Bigsweir Bridge
Carries the A466 between Monmouth and Chepstow across the River Wye, close to Llandogo and St. Briavels.
Occasionally referred to as Bigg's Weir Bridge.
This is an impressive example of an early 19th century cast iron arch bridge, in largely original condition, refurbished in 2010-2011. It is one of a small number of cast iron arch bridges built in the UK before 1830 that having a span of more than 150 ft (Bigsweir bridge has a span of 164 ft). There are thought to have been fourteen bridges in this category, of which six survive.
Designed by Charles Hollis.
1824 'BRIDGE BUILDERS.— A General Meeting of the Trustees of the proposed New Road from St. Awans to Redbrook, will be held at the Squirrel Inn, St. Awans, near Chepstow, Friday the 6th of August, at Seven o’Clock, when persons inclined to Contract for Building a Stone, or Stone and Iron Bridge, over the River Wye, at Biggswear, are requested to attend. Tenders to be sent, sealed, to Mr. R. W. Purchas, Grange, near Chepstow, on or before the of 5th of August next, where plans and specifications may be seen, and any further information may be obtained, on application at the office Mr. C. Hollis, architect, Fenchurch-street. Security will be required for the due performance of the Contract.'[1]
First stone laid in August 1825[2]
Bridge opened in 1827.
J. G. James named Bough and Smith of London as the ironfounders, based on a 'local guidebook', but this seems unlikely, as they appear to be contractors rather than ironfounders. J. G. James described it as unusual, based on the combination of an old-fashioned high-rise deck with modern-looking N-braced spandrels.[3]. A source dated 1854[4], state that the castings came from from Merthyr Tydfil, without identifying the maker. Assuming that a large ironworks was involved, the prime candidates at Merthyr Tydfil were Plymouth Ironworks, Dowlais, Cyfarthfa and Penydarren. At least two of these are known to have made early cast iron bridges.
Charles Hollis succeeded in producing a design which was bold, economical in its use of iron, and proved durable over time (the extent of repair work evidently being quite limited).
The principles of construction are not particularly unusual, but there are details which are unique to this bridge. The basic elements are typical of large bridges of that era, as established by Thomas Telford and William Hazledine, having cast iron arch ribs assembled from a number of segments, the ends of which are bolted to transverse plates; cast iron deck, wrought iron balustrade, and slender cast iron ligaments as spandrels (spandrels being the structures occupying the space between the deck and the arch ribs). Bigsweir Bridge is distinctive for the thinness of its components in relation to its overall size.
Mavesyn Ridware Bridge probably provides the nearest comparison in terms of constructional details. Both have a minimum of superfluous ornamentation, and both have I-beam arch ribs, whose ends are bolted to pierced transverse plates, relatively simple spandrel castings, and diagonal bracings of cruciform section. However, the use of diagonal bracing is more extensive at Bigsweir. Mavesyn Ridware makes greater use of threaded fasteners, and they are more substantial than at Bigsweir. In particular, the bolted connections between the spandrel castings and the arch ribs are minimal at Bigsweir. Bigsweir's arch ribs are 30" deep at the springing, with flanges 5.5" wide and webs approximately 1.5" thick. It appears that I-sction arches were first used on large cast iron bridges by James Walker for Vauxhall Bridge
An example of a pierced transverse plate sandwiched between the flanges of rib segments can be seen nearest the camera in Figs 5, 6 & 7. It extends the full width of the bridge, and adds transverse and torsional stiffness to the arch ribs. These full-width plates are augmented by short plates located midway along each arch rib segment. These are not sandwiched between the ends of the segments, their ends having lugs to bolt to the webs of the ribs (Fig 5). Unlike the full-width plates, the castings incorporate stiffening flanges. One of these plates has been broken, possibly by floating debris.
The quality of the cast iron appears to be good, but aspects of the foundry moulding are curious. On the one hand, the foundry succeeded in producing numerous large castings which had to be dimensionally accurate; on the other hand, the quality of the patternmaking and moulding are certainly not up to the high standards typically seen on cast iron bridges of this era, some of the castings having a relatively rough appearance. However, the standard of fitters' work is worthy of praise: in the absence of large planing machines, a great deal of skilled chipping and filing would have been required, for example where the ends of the arch segments mate with each other and with the transverse plates, and where the diagonal braces are located and bedded in their seatings.
On aspect which made the fitters' work particularly difficult was the designer's requirement to make the outer arch ribs appear as single castings, despite the fact that each is made of nine individual castings. As mentioned, the ends butt up against transverse plates, as seen in Fig 7. They largely succeeded - only in one case is the joint very obvious (Fig 3). Great efforts were also made to disguise the fact that the spandrels between the deck and the arch ribs are assembled from a number of separate castings. They succeeded, to the extent that it is not certain how many individual castings there are, or how they are connected to each other or to the arch ribs. Their connection to the deck plates, using small bolts, can be seen in Fig 12.
The ends of the vertical and diagonal ribs of the spandrel castings terminate in a small 'arch' (Figs 2 & 3). These arched portions appear to embrace semi-circular lugs which are presumably cast integral with the arch rib castings. The lower webs of the spandrel castings are not bedded to the top of the arch ribs: there are gaps which were presumably filled with rust-cement or lead.
Details
Fig 7: Note the chiselled figure 'I3' on the web of the I-beam casting, presumably to facilitate on-site assembly. Also note the two round bars passing through the web. It is not clear what they add to the structure.
Fig 11: Note the chiselled figure 'A2' in the diagonal tie bar (centre of photo). On the underside of the arch ribs' I-beam flanges, to the right of centre, the junction between two beam segments and the transverse plate can be clearly seen. To the left there appears to be another juction, but in fact this seems to be a feature of the casting, perhaps relecting indifferent patternmaking.
Figs 12-14: The main castings of the bridge are relatively thin, and structural stiffness has been added by extensive bracing- diagonal bracing between the arch ribs, and lighter diagonal and horizontal bracing between the spandrel castings.
See also here for excellent photographs of the bridge.
See Also
Sources of Information
- ↑ London Courier and Evening Gazette, 28 July 1824
- ↑ Dorset County Chronicle - Thursday 11 August 1825
- ↑ 'Some Steps in the Evolution of Early Iron Arched Bridge Designs' by J. G. James, Newcomen Society, presented at the Science Museum, 11 May 1988
- ↑ [1] 'Wanderings and excursions in South Wales, with the scenery of the River Wye' by Thomas Roscoe, 1854