'BRUNEL'S SWIVEL BRIDGE', CUMBERLAND BASIN, BRISTOL
INTERIM REPORT 01
Author: David Greenfield Prepared: April 2006
INDEX
Page
1 Introduction 1
2 Summary of main findings 2
3 Design and construction of the 'swivel bridge' - 1849-1850 2
4 Post-construction 3
5 Site inspection 4
6 Discussion [to be completed] 5
7 Conclusions [to be completed] 5
8 Notes and references [to be completed] 5
1 INTRODUCTION
At the first meeting of the Brunel Swivel Bridge Group on 15 March 2006, the author agreed to research the history and form of construction of the bridge, and to assess its historical engineering significance ('heritage value'). A preliminary study of easily-accessible secondary printed sources highlighted a couple of areas of particular concern. Firstly, Angus Buchanan, the highly respected authority on the history and development of Bristol Docks, has concluded that there is some uncertainty whether the bridge alongside north lock is in fact 'Brunel's Swivel Bridge'. Secondly, previous assessments of the bridge's engineering significance have been founded on its reputed, but unproven, early use of plate girders with tubular flanges, and their uniqueness of survival.
Consequently, the objectives of the current phase of research are primarily:
1 to authenticate the bridge as a Brunel-designed structure, and
2 to put the application of tubular flanges into historical context.
The purposes of this interim report are:
1 to present current evidence from documentary sources and site inspections;
2 to identify and discuss issues arising from the evidence; and
3 to highlight gaps in evidence and knowledge.
2 SUMMARY OF MAIN FINDINGS
2.1 In early 1849 I.K. Brunel's Westminster office prepared designs for a 'Swivel Bridge', to carry traffic over the South Entrance Lock which was then under construction. The bridge was completed and operational in October 1849.
2.2 Brunel himself was closely involved in both the bridge design and its subsequent execution.
2.3 Comparisons with the original drawings suggest strongly that the extant bridge now fixed parallel to the North Entrance Lock is the original 'Swivel Bridge', albeit shortened and with minor alterations and repairs, having been relocated from the South Lock.
2.4 The use of tubular-flanged wrought iron girders, such as were utilized here, was in its infancy. Initial research indicates it is highly likely that the bridge is the earliest and oldest-surviving example of such a Brunel-designed structure.
2.5 Initial research suggests that the use of longitudinal pre-stressing 'tie bars' running through the top flanges, as used here, was an innovation.
2.6 Further research is necessary to confirm these findings.
3 DESIGN AND CONSTRUCTION OF THE 'SWIVEL BRIDGE', 1849-1850
In 1844 the Bristol Dock Company commissioned I.K. Brunel to design a replacement for William Jessop's South Entrance Lock, between Cumberland Basin and the river. The main contract was awarded to one Rennie. He sub-contracted the iron-work to George Hennet, who owned substantial engineering and foundry works in Bristol and Bridgwater. The lock was to be spanned by a movable bridge. By 1849, when the lock construction had advanced to the stage where fabrication of the bridge must begin, the Dock Company and all its works had been acquired by the City of Bristol, who managed them through a Docks Committee. Brunel initially intended it to be a 'rolling bridge' but, because of foundation difficulties, in January 1849 he reported to the Committee that he now proposed a 'swing bridge'.
The bridge design was carried out in Brunel's office, 18 Duke Street, Westminster, and drawings and instructions were sent from there to William Bell, Brunel's 'assistant engineer' (resident engineer in all but name) at Bristol docks. The first rough tracing of the proposed 'Swivel Bridge' (so called from then on by Brunel's office) was sent down on 16 February 1849, so that the wrought-iron plates could be ordered. As the design and fabrication advanced, drawings were sent to amend, clarify and refine the details. A written specification (so far not located) was not produced until 3 April 1849, by which time there were only minor design details still to be decided.
The earliest drawing so far located is dated 10 March 1849, showing the plan and elevation of 'Swivel Bridge 76ft Opening'; a copy letter of the same date in Brunel's letterbook refers to this tracing having been sent to Bell that day. The overall length of the bridge as drawn is 121ft 9ins; 87ft 9ins main span plus 34ft for the 'overhanging end' or 'tail' (so called). This and subsequent drawings highlight two areas of particular interest in historical civil engineering terms:
1 the use of wrought iron plate girders with tubular flanges - the top flanges being cylindrical (or, more accurately, inverted pear-shaped), and the bottom triangular; and
2 the use of longitudinal pre-stressing 'tie' rods within the top flanges.
No drawings or correspondence have been found to suggest there were subsequent alterations in the leading dimensions and details.
Robert Brereton, Brunel's chief assistant, signed off the drawings and letters, and was probably responsible for the detailed design. However, it seems likely that Brunel closely monitored progress both in the design and its execution - for example, in April Brereton wrote to Bell that one of the detail drawings was ready and that 'as soon as Mr Brunel has looked at it, it shall be sent'. From time-to-time Brunel's opinions and preferences were forwarded to Bell for action. When it was near time to cast the main bearing wheels in April 1849, Brereton told Bell, 'Mr Brunel likes generally to see these things in the patterns [ie. before casting], you had better get Jones [probably Hennet's works manager] to ask him to look at them when he is in the yard.' In July Brunel himself wrote to Hennet, instructing him to make some alterations; presumably Bell was not authorised to give such instructions.
The bridge was sufficiently complete by 8 October 1849 for it to be swung open and shut. Following some final adjustments, on 29 October the Dock Master reported to the Committee that the bridge had been handed over to him, complete and fit for use. The following day Brereton instructed Bell not to apply the final coat of paint 'till Mr Brunel sees the bridge'. It is not known when Brunel made his inspection, but Brereton told Bell that he intended to make his own inspection during a proposed visit to Bristol on 10 December.
On 2 February 1850 Brunel's chief clerk, Joseph Bennett, sent Hennet's final account to the Docks Committee's secretary, Edward Hinton, for payment. Two days later Bennett wrote to Hinton asking what form of completion certificate the Dock Directors wished Brunel to provide. The construction phase was effectively complete.
4 POST-CONSTRUCTION
George Hennet died in 1857; his second son, Follett Charles, took over the Bridgwater foundry, which traded as Hennet & Spink, later as Hennet, Spink & Else. On 26 August 1863, the firm contracted with the Docks Committee to construct a new bridge over Jessop's North Entrance Lock. Essentially this was a shorter version of the original 'Swivel bridge'; according to the contract drawings it was about 78ft overall; 59ft plus 19ft. The drawings are signed on behalf of Hennet Spink & Else, and by 'Tho Howard, Engineer B. Docks' on behalf of the Committee; presumably Howard, the Docks Engineer from 1855 to 1882, designed the bridge.
Shortly after this, the inadequacies of the remaining Jessop Locks, and the threat of competition from Avonmouth Docks, forced the City Council to prepare plans for improving and enlarging shipping accommodation. Tenders were received in October 1866 for a series of measures designed by Howard, including replacements for the North and Junction Locks. The successful tenderer, William Tredwell, soon began work on a new Junction Lock. However, financial constraints delayed a start on the North Lock until 1868. The 1863 bridge was removed, and reputedly shortened and re-erected over the entrance to Bathurst Basin.
In July 1872 Howard reported that the new North Lock was virtually complete and expected to be operational in October 1872. In the event, he later reported that the lock was opened for traffic on 19 July 1873. Reputedly, during the dock works Brunel's 'Swivel Bridge' was removed from its original site over the South Lock, and was then shortened and re-erected over the new North Lock. So far no original documents have been studied in an attempt to authenticate this, except for an undated drawing titled 'Brunel's Bridge - Proposed Shortening'. This shows preliminary details of how the nose of the bridge could be shortened by 10ft. No drawing has been found that shows how the tail could be shortened.
The Docks Committee had been intending to close the South Lock (by then known as 'Brunel's Lock') and converting it to a public graving dock. However, in November 1873 the Masters of Steam Vessels petitioned the Committee to keep Brunel's Lock open as a second means of entrance and exit. It was not until June 1874 that the Committee directed Howard to prepare estimates for repairing the lock and providing a new bridge over it. Having given an estimate for the bridge of £4,000, he was instructed to prepare plans.
There is a set of undated drawings which show the overall length as 106ft 6in; 76ft 6ins plus 30ft. The bridge is effectively another shortened version of the original 'Swivel Bridge'. Another set of undated drawings, un-dimensioned but otherwise similar to the above, have been signed contractually on behalf of Edward Finch & Co Ltd, of Bridge Works, Chepstow, and by 'Thos Howard, Engineer to the Corporation'. In July 1875 Howard reported that the new bridge was just about to be delivered and erected, and in August 1876 he was able to report that the bridge was complete.
At some point the bridge over the North Lock was converted from manual to hydraulic operation. It was taken out of service in 1968, and swung parallel to the lock. The South Lock bridge still spans across the lock, but has been fixed.
5 SITE INSPECTION
Both the extant bridges have been compared to the relevant drawings, and to each other.
5.1 'Brunel's Swivel Bridge' at the North Lock.
This bridge measures (by pacing) approximately 107ft overall; 77ft plus 30ft. These dimensions would accord with a shortening of about 10ft in the main span, and 4ft in the tail. It is apparent that at some time during its service life the southern top flange over a considerable length was cut out and replaced by a cylindrical steel section of roughly the same diameter; presumably this was a repair after impact damage. Otherwise, the details of the remaining structure appear to agree very well with the drawings of 1849.
5.2 The 'Replica Bridge' over the South Lock.
This measures approximately 94ft overall; 78ft plus 16ft. These dimension would accord with a tail shortening of about 14ft, and it is apparent that the tail section which incorporated the counterweights has in fact been removed. Otherwise, the structure appears to agree very well with the undated drawings signed by Finch and Howard.
5.3 Comparison of the extant bridges
There are several significant differences between the extant bridges:
5.3.1 The web stiffeners on the northern bridge comprise a flat plate, whereas on the southern bridge they appear to comprise a form of bulb-flat; in both cases they are riveted to the web via angles. Unfortunately, the images currently available of the relevant drawings are not clear enough to be able to confirm these details.
5.3.2 That there are longitudinal 'tie bars' within the top tubular flanges of the extant northern bridge is clearly apparent; several holes at 'key' position in the flanges enable the bars to be seen. There are no recognisable signs on the outside of the top flanges of the extant southern bridge to indicate that there are 'tie bars' within.
6 DISCUSSION [to be completed]
Of particular significance in identifying which, if either, of the two bridges is 'Brunel's Swivel Bridge' is the striking similarity between the extant structures and the bridges portrayed in some of the original drawings, allowing for the fact that both bridges have been altered. As noted above,
7 CONCLUSIONS [to be completed]
8 NOTES AND REFERENCES [to be completed]
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