The collapse of the Cypress Street Viaduct during the 1989 Loma Prieta earthquake emphasized the vulnerability of elevated freeway bridge structures. A part of the joint Caltrans and University of California at Berkeley research project, initiated after the earthquake, focused on the outrigger knee joint systems found in bents of elevated freeway bridges. The investigation had two principal goals: to evaluate the behavior of the existing outrigger knee joint systems under a combined transverse and longitudinal loading and to devise and experimentally verify strategies for seismic upgrading of existing outrigger knee joints. The experiments on two half-scale as-built models confirmed the poor performance of existing outrigger knee joint systems observed after the earthquake. Two seismic upgrading strategies, a "ductile" and a "strong" strategy, were proposed and tested on prototype specimens. The "strong" strategy was chosen for the final seismic upgrade design. The variation of this strategy using a steel jacket, was recommended in the seismic upgrade design guidelines.