Seattle Sound Transit Tukwila Segment, Elevated Light Rail

Seattle, Wash.

This 4.2-mi-long precast concrete segmental bridge was designed to complete the 20-mi-long light-rail mass-transit system connecting the Boeing airfield to the City of Seattle Airport. This project’s light-rail alignment faced significant technical challenges, including the minimization of disturbance to major crossing thoroughfares, a large rail yard, and environmentally sensitive waterways. Further complicating the project is the high seismic zone in which it is located. Additionally, this project created unique vibration concerns resulting from the light rail and structural interaction, which had to be minimized to maintain passenger comfort. Because of the unique constraints and contextually required long spans, a precast segmental concrete V-box bridge type was chosen. By using precast concrete, a reliable high-strength concrete could be used (6500 psi) and the dimensions of the box girder were streamlined, allowing for a lighter structure and reduced seismic loads. This resulted in significant cost savings. The choice to use precast segmental concrete V-box girders saved approximately 15% of the original construction cost and accelerated the original construction schedule by six months. For aesthetics, the use of precast concrete allowed for the development of the unique cross-sectional shape of the box girder and architecturally appealing column treatment on this highly visible project.

Precaster: Bethlehem Construction Inc., Cashmere, Wash.
Website: www.bethlehemconstruction.com

Engineer of record: International Bridge Technologies Inc., San Diego, Calif.
Website: www.ibtengineers.com

Deck area: 578,310 ft²
Bridge length: 21,823 ft
Bridge longest span: 350 ft
Bridge width: 265 ft
Precast concrete components:

Typical spans:
109 at 120 ft (most common)
52 at 78 ft to 116.5 ft

Long spans (three-span units):
1 at 142 ft-218 ft-142 ft
2 at 182 ft-268 ft-182 ft
1 at 250 ft-350 ft-250 ft

Station:
7 at multiple = 612 ft total
2205 pieces in total