Claudio Osses, PE, SE; Buckland and Taylor, Huanzi Wang, PhD, PE; AECOM, Richard Patterson, PE, SE; Buckland and Taylor
The Mountlake Triangle Project (MTP) Bridge is a highly-curved 400-foot long cast-in-place (CIP) post-tensioned pedestrian bridge spanning over Mountlake Boulevard between the Seattle Sound Transit light rail station and the University of Washington (UW) campus in Seattle, Washington. Construction of the bridge is scheduled to finish in early 2014. It is anticipated that this daring bridge will become a landmark for both the UW and the City of Seattle.
Design Challenges
The horizontal geometry resembles a highly curved “X” in plan view (see Figure 1), including a forked superstructure at each end of the bridge. The vertical design constraints required the bridge to meet stormwater runoff requirements and match predetermined elevations for three sets of stairs, two elevators, temporary and permanent vertical clearances for traffic, and permanent clearances for pedestrians and bicycles. Steel is usually the option for highly curved bridges. However the client, Seattle Sound Transit, requested the use of concrete for its ease of maintenance. Typically post-tensioning (PT) is not preferred in highly curved bridges due to the difficulties associated with handling the large out-of-plane forces induced. However, for this bridge, post- tensioning was chosen because it allowed a shallow section that met the vertical clearance requirements and produced a high level architectural finish without concrete cracking associated with non-post-tensioned concrete bridges.
The 92-foot minimum horizontal radius of curvature required special PT analysis, detailing, and construction. Local out-of-plane forces caused by the PT can potentially result in the concrete in front of the PT ducts failing. The strut-and-tie method was used to analyze and design the local PT reinforcement. Post-tensioning on a structure with such a tight horizontal curve tends to produce variable PT stresses across the bridge section. These variable PT stresses across the bridge section and between the webs can cause transverse tension in the slabs and undesirable torsional effects if the PT jacking forces are not arranged properly. It was found that using PT forces at each web that were roughly proportional to the height of each web minimized the undesirable variable stress and torsional effect due to PT. Additionally, a staged construction analysis was used to determine the optimal jacking sequence to minimize the undesirable PT effects described, and this optional jacking sequence was used during construction.
With such an unusual geometry, simplified straight line models are incapable of capturing important aspects of the structural behavior of the bridge such as the effect of the post-tensioning on the global bridge response and the stress and load distribution across the bridge section. Complex 3D finite element structural modeling techniques were used to capture the static and dynamic behavior of the bridge.
As seen in Figure 2, in-span hinges were added to the bridge to divide the bridge into three more regular bridge segments, which improved the overall static and seismic bridge behavior and simplified the analysis and design. Uneven span arrangement (end span to mid span length ratio is 0.40) required the use of special design techniques to control live load uplift reactions at the pier and at in-span hinge bearings. Some of the special design techniques used to control uplifting included the use of vertical tie rods, mass concrete at selected box cells, and rigid radial diaphragms at the piers. Also, a special seismic design criterion was developed to satisfy bridge (displacement based) and building (force based) code philosophies for the spans supported by the UW light rail station.
Construction Challenges
A number of challenges to bridge construction were presented by the complex bridge geometry combined with PT and the constraints imposed by the structure location. The bridge is located adjacent to existing structures and utilities and is next to the Seattle Sound Transit light rail station, which is also under construction. Constructing drilled shafts next to an existing aging underground parking garage structure, a spread footing next to a 54″ diameter water main, and several piers on top of the UW station required the use of special construction techniques and planning . For example:
- Soil was excavated to temporarily unload existing structures to allow them to resist construction equipment surcharge loads;
- Secant pile shoring walls and isolating flexible material layers were used to protect utilities; and
- Strategic phasing of the construction of the connection of the bridge to the UW station was used to minimize schedule impacts on the two projects.
The bridge superstructure formwork consisted of a series of adjustable straight formwork modules with tapered shims and plywood panels forming the curve shape. The placement of the duct and web tie reinforcement was carefully done because this reinforcement was critical to resist the PT out-of-plane forces.
The design of the MTP Bridge has pushed the limits for the use of post-tensioning in highly curved bridges, demonstrating that with the proper analysis and detailing, durable and low maintenance post-tensioned concrete can be used for bridges that have traditionally been made of steel.
For more information, please contact Claudio Osses, PE, SE Buckland and Taylor ([email protected]); Huanzi Wang, PhD, PE, AECOM ([email protected]); Richard Patterson, PE, SE, Buckland and Taylor ([email protected]).