In moment resisting frames beams and columns are designed for flexural, axial, and shearing actions due to vertical and horizontal loads. Special proportioning and detailing requirements are applied in these elements for making them capable of resisting against severe earthquakes without significant loss of strength beyond the flexural elastic limit, and avoiding brittle failure (shear mechanisms). The required magnitude of flexural inelastic excursions (local ductility), as known, depend on the dissipative capacity of the global structure. The flexural ductility significantly increases with the transverse reinforcement amount, provided to confine section core and to prevent buckling of compressed longitudinal bars.

Starting from these considerations, in a previous work a first formulation regarding design equations linking transverse reinforcement amount with curvature ductility has been proposed. In this approach the influence of axial load ratio, of concrete and steel strength have been taken into account. The confinement effects within the section core have been evaluated by using the model proposed by Braga et al. (2006). In this paper numerical simulations have been extended to different arrangement of hoops, longitudinal reinforcement ratios and to sections strengthened with FRP wraps. In this way, local strengthening of concrete columns of existing buildings with inadequate confinement level has been treated.

Braga, F., Gigliotti, R., Laterza, M., 2006. Analytical stress-strain relationship for concrete confined by steel stirrups and/or FRP jackets, Journal of Structural Engineering ASCE,  132(9), 1402-1416.