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Case Studies : Cooper River Bridge : Superstructure
The truss members are either box-shaped, as shown in Figure 3.1 (for compression forces) or H-shaped (for tension), as shown in Figure 3.2. In both cases, the overall width of each member is a uniform 30", and the sections are built-up by welding large, thick steel plates together. Figure 3.3, Figure 3.4, and Figure 3.5 show the preheating and submerged arc-weld process taking place for one of the H-shapes. Inspection and testing of shop welding was performed with non-destructive radiographic, magnetic particle and ultrasonic tests. (ref. 5) In addition to shop welding by semi-automatic procedures, computer-controlled cutting and drilling are used to cut down on labor costs. Each truss panel is assembled on the shop floor to guarantee tolerances, before being painted and shipped to the bridge site.
Individual truss panels are prefitted on shore and then brought to site by barge, where they are lifted into place with cranes mounted on barges, as shown in Figure 4.11. Because the truss is continuous over three spans, the balanced cantilever method, illustrated in Figure 4.9, of bridge construction is used (See ref. 3 for more details of this and other erection methods). Erection begins at the two interior piers and continues in both directions, cantilevering out from the pier. If this method were not used, or if the truss were not continuous, temporary steel falsework would be required to provide intermediate support as erection proceeded out from the pier. This would obstruct river traffic and would be objectionable to the client. At all times, the partially erected structure must be stable and capable of resisting the loads it is subjected to, including the possibility of strong winds and earthquakes.
The large lifting capacity of the floating cranes allows multi-panel segments to be placed in one lift, saving considerable construction time. This would not be possible had the contractor used conventional travelling derricks (mounted on the ends of the cantilevers.) When the anchor spans are complete, the remaining center span panels are assembled on shore, barged to the site, and lifted into place. These pieces weigh about 176 tons each. Following completion of the truss structure, the floorbeams and stringers are bolted into place, the concrete deck slab is placed, and the trusses are painted with two coats of paint -- an intermediate coat of high-build epoxy and a top coat of blue urethane (ref. 5). See the Truss Erection Images for an illustrated description of the construction of the superstructure. No figures for this section. |
