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/622975
PROJECT PEARL HARBOR MEMORIAL BRIDGE Stay-cable installation for the new bridge by Wade S. Bonzon, Figg Bridge Engineers Inc. The new $635 million, 10-lane Pearl Harbor Memorial Bridge is the engineering and aesthetic highlight of the $2.0 billion, Interstate 95 (I-95) New Haven Harbor Crossing (NHHC) Corridor Improvement Program, now nearing completion in New Haven, Conn. The concrete segmental main-span unit features a 515-ft-long span over the Quinnipiac River with symmetrical back spans of 249 ft. The structure consists of twin box-girder superstructures, each supported by two planes of stay cables. Sixty-four stay cables support each girder, with the middle towers sharing cable anchorages for both box girders. To avoid Federal Aviation Administration flight path restrictions, this concrete segmental extradosed cable-stayed bridge features shorter towers than normally found on a conventional cable-stayed bridge, with correspondingly flatter staycable angles. In addition, the extradosed structure type allowed for a shallower section depth than a conventional boxgirder superstructure, providing necessary vertical clearances for shipping traffic. The shallow-angled stay cables apply greater load along the axis of the bridge compared to a conventional cable-stayed bridge. The structure also features a stiffer deck system than most typical cable-stayed bridges, making fatigue stresses due to live-load effects on the cables less of a concern. While the stresses in conventional stay cables are limited to 45% of the strands’ minimum ultimate tensile strength (MUTS) for American Association of State Highway and Transportation Officials' AASHTO LRFD Bridge Design Specifications Service Load Combination I, the contract documents for this project specified that the extradosed cables could be stressed up to 55% of MUTS for the same load combination. Similar increases in stresses are allowed for other AASHTO load combinations. All of the stay cables are comprised of 48 individual 0.6-in.-diameter, 7-wire, low relaxation strands, which are each greased and encapsulated in a tight fitting HDPE coating. A 9-in.-diameter, high-density polyethylene (HDPE) sheathing pipe provides the outermost layer of protection to the strands. All stay-cable strands were stressed individually with a monostrand jack. To ensure that all strands in a stay were stressed to the same force, the elongation method was used to control strand-bystrand stressing forces. In theory, if all strands are elongated the same amount they will all carry the same force. In practice, this required careful quality control during strand preparation, installation, and stressing. All strands needed to have their protective sheathing removed in the anchorage regions to be properly gripped by the three-part anchor wedges. The stripped length was carefully controlled so that exposed strand portions remained fully within the protective “wax box” of the anchorage, which was filled with protective grease after completion of stay stressing. After HDPE coating removal, each strand was scored with a small v-notch reference mark in the unstressed length near each end. These reference marks were used to match elongations during stressing, so the distance between marks for each strand of the stay cable needed to be consistent to within less than +/-1⁄8 mm. Depending on the stay location, distances between reference marks varied from 114 to 221 ft. To achieve the required precision and consistency of the reference mark placements, sheet metal layout trays were set up on the bridge deck. Each tray was fabricated so that two rows of 24 strands each could be laid onto the tray snugly together, thus keeping all of the strands straight and parallel with each other in the tray. Reference marks were aligned across each row of strands using a heavy steel angle as a straightedge. Independent checks of the cut strand lengths, sheathing removal lengths, and reference mark locations were performed by the contractor and the inspector.