Bi-axially Loaded RC Beam-Column Joints and Haunch Retrofit

Abstract

The issue of reinforced concrete (RC) beam--column joints in existing frames, built without regards on seismic provisions, is considerably actual. In fact, beam--column failure has been often mentioned as one of the causes of buildings' collapses post--surveying strong earthquakes. The last witness belongs to Turkey's event of February 2023. In this paper, an experimental investigation on RC beam--column joints under bi--axial load condition is presented. Three full--scale beam--column joints were tested both in as--built and retrofit condition. Haunch retrofit was employed for the latter. Tests were carried out using hexagonal displacement protocol which is essentially to apply load in one direction double with respect to the other. Results obtained for as--built condition were included in a database of corner joints tested bi--axially and discussed in the light of the impact of bi--axial load on joint's shear strength. Numerical analyses using brick finite elements were employed to thoroughly evaluate the coupled structural response. Bi--axial shear demand proved to be detrimental but mitigation effect of joint's confinement was proved as well therefore conclusive statement on the subject was not reached. Haunch retrofit proved to satisfactorily reduce the shear demand at joint. Experimentally--derived internal forces were used to validate Strut-and-Tie models of the joints. Such design tool is recommended for practice cases adopting haunch retrofit.