Civil Engineering Reference
In-Depth Information
7.4.4 Cast-in items
Although bearings are designed to be replaceable, some items such as dowels are cast
permanently into the concrete, and in some installations upper or lower plates may also
be cast in. Many bearings on box section bridges are protected by long overhanging
side cantilevers when there is little chance of moisture being present regularly. In other
situations, where deck expansion joints may allow water onto the bearing shelf, where
bearings are not protected from rain by side cantilevers, in situations where the bearing
would be subject to salt-laden spray from the sea, or from an adjacent carriageway, or
if it is impossible to repaint regularly, stainless steel should be employed for such cast-
in items. Electrolytic corrosion will occur at the interface between carbon steel and
stainless steel in the presence of water. If water will be present, the two steels should
be separated by suitable washers.
7.5 Elastomeric bearings
These consist of natural rubber or Neoprene slabs vulcanised between steel plates,
Figure 7.2. They cater for rotation by differential compression across the width of the
slabs, and for displacement by shearing of the slabs. The bearing can be designed to
suit the particular bridge by providing the appropriate plan area to carry the load, an
adequate total thickness of elastomer to cater for the displacement, and by assembling
the appropriate number of layers of a specifi ed thickness to cater for the required
rotation. Rubber bearings are regularly made in sizes up to 700 mm square, when they
may carry loads up to 8 MN. Such bearings may be assembled in series to carry greater
loads.
The allowable working compressive stress of the bearings lies generally in the
range 12-17 MPa with bearings of larger plan area working at the higher stresses.
The allowable shear displacement of a bearing is approximately 70 per cent of the
thickness of rubber. However, the design of these bearings is complex, as the capacity
Figure 7.2
Elastomeric bearing (Image: CCL Stressing Systems Ltd)
