Structural strengthening of square spread footings using circular external prestressing

Footing enlargement method is widely used to increase the load carrying capacity of obsolete spread footings. The traditional method involves addition of new concrete and dowel bars, along with splicing of steel bars, which is labor intensive and impractical. This paper presents a new strengthening system that utilizes an unbonded post-tensioning system. The original square footing was turned into a circular footing, using ordinary reinforced concrete. The new system is more applicable, as the connections are achieved by the confinement actions provided by the prestressing strands. To investigate the proposed system, finite element models were built and analyzed in ABAQUS. The original as well as all strengthened footings failed in punching shear, and the proposed system proved to be efficient in improving the punching shear capacity. The investigated parameters include amount of prestressing force, shear-span to depth ratio, eccentricity of strands, flexural reinforcement ratio, and notch length. The finite element results suggest that the amount of prestressing force had the most significant influence on the punching shear behavior of strengthened footing. For original spread footings, the empirical model for punching shear presented in Eurocode 2 (a_cri = d/2) was found to serve as a good analytical model, as the predictions from this model yielded the best agreements with the FEA results. For the strengthened footings, an analytical model was proposed by modifying the empirical model adopted by Eurocode 2. A new factor η was introduced, which reflects the effects of all investigated parameters.