![]() Since a cable carries only direct stress, the resultant axial force T on all sections must act tangentially to the longitudinal axis of the cable (Figure 6.2b). The lower modulus of the cable is due to the uncoiling of the wire’s spiral structure under load. Steel cables have a modulus of elasticity of approximately 26,000 kips/in.2 (179 GPa) compared to a modulus of 29,000 kips/in.2 (200 GPa) for structural steel bars. Ultimate elongation of 7 or 8 percent compared to 30 to 40 percent for structural steel with a moderate yield point, say, 36 kips/in.2 (248 MPa). Photo 6.2: Cable-stayed bridge over the Cooper River in Charelston, South Carolina. While the drawing of wires through dies during the manufacturing process raises the yield point of the steel, it also reduces its ductility. The twisting operation imparts a spiral pattern to the individual wires. Based on these parameters, the designer applies cable theory to compute the end reactions, the force in the cable at all other points, and the position of other points along the cable axis.Ĭharacteristics of Cables Cables, which are made of a group of high-strength wires twisted together to form a strand, have an ultimate tensile strength of approximately 270 kips/in.2 (1862 MPa). In a typical cable analysis the designer establishes the position of the end supports, the magnitude of the applied loads, and the elevation of one other point on the cable axis (often the sag at midspan see Figure 6.2a). A number of sports arenas, including Madison Square Garden in New York City, are roofed with a cable system of this type. © Universal History Archive/UIG via Getty Imagesĭesigner creates an efficient structural form for gravity loads that requires only vertical supports around its perimeter. Roof supported on a net of steel cables spanning between massive, sloping, reinforced concrete pylons. Photo 6.1: Terminal building at Dulles air. Cables and arches structural analysis pdf free#By creating a self-balancing system composed of members in direct stress, theįigure 6.1: Cable-supported roof composed of three elements: cables, a center tension ring, and an outer compression ring.įigure 6.2: Vertically loaded cables: (a) cable with an inclined chord-the vertical distance between the chord and the cable, h, is called the sag (b) free body of a cable segment carrying vertical loads although the resultant cable force T varies with the slope of the cable, ∑Fx = 0 requires that H, the horizontal component of T, is constant from section to section. The small center ring, loaded symmetrically by the cable reactions, is stressed primarily in direct tension while the outer ring carries mostly axial compression. For example, Figure 6.1 shows a schematic drawing of a roof composed of cables connected to a center tension ring and an outer compression ring. To take advantage of the cable’s high strength while minimizing its negative features, designers must use greater inventiveness and imagination than are required in conventional beam and column structures. Providing an efficient means of anchoring the large tensile force carried by cables. Preventing large displacements and oscillations from developing in cables that carry live loads whose magnitude or direction changes with time. To use cable construction effectively, the designer must deal with two problems:ġ. Because of their great strength-to-weight ratio, designers use cables to construct long-span structures, including suspension bridges and roofs over large arenas and convention halls. Use the general cable theorem to establish a funicular shape of an arch for which forces are in direct compression along the arch, resulting in an efficient minimum weight arch design.Īs we discussed in Section 1.5, cables constructed of high-strength steel wires are completely flexible and have a tensile strength four or five times greater than that of structural steel. Study the characteristics, types, and behavior of cable and arch structures.Īnalyze determinate cable structures and calculate support reactions by two methods, namely by equations of static equilibrium and by the general cable theorem, as well as determine the cable forces at specific points along its length.Īnalyze determinate three-hinged arches and trussed arches.
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