## 4 Conclusions  
In this study, the assessment of different retrofitting approaches on the structural performance of RC buildings against progressive collapse was studied by comparing the eight different cases. The following conclusions are drawn from the results of the analysis:  
* Overall, in structures reinforced with various braces, the use of a diagonal bracing system has performed better than other braces and has been able to maintain the level of immediate occupancy performance, and the distribution of plastic hinges in structures has been improved. The most important limitation of this method is that the brace's frame has a better performance against progressive failure. However, 2D models do not redistribute forces properly and affect other frames.
* In X-braced frames, the plastic hinge creation occurred on the braced element inside the collapse range, which is considered compression failure of the brace.
* In diagonal bracing, the plastic hinge formation has begun to develop in brace members. As a result, plastic hinges formed in columns and beams and were disseminated over particular building members within the immediate occupancy area.
* The inverted V-braced arrangement increases the constraint level at the beam end, allowing catenary action. In addition, the horizontal braces distributed certain gravitational stresses to the surrounding structures, which were then carried to the foundation via the vertical members.
* In structures reinforced with viscous dampers, the results show that dampers in certain stories have lower performance than other methods.
* In the viscously damped frame in the central bay, the plastic hinges in beams on the third and fourth stories reach within range of collapse, and that the application of a viscous damper prevented oscillation and shock to the structure.
* Using the viscously damped frame in two inner bays enhanced the structure's tensile performance, and improved the performance of the beams compared with the viscously damped frame in the central bay.
* The use of CFRP to retrofit the results shows that it improves the structure's overall performance, increases its chain performance, and improves the redistribution of forces in the structures. The advantages of this method are that it can be easily implemented in the whole structure and does not cause architectural problems. On the other hand, the disadvantage of this method is that it could create much force after removing the column; this issue could be problematic with increasing the possibility of damage in the structure.
* In general, various retrofitting schemes can be applied to strengthen the structure and increase the resistance of the structures under progressive collapse. In this regard, their combined use could be utilized furtherly to redistribute forces more quickly and achieve the best performance in the structure in terms of chain performance under progressive collapse.  
Conceptualization, B.H.A.; methodology, B.H.A.; software, B.H.A.; validation, M.B and B.H.A.; formal analysis, B.H.A.; investigation, B.H.A. and E.M.G.; resources, B.H.A. and M.B; writing--original draft preparation, B.H.A. and M.B.; writing--review and editing, E.M.G.; visualization, B.H.A.; supervision, E.M.G.; project administration, E.M.G.; funding acquisition, B.H.A. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
The authors declare no conflict of interest.