Mills Hill Bridge, Chadderton
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near Chadderton and Middleton, Lancashire
Also known as Bridge 69b, Scowcroft Farm Bridge, and locally as Th' Iron Dongle. The name Mills Hill Bridge is assigned to it in 'Britain's Historic Railway Buildings' by Gordon Biddle.
General
The attractive and historically important skewed iron bridge carried the Manchester and Leeds Railway between Manchester and Rochdale over the Rochdale Canal, adjacent to Lock 61 (Scowcroft).
Whoever was responsible for saving this bridge from demolition has done a great service to the memory of the designers, foundry pattern makers and moulders, fitters, erectors and stonemasons responsible for its construction.
According to one source, a girder bridge was added immediately alongside in the late 19thC to provide quadruple tracks. In the 1970s the line reverted to double tracks and the original bridge was taken out of use and had its deck removed.[2]. This is supported by an advertisement placed in 1903 by the L&YR for tenders for the construction of bridges, etc., for widening the line near Scowcroft Farm, between Middieton Junction and Castleton.[3]. However, the 25-inch O.S. map surveyed in 1915 (published 1922) shows only two tracks, on the west ('new') side of the bridge, with no tracks on the east side. The 1889-91 map shows only the old bridge. This suggests that the track was not quadrupled, but that the old bridge was taken out of service, and presumably saved for posterity.
Geograph entry here.
Note: 1 mile to the north, the same railway crosses the same canal, also at a considerable degree of skew, but this time on a masonry arch bridge. See Rochdale Canal, Bridge 68B.
Historical Information
Grade II listed (No. 1356431). Historic England listing here. For some reason, this wrongly gave the construction date as 1863. In fact, the line had been built over the Rochdale canal by 1839, and the design is consistent that of several similar bridges on railways built by George Stephenson and Robert Stephenson in the late 1830s (see below). The bridge was mentioned in September 1838, in an article about a trial run on part of the line: ' The Rochdale canal is next crossed, for the first time, about a quarter of a mile further, by a cast-iron trussed-beam bridge, with an arch of 75 feet span; the rails being about 20 feet above the surface of the water. '[4]. Further, in July 1839:-
From Mills hill the train started, after a halt of four minutes, at seventeen minutes before one o'clock. The line here crosses the valley of the Irk on an embankment of nearly sixty feet in height. The eye of the passenger is delighted in the progress by very interesting views of a well-cultivated and varied country; but it is soon attracted by a large iron bridge, by which the train crosses the Rochdale canal; it is a skew of 73 1/2 feet span, and was built by Messrs. Radfords and Co, to whose workmanship it does the highest credit.[5]
Thomas Longridge Gooch is credited as the designer.
Design and Construction Details of the Cast Iron Bridge
Many of the details of construction are shown in the photographs on this page. See also here (historicbridge.org website) for numerous photos showing details.
Overview
It is rare example of a cast iron bowstring (tied arch) railway bridge with a suspended deck, and is almost certainly the earliest surviving example of this type.
Each side of the bridge comprises a pair of cast iron girders, spaced several feet apart. These are joined to each other by transverse X-braces, bolted to the vertical ribs of the girders.
Each of the cast iron girders embodies a 'Tudor' arch as its principal member, and the 'string' of the 'bow' is a pair of tensioned rods for each girder. Each of these tensioned wrought iron rods comprises several round bars joined by iron spools with tapered cotters. Hammering in the tapered cotters would provide a degree of pre-tensioning. Decorative castings are attached to the bottom of the girders.
The transverse X-braces have a vertical central hole through which a wrought iron rod passes. From these rods are suspended fish-bellied cast iron cross beams which supported the deck. Details of construction of a somewhat similar bridge can be seen in drawings of Stephenson's 1837 bridge which carried the London & Birmingham Railway over Regent's Canal near Chalk Farm[6]. See Fig 2.
At some point the deck supports were augmented by the insertion of additional cross beams between the original cast iron beams. These additional beams are of riveted wrought iron or steel construction. They may have been added to increase carrying capacity, or because of concerns about relying on cast iron beams subjected to tension from bending.
Decorative Castings
It will be seen from Figs 4, 5 & 6 that the thin, decorative castings below the girder are missing from the right hand side. However, this 2013 photograph shows that the castings were still present on the RHS in 2013. The castings on the diagonally opposite location have been strapped on to secure them. Presumably the bolts are failing due to corrosion.
The decorative castings may have shielded the wrought iron tie bars from driving rain, but nevertheless they have suffered severs corrosion in places (fig. 5).
As decorative castings, they hid some of the functional features, and they also gave the bridge a more rigid appearance, an impression aided by the masonry corbels (Fig. 7) which ostensibly supported the castings. By covering the wrought iron tie bars, the castings would have tended to moderate the rate of change of temperature of the bars (which would heat and cool more quickly than the girders under the effects of ambient temperature and sunshine). However, it is doubtful that this benefit was either intentional or very significant).
The decorative castings are fine examples of the foundrymens' art, but they are brittle. The castings are complex and very thin, and the type of cast iron used was selected for its molten flow characteristics, and was inherently brittle. Figs 8 & 9 show the reverse side of some of these castings. Note the thin flanges used to bolt the sections together.
In Fig 6, the exposed ends of the decorative castings appear to be of appreciable thickness. In fact they are thin, and what we see are flanges which were used to attach the now missing castings.
Main Girders
As previously indicated, the arch ribs and the wrought iron tie bars serve to take the compressive and tensile strains respectively which arise from the self-weight and traffic loading. The arches get limited assistance from the top chords (where the tops of the small gothic arches join together), but these are less rigid than they appear: thin iron plates are bolted on to give a deceptive appaearance. Gaps behind the plates can be seen at the top in Fig 12. It is possible that these plates are austere replacements for decorative castings. Samuel Charles Brees provided detailed drawings of the bridge, which show ornate castings mounted on top of the girders' ribs [7]. Note: the corresponding plates on the somewhat similar Todmorden - Gauxholme No. 2 Viaduct have been removed.
Each girder is made in several pieces, joined by bolted flanges (Fig 10, for example). The joints are very discrete on the outside (Figs 11 & 12).
There is no bottom chord as such, the tie rods providing this service. Although the vertical ligaments coming down below the arch are joined by a horizontal bar, the bar is very slender, and is not continuous. It was used to attach the decorative castings. The vertical ligaments protruding below the arch do have a role - the X-bracings (stretchers) are bolted to them.
Clearly the girders must be reinforced at the ends where the wrought iron tie bars are fixed, but these ends are not accessible. Fig 13 shows part of the end of a girder, and it will be seen that the top chord is braced by a diagonal ligament. See here for a photograph of the corresponding location on Gauxholme No. 2 Viaduct.
X-Bracings (Stretchers)
These are used to join pairs of girders together, and also serve to attach the suspension rods for the deck beams. The stretcher castings are located by a tongue and groove arrangement and by bolting, using the raised pads on the girder castings seen in Fig. 14. The mating surfaces of these features needed to be accurately located and sized, and the requirement could have been readily met if the makers had a large planing machine. However it is by no means certain that the foundry would have possessed such a machine in 1838.
The wrought iron suspension rods are fixed in the stretchers and in the bosses of the cross beams using tapered cotters. These would also be used to adjust the tension in the rods.
Cross Beams
The cross beams originally supported longitudinal beams (probably wooden) supporting the rails via sleepers. As previously mentioned, the original cast iron beams were supplemented by riveted wrought iron or steel beams. Fig 16 shows the alternating arrangement of old and newer beams. Due to the large degree of skew of the bridge, the cross beams near the abutments were partly supported by the masonry (Figs 17 & 18), so additional beams did not need to be inserted here.
The ends of the additional horizontal beams turn upwards between the pairs of girders, and terminate in riveted crossheads which sit on top of the main girders to take the weight. Installation was not a simple(!) matter of floating the prefabricated beams below the bridge and lifting them into place, because the wrought iron tie bars were in the way. It is hardly likely that the tie bars would have been removed to facilitate the installation, as this would have precluded the passage of trains. Examination of the slots in these beams which accommodate the tie roads suggests that the beams were installed as at least five sections (one horizontal, two vertical, two elbows) and joined by fishplates riveted in situ. See Figs 6, 15 & 19.
The Adjacent Bridge
At some point a wrought iron or steel lattice girder bridge was installed immediately alongside. Some details of the structure can be seen in Figs 20 - 23.
Masonry Abutments
There is a small skewed arch in the northern abutment. The original arch is just visible in Fig 4, while Fig 24 shows the arch in the 'new' abutment.
Samuel Charles Brees provided drawings of the bridge, which show castellated turrets, and a cross carved in the stone face below the turret[8]. Reference to Figs 25 (south east corner) and 26 (north east corner) shows no sign of a cross, but there is some evidence to suggests that turrets had been included. The main evidence is limited to a gap in the dado and a faint curved 'U' mark in the stone, corresponding to a corbel in the sketch. Also, the plain rectangular cap stones seem out of keeping with the generally ornate detailing of the bridge. If turrets were present, they would probanly have to be removed on the west side to accommodate the new girders, so it is possible that those on the east side would be removed to maintain symmetry.
Bridges of Similar Construction
The larger Todmorden - Gauxholme No. 2 Viaduct is of similar construction, but has been much altered. This had a twin - part of Whiteleys Viaduct - but the bowstring girders were replaced in 1939 by steel plate girders. The High Level Bridge, Newcastle, the best-known example of Stephenson's bowstring suspended deck bridges, was built later, being opened in 1848.
J. G. James mentions several further examples of 'Stephenson's' cast iron bowstring bridges with suspended deck[9]:-
- A bridge over the Grand Junction Canal at Buckby Wharf, near Weedon, opened 1837, demolished 1933.
- A skew bridge carrying the Eastern Counties Railway over Regent's Canal, 1838-9, made by the Horseley Co. Chalk Farm bridge?
- North Midland Railway at Derby, 1839-40: not confirmed.
- Redbridge - small bridge over the Roding, 1841, demolished 1924. Photo here.
See Also
Sources of Information
- ↑ [1] 'Railway practice: A Collection of Working Plans and Practical Details of Construction in the Public Works of the Most Celebrated Engineers Comprising Tunnels and Tunnel Fronts, Turnpike Road Bridges...' by S. C. Brees, 1838, p.193
- ↑ 'Britain's Historic Railway Buildings' by Gordon Biddle, Oxford University Press, 2003
- ↑ Manchester Courier - Tuesday 13 October 1903
- ↑ Blackburn Standard - Wednesday 19 September 1838
- ↑ Leeds Mercury, 6 July 1839
- ↑ [2] 'Railway practice: A Collection of Working Plans and Practical Details of Construction in the Public Works of the Most Celebrated Engineers Comprising Tunnels and Tunnel Fronts, Turnpike Road Bridges...' by S. C. Brees, 1838
- ↑ [3] Second Series of Railway Practice by S. C. Rees, 1840, Plate 46
- ↑ [4] Second Series of Railway Practice by S. C. Rees, 1840
- ↑ 'The Evolution of Iron Bridge Trusses to 1850' by J. G. James, Newcomen Society & Institution of Civil Engineers, 14 Jan 1981