ASPIRE is a quarterly magazine published by PCI in cooperation with the associations of the National Concrete Bridge Council. The editorial content focuses on the latest technology and key issues in the Concrete Bridge Industry.
Issue link: http://www.aspiremagazinebyengineers.com/i/485551
The erection method had to be selected at the outset, since temporary loading and construction means and methods for this type of structure define the final dead load condition of the completed bridge. In addition, the slender dimensions of the edge girders, the short towers, and the cable saddles made it critical that the cantilever construction did not overstress the deck or allow the cables to slip through the saddles as the segments were cast on opposite sides of the towers. Each construction operation needed to be coordinated and integrated into the design. Pylon Foundations The reference concept provided by the owner included a large circular foundation supported on eight 10-ft-diameter shafts. Preliminary analysis showed that much of the mass supported by the shafts was in the footing itself, so that by reducing the size of the footing the number of shafts could be reduced. The figure shows the reference concept and the final design. The shape was made more oval and the number of shafts was reduced to six 10-ft-diameter shafts. This resulted in a significant reduction in concrete quantity, lowered seismic demands, reduced the size of the cofferdam required to construct the footings, and shortened the construction duration for the main foundations. The main controlling load cases evaluated for the foundation included seismic, wind, ship impact, and light rail vehicle (LRV) live loads in combination with the dead load of the structure. The shafts are embedded in an extremely hard deposit known as the Troutdale Formation. This formation is a dense, compact granular material that provides excellent support for the bridge. O-cell shaft capacity tests were performed to verify their ability to support the structure. This testing showed that the end bearing and side friction of the shafts exceeded even the higher predictions and, therefore, the final shaft tip elevations were reduced further based on the test results. Knowledge of the strength of local subsurface conditions allowed the design to use capacities considerably higher than classical values for both end bearing and side friction. Liquefiable Soils Preliminary geotechnical investigation performed by the owner during the preliminary design phase indicated that there were potentially liquefiable soils on the west side of the project. In addition, the location of the bridge was an old dock and dumping area for a shipyard that once operated on the site, and investigations showed the fill had hazardous materials. The bidding documents indicated that ground improvement might be required to stabilize the area in the event of an earthquake. In lieu of ground improvement strategies that would require handling a large volume hazardous material, the design-build team evaluated structural solutions, and established demands from the predicted soil liquefaction on the bridge displacements and foundation strength and ductility demands. The design criteria required evaluating a 475- year return operating and a 975-year return extreme earthquake. For the lower-level earthquake the bridge must remain operational with repairable damage, and for the higher-level earthquake the structure must not collapse. Evaluations were performed using a three-dimensional nonlinear time history model with both liquefied and non-liquefied soils in order to bracket results. The predicted displacement demands were applied to the foundations along the length of the structure in order to design the foundation-superstructure frame for the liquefied design case. This solution was compared to a ground improvement condition, and found to be both superior in terms of performance and less expensive for construction, even without considering the environmental impacts of moving large volumes of hazardous materials. Midspan Design Due to the maximum bridge slope allowed for the Americans with Disabilities Act and the navigation clearance required, there was not adequate clearance below the bridge to permit the midspan cable anchors to be below the edge girder in the middle 150 ft at the midspan of the structure. This issue was left open in the RFP because the realization of height limitations came late in the project development process.