Civil Engineering Reference
In-Depth Information
of friction coeffi cients between the surfaces in contact. On the East Moors Viaduct
for instance, it was found that the coeffi cient of friction between the top plate of a
few of the bearings and the mortar skim was inadequate. Consequently, a pattern of
grooves was machined into the plate to enhance this friction, avoiding the need for a
mechanical connection to the segment.
When a mechanical link between segment and bearing is required, the connecting
bolts must be sized to carry only the residual horizontal force to be transmitted between
the deck and the bearing, once friction has been deducted. The simplest method of
making the connection is to equip the bearing with an additional top plate to which
it is bolted. The segment is placed onto the accurately levelled bearing with an epoxy
resin bonding it to this top plate. However, this is an expensive way of solving the
problem due to the presence of the additional plate which must be thick enough to
accommodate the bolts and must be well protected against corrosion as it cannot be
changed.
Alternatively, a light steel frame to which are welded threaded anchorage sockets
for the bearing bolts is placed on the soffi t form for each of the bearing downstands in
the casting cell, and cast into the segment. As the pier segment is removed to storage,
a template is made up recording accurately the relative position of the anchorage
sockets in both plinths. This has been done by placing a plank of timber across the
two downstand plinths and hammering it so that the position of the bolt-holes is
imprinted, but other methods may be devised.
This template is used to locate the bearings on the pier head; the dowels anchoring
the bearing to the pier head may then be cast in. When the segment is presented, it
can be bolted directly to the bearings. This procedure is very quick as the precise
orientation of the pier segment has been predetermined, but requires careful working
and supervision.
A further option is to cast sockets into the bottom of the segments as described
above. The dowels anchoring the bearings to the pier are then left in empty pockets
in the pier head, Figure 15.20. When the pier segment is presented, it is placed on its
temporary supports and the bearings are bolted up to it. It is then orientated precisely,
and the pockets on the pier head containing the bearing dowels are grouted up. This
method is less demanding but slower.
15.5.3 Temporary prestress for cantilever construction
The resin used for jointing segments has an open time typically of 45 minutes. As it
is not possible to install the permanent Stage 1 prestress within this time, each new
segment must be attached to the cantilever by temporary prestress bars that can be
installed in a matter of minutes.
A minimum of four bars should be used to provide the redundancy that allows
one bar to fail without loss of the segment. The bars used for temporary stressing
should be reasonably ductile. Generally the grade adopted should not be stronger than
835/1,030, these being respectively the 0.1 per cent proof stress and characteristic
ultimate strengths in MPa. The Macalloy bar of this grade has a minimum elongation
at rupture of 6 per cent.
Temporary prestress may be anchored on internal blisters beneath the top slab,
cast together with the segment, Figure 15.21. This is certainly the cheapest form of
anchorage, but may become complicated if there are also top anchor blisters for the
