2633 Design Procedure

After the type of structural system and the span arrangement are decided, the design of suspension bridge superstructure is performed according to the procedure shown in Figure 26.10 [10].

26.3.3.1 Design Loads

Accurate evaluation of dead load is important in suspension bridge design because its contribution to the main components of the bridge is large. Design live load can be reduced from a probabilistic viewpoint with increasing span length and road width. The dynamic magnification due to running vehicles is usually negligible for design of the main cables, towers, and stiffening girder of a large suspension bridge owing to its long span and low natural frequencies.

The dynamic analysis is inevitable in seismic and wind-resistant design of flexible suspension bridges. Although the seismic forces induced in the superstructure are relatively small due to its long natural period, attention should be paid on possible large displacement of the stiffening girder and great forces transferred to the supports under seismic loading. Aerodynamic stability often dominates the design of a suspension bridge and some appropriate cures may be needed (cf. Chapter 9 of Section II) [11].

FIGURE 26.10 Design process of a suspension bridge superstructure. Copyright 2005 by CRC Press

26.3.3.2 Main Cables

The final form of the main cable of a suspension bridge is a parabola. Sag/span ratio is determined to optimize construction cost of the entire bridge, and sometimes in consideration of the aerodynamic stability as it affects the dynamic characteristics of the entire structure. In general, the sag/span ratio is around 10 irrespective of span length.

As mentioned in Section 26.2.2, the parallel-wire cable is exclusively used in very long-span bridges, whereas the spiral ropes or LCRs have been sometimes adopted in moderate- or short-span suspension bridges.

26.3.3.3 Hangers

Hanger cables are arranged to be either vertical or diagonal (forming a truss). The latter aimed at increasing the damping properties and stiffness of the structure, and was first adopted in the Severn Bridge in the United Kingdom in 1966, followed by two large suspension bridges in Europe. But it was abandoned afterwards due to the fatigue injuries in the Severn Bridge.

The spiral or strand ropes have been widely used on the hangers of suspension bridges. Exceptions are the LCRs as in the Little Belt Bridge in Denmark and the PWS in the latest Japanese suspension bridges (Akashi and Kurushima). The hangers are connected to the main cable either by pin or by laying around the cable.

26.3.3.4 Main Towers

To meet the condition that the longitudinal movement of the main cable at the tower top should be theoretically allowed in a suspension bridge, towers are classified into rigid (saddle on rollers at the tower top), rocking (hinged at the tower base), and flexible types. Nowadays, the rocking type and flexible type are used in short-span and long-span bridges, respectively. In the case of the intermediate tower(s) of a multispan suspension bridge, the main cables may be fixed by anchoring them to the top of the rigid tower. The tower height above the deck is governed by the sag of the main cable.

Except the A-shaped tower used on some self-anchored suspension bridges or pedestrian bridges, the main towers consist of basically two vertical shafts connected to each other with horizontal struts at the top and underneath the bridge deck. Between these struts and between the lower strut and tower base, two shafts are often connected by additional strut(s) (portal-type tower) or diagonal members (braced-type tower). The vertical shafts can be slightly inclined. The member axis of inclined shafts typically coincides with the center line of the main cable at the tower top.

The main towers of suspension bridges may be made of either steel or concrete. Although almost all stiffening girders are steel structures, reinforced concrete towers have been prevalent in the recent European and Chinese bridges. Since the flexible towers are subject to both compression and bending, strength as well as elastic stability analysis shall be carefully conducted.

The steel tower shaft has a closed section consisting of stiffened plates. The outline of the cross-section is a rectangular, T-, or cross-shape. The latter two shapes are composed of multicells. The tower base where axial force, lateral force, and bending moment are transmitted into the foundation is either grillage (bearing transmission type) or embedded type (shear transmission type). On the other hand, the shaft of concrete towers has a rectangular hollow section. Since the main towers of a suspension bridge is outstanding and symbolic, esthetic consideration should be taken into account in its configuration.

26.3.3.5 Stiffening Girder (Bridge Deck)

The role of the stiffening girder is not only to give stiffness to the entire structure but to distribute the loads applied after the completion and transfer them to the main cables through hangers. However, since the stiffness provided by cable tension is quite large in long-span suspension bridges and almost all the dead load is sustained by the main cables, the rigidity of the stiffening girder may be not so large even if the span length is long. On the other hand, because the torsional rigidity of the stiffening girder is significant to ensure the flutter stability of a suspension bridge under wind, it is intended to constitute closed box effects structurally even if the girder is a trussed structure, by providing with both upper and lower lateral bracings. The ratio of fundamental natural frequencies in torsion and in bending is recommended to be 2.0 or higher.

Although short-span bridges are often provided with I-girders, the stiffening girder of a long-span suspension bridge is usually either a truss girder or a box girder. Its design is mainly governed by the function such as traffic requirement, the necessary stiffness, and aerodynamic stability, as well as the site conditions of erection and maintenance. Selecting an aerodynamically stable cross-section has usually priority. Use of either streamlined box section or the section with openings such as a truss of small solidity factor and steel open-grating floor will meet this requirement. Three typical examples of the stiffening girder are shown in Figure 26.11. Although a truss girder has been used on double-deck

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