Failure analysis and retrofitting of reinforced concrete beams in existing moment resisting frames

During the structure lifetime, the design loads can modify because of a changing in the use destination. In common practice, any architectural change of existing buildings, which modifies the magnitude of loads, requires the evaluation of the load-carrying capacity of structural members. To this aim a nonlinear analysis considering the rotation capacity of critical regions until the failure of the structure can be advantageous compared to a simple elastic analysis. Plastic deformations of ductile elements allow for attaining a more accurate evaluation of the ultimate load, once the strength of brittle mechanisms has been averted. Exploiting the actual rotation capacity of critical regions, where plastic hinges form, this paper presents new analytical closed-form equations to evaluate the ultimate load and the failure mode of an interior span of a multi-span moment resisting frame, as impacted by the strength of beam- column joints and the elastic and post-elastic structural response of adjacent elements. Taking advantage of derived equations, a retrofitting design procedure to identify the proper structural strengthening of critical regions is proposed. The accuracy of the method is checked through a comparison with numerical results. The procedure represents a new useful tool for engineers for the local strengthening of existing reinforced concrete buildings.