Will the rails moving ten inches up and down, and five inches from side to side make cross-Lake trains on the I-90 floating bridge possible? Sound Transit staff report that they have a solution ... what has worked for the SkyTrain bridge just north of Seattle in the suburbs of Vancouver, British Columbia
For over a decade, Sound Transit in the Seattle area has been planning one of its Link light rail segments to cross the I-90 floating bridge spanning Lake Washington between Seattle and Mercer Island.
The configuration of the track bed off center on the northernmost of the two parallel floating bridge segments is shown in the figure above. This alignment takes over what is called the Center Roadway, now dedicated to reversible HOV lanes. A technical report from KPFF Consulting Engineers in November 2005 provides evidence that the off-center placement of tracks on one of the bridge segments as shown won't sink the bridge or otherwise hurt it.
Reduced weight in the track design is important, so engineers have specified "continuous welded rail (CWR) with direct fixation track on doweled concrete single fastener plinth with restraining rail to reduce the dead load on the bridge."
A key question is whether light rail trains can successfully operate on a bridge that moves. The report calculates the movement of the track bed that results at the point where the floating bridge is attached to the Lake Washington shoreline in Seattle (excerpt from the report here).
That movement described in the KPFF technical report indicates the magnitude of the design challenge that this plan represents -- vertical rail movement (up and down) of ten inches, and horizontal rail movement (side to side) of five inches. This movement occurs at a roadway expansion joint on the transition segment between the part of the bridge that is floating, and the part that is attached to the Seattle shoreline, shown in the following figure from the technical report.
The movement occurs because of the weight of the moving trains, trucks, cars, and buses on the bridge, plus the effect of wind. This much movement would occur on a very stormy day, the kind that occurs about once per year. On a calm day, the amount of movement would be about an inch less on each of the two dimensions.
The report clearly puts the decision on whether this much movement is possible with light rail passenger trains with "Sound Transit's rail designers."
An excerpt from the KPFF Report is available for download here in pdf (1 megabyte).
A determination on an acceptable solution for moving tracks was reported by Sound Transit staff at the agency's Board of Directors meeting of April 27, 2006. In summary, consultants to Sound Transit assess that a design of tracks on springs successfully engineered for the cable-stayed bridge crossing for SkyTrain rail transit over the Fraser River in New Westminster, British Columbia, Canada will suffice to cover all of the predicted motion described by KPFF. An excerpt with illustrations from the report to the Sound Transit Board is here in pdf.
Aside from the issue of technical feasibility, another question about light rail on the I-90 floating bridge is the reduction in vehicle, freight, and passenger capacity that this use of right-of-way represents. This issue is described here.
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Last modified: July 27, 2006