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Nunn's Bridge, Fishtoft
Hobhole Drain, Cut End Road, Fishtoft, Lincolnshire, UK
Nunn's Bridge, Fishtoft
associated engineer
LG Mouchel & Partners
Gilbert Elliot Ernest Buchner
date  1947 - 1948
era  Modern  |  category  Bridge  |  reference  TF366415
ICE reference number  HEW 1743
photo  Tensioning in 1947, courtesy Andy Carrott, Engineering Manager, Witham Fourth District Internal Drainage Board
Nunnís Bridge carries a minor road across Hobhole Drain, 4.8km south east of Boston in Lincolnshire. Its unassuming appearance belies its importance as the first prestressed concrete bridge in Britain to be constructed in situ. It remains in good condition and still carries road traffic.
A three span brick bridge at this location was constructed around 1805. In 1940, the UK's military authorities constructed demolition chambers in the bridge structure as part of wartime defensive measures.
In January 1945, a widening horizontal crack some 12.8m long was noted in the south parapet wall at the east end of the bridge. A 50mm water main through the structure, close to a demolition chamber, had burst and water was seeping through the brick arch joints. The roadway over the eastern arch had sagged and there was up-thrust near the east abutment.
In March 1945, trial pit excavation at the east pier revealed its base had moved laterally 90mm towards the centre of the drainage channel. The Witham Fourth District Internal Drainage Board resolved to rebuild the bridge as a single span. On 19th August 1946, Holland County Council closed the bridge and demolition work commenced.
The present bridgeís pioneering design of post-tensioned concrete was developed by L.G. Mouchel & Partners, with prestressing design by Dr Karl Walter Mautner (1881-1949) of the Prestressed Concrete Company.
The bridge was built using direct labour. The drainage boardís engineer from 1938 to 1972, Gilbert Elliot Ernest Buchner (1907-75), supervised the construction and produced the initial design drawings. The two piers of the existing brick bridge were used as temporary formwork supports and demolished later.
The old brick abutments have been trimmed back at the top but remain in situ as part of the new work. Steel sheet piles 5.5m in length were driven in front of the brickwork to form a cut-off wall. Individual precast reinforced concrete piles, 305mm square in section and 8.9m long, were installed in front of the sheet piles, ten at each abutment. The toes of the concrete piles are some 5.6m below the foundations of the exiting abutments and the cut-off wall about 3.8m below.
Vertical reinforced concrete abutment walls rise above the piles to deck height, with a cutback to accommodate the ends of the deck superstructure. The space between the new concrete and the brickwork is filled with mass concrete and Ďplumsí (rocks or blocks of old concrete).
The concrete structure has a clear span of 21.9m and an overall deck width of 6.1m, with a 5.2m roadway. Its five main I-section beams are 1.1m deep and 380mm wide. Each is reinforced with 12 prestressing cables arranged in a complex pattern of catenaries.
The cables are post-tensioned and 32mm in overall diameter. Each one consists of 12 round 5mm high tensile wires, spaced around the circumference of a wire helix. The whole thing is then spirally wrapped and sheathed with a 127mm wide steel strip.
The main beams are interlinked by five equally spaced reinforced concrete stiffeners, and support the integral deck slab. The maximum total depth of stiffeners plus deck slab is 1.4m, and the stiffeners are 305mm thick at the abutments but 229mm thick elsewhere. The deck was surfaced with 76mm of tarmacadam. Hand railings consist of precast reinforced concrete posts with three rows of galvanised steel tube rails.
Where the concrete superstructure meets the abutments, steel bearings between the end stiffener and the abutment wall take up any movement. One end of the bridge has a roller bearing, the other a rocker bearing.
Nunnís Bridge was completed around the end of February 1948, and the final cost was estimated at about £9,800. Holland County Council (later Lincolnshire County Council) took ownership in about 1949.
At the time of the bridge's construction, Hobhole Drain's profile was changed so that it was some 9m wide at its level base, rising at a batter of 1 and 3 at the sides. In the 1980s, it was deepened by constructing reinforced concrete channels beneath the bridge. The location of the channels corresponds to the areas between the piers of the original 19th century structure.
The bridge has remained in sound condition and requires little maintenance. In the mid 1990s, it was subject to rigorous structural analysis and assessed as capable of carrying 40 tonne vehicles.
Contractor: direct labour
Research: ECPK
"Historic concrete: background to appraisal", eds James Sutherland, Dawn Humm and Mike Chrimes, Thomas Telford Ltd, London, 2001
Reconstruction of Nunnís Bridge, design drawings, 1946
Witham Fourth District Internal Drainage Board, selected minutes of committees, 1945 and 1946
Additional information kindly supplied by Andy Carrott, Engineering Manager of Witham Fourth District Internal Drainage Board (minutes), and Richard Waters, Principal Engineer at Lincolnshire County Council (bridge drawings)
reference sources   CEH E&C

Nunn's Bridge, Fishtoft