Seismic Retrofitting of Outrigger Knee Joints

Abstract

Experimental tests were conducted on one-third-scale outrigger knee joint specimens to define the vulnerabilities of existing outrigger bents under seismic in-plane loading and to develop appropriate retrofit measures to address the identified vulnerabilities. The specimens incorporated deficiencies present in the outrigger knee joints in the WA-99 Spokane Street overcrossing in western Washington State. The as-built specimens exhibited poor behavior with failure at low ductility levels because of shear distress and reinforcement bond failures within the joint. Threshold principal tension stress values describing the expected condition of the joints were established and compared with values obtained by other researchers. Elbow-shaped circular steel jacketing around the joint and beam region was used to retrofit the as-built specimens. The retrofitted specimens formed a plastic hinge in a gap introduced at the top of the column with improved ductility and energy dissipation capacities when compared to the behavior of similar as-built specimens. Retrofitted outrigger knee joint systems can be expected to achieve ductility levels of at least 5 as well as drift capacities exceeding 6%.