Specialized Track Bridge Structure for Light Rail Construction on Floating Bridges

Abstract

Sound Transit's East Link Light Rail project carries light rail transit (LRT) to east King County Washington over the existing Homer M. Hadley Memorial Bridge. Although there are examples of similar projects on cable-stayed and suspension bridges, there is no precedent for LRT on floating bridges. The design of a specialized track bridge structure is required to allow light rail vehicles to travel across the bridge's four expansion joints. The Curved Element SUpported RAil (CESURA) concept employs a novel method to overcome the motions at the interfaces between the fixed and floating portions of the bridge. This system passively adjusts vertical and horizontal alignments to allow the rails to undergo the movements intrinsic to the nature of the floating structure. Because of the complexities of the CESURA system, the non-standard components that comprise its major elements, and the unknown interactions between these parts, extensive experimental research data was needed to validate and calibrate computer models which will be used to produce the final design. A full-scale test of a part of the system was conducted in the University of Washington's Structural Engineering Laboratory. The tested system consisted of approximately one quarter of the CESURA system. The constructed specimen was subjected to exaggerated geometric configurations compared with those that would arise during normal operation, and it was tested under both vertical and horizontal forces. The specimen behaved largely as anticipated and revealed no major shortcomings. The behavior of the system was found to be largely independent of the imposed geometric configuration. Both the re-centering capabilities and geometric design of the system were also investigated by examining the effect of specific elements of the current design. The system was found to be highly sensitive to the construction process. Component tolerances should be adjusted to facilitate their precise location within the system. Specific fabrication methods should be adjusted to prevent the undesirable construction sequence that was necessary in the component testing. Instrumentation of the full-scale prototype test, to be conducted at the Transportation Technology Center, Inc. (TTCI) facility in Pueblo, Colorado in 2013, should be planned carefully to avoid the issues encountered during the testing program.