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FIGURE 26.11 Examples of stiffening girder of modern suspension bridge: (a) Severn Bridge; (b) Tsing Ma Bridge and (c) Akashi Kaikyo Bridge.

FIGURE 26.11 Examples of stiffening girder of modern suspension bridge: (a) Severn Bridge; (b) Tsing Ma Bridge and (c) Akashi Kaikyo Bridge.

bridges, adoption of big fairings on both sides of the cross-section and longitudinal openings on upper and lower decks features the Tsing Ma Bridge in Hong Kong (Figure 26.11b).

Even if the stiffening girder is simply supported, both supports are designed as movable shoes in longspan suspension bridges; that is, the stiffening girder is suspended by tower links at the tower and end links at the abutment. The wind bearings preventing transverse displacement of the girder are installed on the horizontal strut of the tower and on the abutment. When no vertical support is provided at the tower, the lateral support at the tower can be accomplished by applying vertical sliding bearings between the girder and the inner surfaces of the tower shafts as in the case of the Great Belt Bridge.

In many long-span suspension bridges, the stiffening girder is fixed to the main cables at the center of main span by either rigid clamps or diagonal stays, which are called the center tie and the center diagonal stay, respectively. These aim at constraining the asymmetric vibration mode of torsion and the relative longitudinal displacement between the cables and the girder, resulting in the reduction of secondary bending stress in short hangers and the increase of longitudinal restoring force of the girder. In the case of the diagonal stay, it may be proportioned to fail at a certain service limit state.

26.3.4 Stress Ribbon Bridge

Stress ribbon bridges are a kind of earth-anchored suspension bridges (Figure 26.12). The stretched cables are encased in the concrete slab which is used as the bridge deck. As understood from Equation 26.2, the cable tension due to the dead load and prestressing, in addition to bending rigidity of the deck slab, contribute to the stiffness of the entire bridge. Although the sagged form is inevitable in the stress ribbon bridge, the maximum longitudinal slope is limited from the function of the bridge. On the other hand, very large tensile force in the cable is associated with increase of the cost. Therefore, this type of structure has been applied so far to the pedestrian bridges having short or moderate span length, although the structure is efficient, very slender, and elegant. In their design, care should be taken in forming a smooth transition between the slab and the abutments [12].

26.3.5 Erection 26.3.5.1 Towers

The control of erection accuracy in holding verticality is very important in tower construction. Steel towers are erected by using tower cranes and creeper traveler cranes. When open water area is available, floating cranes are sometimes used together. Because very tall and slender steel towers are vulnerable to wind-induced vibrations, in particular during the free standing stage of erection, some damping devices

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FIGURE 26.12 An example of stress ribbon bridge.

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FIGURE 26.12 An example of stress ribbon bridge.

FIGURE 26.13 Operating principle of aerial spinning. (Source: Honshu-Shikoku Bridge Authority, Technology of the Seto-Ohashi Bridge, Japan, 1989, with permission.)

FIGURE 26.13 Operating principle of aerial spinning. (Source: Honshu-Shikoku Bridge Authority, Technology of the Seto-Ohashi Bridge, Japan, 1989, with permission.)

are often installed. In the case of the Akashi Kaikyo Bridge, a set of tuned mass dampers are left as permanent fixtures inside the shafts. In constructing concrete towers, the towers are slip-formed in a continuous operation, employing self-climbing forms, tower cranes, and concrete-pumping buckets.

26.3.5.2 Main Cables

The erection of parallel-wire main cables of a suspension bridge is executed by either aerial spinning (AS) method or prefabricated strand (PPWS) method. In the former, the total cable section is assembled on site from individual 5 mm wires pulled across an anchorage to the other anchorage over the tower saddles. The conventional sag-control method spans individual wires in free-hang condition and the sag of each wire is individually adjusted to ensure the wires to be of equal length (Figure 26.13). The problem of this method was the sensitiveness of the quality of the cables and the erection period to such site working conditions as wind environment and spinning equipment.

A new method that is called the tension-control method was developed in Japan (Figure 26.14) and has been applied not only to the Japanese bridges but also to the Second Bosporus and Great Belt bridges. Although adjustment of individual strands is still required, the idea of this method is to keep the tension in the wire constant during cable spinning to obtain uniform wire lengths.

On the other hand, the PPWS method was introduced to reduce the labor and the weather sensitivity and consequently to speed up erection work. Although the limitation of this method may lie in the weight of the strand and the size of the reel, it was successfully applied to the world's longest Akashi Kaikyo Bridge, requiring strands with a length of 4000 m and a strand plus reel weight of 95 tons.

Now that the AS and PPWS methods have been improved step by step and experience has been gained, the method undertaken depends on the cost and period of construction at the specific site or area.

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