Design of Light Rail on the I-90 Floating Bridge across Lake Washington

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Installing light-rail transit across a floating roadway bridge presents many unique challenges, foremost of which is how to design the multidimensional moving joints at both ends of transition spans between the fixed and floating structures. 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 Parsons Brinckerhoff, Inc. (now WSP | Parsons Brinckerhoff), 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 (see Figure 1). 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.

Figure 1 – Bridge Axes and Movements

Ability to Accommodate Many Movements

The schematic features of the track bridge are illustrated in Figure 2. Two steel “wings” span the bridge (hinge) joints connecting the fixed and transition segment bridge decks (and the transition and floating segment decks). These wings form triangular secondary planes located such that they each have one (long) edge perpendicular to the hinge and a vertex on the hinge axis. A curved element called the “yoke” is placed on each wing, and the track is mounted on “bearer bars” supported by the yoke.

  • When the hinge angle is zero and the wings are lying flat, an observer looking from the side in the direction of the hinge axis will see the yoke as a straight line.
  • As the hinge angle increases, the wings will incline inwards as the long edges are forced upwards. The observer would then see a developing curvature as the yoke rises on the sloping wing, and the bearer bars would appear to the observer to lie on a smooth yet continuously variable curve that is tangential at either end to the incoming/outgoing tracks.
  • When the hinge angle is positive upwards, the track is in a segmental vertical sag curve.
  • When the hinge angle is negative downwards, the track is in a segmental vertical crest curve.

In this way, the track is supported across each moveable joint in a continuous and automatically conforming alignment and profile.

Figure 2 – Schematic Features of the CESuRa Track Bridge

Innovative Design of CESuRa Components

The components of the CESuRa track bridge joint are illustrated in Figure 3. The rail and guard rail subsystems are supported by the bearer bars: the 115RE continuous welded (running) rails are supported by bolting two direct-fixation (DF) track fasteners to each bearer bar, and the two 8-inch tall by 8-inch wide by ½ inch thick steel angle guard rails are pinned to the bearer bars just inside the DF fasteners.  The 17 variable-length bearer bars are supported near their ends by friction pendulum bearings mounted in a curved pattern on a pair of wings approximately 42.5 feet long. These 17 bearings mounted as a group on each wing in a semi-circle are referred to as the “yoke,” shown in Figure 2. 

Each wing is stiffened by an upturned edge beam, and the wing and edge beam together form a continuous steel box fabricated from A572, Grade 50, welded steel plate. Each wing is supported by three steel laminated elastomeric bearing configurations. Each bearing experiences rotational movement in multiple planes as the bridge moves, plus compression under traffic. The bearings at the transition span ends are allowed to slide longitudinally but are constrained laterally. In an extreme event where the rotation and/or translation exceeds the bearing design limits, the bearings or mountings act as structural “fuses” by fracturing, protecting the bridge from damage.

Figure 3 – Features of the CESuRa Track Bridge

The design uses longitudinally free fasteners on the track bridge. The rail clip is a tension clamp that is bolted in place, with plates and pads under the rail foot and between the rail and the rail clip to permit “free” (low-friction) sliding of the rail with thermal and bridge movement. The rails will be free to move longitudinally over the track bridges to accommodate changes in Pitch and Surge. Rail expansion joints will be installed on the first floating span on the lake side of each interior joint. The design accommodates lake level changes of +/- 18 inches from the median “neutral” position.

Confirmation through Full-Scale Prototype Testing

Full-scale testing was conducted at the Transportation Technology Center (TTC) outside of Pueblo, Colorado, in the summer and fall of 2013. Balfour Beatty Rail Infrastructure, Inc. built the Sound Transit Test Track (STTT), a customized 5,000-foot test track designed by the consultant team during Phase 1 (see Figure 4), and Jesse Engineering in Tacoma, Washington, fabricated two full-scale prototype track bridges that were shipped to the TTC for installation, instrumentation, and testing.

Figure 4 – The STTT looking north towards the Exterior Joint, viewed from the cab of a test train over the Interior Joint. Track Bridge spacing and track geometry replicated the West Approach of the I-90 bridge.

Figure 5 – Light-rail vehicles shown crossing the prototype Exterior Joint installed in the STTT

Two Sound Transit light-rail vehicles (see Figure 5) were shipped to the TTC for use in the testing. One was extensively instrumented to measure and record wheel, bogie and carbody movement and acceleration. The vehicles’ performance over the prototype track bridges was demonstrated to be well within track safety and rider comfort criteria for all bridge configurations from 5 to 55 MPH, confirming that the track bridge system could function at operating speeds of up to 55 MPH for design level I-90 bridge movements.

Lessons Learned for Construction and Installation

The results of the full-scale prototype testing program confirmed that the proposed design met the track bridge system technical requirements and provided valuable experience in fabrication, installation, adjustment and maintenance of the prototype track bridges.

Lessons were learned in the track bridge fabrication, construction, shipping and installation, and in the multiple testing phases of the program, all of which will benefit and improve the final fabrication, shipping and installation of the eight production track bridges to be installed on the I-90 bridge, most likely in 2019. After testing was completed in Pueblo, the two prototype track bridges were shipped back to Seattle, disassembled and stored so that the parts can be reused in the production of the final production units. WSP | PB is currently completing Issue for Construction documents, anticipating the start of construction in the spring of 2017.

Note: The project received a WSP | Parsons Brinckerhoff 2014 US Project of the Year Award.  This article was excerpted and adapted from a paper submitted to the Transportation Research Board for presentation and publication in January 2017.

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