How to Put Light Rail on a Floating Bridge: Experimental Evaluation of a Novel Approach

Sound Transit, the agency that owns and operates the light rail transit system in the Seattle, Washington, area, is extending light rail transit service to east King County over the existing Homer M. Hadley Memorial Bridge. The East Link Extension Project presents unique technical challenges: the movements of the floating structure pose problems for a fixed rail track and, although there are examples of similar projects on cable-stayed and suspension bridges, there is no precedent for light rail on a floating bridge. A novel method—the curved element supported rail concept—has been developed to accommodate the multidimensional movements that exist at the joints between fixed and floating portions of the bridge. This paper presents a discussion of the results of full-scale tests of a portion of the overall system. The purpose of these tests was to assess the efficacy of several key components of the new system in advance of full-scale prototype testing. Overall, the system behaved as it was designed to. The tested components provided the necessary movement capabilities; stresses did not exceed 10% of yield; and deflections, which could cause alignment issues and poor rider comfort, were well within operating requirements. The tests revealed that the design of the guardrail, and in particular the way in which it is fastened to the system, is one of the most important design decisions facing the successful implementation of the system. In addition, the tests helped refine the testing needs and requirements for the follow-on prototype testing program.