Civil Engineering Reference
In-Depth Information
Figure 15.43
Forces on pier head
which is likely to be similar to the launching load, in the absence of gradient effects.
However for fl exible piers with pinned bearings, or piers with laminated rubber
bearings, the launching load is likely to be the higher than the service load.
15.8.13 The pier head
The pier head must be designed to accommodate the jacks necessary to lift the deck in
the event of a jammed rubber pad. As they may be required at any stage of the launch,
these jacks must be situated close to the axis of the web. If the launch bearings have
been set off centre on the pier as described above, it is often possible to accommodate
a single jack per web in front of and in line with the launch bearings, Figure 15.41.
Alternatively, two smaller jacks may be used, fore and aft of the launch bearings.
Accommodating these jacks often leads the pier head to be elongated, which may look
rather inelegant. When it is not desirable to increase the size of the pier head in this
way, the jacks may be carried by falsework, either resting on the pier foundation, or
stressed to the pier head. However, it is not economical to provide falsework for every
pier for an eventuality that may not occur. Consequently, this falsework needs to be
designed so that it may be erected quickly in the event of need, minimising delays to
the construction programme.
The pier head must also be designed to accommodate the jacks required to transfer
from the launch bearings to the permanent bearings. As the deck generally includes
diaphragms at pier positions, these latter jacks may be situated in-board of the permanent
bearings. These jacking positions may subsequently be used for maintenance of the
permanent bearings. When the pier consists of two columns, the transfer jacks may be
accommodated on falsework.
Every third pier is equipped with guides to avoid the possibility of the deck drifting
sideways during the launch.
15.8.14 Launching procedure
As each segment is completed, it is attached to the previous segment by the fi rst stage
prestress and the whole deck is launched forwards, freeing the casting area.
