Civil Engineering Reference
In-Depth Information
of 5 per cent. Friction coeffi cients of less than 0.5 per cent have been measured and
holding-back systems are best designed on the assumption that friction is zero.
15.8.12 Effect of launching on the piers
As the bridge is launched, it applies a friction force to the top of the pier head, creating
an overturning moment on the pier and its foundation. The friction force should be
assumed to be 5 per cent of the deck reaction which is greatest on the leading pier. If
the deck is on a longitudinal gradient, the reaction between the deck and the pier will
be inclined to the vertical, giving a horizontal reaction on the pier head, Figure 15.43.
This will be in addition to the friction.
In order to reduce the overturning moment on a pier during the launch, the launch
bearings may be set behind the pier axis, providing a moment in the opposite direction
to the friction, Figure 15.41.
Occasionally the sliding bearings may jam, principally due to a rubber pad being
introduced upside down. If this is not spotted quickly, the pier could be severely
damaged by the power of the launching jacks. It is essential that there is a system that
stops the launch if the horizontal force at the head of any pier exceeds some pre-set
limit. This may be achieved by surveying the defl ection of each pier head during
the launch, or by measuring the distance between each pier head, so any anomalous
defl ection of a single pier is quickly identifi ed.
If the permanent deck bearings are sliding, the pier must be designed for a horizontal
load in service of approximately 5 per cent of the reaction due to self weight + fi nishes,
Figure 15.42
Kap Shui Mun Bridge, Hong Kong: launching of approach span (Photo: Benaim)
