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/439391
CONCRETE BRIDGE TECHNOLOGY Manhattan West Platform Challenges by Phil Marsh, McNary Bergeron, and Andrea Travani, Rizzani de Eccher USA This new ASPIRE™ department highlights some of the detailed engineering topics and key issues that significant projects bring to light. The editors asked the engineer of record and contractor on the Manhattan West project in New York City to detail six specific challenges they faced in achieving the owner's goals. For an overview of the project, see the article beginning on page 24. The Manhattan West platform project had a number of significant challenges that had to be addressed during the design process. These were unprecedented and took careful calculations to meet the unique design constraints required of the precast concrete components. Six of the key challenges that had to be met were: 1. Compressive Stress Limits. Because of the long span and heavy design loads, the beams were highly post-tensioned. Design calculations showed that after a beam was completed, the compressive stress at the bottom of the beam would be 5.6 ksi, which was very close to the initial compressive stress limit of 5.7 ksi (0.6f’c). During post-tensioning operations, the reduced cross-sectional area due to the ungrouted ducts would have caused the compressive stress limit to be exceeded. Therefore, a 360-kip counterweight was placed on the beam near midspan to temporarily reduce the precompression in the bottom slab until grout was placed in the ducts and could cure. After the grout had become an effective part of the girder cross section, the counterweight could be removed, and the stress in the bottom slab remained just below the limit. At this initial condition, the top slab was also in compression, but at a stress between 1 and 2 ksi. In the final, fully-loaded design condition, the stresses reverse with the top slab being in a high compression of 5.5 ksi, once again, very close to the limit, and the bottom slab being near zero compression. The high compression in the top slab became an important design consideration when detailing the openings for future columns. 2. End-Anchor Segment Integration. The end-anchor segments were 4 ft 2 in. long, weighed 56 tons, and contained twenty 37-strand anchors, six 31-strand anchors, four 9-strand transverse tendons, twelve 1.25-in.-diameter permanent post-tensioning bars, eight 3-in.-diameter bars for lifting, and bearing recesses. In all, 9000 lb of reinforcement was required in each end-anchor segment, with more than 200 form-savers per segment. That was a lot of material to fit into these relatively small end segments, which had to be kept as short as possible due to shipping-weight limits. All of these elements had to be carefully integrated to make it possible to successfully build these segments. A lifting shoe with a 900-ton design capacity was stressed down to each anchorage group of an end segment. Each shoe was attached to the beam with four bars that each had a capacity of 450 kips.The four shoes provided a total lifting capacity of 3600 tons, which was 150% of weight of the 2400-ton beam. The increased capacity was required by the railroad to ensure no problems were encountered lifting the loads over live rail traffic. The distance between the beam seat and the bottom of the end segment was about 12 in. That space was needed for two 380-ton temporary jacks as well as the permanent bearing. Spherical bearings were used on the project because of their compactness. The largest bearings were designed for a 2500-ton ultimate load.