Bridges, Tunnels and other Transportation Structures
The Northgate Link project extends Sound Transit’s light rail train (LRT) alignment to the Northgate neighborhood in Seattle. Included in this project is a 2,500-ft long aerial guideway running along I-5 with an elevated station adjacent to the Northgate Mall. The project site was designated as Site Class C & D and Seismic Design Category D. The high seismicity and geometric constraints of the project area, coupled with the complex details required to frame precast members into the cast-in-place (CIP) elements that must support them, forced the design team to creatively solve these challenges to meet the needs of the project.
To align with the project’s architectural requirements, trapezoidal shaped girders framed into trapezoidal bent caps. The precast girders with CIP track slab utilized integral construction techniques to create continuous guideway segments of up to six spans. Framing precast girders into irregularly shaped bent caps created multiple detailing challenges. To maintain the profile of the girders at their interface with the expansion bents, the girders were detailed with dapped ends supported either on top of a truncated bent cap or on blisters extruding from the bent cap. This configuration required extensive detailing to adequately support the large shear forces generated in both the girder dapped ends and the support blisters.
The heavily reinforced guideway columns required large amounts of confinement reinforcing within the plastic hinges to resist earthquake loading, and to provide acceptable seismic structural behavior, the superstructure must resist the plastic moments of the column. However, the girder section with extended prestressed strands was not adequate to resist these demands alone. Special continuity details incorporating a combination of extended girder strands and post-tensioned bars and strands were developed to provide the capacity required after continuity. Pipe-pin restrainers providing shear transfer without rotational rigidity were also utilized in column to cross-beam joints at various integral bents, reducing the moments generated in the substructure and the torsion experienced by the cross-beams.
Crossing twice over a four-lane roadway required multiple cantilever and straddle bents to carry the guideway. The design team advanced a concept using the straddle bents to support construction activity in addition to the weight of the precast girders and poured track slab, thus eliminating the need for temporary supports that would interfere with traffic operations. Utilizing these cross-beams for temporary works in conjunction with the use of precast girder elements increased the speed of construction and lowered the impacts to traffic and adjacent businesses.
The two-level elevated station was designed and detailed to comply with competing code constraints, requiring ductile behavior in the mezzanine and platform beams to comply with the International Building Code (IBC), while also providing adequate ductility in the columns to satisfy AASHTO LRFD Bridge Design requirements. In addition to analyzing the behavior under seismic and LRT loading, the station analysis included construction sequencing, allowing the design team to phase the construction and stressing of the station elements and to identify the requirements for shoring the bents, platform, and mezzanine during construction.