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To Cross the Seine | |
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Perforated Metal | |
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To Cross the Seine Truss Performance The two parallel vertical truss planes are spaced 16 feet (5 meters) apart, creating inside and outside volumes, placing half the pedestrian surfaces outside the structure, for unobstructed views. This configuration makes the bridge quite slender transversely, but it is stiffened by the continuity through the lateral spans. The shear strength of the truss is provided by the parabolic geometry of the booms for uniform loads, and by the radiating obelisks for partial loads. The compliant bending stiffness of the elements provides shear stiffness without compromising the differentiation of the tension and compression booms. The deflections of the apparently slender structural depth are controlled by the moment continuity at the supports. This truss system was invented for the specific context of the Paris Seine landscape. The structure is intentionally flexible, and reacts pleasantly to the wind and pedestrian movement. Tuned mass dampers control the accelerations for safety and comfort. The deck has intentional porosity to avoid the unusual Scanlan coefficients which are characteristic of this topology in section. The main span across the river is 620 feet (190 meters); a shorter span on each end crosses the quayside roads. At the abutments, a pair of mannerist-style, slotted, vertical tension plates and bilinear struts transfer forces from the truss to the foundations. Anchoring the Ends The foundations are reinforced concrete anchored in the underlying limestone. Prestressed ground anchors are incorporated into a design that allows visual inspections and will facilitate eventual replacement. The foundations were poured near the quay at each end of the bridge. A compact milling machine was used because of the poor bearing capacity and the limited access to the lower quay level. The cut passed through the river sediment down into the limestone below. The cut was stabilized with thixotropic bentonite mud. Steel reinforcing cages were lowered into the mud, and the concrete was poured though a tube to the bottom. The steel cables that precompress the tension foundation were cemented into steel tubes, which were themselves cemented into individual holes bored through the limestone. This foundation system was permanently equipped with devices to measure stresses over time. The main tension plates of the superstructure were anchored on a hammer head, which applies a compression force to the back of the concrete ground beams. The hammer head was the first part of the steel superstructure to be delivered to the site so the concrete substructure could be cast around it. The jacks, at the steel/ concrete interface, were used to fine-tune the altitude of the center of the bridge, compensating for slight movements of the foundations and the fabrication tolerances. Prefabricated steel castings were used where different laminated steel sections met at nodes, at the crossing of the compression and tension booms, and in the changes in direction of the transformer. The occasional defects that can arise in both the welding and casting processes were identified by ultrasonic and X-ray testing, and in critical locations they were cut out and repaired. Fabricating the Components The fabrication of the steel structure was carried out in workshops in the cities of Lauterbourg Nantes and Charleville-Mézières. Lengths of laminated steel and different shapes of cast steel were assembled by manual and semiautomatic welding. >>> Discuss this article in the Architecture Forum...
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