to fully rigid connections. This phenomenon can be justified by the failure mechanism that develops in the connection’s components rather than the connected beam, preventing the catenary mechanism development.
II. The maximum rotation capacity versus connection depth for almost all beam-to-column connection categories significantly surpasses the DoD’s recommended acceptance criterion. The suggested acceptance criteria are on the conservative side as it only considers pure flexural resistance. Therefore, connection depth alone is not a reliable indicator to predict the rotational capacity of beam-to-column connections.
III. The stiffness in fully rigid and semi-rigid connections generally experiences a decrease by increasing inter-storey drift angle. On the other hand, the flexible connections have a potential to develop the initial stiffness as the inter-storey drift angle increases. Such behavior can be explained by the geometry of these types of connections that allows rotation at preliminary steps, while the stiffness can be developed at higher drift angles depending on tensile capacity of connections’ components and stiffness hardening.