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The very longest bridges in the world, up to about 2000 m span, are of the suspension type illustrated inFig. 1.40. In suspension bridges, the main cables are in catenary and the deck hangs from them applying a substantially uniform loading. They are more expensive to construct than cable-stayed bridges as they are not particularly suited to staged construction and the initial placing of the cables in position is onerous. Further, it is sometimes difficult to cater for the horizontal forces generated at the ends of the cables. For these reasons, cable-stayed construction is generally favoured except for the very longest bridges.

1.5 Articulation

Bridge design is often a compromise between the maintenance implications of providing joints and bearings and the reduction in stresses which results from the accommodation of deck movements. While the present trend is to provide ever fewer joints and bearings, the problems of creep, shrinkage and thermal movement are still very real and no one form of construction is the best for all situations.

The articulation of a bridge is the scheme for accommodating movements due to creep, shrinkage and thermal effects while keeping the structure stable. While this clearly does not apply to bridges without joints or bearings, it is a necessary consideration for those which do.

Horizontal forces are caused by braking and traction of vehicles, wind and accidental impact forces from errant vehicles. Thus, the bridge must have the capacity to resist some relatively small forces while accommodating movements.

Fig. 1.40 Suspension bridge

In-situ concrete bridges are generally supported on a finite number of bearings. The bearings usually allow free rotation but may or may not allow horizontal translation. They are

generally of one of the following three types:

1. fixed—no horizontal translation allowed;

2. free sliding—fully free to move horizontally;

3. guided sliding—free to move horizontally in one direction only.

In many bridges, a combination of the three types of bearing is provided. Two of the simplest forms of articulation are illustrated in Figs.1.41(a)and(b)where the arrows indicate the direction in which movements are allowed. For both bridges, A is a fixed bearing allowing no horizontal movement. To make the structure stable in the horizontal plane, guided sliding bearings are provided at C and, in the case of the two-span bridge, also at E. These bearings are designed to resist horizontal forces such as the impact force due to an excessively high vehicle attempting to pass under the bridge. At the same time they accommodate longitudinal movements, such as those due to temperature changes. Free sliding bearings are provided elsewhere to accommodate transverse movements. When bridges are not very wide (less than about 5 m), it may be possible to articulate ignoring transverse movements such as illustrated in Fig. 1.41(c).

Fig. 1.41 Plan views showing articulation of typical bridges: (a) simply supported slab;; (b) two-­

span skewed slab;; (c) two-­span bridge of small width

When bridges are not straight in plan, the orientation of movements tends to radiate outwards from the fixed bearing. This can be seen in the simple example illustrated in Fig. 1.42(a).

Creep, shrinkage or thermal movement results in a predominantly longitudinal effect which causes AB to shorten by 1 to AB'. Similarly, BC shortens by 2to BC'. However, as B has moved to B', C' must move a corresponding distance to C . If the strain is the same in AB and BC, the net result is a movement along a line joining the fixed point, A to C. Further, the magnitude of the movement |CC |, is proportional to the radial distance from the fixed point,

|AC|. The orientation of bearings which accommodate this movement is illustrated inFig.

1.42(b). Similarly for the curved bridge illustrated in plan inFig. 1.42(c), the movements would be accommodated by the arrangement of bearings illustrated in Fig. 1.42(d).

Bearings are generally incapable of resisting an upward ‘uplift’ force. Further, if

unanticipated net uplift occurs, dust and other contaminants are likely to get into the bearing, considerably shortening its life. Uplift can occur at the acute corners of skewed bridges such as B and E inFig. 1.41(b). Uplift can also occur due to applied

Fig. 1.42 Plan views showing articulation of crooked and curved bridges: (a) movement of crooked bridge;; (b) articulation to accommodate movement;; (c) movement of curved bridge;; (d) articulation to accommodate movement

Fig. 1.43 Uplift of bearings due to traffic loading

Fig. 1.44 Uplift of bearing due to transverse bending caused by differential thermal effects

loading in right bridges if the span lengths are significantly different, as illustrated in Fig. 1.43.

However, even with no skew and typical span lengths, differential thermal effects can cause transverse bending which can result in uplift as illustrated inFig. 1.44. If this occurs, not only is there a risk of deterioration in the central bearing but, as it is not taking any load, the two outer bearings must be designed to resist all of the load which renders the central bearing redundant. Such a situation can be prevented by ensuring that the reaction at the central bearing due to permanent loading exceeds the uplift force due to temperature. If this is not possible, it is better to provide two bearings only.

1.6 Bearings

There are many types of bearings and the choice of which type to use depends on the forces and movements to be accommodated and on the maintenance implications. Only a limited number of the more commonly used types are described here. Further details of these and others are given by Lee (1994).