A technical solution to this technical challenge has been proposed, analyzed, modeled, designed and prototype-tested in the context of the east link extension, a light rail transit project being carried out by Central Puget Sound Regional Transit Authority (Sound Transit).

In early 2011, Sound Transit selected a consultant team led by WSP, with Balfour Beatty, SC Solutions, Inc., and the Transportation Technology Center, Inc. (TTCI) as principal subconsultants, to tackle one of the more challenging aspects of the East Link Extension project: a light rail track bridge system on an Interstate 90 floating bridge across Lake Washington (the I-90 bridge). The approximately 5,700-foot long bridge comprises concrete box girder fixed segments (approach spans) supported by deep foundations, a floating segment supported by pontoons, and transition segments connecting the fixed and floating segments.

Two initial activities occupied the first few weeks of the project: a literature search to identify any other possible technical solutions that might exist for installing rail transit across a floating bridge, and a technical workshop led by key members of the consultant team, with key staff from Sound Transit and the Washington Department of Transportation (WSDOT), owner of the I-90 bridge. The literature search identified and documented a variety of rail joint configurations for handling various bridge movements, primarily in one direction or axis of rotation, but none for handling the combinations of movements and rotation of a floating bridge. 

Innovative CESuRa Concept for Track Rail Bridge

At the technical workshop, Mr. Andy Foan, Chief Engineer of Balfour Beatty Rail (UK) and one of two Balfour Beatty participants at the workshop, introduced the innovative Curved Element Supported Rail (CESuRa) track bridge concept. Upon thorough analysis and evaluation in Phase 1 of the project, the CESuRa concept was determined to be superior to other concepts studied, particularly in terms of its ability to support multiple movements and rotations at the same time, and was selected to proceed to design and testing in Phase 2. 

The I-90 floating bridge experiences the following normal movements . The required design ranges are shown in parentheses.

  • Surge - longitudinal x-axis movement of the floating segment and the track, handled through rail expansion joints.
  • Roll (+/- 0.7 degrees) - longitudinal x-axis rotation of the floating segment resulting from load and weather.
  • Yaw (+/- 0.1 degree) - vertical y-axis rotation caused by Sway, which is lateral z-axis movement of the floating segment resulting from load and weather.
  • Pitch (+/- 0.5 degrees) - transverse or lateral z-axis rotation of the floating segment caused by Heave, which is vertical y-axis movement resulting from changes in lake level.