2511 Bearings Expansion Joints and Railings

25.12.1 Introduction

Aside from the main components such as the girders or the floor structure, some other parts such as bearings (shoes), expansion joints, guard railings, drainage paths, lighting, and sound-proofing walls also make up the structure of a bridge. Each plays a minor part but provides an essential function. Drains flush rain water off and wash away dust. Guard railings and lights add to the esthetic quality of the design as well as providing their obvious original functions. A sound-proofing wall may take away from the beauty of the structure but might be required by law in urban areas to isolate the sound of traffic from the surrounding residents. In the following section, bearings, expansion joints, and guard railings are discussed.

25.11.2 Bearings

Bearings (shoes) support the superstructure (the main girders, trusses, or arches) and transmit the loads to the substructure (abutments or piers). The bearings connect the upper and lower structures and carry the whole weight of the superstructure. The bearings are designed to resist these reaction forces by providing support conditions that are fixed or hinged. The hinged bearings may be movable or immovable; that is, horizontal movement is restrained or unrestrained (horizontal reaction is produced or not). The amount of the horizontal movement is determined by calculating the elongation due to a temperature change.

During the 1995 Kobe earthquake in Japan, many bearings were found to have sustained extensive damage due to stress concentrations, which are the weak spots along the bridge. The bearings may play the role of a fuse to keep damage from occurring at vital sections of the bridge, but the risk of the superstructure falling down goes up. The girder-to-girder or girder-to-abutment connections prevent the girders from collapsing during strong earthquakes.

Many types of bearings are available and some are shown in Figure 25.54 and briefly explained in the following:

1. Line bearings. The contacting line between the upper plate and the bottom round surface provides rotational capability as well as sliding. These are used in small bridges.

2. Plate bearings. The bearing plate has plane surface on the top side which allows sliding and a spherical surface on the bottom allowing rotation. The plate is placed between the upper and lower shoes.

3. Hinged bearings (pin bearings). A pin is inserted between the upper and lower shoes allowing rotation but no translation in the longitudinal direction.

4. Roller bearings. Lateral translation is unrestrained by using single or multiple rollers for hinged bearings or spherical bearings.

Side plate Upper shoe

Side plate Upper shoe

Anchor bolt Lower shoe

Anchor bolt Lower shoe

Iii, va

Bearing surface

Bearing surface

v Lower shoe

Side

Upper shoe Wock

Side

Upper shoe Wock

Seal ring

Bearing plate

Upper

Seal ring

Bearing plate v Lower shoe

(i) Surface bearing (ii) Point bearing

Upper

Lower shoe

(i) Shear type

Lower shoe

(ii) Bearing type

(i) Shear type

(ii) Bearing type

- Upper shoe 'Pin

- Lower shoe

FIGURE 25.54 Types of bearings: (a) line bearing; (b) plate bearing; (c) hinged bearing; (d) multiple roller bearing; (e) single roller bearing; (f) spherical bearing; and (g) pendel bearing (JASBC 1984).

Anchor base -

5. Spherical bearings (pivot bearings). Convex and concave spherical surfaces allow rotation in all directions and no lateral movement. The two types are: a point contact for large differences in the radii of each sphere and a surface contact for small differences in their radii.

6. Pendel bearings. An eye bar connects the superstructure and the substructure by a pin at each end. Longitudinal movement is permitted by inclining the eye bar; therefore, the distance of the pins at ends should be properly determined. These are used to provide a negative reaction in cable-stayed bridges. There is no resistance in transverse direction.

7. Wind bearings. This type of bearings provides transverse resistance for wind and is often used with pendel bearings.

8. Elastomeric bearings. The flexibility of elastomeric or lead rubber bearings is that they allow both rotation and horizontal movement. Figure 25.55 explains a principle of rubber-layered bearings by

FIGURE 25.55 Properties of elastomeric bearings (Bridgestone Co.).

comparing with a unit rubber. A layered rubber is stiff, unlike a unit rubber, for vertical compression since the steel plates placed between the rubber restrain the vertical deformation of the rubber, but flexible for horizontal shear force like a unit rubber. The flexibility absorbs horizontal seismic energy and are ideally suited to resist earthquake actions. After the disaster of the 1995 Kobe Earthquake in Japan the elastomeric rubber bearings are becoming more and more popular, but whether they effectively sustain severe vertical actions without damage is not certified.

9. Seismic isolation bearings. Many different types of seismic isolation bearings are available, such as elastomeric isolators and sliding isolators. When installed on the bridge piers and abutments, the isolation bearings serve both vertical bearing devices for gravity loads and lateral isolation devices for seismic load. The basic purpose of isolation devices is to change the fundamental mode of vibration so that the structure is subjected to lower earthquake force. However, reduction in force may be accompanied by an increase in displacement demand that shall be accommodated within the isolation system and any adjacent structures.

Cutter slit Pavement

Slab.

30-50 mm

. Waterproof slit Stopper

30-50 mm

20 mm

Girder

. Waterproof slit Stopper

0 0

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