# Unified Simplified Capacity Model for Beam-Column Joints into RC Moment Resisting Frame
## 1 Introduction  
The structural behavior of beam-column joints represents a key aspect for both new and existing reinforced concrete (RC) buildings. Recent seismic events have shown the fragility of these components in existing RC buildings. Therefore, the beam-column joint must be accurately designed into RC moment resisting frames. Many existing buildings were constructed at a time when no anti-seismic regulations [1, 2] were available, making them strongly vulnerable under seismic actions [3, 4, 5]. Beam-column joints in RC buildings are circumscribed portions where high loads transfer between the connecting beams and columns. In fact, the internal stresses increase due to local effects, which govern the equilibrium of beam-column joints [6, 7]. This aspect becomes strongly crucial in the seismically resistant frames, where the post-elastic capacity of RC members (non-linear effects) is mobilized due to the large demand [8, 9]. The major dissipation could be concentered in these portions, promoting brittle failure modes when the RC joints are poorly designed. This phenomenon appears to be very recurrent in existing RC buildings, jeopardizing the strength resources of entire structures [10, 11]. For this reason, the modern building codes [1, 2, 12] pay close attention to the design of these structural components. In fact, in the new buildings, they shall be correctly designed [13, 14] in order not to activate brittle failure modes [15, 16, 17]. Under dynamic actions, the failure mode of beam-column joints is governed by two main mechanisms: debonding and shear failure mechanisms, which are generally brittle [18, 19, 20]. RC joints can be modeled in general as bidimensional elements [21, 22]; however, this approach has some drawbacks, especially in the numerical modeling [23, 24]. When a beam-column joint is subjected to seismic action [25, 26], the beams connecting into the joint are subjectedto a bending moment in the same direction. The steel bars of the same layer at opposite sides of the RC joint panel are pulled or pushed in the same load direction [27; 28]. The internal stresses strongly increase due to these local effects [29; 30]. When the strengths of materials (concrete and steel) are lower than the imposed stresses, the equilibrium condition cannot be guaranteed, and the RC joint cracks and collapses [31; 32]. The beams could lose their load capacity if the steel bars slip inside the joint region. Furthermore, the cyclic load provides diagonal pull and push actions, and the concrete joint panel diagonally cracks [33; 34]. For this reason, the stirrups provide a beneficial effect on the ultimate behavior of the RC joint [35; 36; 37]. The shear strength and the passive confinement effect increase the load capacity of the RC joint panel, preserving the RC joint from a premature brittle failure mode. According to the modern building codes, a proper amount of stirrups allows the internal stresses to be properly transferred between the main structural elements (converging beams and columns). Unfortunately, the existing building stock shows a deficiency of stirrups in joint panels, due to the old design criteria often being based on gravitational condition only [38; 39]. The current research aims to develop cost-effective strengthening strategies [40; 41] by means of ordinary and innovative techniques [42; 43; 44]. Ordinary strengthening systems, namely steel or RC jackets, have been largely studied in the past [45; 46]. The benefits of several innovative materials, if compared with the past systems, were shown in several scientific works [47; 48]. These systems have been largely applied in engineering practice due to many benefits: mechanical performance, light weight, corrosion resistance, and very fast constructability [49; 50]. The efficiency in terms of structural capacity of RC joints has been experimentally demonstrated [51; 52].