Seismic Rehabilitation of Steel Frame Connections Through Asymmetrically Weakening the Beam

Poor seismic performance of steel frame connections were proved after the Northridge earthquake (1994) which led to fundamental change in acceptance criteria of the steel frame connections. New design codes consider the plastic hinge to be located far from the column face to prevent brittle fractures at or near welds. In these cases there is a need to retrofit the existing steel frame buildings which have been designed according to Pre-Northridge seismic codes. In current study, several experimental tests were conducted to investigate the influence of two retrofit techniques on cyclic behavior of a Pre-Northridge connection. The techniques involved intentionally weakening the beam to trigger the plastic hinge to a predefined location in the beam. The retrofit scenario was implemented through reduction and annealing the beam bottom flange. For the Pre-Northridge connection a tear damage mechanism was observed near the welded joint accompanied by a sudden drop in connection strength, while the retrofitted specimens were able to relocate the plastic hinge away from the column face. In reduced specimens, top and bottom flanges of the beam showed local buckling near the joint at the end of loading, however in annealing technique low stress demands in near weld region was resulted. It was concluded that area reduction of the beam would make the connection prone to out of plane buckling.