Context 139 - May 2015

C O N T E X T 1 3 9 : M A Y 2 0 1 5 15 appropriate way forward. However, when the bridge had been refurbished previously changes had been made to the original Dredge 1836 design which diminished the extent of original remaining material. There was also a clear requirement that the bridge should function to modern structural design codes, ensuring that the structure would be safe in use and require minimal future maintenance. Agreement was reached that many of the later replace- ment elements, such as the bridge deck, which were seen to have less heritage significance, would be replaced with new fabrications to replicate the existing design. They would be uprated to meet current design codes. Original elements such as tie bars, bosses, parapets and chain links would be retained where possible. Listing building consent was gained in August 2013 for the works. Balfour Beatty was appointed as main contractor to undertake the project works, with its specialist metalwork contractor Centregreat Engineering. Dismantling of the bridge began in February 2014, following a full documented survey. The first sections of the bridge were taken to the contractor’s workshop in Cardiff for component dismantling and assessment. The temporary works that had previously been installed supported the existing structure during this time. Each dismantled component was individually tagged with a reference number and a photo record in a specialist database loaded on to mini iPads.This digital record was updated with an assessment and treatment information as each component was refurbished. On completion this database would provide an accurate record of each component that was removed from the structure, becoming a vital part of the historic environment record. Each component was abrasively cleaned to SA2.5 (a Swedish standard for abrasive cleaning) to remove paint layers and corrosion products, and then assessed for defects. The wrought-iron chain links were seen as a very significant part of the original bridge structure. Dredge had designed the chains with a reducing number of links towards the centre of the bridge, dramatically reducing the overall weight of the bridge structure compared with that of his competitors. The design of the refurbished bridge allowed for new mild-steel chain links to be fitted to the inner less visible positions, with the original links being fitted to the outer positions of the chain. Each link was accurately measured to determine if it had been stretched in situ , and where acceptable was cleaned and further assessed to identify material defects and evidence of previous repairs. Following the application of a zinc-rich paint system, the links were fitted into sub-component assemblies with new inner links and pins ready for rebuilding on site. New bridge deck sections were fabricated to resemble the existing sections.Where possible sections such as the edge longitudinal beams were made in flat plate to match the original design intent rather than the later replacement 1 Additional defects in chains included splits within the wrought iron, deformation of the link and physical damage. 2 As it was dissembled, each component or series of components was labelled with a penny washer stamped with a unique ID. This tag and its connecting wire were sufficiently durable to withstand the abrasive cleaning and painting process, and remained tied to the component until reassembly. 3 Chain links following abrasive cleaning to SA2.5. Each element was inspected to identify potential defects. Here the link has been rewelded. It was rejected for re-use. 4 The cleaning of the links allowed additional information to be gained regarding the bridge structure, as a number of makers’ marks were found on the components. Approximately 10 per cent of the links were stamped with a crown and SC markings.These markings are noted in Rivington’s Notes on Building Construction (1915) as being the brand of Messrs Bradley of Stourbridge, SC Crown Iron. 5 Additional markings were found on other links which appeared to readVB Bristol Scrap.This stamp may have indicated that the iron was salvaged or scrap iron that had been recycled for use as new chain links used for repairs. 6 An original Dredge patentee plaque, fitted to the chain tower. 1 3 4 5 6 2