Five-storey prototype RC frame structure (dimensions in mm): (a) 3D model; (b) plan view

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... 50011-2010(SAC, 2010b). The structural layout is shown in Figure 1. The height of the first storey was 4050 mm and the height of the remaining stories was 3600 mm. ...

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... two-bay column-beam-slab test specimen substructure was isolated from the first storey of the prototype structure in both horizontal directions, as highlighted by the red dotted box envelope in Figure 1(b). In a real situation in the prototype structure, the test area is potentially horizontally restrained from surrounding frames, in which three perimeter edges of the test substructures are free edges, while one side in the Y-direction is restrained. ...

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... vertical load-displacement curves of the loaded middle joint for specimens B1, S1 and S2 are shown in Figure 10. The displacement refers to the vertical displacement of the middle joint (middle column stub), and the load is the corresponding external load applied to the middle joint. ...

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... displacement refers to the vertical displacement of the middle joint (middle column stub), and the load is the corresponding external load applied to the middle joint. The crack patterns and failure mode process described in the previous section and the load-displacement curves indicate that the RC frame structure had two stages of resistance against progressive collapse: a primary mechanism (the beam and compressive membrane mechanism) and a secondary mechanism (the catenary and tensile membrane mechanism), as shown in Figure 11. In the primary mechanism stage, the sections of the beam and slab near the middle column were subjected to the positive bending moment, and the neutral axis of the section moved upward because of concrete cracks at the bottom. ...

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... four horizontal displacements of edge columns A, B, C and D for specimens B1, S1 and S2 (see Figure 5 for the positions of the displacement meters) as functions of the vertical displacements of the middle columns are shown in Figures 12-14. In the primary mechanism stage, owing to the compressive arch action in beams, the edge columns deformed, exhibiting horizontal displacement towards the outside of the frame. ...

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... it was asymmetric in the X-direction of specimen B1, the beam with a short span in the X-direction exhibited an obvious compressive arch action phenomenon under the beam mechanism, and the column deformed to the outside of the frame. The maximum horizontal displacement towards the outside of column A was approximately −1.78 mm, and that for column C was −2.5 mm ( Figure 12). The vertical displacement D c (point C) of the X-direction beam was 80 mm. ...

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... the middle-column vertical displacement was 220 mm, the resistance mechanism for the Y-direction beam was completely transformed into the catenary mechanism stage. Figure 13 shows that the horizontal displacement of column A in specimen S1 developed rapidly and was significantly larger than the displacement of column C. The reason is that the shortspan beam and slab deformations were larger than the deformations in the long span in the X-direction, where the edge column in the short span (column A) provided greater resistance and deformation. The asymmetric (unequal-span) design in the X-direction had a reduced ability to resist compressive collapse because of a lack of synergy between the two unequal spans. ...

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... test of slab effects on reinforced concrete spatial frame substructures Du, Bai, Teng et al. displacement was 230 mm, the resistance mechanisms for the Y-direction beam and slab were completely transformed into the catenary and tensile membrane mechanism stages. Figure 14 shows that the primary mechanism stage (compressive arch action and compressive membrane action) was dominant during the whole test for specimen S2. The maximum horizontal displacement of column B to the outside of the frame was approximately −6 mm, and that of column C was −12.5 mm. ...

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... in Figure 6. Positive and negative strains are denoted as tension and compression strains, respectively (Figures 15-17). To depict the relationship between the progressive-collapse resistance and the material strain clearly, the specimen vertical displacement curves (grey lines) are given with the material strain in Figures 15-17. ...

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... and negative strains are denoted as tension and compression strains, respectively (Figures 15-17). To depict the relationship between the progressive-collapse resistance and the material strain clearly, the specimen vertical displacement curves (grey lines) are given with the material strain in Figures 15-17. ...

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... frame specimen B1 Figure 15 presents the longitudinal reinforcement steel strain changes at the beam sections of specimen B1. As shown, in the beam mechanism stage, owing to the bending moment, the reinforcements at the bottom of sections A\F and B\E were under compression and the reinforcement at the top of the sections was under tension. ...

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... bottom reinforcements in sections C\D were fractured first, and then the other sectional reinforcements were fractured. Comparing the strains of sections C\D in the X-and Y-directions (see Figures 15(c) and 15(f)) reveals that the X-direction short-span beam required a greater deformation to coordinate with the other beam deformations, resulting in a greater strain and stress on the reinforcements; thus, the unequal-span design indirectly reduced the resistance of the structure against progressive collapse. ...

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... to the symmetry in the Y-direction, only the slab reinforcement strains on the south side are presented herein. The figure shows that under the synergistic action of the beam and slab, the behaviour of the slab reinforcement was consistent with the top reinforcement rebar in the beam; both of them were under tension, in contrast with the compressive behaviour of the reinforcement at the top of sections C\D in beam B1 (see Figure 15(c)). This indicates that the height of the compression side in sections C\D was smaller than the thickness of the slab. ...

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... the beam mechanism stage, the reinforcements were in the tension state in sections A\F in both directions, indicating that the compressive zones in these sections were only located in the Figure 18) of specimen S1 increased rapidly and exceeded the yield line, while the strain gauges in zones 2 and 3 were in elastic states. This result indicates that under the beam mechanism, the resisting capacity of specimen S1 was mainly due to the reinforcement of the beam and the reinforcement of the slab near the beam. ...

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... to the reinforcement strain development in the slab, the slab yield line of specimen S1 was obtained, as shown in Figure 19. The positive yield line at the bottom divided the slab into eight parts (tension net): four triangular blocks (Aarea) and four fan-shaped blocks (B-area). ...

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... decompose the contributions of the compressive membrane action of the slab and the compressive arch action of the beam developed in the RC frame-slab structure at the primary mechanism stage, the load resistance of specimen S1 from the beams and slab was decomposed. As shown in Figure 21(a), 62% of the load resistance was provided by the beams when S1 was at the initial stage. As the displacement increased, the contribution of the load resistance from the compressive membrane action in the slab was increased. ...

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... The peak load capacity of the secondary mechanism of S1 (F d ) was 75% greater than that of B1, mainly owing to the tension force of the reinforcement in the slab. Notably, only the slab reinforcement near the middle beams (zone 1 in Figure 18) provided a large catenary force to balance the applied external load as the unevenly distributed deformation in the slab. Therefore, although the slab reinforcement quantity was 2.3 times larger than that of the beam, the improvement in F d was far smaller. ...

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... although the slab reinforcement quantity was 2.3 times larger than that of the beam, the improvement in F d was far smaller. As shown in Figure 21(a), when S1 reached transition point (c), approximately 26.0% of the load was resisted by catenary action in the beam, and the remaining 74.0% was resisted by tensile membrane action in the slab. Subsequently, the contribution of the load resistance from catenary action in the beam was increased. ...

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... first peak bearing capacity of the primary mechanism (F b ) of S2 was 30% greater than that of S1 and 210% greater than that of B1 because the increase in the slab thickness led to an increase in the moment of inertia of the beam-slab section. As shown in Figure 21(b), 84% of the load resistance was provided by the beam when S2 was in the initial stage. When S2 reached its first peak load (F b ), approximately 22% of the load was resisted by the beam, and the other 78% was resisted by the slab. ...

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... is probably due to a higher shear transfer through the slab in S2. As shown in Figure 21(b), beyond 154 mm, the contribution of the beam increased significantly because of the punching failure that occurred in the slab. ...