| The seismic vulnerability of outrigger knee joint systems in older bridges was demonstrated in the 1989 Loma Prieta, California earthquake and in the 2001 Nisqually, Washington earthquake. This research was conducted to obtain a better understanding of the seismic behavior of existing outrigger bents that incorporate deficiencies as present in the Spokane Street Overcrossing in western Washington State and to develop and evaluate the effectiveness of retrofit measures for improving their performance in an earthquake.; In this study, seven knee joint specimens were tested under simulated seismic loading. Three as-built specimens were tested, two under in-plane loading and one under out-of-plane loading. The as-built specimens failed at low ductility levels due to shear distress, low torsional strength of the beam, and reinforcement bond failures within the joint. Retrofit measures using steel jacketing were developed and applied to the remaining four knee joint specimens. Three of the specimens were retrofitted with circular steel jacketing around the joint and the beam; two were tested under in-plane loading and the third under out-of-plane loading. The fourth retrofitted specimen represented a knee joint in a split outrigger bent and was upgraded with D-shaped steel jacketing and tested under in-plane loading. All retrofitted specimens performed well, achieving enhanced strength, ductility, drift ratio, and energy dissipation capacities.; Threshold principal tension stress values for evaluating existing outrigger joints with similar steel arrangement to those present in the Spokane Street Overcrossing were established. Principal tension stress values of 4.5 f'c psi (0.38 f'c MPa) and 6.0 f'c psi (0.50 f'c MPa) were set as limits beyond which joint shear cracking and joint failure, respectively, are expected.; The retrofit measures proposed in this study incorporate the use of an elbow-shaped steel jacket around the beam and the joint. Design and detailing guidelines for retrofitting outrigger bents were proposed, including for split outrigger bents. The guidelines include equations for the jacket thickness required to form a stable force transfer mechanism between the beam and the column reinforcement, to avoid anchorage failure of the column longitudinal bars, and to restrain the hooks on the beam bars. |
