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This is a 180 ft span girder bridge across the River Mersey in Warrington. It is of particular interest because it retains an unusual design of wrought iron box girder from the early 1850s.
It was erected in 1853-4 for the Warrington and Stockport Railway.. The designer was John Lister, and the ironwork was constructed by the nearby Bank Quay Foundry Co. The W&SR was later absorbed by the London North-Western Railway.
By 1908 the bridge was no longer suitable for the weight of locomotives needing to use it, and it was decided to strengthen it, and also to widen it to current loading gauge standards. This had to be done while maintaining traffic on the railway. The work was the responsibility of William Dawson, who presented a Paper on the subject to the I.C.E. in 1911..
The 1911 Paper also describes the design of the original bridge. There were three large wrought iron plate girders, 190 ft long between bearing centres, with a camber of 9". The central girder was of the box type, 16 ft 9" high, with an unusually wide botton flange (6 ft). The outer girders were of the single plate web type, 13 ft high. Walkways were provided outboard of the outer girders, supported on wrought iron cantilevered brackets. The railway lines were carried on 9" x 9" timber beams, spanning the full width over the girders and bolted to the bottom of the girders.
An interesting feature was the use of iron castings riveted to the top flange to accommodate compression loading. The castings were each 8 ft long, of semi-circular section with horizontal flanges for bolting to the wrought iron top flanges, with a wall thickness of 2.25". The castings had vertical flanges at each end, allowing them to be riveted or bolted together. The castings are visible on top of the central girder in the high resolution photograph here. See above for lower resolution version.
One of the options considered for strengthening the bridge was the provision of cast iron piers set in the river 120ft apart. Dawson had applied a similar concept when he strengthened the Conway railway bridge in 1899. There were various objections to this, including the fact that to some extent the top flange would be subject to tension. Another shortcoming is that it would not have addressed the narrow passages between the girders which represented a danger to track workers.
The method adopted for strengthening and widening involved replacing the two outer wrought iron girders by steel trusses of the Linville type. The original central box girder was retained, but its loading was partly transferred to the two outer girders by means of overhead lattice girders, in order to reduce the stresses in the bottom flange. This high resolution photo shows the arrangement of the girders. The iron castings on the top of the central box girder are visible.
Another Geograph photo and map here.
During the reconstruction work, traffic used one side of the bridge while on of the new 135-ton girders was assembled on the other side, using an overhead travelling crane running on rails at 10 ft gauge. It was assembled with the same large 9" camber to match the retained centre girder. On completion the new girder was temporarily slid across towards the centre girder to allow removal of the old outer girder.
To remove the old girder it was necessary to move it inboard to align with the existing railway tracks. This was a delicate operation due to the limited transverse stiffness of the girder (the top flange being only 2 ft wide). Removal involved supporting the western end on a low railway wagon, and then bringing a pair of barges under the eastern end, with suitable staging provided to lift the end as the tide rose. A locomotive then pulled the girder across. As the tide fell, support of the eastern end was transferred from the barges to another truck running on temporary trestles. Extra precautions were taken because of the lack of transverse stiffness of the old girder. These ate into the limited time for movement allowed by the tide. However, it was found the girder was much stiffer when supported on the barges, so the precautions were dispensed with on the second girder, and the crossing took just 8 minutes instead of 45.
The work also involved widening the abutments to accommodate the wider spacing of the girders. Also, before altering the second side of the bridge, it was necessary to raise that portion - 320 tons of old centre girder, old outer girder, floor, and rails - by 7", and slew the whole assembly across by 2 ft 7", without disconnecting the rails. The assembly was jacked up and lowered onto greased rails on the abutments, and the whole jacked across. This was done on a Sunday, between trains.