2642 Configuration and Design of Structural Components

26.4.2.1 Stay Cables

Since the types, corrosion protection, and vibration control of cables were already mentioned in Section 26.2, just the anchoring techniques of cables in cable-stayed steel bridges will be reviewed herein.

The structural details to anchor stay cables at the main girder or the tower shall be designed so as to smoothly transmit cable tension to the main structural component, to be as simple and compact as possible, to be easy in fabrication and maintenance, and to have good appearance. Care shall be also taken on the fatigue and secondary stresses of both cables and anchorages. Although the types of structural details are manifold, the typical ones are given in the Sections 26.4.2.1.1 and 26.4.2.1.2.

26.4.2.1.1 The Stay Anchor at the Girders

1. Spray saddle/anchor girder type — The combination of spray saddle and anchor girder has been used for thick cables, but is not prevailing now.

2. Anchor girder (block) type — The anchor girder or the anchor block is inserted in the main girder by welding or bolting.

3. Bracket type — The bracket that anchors the stay cable is projected outside the main girder.

4. Pipe anchor type — The pipe that anchors the stay cable is incorporated with the web of a main girder.

5. Gusset type — The cable socket is connected to the gusset extended from the web of a main girder.

26.4.2.1.2 The Stay Anchor at the Tower

In the earlier bridges having a small number of thick stay cables, the hinged or fixed saddle over which cables are continuous was used. But nowadays, use of thin stay cables is prevailing and the typical types of anchorage are as follows:

1. Saddle type (e.g., Zarate-Brazo Largo Bridge, Yokohama Bay Bridge).

2. Anchor girder type (e.g., Speyer Bridge, Tempozan Bridge).

3. Bearing plate type (e.g., Kurt-Schumacher Bridge, Torikai-Ninnaji Bridge).

4. Pin-socket type (e.g., Stroemsund Bridge, John O'Connel Memorial Bridge).

26.4.2.2 Towers

Since the action of stay cables becomes inefficient with decreasing inclination, the stay inclination is usually taken as 25 to 65°, or the tower height of a cable-stayed bridge is around 1 to 1 of the main span and hence much taller than that of a suspension bridge with equivalent scale.

Almost all towers of the recent cable-stayed bridges are made of concrete though steel towers were used on the earlier steel bridges and presently almost all Japanese bridges have steel girders. Now concrete towers can be used for very long-span cable-stayed bridges such as the Normandy Bridge over the Seine (850 m span) and the Su-Tong Yangtze River Bridge (the world's longest span 1088 m) now under construction in China, with advances in concrete construction and formwork technology. In the Stonecutters Bridge (1018 m span) now under construction in Hong Kong, the top one third of 290 m high single-pylon tower will be a composite steel/concrete structural section. The steel skin will be fabricated from stainless steel for reasons of appearance and durability.

Development of bending moment in pylons was prevented in the earlier bridges by the use of rocker or sliding saddles and pinned tower feet, but those of recent bridges except for very short spans are mostly made integral with the pier or sometimes with the girder.

Wider choice is available in the tower form of a cable-stayed bridge as compared with a suspension bridge. This feature is advantageous in view of esthetical design. The tower form in general has distinctive characters, attaining increasing levels of refinement and rationality as time passes. Needless to say, the design of the tower must adapt to the stay cable layouts and to the topography and geology of the bridge site, and carry the forces economically.

FIGURE 26.20 Various forms of cable-stayed bridge tower.

The typical forms of cable-stayed bridge towers are shown in Figure 26.20. The single pylon adapting to a single-plane cable system has to be usually situated within the central median of the roadway and hence an additional width of deck is required for the necessary clearance to traffic. The inverted Y-shape tower is used for both single-plane and double-plane cable systems, while A- or inverse V-shape towers adapt to a double-plane cable system. The H-shape tower is the most logical form structurally for a two-plane cable system. In the case of the A or inverted Y towers, the spacing of two shafts under the deck is often narrowed to reduce the size of the foundation. These diamond-shape towers have been used for long-span cable-stayed bridges such as the Tatara (Japan), Yangpu (China), and Baytown (United States), but the visual aspect of securing the strength of the portion under the deck should be carefully pursued.

26.4.2.3 Bridge Deck

In cable-stayed bridges, the solid-web girders precede the truss girders except for double-deck designs for both structural and esthetic reasons. These girders or bridge deck are subjected to not only bending and torsional moments as well as shear forces but also axial forces. The closer spacing of cables in a multistay system allows a more slender bridge deck. Although the stability of the girder as a beam-column is generally not yet a serious problem owing to elastic and almost continuous support by stay cables so far, the overall stability of the structure must be checked by using a nonlinear, second-order analysis for super-long-span cable-stayed bridges [14].

As for the cross-section of the steel girder in a cable-stayed bridge, there seems two ways at the moment. One is a single or double box as seen in many long-span cases. Orthotropic steel plate deck is incorporated with the steel girder and the circumscribed shape of the cross-section is usually trapezoidal or hexagonal, mainly for aerodynamic reason. Another way is the use of shallow-plated edge girders connected with cross-girders. For wider bridges and longer span bridges than 500 m, Leonhardt and Zeller [15] suggest the use of similar cross-sections of all steel structures with an orthotropic steel deck, and that no box girder is needed and simple edge beams are sufficient. However, a box girder with high torsional rigidity should be used when the stay cables are arranged in a single plane along the center line of the bridge deck.

The use of a composite steel/concrete girder on cable-stayed bridges may be categorized into the latter. Prefabricated or in situ concrete slabs are connected with longitudinal edge girders and cross-beams in steel by stud shear connectors. The compressive forces from the stay cables can be sustained by the concrete slab. The Yangpu Bridge in Shanghai with a long main span of 602 m is provided with doubleweb edge girders. Figure 26.21 shows two typical examples of steel girders used in large cable-stayed bridges.

One trapezoidal or two rectangular box designs are most popular in prestressed concrete cable-stayed bridges. But as in the case of the composite girder mentioned above, the longitudinal edge beams

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FIGURE 26.21 Examples of steel girder of cable-stayed bridge: (a) composite girder of the Nanpu Bridge and (b) steel box girder of the Tatara Bridge.

connected by the transverse beams and incorporated with the stay anchors are sometimes employed. Even in the prestressed concrete girders, wind nose may be added to the cross-section when the span is long.

When the ratio of the side span to main span length is small, the use of steel girders in the main span and continuously extended concrete girders in the side spans is one of the solutions to attain the rational and economical design. If the situation allows, it is advantageous to provide with intermediate supports for the side-span girders in this case.

Most of the truss girders for cable-stayed bridges are employed in double-deck cases, as exemplified in several Japanese designs. Among them, the Hitsuishijima and Iwakurojima bridges (Figure 26.22) were designed to carry four lanes of roadway traffic on the upper deck and ordinary type railway tracks as well as double Shinkansen rail tracks on the lower deck. Yokohama Bay Bridge, which carries six lanes of roadway traffic on the respective decks, is unique in that the upper chord of its truss is a shallow steel box section stiffening the girder system and concealing power and communication cables. The truss girder of the Higashi-Kobe Bridge is shallow and of Warren type without vertical members, so it appears slender for a double-deck road bridge.

26.4.2.4 Vertical Support of Girder

A variety of supporting conditions have been adopted in modern cable-stayed bridges [16] because the selection of the supporting conditions for longitudinal movement is rather adaptable owing to the existence of stay cables and flexible towers. Less constraint in the bridge axis direction yields longer natural periods of the corresponding motion and thus reduces the seismic inertia forces. However, less constraint on the longitudinal movement of the girder may cause large bending moment in the towers and larger displacement of the girder. The effect of temperature change should be also taken into account. Figure 26.23 illustrates the potential combinations of supporting conditions for three span cases that are most prevalent in long span cable-stayed bridges.

Highway

Highway

FIGURE 26.22 Typical cross-section of the Seto Bridge.

FIGURE 26.22 Typical cross-section of the Seto Bridge.

Type (a) or (e) in the figure, where one support is made as a fixed hinge and all other supports are longitudinally movable, have been widely used for bridges with medium or short span length because temperature effects are released and the seismic force applied to the substructure is relatively small. With increasing span length, design and construction of the pier fixed to the girder become difficult due to the increase of seismic reaction. The supporting condition (e) has been preferred to (a) in Japanese bridges because the expansion of the girder end under temperature change is smaller and the size of the tower foundation is relatively large for reasons other than earthquake effect. The supporting condition (a) may, however, be preferable when the height of the tower below the bridge deck is large and the end support is on the abutment.

When large clearance height is required, the bridge deck is often provided with fixed hinges at both flexible towers as in (c) or is rigidly connected to these towers. The stresses due to temperature change can be released owing to the flexibility of the towers and seismic reactions can be sustained by both tower piers. Some difficulties may arise, however, in giving the towers both the flexibility to absorb deformation of the girder due to temperature change and the stiffness to cope with seismic effects. The additional thrust induced in the girder should be also borne in mind in this case.

Prevalent in long-span cable-stayed steel bridges in Japan are the various devices connecting the girder elastically with towers or abutments ((b), (d), or (h) in Figure 26.23). In the Meikoh-West bridge and some other bridges, elastic restraint was provided by attaching horizontal cables between the girder and the tower, while large belleville springs were fitted to the rocker bearings at each end pier of the Hitsuishijima and Iwakurojima bridges. The purpose of these devices was to reduce and distribute seismic forces, to control the longitudinal movement of the girder, and also to find a compromise with temperature effect.

The advantage of all movable support types, (f) and (g) in Figure 26.23, is to reduce the seismic inertial force of the girder by attaining very long natural periods of longitudinal sway motion. The so-called floating type (g), where the vertical support at the tower is omitted, leads to a noticeable reduction of the bending moment in the girder at the towers. However, care should be given to excessive displacement of the girder and proneness of instability of the towers. In the Higashi-Kobe Bridge, newly developed vane-type dampers were installed on the end piers as stopping devices against unexpectedly severe earthquakes. The use of short tower links in the Yokohama Bay Bridge or thick rubber shoes on the side span of the Ikuchi Bridge was also aimed at optimizing seismic design under the given conditions.

Elastically fixed at each end of ^ girder

Type of supporting conditions

Fixed at one end of girder

Elastically fixed at each end of ^ girder

Fixed at tower piers

Elastically fixed at tower piers

Fixed at one tower pier

Movable at all supports

FIGURE 26.23 Different supporting systems of cable-stayed bridge.

Floating

Floating but elastically constrained at tower piers

(Elastically) fixed at all supports

Fixed outside

FIGURE 26.23 Different supporting systems of cable-stayed bridge.

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