Civil Engineering Reference
In-Depth Information
Each sliding bearing must be attended during the launch to insert the laminated
rubber pads at the back of the bearing and to collect them as they are ejected at the
front of the bearing. If the bridge is long this can require a considerable labour force,
and traditionally contractors mobilise their offi ce staff for this duty.
The hydraulic pressures of the launching jacks and thus the launching force must
be monitored continuously and compared with a predicted force. Any anomaly in the
force must be investigated and explained. For instance, if a rubber pad jammed on a
pier, it may be possible to register the increased force of the jacks before damage has
been done. However, if there are many piers, the increased force in the launching jacks
due to one jammed bearing may not be noticed before the pier has been damaged.
Consequently, this is only a back-up to the security system described in
15.8.12
.
If the deck is launched over a vertical curve, the launching force may rise to a
maximum and then reduce as the weight of the deck on the downhill side increases.
Clearly this will only happen if the downhill gradient is in excess of the friction
coeffi cient. However, as has been noted above, the design of the launching procedure
must take into account the possibility that the friction coeffi cient may be only marginally
above zero.
The launching nose is usually designed so that it may be dismantled in sections. This
avoids having to accommodate its full length in an excavation behind the abutment at
the end of the launch.
Once the deck is complete and in its fi nal longitudinal position, the second stage
prestress must be installed, and it must be jacked up pier by pier to install the permanent
bearings. Generally these operations may be carried out simultaneously.
15.8.15 Alternative launching method
An alternative launching method, patented by Bouygues, may be used when launching
up a steep gradient, where the design launching jack force using the traditional method
would be
W
×5 / 100 +
W
×
x
/100, where
W
is the weight of the deck and
x
the gradient
in percent.
In this method, horizontal roller bearings are placed on the abutment and on each
pier. Lifting jacks on each pier raise the deck by an amount that corresponds to
x
per
cent of the horizontal stroke. The rollers are pulled back to the beginning of their
travel, and placed into contact with the deck. The lifting jacks are released, transferring
the weight of the deck to the rollers and the launching jacks at the abutment push the
deck horizontally by one stroke. The lifting jacks then lift the deck off the rollers and
the cycle is repeated. The friction of roller bearings is more predictable than PTFE,
and is reported to be 0.5 per cent.
15.8.16 Second stage prestress
The second stage prestress may be internal or external. In any event, it is economical
to plan for long tendons. If the bridge is less than 150 m long, tendons from end to
end should be adopted. For longer bridges with internal prestress, tendons two or
three spans long are usually used. Such tendons are often anchored at pier diaphragms,
overlapping to provide continuity of effect. External tendons in excess of 400 m long
have been used successfully by Benaim on the Broadmeadow and Blackwater bridges
(
15.8.5
).
