The Seattle Sound Transit Light Rail Project, Tukwila Segment, consists of 5.2 miles of twin track light rail line, including of 4.6 miles of elevated precast segmental guideway. This project will complete a critical link in the regional light rail system, directly connecting the existing light rail network with SeaTac Airport. More than just a transit link, though, the Tukwila segment is a highly visible portion of the alignment, and strong emphasis was placed on creating a well integrated urban design.
This focus found its expression in two principal ways: sensitivity to context and attention to aesthetics. The structure will pass over several environmentally sensitive areas, a consideration that drove the decision to elevate many portions of the guideway. In other locations, the alignment will pass through tightly congested urban areas, as well as crossing over a wide river and a busy Interstate. The single solution to these varied requirements was a precast segmental concept, which could be modified to meet the demands of each location. A top-down construction scheme with an overhead gantry also minimized the impact of construction along the alignment.
Attention to aesthetics began with the development of a unique tapered cross section, which both reduced the weight of the structure while creating a light, clean appearance. From this basic concept, details were developed to visually integrate all of the functional elements. The result is an elegant guideway that was achieved without sacrificing economy.
The project team overcame many technical hurdles in the design phase, foremost of which was the stringent seismic design. The Seattle area is seismically vulnerable, and the guideway was designed for multiple levels of seismic performance. This was made more challenging by the widely varying soils along the alignment. The team also incorporated four long-span structures into the design, which were designed as balanced cantilevers. These spans include crossings over an interstate highway, a rail yard, and the 350-ft wide Duwamish River.
IBT, as a sub-consultant to Hatch Mott Macdonald, was responsible for the design of the elevated guideway, including superstructure, substructure and foundations as part of the design-bid-build contract.
The contract was successfully bid upon by PCL Civil Constructors, Inc. in February 2005. Since that time, and additional extension has been added to the contract, bring the total amount to
approximately $270 million. Construction of the project was opened to passengers in early 2008.
Owner: Central Puget Sound Regional Transit Authority
Contractor: PCL Construction Services
Prime Consultant: Hatch Mott MacDonald
Bridge Design Consultant: International Bridge Technologies, Inc.