50 Years Ago
The Irwell Valley Bridge is a single
span road bridge, over 200 ft long,
comprising seven trapezoidal box
girders. Each girder was fabricated
at the works in four parts. This
is a good example of steel prefabrication
involved in the shop fabrication
are well illustrated in the accompanying
Each girder was fabricated
so that when finally positioned on
30 NSC
and the various processes
photographs and drawings.
Nov/Dec 18
site it had a camber of 21/8 in. The
two middle sections of the girder
are each 45 ft long and weigh
36½ tons, whilst the two outer
sections with their skew ends
are each 64 ft long and weigh 46
tons. The box girders are 10 ft 10
in deep, 12 ft wide at the top and
8 ft at the bottom. The web plates
are ½ in thick throughout but the
flange thicknesses vary as follows:
the middle sections have top flanges
1 in thick and bottom flanges of
2 in plate; the corresponding thicknesses
for the end sections are ¾ in
and 15/8 in. The flanges for the end
sections are made from two plates
butt welded together but the middle
section flanges are in single
plates. The bottom flanges in all
cases are of NDII steel to BS.2762.
Material preparation and
assembly
All web and flange plates were
flame cut to width and bevelled
as required, in one pass, on a specially
constructed double-headed
burning bench. Indicators were
Reprinted from Volume 5 No. 3
July 1968
incorporated to enable the plates
to be marked to length. Longitudinal
and transverse stiffeners,
6 in bulb flats and 7 in by 3½ in
by 7/16 in angles respectively, were
sub-assembled to webs and flanges
in assembly jigs.
All flange stiffeners were welded
in with 5/16 in fillet welds on both
sides and web stiffeners were similarly
attached with 3/16 in fillet
welds. Two welders worked simultaneously,
one on each side of a
stiffener, working from the centre
outwards, to ensure that all the
welds were balanced and to avoid
distortion. After sub-assembly, all
parts were blast cleaned and given
one coat of etch primer.
Final assembly
For the final assembly of each
section, a former jig was used to
keep the webs and flanges firmly
in position and at the same time
kept the ends in the correct relationship.
As a further aid to assembly,
holes were drilled in the
transverse angle stiffeners so that
the webs and flanges could be tack
bolted together: the holes were
filled with galvanized bolts before
the inside of the box was painted.
To ensure that on site the ends
matched, an inside former frame
was also used to maintain the correct
shape and overall dimensions
of the ends of the boxes. The assembled
box was strongly tacked
before being transferred to the
manipulator jig for final welding.
Weld details were modified at the
corners of the boxes to give a more
balanced weld and to allow the
two corners to be welded whilst
Fabricating the
Irwell Valley Bridge