Modelling load-transmission mechanisms in axially loaded RC columns retrofitted with steel jackets

The use of steel jacketing technique is a common practice for retrofitting existing reinforced concrete (RC) columns, as it allows increasing load-carrying capacity and ductility of the member. When the external jacket has no-end connections – i.e. the jacket is indirectly loaded- the load sustained by the column is transferred from the inner RC core to the external jacket through shear stresses along the contact surface. The assessment of this mechanism is quite complex, due to the marked non-linear behaviour of constituent materials and to the calibration of a proper shear stress-relative slip constitutive law of the concrete-to-steel interface. In this paper, a step-by-step analytical approach is proposed to assess the load transfer mechanisms in steel jacketed RC columns loaded in compression. The model is able to predict slip and shear stresses along the core-to-jacket contact interface and then the load-carrying capacity of strengthened members. Shear stress-slip constitutive law on the interface is initially assumed linear and a suitable stress-strain constitutive model of confined concrete is considered. Yielding of steel angles is also taken into account into the process by assuming an bilinear law with strain hardening. The progressive damage of concrete is also considered by dividing the examined member in portions and splitting governing equations. Finally, comparisons are performed in order to validate the proposed procedure against experimental results available in the literature, showing good agreement.