19
Cable-Stayed Bridges
19.1 Introduction
19.2 Configuration
General Layout •Cables •Girder •Tower
19.3 Design
Permanent Load Condition
•Live Load •Thermal
Loads •Dynamic Loads 19.4 Superlong Spans 19.5 Multispan Cable-Stayed Bridges 19.6 Aesthetic Lighting 19.7
Summary
Since the completion of the Stromsund Bridge in Sweden in 1955, the cable-stayed bridge has evolved into the most popular bridge type for long-span bridges. The variety of forms and shapes of cable-stayed bridges intrigues even the most-demanding architects as well as common citizens. Engineers found them technically innovative and challenging. For spans up to about 1000 m, cable-stayed bridges are more economical.
The concept of a cable-stayed bridge is simple. A bridge carries mainly vertical loads acting on the girder, Figure 19.1. The stay cables provide intermediate supports for the girder so that it can span a long distance. The basic structural form of a cable-stayed bridge is a ries of overlapping triangles comprising the pylon, or the tower, the cables, and the girder. All the members are under predominantly axial forces, with the cables under tension and both the pylon and the girder under compression. Axially loaded members are generally more efficient than flexural members. This contributes to the economy of a cable-stayed bridge.夏日缤纷
At the last count, there are about 600 cable-stayed bridges in the world and the number is increasing rapidly. The span length has also incread significantly [2,7].
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Some milestones: the Stromsund Bridge in Sweden, completed in 1955 with a main span of 183m is usually recognized as the world’s first major cable-stayed bridge; the Knie Bridge (320 m) and Neuenkamp Bridge (350 m) in Germany, Figure 19.2, were the longest spans in the early 1970s,until the Annacis Island–Alex Frar Bridge (465 m) was completed in the mid 1980s. The 602-m-span Yangpu Bridge was a large step forward in 1994 but was surpasd within about half a year by the Normandie Bridge (856 m), Figure 19.3. The Tatara Bridge, with a center span of 890 m, is the world record today. Several spans in the range of 600 m are under construction. Longer spans are being planned.
Man-Chung Tang
T.Y. Lin International
FIGURE 19.1Concept of a cable-stayed bridge.
砂锅豆腐的做法家常
FIGURE 19.2Neuenkamp Bridge.我的心愿作文400字
FIGURE 19.3Normandie Bridge.
19.2.1General Layout
At the early stage, the idea of a cable-stayed bridge was to u cable suspension to replace the piers as intermediate supports for the girder so that it could span a longer distance. Therefore, early cable-stayed bridges placed cables far apart from each other bad on the maximum strength of the girder.This resulted in rather stiff girders that had to span the large spacing between cables, in addition to resisting the global forces.
The behavior of a cable-stayed girder can be approximately simulated by an elastically supported girder. The bending moment in the girder under a specific load can be thought of as consisting of a local component and a global component. The local bending moment between the cables is proportional to the square of the spacing. The global bending moment of an elastically supported girder is approximately [5]
(19.1)
where a is a coefficient depending on the type of load p, I is the moment of inertia of the girder,and k is the elastic support constant derived from the cable stiffness. The global moment decreas as the stiffness of girder , I, decreas.
尺的英文
Considering that the function of the cables is to carry the loads on the bridge girder, which remains the same, the total quantity of cables required for a bridge is practically the same indepen-dent of the number of cables, or cable spacing, Figure 19.4. But if the cable spacing is smaller, the local bending moment of the girder between the cables is also smaller. A reduction of the local bending moment allows the girder to be more flexible. A more flexible girder attracts in turn less global moment. Conquently, a very flexible girder can be ud with cloly spaced cables in many modern cable-stayed bridges. The Talmadge Bridge, Savannah, Figure 19.5, is 1.45 m deep for a 335m span, The ALRT Skytrain Bridge, Vancouver, Figure 19.6, is 1.1 m deep for a 340 m span and the design of the Portsmouth Bridge had a 84-cm-deep girder for a span of 286 m.
Becau the girder is very flexible, questions concerning buckling stability occasionally aro at the beginning. However, as formulated by Tang [4], Eq. (19.2), using the energy method,
(19.2)
where E is modulus of elasticity, I is moment of inertia, A is area, L is length , w is deflection, and ( )′is derivative with respect to length s. The buckling load depends more on the stiffness of the cables than on the stiffness of the girder. Theoretically, even if the stiffness of the girder is neglected, a cable-stayed bridge can still be stable in most cas. Experience shows that even for the most flexible girder, the critical load against elastic buckling is well over 400% of the actual loads of the bridge.
FIGURE 19.4
Cable forces in relation to load on girder.
贝塔斯瑞
P cr EIw ds EC Ac Lc Ps Pc w ds ()=′′+∗∗
()′ ∑∫∫22
The recently adopted design requirement that all cables be individually replaceable makes cloly spaced cables more desirable. It is usually required that one cable can be detensioned, dismantled,and replaced under reduced traffic loading. The additional bending moment in the girder will not increa excessively if the cable spacing is small.
Availability of ever more powerful computers also helps. The complexity of the analysis increas as the number of cables increas. The computer offers engineers the best tool to deal with this problem.
Harp, radial, fan, Figure 19.7, or other cable configurations have all been ud. However, except in very long span structures, cable configuration does not have a major effect on the behavior of the bridge.
A harp-type cable arrangement offers a very clean and delicate appearance becau an array of par
allel cables will always appear parallel irrespective of the viewing angle. It also allows an earlier FIGURE 19.5Talmadge Bridge.
FIGURE 19.6ALRT Skytrain Bridge.
FIGURE 19.7
Cable arrangements.
start of girder construction becau the cable anchorages in the tower begin at a lower elevation.The Hoechst Bridge and the Dames Point Bridge are examples that fully utilized this advantage.登机行李箱尺寸
A fan-type cable arrangement can also be very attractive, especially for a single-plane cable system.Becau the cable slopes are steeper, the axial force in the girder, which is an accumulation of all horizontal components of cable forces, is smaller. This feature is advantageous for longer-span bridges where compression in the girder may control the design. The Nord Bridge, Bonn,Figure 19.8, is one of the first of this type.
A radial arrangement of cables with all cables anchored at a common point at the tower is quite effici
ent. However, a good detail is difficult to achieve. Unless it is well treated, it may look clumsy.The Ludwighafen Bridge, Germany, Figure 19.9, is a successful example. The Yelcho Bridge, Chile,with all cables anchored in a horizontal plane in the tower top, is an excellent solution, both technically and aesthetically.
When the Stromsund Bridge was designed, long-span bridges were the domain of steel construc-tion. Therefore, most early cable-stayed bridges were steel structures. They retained noticeable features from other types of long-span steel bridges.
In the 1960s, Morandi designed and built veral relatively long span concrete cable-stayed bridges. His designs usually had few cables in a span with additional strut supports at the towers for the girder. They did not fully utilize the advantages of a cable-stayed system. The concrete cable-stayed bridge in its modern form started with the Hoechst Bridge in Germany, followed by the Brotone Bridge in France and the Dames Point Bridge in the United States, each reprenting a significant advance in the state of the art.
FIGURE 19.8Nord Bridge.keep过去分词
FIGURE 19.9
Ludwighafen Bridge.
