Investigation of nonlinear behavior of reinforced concrete moment frames retrofitted with steel haunch bracing under progressive collapse

Progressive collapse is a nonlinear phenomenon that starts from the damage of a part of the structure and ends with the whole structure and its total damage. Therefore, the study of this phenomenon in high-rise and important structures while securing it will ensure the design process of the structure. Progressive collapse usually occurs in the structure due to the loss of one of the main members of the structure, which is usually the desired column. With the sudden removal of a column in the structure, in the node where the column is removed from the structure, there is a displacement that has a seismic nature. In this research, the effect of using steel haunch bracing with different locations in concrete moment frames is investigated under progressive collapse. The purpose of this evaluation was to determine the vulnerability of concrete structures retrofitted with steel haunch bracing as well as the effect of steel haunch bracing in spans and height of the structure, the place of eliminated element (edge or middle) and the number of the story in which the member was removed in the progressive collapse, is studied. A noteworthy point in different cases of component removal is in the wide changes of stresses and change of node displacement and lack of formation of plastic hinges in the members, which can be explained by the proper distribution and placement of steel haunch bracing in spans and height of the structure in the place of elimination of members in the model, there are more structural elements and this issue strengthens the catenary action of the members to transport and transfer load and also the immediate stabilization of the situation.