Introduction
The UFC 4-023-03 guideline defines progressive collapse as: "The spread of an initial local failure from element to element, resulting in the collapse of an entire structure or a disproportionately large part of it." Despite various definitions for this phenomenon, its fundamental characteristic is the disproportionateness of the final collapse relative to the initial local failure. Although the term "progressive collapse" had been used several times before 1968 to describe certain types of structural failures, the first significant attention to this phenomenon—as it is known today—was paid following the partial collapse of the Ronan Point tower in London. After the events of September 11, scientists and engineers refocused their attention on this phenomenon and related issues. In this context, investigating methods to increase the resistance of existing structures against progressive collapse is of particular importance.
Strengthening and retrofitting techniques that specifically focus on controlling the propagation of collapse in a structure can be classified into two main groups: in the first group, the structure is strengthened by adding new load transfer paths, and in the second group, by improving existing load paths. In the first case, new load paths are provided by installing structural members (braces, cables, etc.). These new members may be similar (in shape and function) to the main members, or they may be completely different from the existing members. In the second case, structural members (beams, columns, and slabs, and sometimes non-structural elements such as infill walls) are improved to resist progressive collapse following the initial failure.
The main objective of this research is to investigate and compare the capability of using horizontal braces and strengthening connections with CFRP in increasing the structural resistance against progressive collapse. For this purpose, a two-dimensional reinforced concrete frame with 8 stories and three spans has been utilized. The connections of this frame have been strengthened sufficiently to shift the location of plastic hinge formation to the end of the strengthened zone. Furthermore, in other models, the top story of the frame has been strengthened using different horizontal braces.

Methodology
A two-dimensional reinforced concrete frame with 8 stories and three bays, which had previously been strengthened with CFRP by Niroomandi et al in 2010 for evaluating the seismic behavior factor, was utilized. The beam-column joints of this frame were strengthened with web-bonded CFRP sheets, to the extent that the location of the plastic hinge was shifted to the end of the strengthened region. In addition to shifting the plastic hinge location, strengthening the connections with CFRP increases the flexural strength of the connection, and the over strength resulting from the strengthening has been extracted as a moment-rotation curve by modeling in ABAQUS. The over strength resulting from the connection strengthening was applied in the structural model in SAP using NLlink elements. Furthermore, in other models, the top story of the frame was strengthened using different types of horizontal steel braces. Finally, progressive collapse analysis was performed on frames using nonlinear dynamic and pushdown analysis.

Results and discussion
To investigate the progressive collapse resistance, the vertical displacement of structure versus the load factor was plotted for all structures using pushdown analysis. The results indicate that the highest increase in the load factor belongs to the structure strengthened with X-bracing, followed by the structure with tensile brace. A closer look at the displacement-load factor curves reveals that the slope of the curves for braced structures is higher than that of the original structure in the linear region; in other words, adding braces has increased the stiffness of the structure against vertical displacements. In the CFRP-strengthened structure, no significant change was observed in the slope of the initial region compared to the original structure.To investigate the dynamic effects of sudden column removal nonlinear dynamic analysis was performed. The results show that column removal in the 6th and 7th stories of the original structure and column removal in the 7th story of the CFRP-strengthened structure caused the rotation of plastic hinges to exceed the Collapse Prevention limit. Due to the weakness of steel braces in compression, column removal in the first four stories of the structure with compression bracing caused the performance level of the brace to exceed the Collapse Prevention limit. Nevertheless, the compression bracing reduced the vertical displacement of the structure. In all cases, the greatest reduction in vertical displacement compared to the original structure belongs to the structure with X-bracing.

Conclusion
The results of this study indicate that both investigated strengthening methods can increase the resistance of concrete moment resisting frames against progressive collapse; however, the extent and manner of their effectiveness differ. Strengthening connections with CFRP is primarily effective in improving the flexural capacity of the connection and reducing damage, showing suitable efficiency particularly in lower stories, although the need for a large number of CFRP layers is considered a limitation of this method. In contrast, the use of horizontal braces, especially X-bracing and tensile bracing, was identified as the most effective strategy for increasing vertical stiffness, reducing displacements, and creating alternative load paths following column removal.