Civil Engineering Reference
In-Depth Information
drawings must make clear the areas of the deck where materials may be stored,
and the construction loads assumed;
a tolerance on the weight of the deck, leading to an additional load of 2.5 per
cent of the self weight on the side of the leading segment;
the overturning effect of the vertical component of normal construction wind
load, including any differential effects on the two arms of the cantilever to
allow for gusting.
•
•
b) In normal construction, nominally in balance:
construction load as above on the side of the trailing segment;
a tolerance of 2.5 per cent on the weight of the deck on the side of the trailing
segment;
the traveller on the trailing side moved forwards;
the overturning effect of normal construction wind as above.
•
•
•
•
c) Disaster scenario:
the complete loss of a segment on either side;
the dynamic effects of this loss. The loss of a segment by failure of the
supporting shutter during casting is unlikely to be an impulse and an upwards
dynamic force of 25 per cent of the segment weight is suggested. The holding-
down bars of the traveller should be designed with suffi cient redundancy to
make it inconceivable that it should fall with the segment;
the 2.5 per cent tolerance on weight as above;
construction load as above;
the overturning effect of the maximum credible wind.
•
•
•
•
•
For cases (a) and (b), the falsework providing stability should be working at normal
stresses or load factors, with up to 25 per cent overstress allowed on foundations (but
not on falsework) for the effect of wind. Any tension on bored foundation piles should
not exceed their weight, suitably corrected for the presence of water, and suitably
reinforced.
For a deck prestressed down to the pier, all temporary bearings should remain
compressed. However, it must also be checked that there is no danger of the deck
slewing under the effect of wind, which could be a risk if the two temporary bearings
on one side are close together and the temporary bearings on the other side are barely
compressed.
For load case (c), the falsework, column and foundations must be checked at the
ULS for overturning and for slewing, with load factors of 1.4 on wind, and 1.5 on the
weight of the traveller and on construction loads.
When the deck is built into the piers, the load cases (a) and (c) must be considered;
load case (b) is generally not relevant.
The weight of the travellers should be estimated with caution as the majority
consists of formwork, access platforms and other 'secondary' items which are often
inadequately allowed for early in the project.
15.4.5 Building the end spans
End spans should ideally be little more than half the length of the main span. Thus
when the balanced cantilever is complete, the end span traveller is at the face of
