## 10 Progressive Collapse Resistance of Composite Structures: State-of-the-Art
Furthermore, [PERSON] and [PERSON] (2023) [164], examining composite structures consisting of CFT columns and H-shaped steel beams, compared six beam-column connections for collapse resistance during a middle-column loss scenario. While the connections showed minimal differences under normal conditions, they vary greatly under extreme loads. A design concept to improve collapse capacity was proposed and validated through finite element analysis. Characteristic results using ABAQUS software ([https://www.3 ds.com/products/simulia/abaqus](https://www.3 ds.com/products/simulia/abaqus)) are shown in Figure 9.  
[PERSON] et al. (2017) [165] discussed the seismic collapse capacity for the case of high-rise CFT moment-resisting frames subjected to extreme earthquakes beyond design levels. Ground motions with a flat velocity spectral shape were selected to minimize record-to-record uncertainty. A numerical approach using fiber elements with stiffness and strength degradation in stress-strain models was developed. Incremental dynamic analyses were conducted to assess the P-Delta and degradation effects on local and global collapse mechanisms. The results showed drift concentration at lower stories triggering side-swap collapse controlled by post-buckling strength deterioration of CFT columns. Moreover, [PERSON] et al. (2020) [23] concentrated on the ability of composite building structures to withstand accidental loads, such as earthquakes, fires, or explosions, by depending on the beam and catenary mechanisms of steel beams to prevent collapse. They also examined a concrete-filled square steel tubular column attached to a steel beam with a bolted-welded hybrid joint in a scenario where the central column was eliminated. They computed the collapse resistance and mechanism under vertical loads, demonstrating that the steel beam  
Figure 9: Finite element model of structural connections analyzing collapse resistance mechanisms in composite structural systems (adapted from [PERSON] and [PERSON] (2023) [164]). can still provide resistance even after the failure of a short-span beam. They recommended the utilization of a welded haunch joint to enhance collapse resistance, offering valuable insights for engineering design to avert progressive collapse. A numerical simulation method was proposed by [PERSON] and [PERSON] (2022) [166] to integrate multi-scale element modeling with simplified beams and columns, reducing modeling elements and increasing computing efficiency. This method allowed for the accurate simulation of material damage and fractures in critical areas. Testing on CFST column composite beam frames showed that the new simplified multi-scale modeling approach was efficient and accurate. Different column-loss scenarios and the number of stories were studied to improve the composite frame's resistance to progressive collapse. Two improvement alternatives were proposed and analyzed to strengthen the frame's capacity. [PERSON] et al. (2021) [167] simulated the hysteretic behavior on a single-story, single-span frame with concrete-filled steel tube columns and prestressed concrete-encased steel beams. Parameters for stiffness degradation, strength degradation, and pinching behavior were determined. An analytical model was developed for out-jacketing frames with single-span, multi-story layouts. Seismic response analysis was conducted for different seismic fortification levels, with the results showing that certain frames needed improvements for seismic resistance. The mechanical behavior and collapse potential of out-jacketing frames were determined from the findings of the nonlinear time-history analysis, leading to the development of pertinent seismic design recommendations.