NSC 31
Sept 19
together to form 56ft long boxes complete with
cantilevers ready for lifting into the bridge.
The greatest care is taken in this assembly,
by fabricating each box against its preceding
and succeeding neighbours, to ensure that the
boxes, which weigh from 120 to 160 tons each,
will fit without difficulty when offered up at
the bridge head.
Fabrication of the V-Shaped stiffeners is
worthy of special mention. So far in Britain
it had been possible to produce such members
only by pressing and in lengths limited by the
size of the press. Because of the advantages
of continuous longitudinal stiffeners, however,
the steelwork fabricator has developed a
rolling process by which they can now be
produced in any desirable length. This
process involved the adaptation of machinery
previously used for producing similar profiles
but in lighter gauge material and of smaller
girth. The V-stiffeners for Erskine Bridge
are of ¼in thick high yield stress steel with a
depth of 9in and a girth of 24in.
Erection
Erection of the steelwork is so planned that
the centre will be reached from both ends
simultaneously. This meant starting on the
north abutment where the first two spans
were erected by specially designed gantry
cranes on falsework and welded up so that the
remainder of the boxes could be erected by the
cantilevering process.
Each succeeding box is lifted onto trolleys on
the second span and winched out to the bridge
head where specially designed launching
girders are used to lower it and then to
support it in position while it is welded to the
preceding section.
The launching girders, developed from those
used for the Wye Bridge, are then moved
forward ready to receive the next box.
Before the last section of any approach span
can be positioned, the cantilevered part has
to be jacked up with the aid of a temporary
prop to counteract the sag of about 4ft so that
the next section will pass over the top of the
supporting pier.
When the anchor and main spans are erected,
special steps will be taken because the box
is not strong enough to span the distances
concerned. In the 360ft long anchor span, a
temporary intermediate pier will be erected
to limit the effective span to about 240ft. In
the case of the main span, however, where
the deck girder has to cantilever out 500ft
to the centre, use will be made of permanent
cables. When the girder cantilevers about
190ft, the steel mast will be erected over
the main pier and some of the cable strands
will be attached in temporary positions to
support the steelwork while erection proceeds.
As soon as the bridge head has advanced
sufficiently (about 375ft from the main pier),
the remainder of the strands will be attached
in their final positions and the rest of the box
sections can be cantilevered out.
On closing the bridge, the temporary cable
strands will be fixed in their permanent
positions.
Elevation and plan
Typical cross section
A view of the launching girder
50 Years Ago