Strengthening Strategy for the Shear Capacity in Existing Concrete Structures

This paper presents an effective strengthening strategy to increase the shear capacity of existing reinforced concrete structures. The solution is derived by using the upper-bound theorem of the theory of plasticity, a well-known approach in the design of new structures. It is shown how the solution can be effectively used to not only derive the shear capacity, but also to find out where strengthening reinforcements should be placed. The strategy is exemplified by a strengthening project wherein the cross beams on a three-span concrete bridge have insufficient local and global shear capacity. The cross beams are strengthened with unbonded steel bars, which are installed in drilled holes. First, it is shown how the formulation of kinematically admissible failure mechanisms are easily used in the assessment of the capacity of existing structures. Second, it is shown how the solution is effectively used to find the ideal placement of the steel bars. Finally, the paper presents some considerations on the strengthening method. It is argued that it is necessary to prestress the unbonded post-installed steel bars in order to ensure their concurrent contribution with the existing reinforcement and sliding in the concrete.