Civil Engineering Reference
In-Depth Information
Expansion Joint
Bridge Deck
Abutment
Elastomeric
Bearing
Viscous Damper
Figure 6.2. A viscous damper and elastomeric bearing connecting a girder bridge
to its abutment.
let us pause for a minute to explain that some bearings have ordinary uses
unconnected to extreme events. Even without regard to earthquake threat,
engineers must install bearings between the superstructure and the sub-
structure. In a girder bridge, the bearings transmit the weight of the bridge
span and vehicle loads to the column, which in turn transmits it to the
ground. Some bearings help reduce traffic vibration and allow expansion or
contraction from heat variations.
Now let us return to earthquake regions: here the bearing has to take
on additional duty. During an earthquake, it must structurally isolate the
girder from the abutment on which it rests. This is called a
seismic isolation
bearing.
Imagine the bearing as a box connecting the underside of the girder
to the abutment. In one example, called an elastomeric bearing, the box is
flexible or rubbery. As the ground moves northward, it deforms the bearing,
stretching it in the northward direction. The acceleration of the girder is
reduced accordingly. The bridge is then partly isolated from the impact.
Alternatively, the bearing can consist of two plates, one atop the other,
separated by steel rollers that allow sliding. The upper plate is connected
to the girder and the lower one to the abutment. When the ground moves
northward, the upper plate will not move as much as the lower plate. The
rollers reduce the bridge's impact against the abutment.
