Flexural response of external R.C. beam-column joints externally strengthened with steel cages

An experimental and theoretical research referred to the flexural behavior of external R.C. joints strengthened with steel cages constituted by steel angles and battens is presented. The subassemblage (beam, column and joint) was subjected to a constant vertical load acting on the column and to a monotonically increasing lateral force applied at the tip of the beam. The control specimen is without strengthening system and it was designed with weak column and strong beam and overstrength in the joint region. Strengthening cases here studied refer to steel caging in the column and both in the beam and in the column. Cyclic response in term of load-displacement curves, crack patterns and observation of failure mode types were the main results of the experimental results. Results highlight the effectiveness of the external steel cages as strengthening system, which increases the flexural strength and allows one to move the failure mode from the column to the beam. A simplified analytical model is proposed that can be used for pushover analysis and is able to reproduce the flexural behavior of external beam-column joints under monotonic loading. The model includes shear-to-moment interaction for beams and columns and confinement effects induced by external steel cages. The model is based on the determination of elastic-plastic behavior of the beam and of the columns, the latter being obtained from knowledge of the ultimate moment associated with the design axial force and deduced through a simplified moment-axial force domain. The joint was modeled with a strut and tie model in which concrete crushing and yielding of stirrups are the only limit states considered. Finally, a comparison between experimental load-deflection curves and analytical prediction is made showing good agreement.