Civil Engineering Reference
In-Depth Information
Figure 15.41
Launch bearings and pier head
15.8.10 The sliding bearings
The sliding bearings consist of concrete blocks covered with a polished stainless steel
sheet. They have lead-in and exit tapers at each end. To allow the deck to move over
them, pads consisting of two layers of rubber laminated with a central steel sheet,
bonded to a PTFE lower surface are fed between the deck and the rear end of the
bearings, Figure 15.41. The pads are collected as they are expelled from the front of
the bearing and re-fed in at the rear. The bearings and rubber pads should be designed
to work at 12 MPa under the nominal deck reaction, giving a stress of 18 MPa under
the enhanced reaction described in
15.8.6
. The bearings should be as short as possible,
as small deck rotations have to be accommodated, particularly at the front and rear
of the launch. Making the bearings too wide, however, may increase the bending
moments on the webs.
15.8.11 The launching jacks
The deck is usually moved forwards by one of two systems. In the fi rst, two hollow ram
jacks reacting against the abutment pull on cables that pass beneath the casting area
and are anchored to the newly cast deck. Two jacks are used so that when one retracts,
the other may hold the deck. Alternatively, a lifting jack located on the abutment reacts
against the bridge deck through a sledge device that is equipped with the pushing
jack. Again two devices working in tandem are normally used. Benaim designed the
launching methods for the approach spans either side of the 450 m span cable-stayed
Kap Shui Mun Bridge in Hong Kong, with a different contractor for each side. Although
the two structures were identical, the two contractors adopted rival launching systems.
There cannot be much to choose between them! The concrete approach spans used the
fi rst sections of the steel main span as a launching nose, Figure 15.42.
The jacks must have suffi cient force to overcome the sliding friction added to any
gradient effects. If the deck is launched downhill, it is essential to have hold-back
systems. The friction experienced during launching is very variable. Generally it can
be expected to be of the order of 2.5 per cent on the slide bearings and 10 per cent or
more in the casting area. However the jacks should be designed for an average friction
