Sound Transit Link
Light Rail – Seattle, Washington

The Sound Transit Link Light Rail project encompasses a 5-mile (8 km) stretch of light rail, extending from Boeing Field to the Seattle-Tacoma Airport. It comprises 4 miles
(6.4 km) of elevated guideway supporting twin tracks with continually welded rails atop the superstructure. Along the alignment, three stations are strategically placed, all within an environmentally sensitive area.

The superstructure consists of a precast segmental box girder with typical spans of 120 ft (36.6 m). Three long span structures with spans varying from 220 ft to 350 ft (67 m to 106.6 m) were required for special crossings over the I-5 highway, the BNSF railroad, and the Duwamish River. Construction from the top minimized impact on traffic and the sensitive environment.

A unique V-shape triangular cross section was developed specifically for this project.
This design optimized superstructure quantities and reduced the visual impact when observed from ground level. The forming of the superstructure segments was simplified by the elimination of the diaphragms at the PT tendon deviations. Exterior diaphragms were used at the pier segments for transverse stability and to transfer seismic forces to the buffers on top of the piers. The lower part of the diaphragm was kept horizontal to eliminate the need for bearing plinths.

The columns were designed to match the shape of the outside diaphragms at the pier segments so that the superstructure appears to be supported by a “cradle.” Light curved parapets were used on each side of the guideway. Most columns are supported on single 10 ft (3 m) diameter drilled shafts to reduce the impact of construction on the right-of-way. The bid for the successful contractor was US$25 million under the owner’s estimate.

In recognition of the firm’s technical expertise in bridge engineering, this project was honored with the prestigious 2009 ACEC Engineering Excellence National Recognition Award.

Additional Awards:

• 2008 WA Aggregates & Concrete Assoc. – Excellence in Concrete Construction
• 2009 ASBI – Bridge Award of Excellence
• 2009 ACEC California – Engineering Excellence Honor Award
• 2009 ACEC – Engineering Excellence Small Firm Honor Award
• 2009 PCI – Excellence in Engineering and Construction Design Award
• 2010 PCA – Concrete Bridge Award of Excellence