SUMMARY
Cars have crossed Lake Washington on the Interstate 90 Homer M. Hadley Memorial Floating Bridge for decades, but running light rail across a structure supported by 18 floating pontoons was a problem no one had solved. Trains traveling at 55 mph need continuous, stable rail, while a floating bridge moves in multiple directions as lake levels, traffic loads, wind and waves change. Sound Transit’s East Link Extension depended on that crossing, and the resulting Crosslake Connection links the 2 Line with the 1 Line to create a fully integrated regional light rail system. Passenger service opened March 28, 2026, and the project was inducted into the Transportation Choices Coalition Hall of Fame the same year.
Before design could move forward, owners needed confidence that light rail on a floating bridge was feasible. The Washington State Department of Transportation (WSDOT) and the Washington Legislative Transportation Committee retained Structural Integrity Associates (SIA) to lead an independent review team of industry and discipline experts. The team weighed seismic vulnerability, wind and storm effects, track and expansion joint design, passenger comfort and stray current mitigation, while SIA guided consensus discussions between Sound Transit and WSDOT on operations and maintenance and briefed the Joint Transportation Committee of the Washington State Legislature. SIA’s own feasibility studies identified critical design elements early to minimize future risk.
The answer to a moving bridge is the track bridge, a first-of-its-kind system installed at each end of the transition span. Rail rides on bearer bars that rotate about three axes, supported by 34 friction-pendulum isolation bearings, so the rail profile can follow the bridge without significant deformation. SIA’s advanced engineering team performed the analysis that developed and validated the CESURA track bridge system. Engineers built a detailed 3D finite element model of the floating bridge, including its fixed, transition and floating spans, anchors, and nonlinear springs that simulate variable buoyancy, then confirmed that the construction sequence would not overstress the rails. Nonlinear dynamic time-history analysis ran a light rail vehicle model over the track through a range of storm conditions and train speeds to evaluate ride quality and derailment potential, with the vehicle model validated against full-scale testing.
On the east and west approaches, SIA’s earthquake engineering specialists completed a seismic vulnerability study, independently validated the seismic design against Sound Transit and WSDOT criteria, and used parametric studies to optimize the retrofit. Parametric studies of the track attachment, coordinated with laboratory test data, rounded out the work. From early feasibility through final design validation, the Crosslake Connection shows how SIA’s civil infrastructure practice helps owners of bridges, tunnels and rail transit solve problems without precedent. Contact SIA to discuss your next transportation project.