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London Bridge (1972)
River Thames, City of London, UK
London Bridge (1972)
associated engineer
Mott Hay & Anderson
date  November 1967 - 1972, opened 16th March 1973
UK era  Modern  |  category  Bridge  |  reference  TQ328805
ICE reference number  HEW 261
photo  London Metropolitan Archives
The three slim spans of prestressed concrete we call London Bridge today are the descendants of a venerable family of bridges on this site, stretching back nearly two millennia to the founding of London by the Romans. The modern bridge's hollow box girders are ideal for carrying essential infrastructure. The bridge is in constant use and good condition.
The Romans chose this spot for its favourable ground conditions on the north bank and because this was the tidal limit of the Thames at that time. They built the first of a series of timber bridges that served the city until the first stone bridge was constructed 1176-1209 — Old London Bridge, one of the sights of Europe, with its tall houses and shops. It was built a little upstream (west) of its timber predecessor.
Old London Bridge was the only bridge on the Thames for 500 years and Bridge House Estate, which controlled it, grew rich on the rents, tolls and property bequests that came its way. Old London Bridge was replaced in 1831 by a five-arched masonry bridge designed by John Rennie (1761-1821), paid for largely by Bridge House Estate from its reserves. London Bridge (1831) was sited a little further upstream again.
However, by the late 1950s, the Rennie bridge was judged incapable of coping with modern traffic volumes. It was also sinking into the river bed at 3mm per year. The City of London Corporation decided to replace it with a new bridge on the same alignment and the London Bridge Act was passed in 1967 (royal assent 16th February) to enable this to happen.
Mott Hay & Anderson was appointed engineer for the project, with William Holford & Partners as architectural adviser. On 5th October 1967, contractor John Mowlem & Co Ltd was awarded the £4,066,000 contract for a three-span bridge. Work commenced on 6th November 1967.
The 1972 bridge follows the line of Rennie’s structure, fitting in with the infrastructure and buildings around it. The team faced the challenge of erecting it while simultaneously demolishing the old bridge, keeping open two waterways (each 30m wide) and maintaining access for vehicles and pedestrians. Only a limited site area was available at any one time.
The contract specified that the arches of the old bridge had to be demolished evenly to avoid overstressing the piers and foundations. This was carried out while working in spring tidal flows of about 9.3km per hour (5 knots) — increased by temporary obstructions during the works. An overhead gantry truss was erected, weighing 1,900 tonnes and running the full length of the bridge. It supported the arches during demolition, and later used to hoist precast sections.
The new bridge was constructed as four longitudinal sections, joined together once all were complete. The outer sections were constructed first, one after the other, either side of the old structure, which continued to carry traffic. Then the traffic was diverted to the new sections while Rennie's bridge was demolished and the centre sections constructed.
Each section consists of a three-span superstructure 'rib supported on shaft foundations at the river piers and abutments. Sheet piled cofferdams were driven, and shafts sunk below the old bridge and hand dug some 25m into the river bed. The shafts are lined with mild steel rings, the diameter of which steps out with depth from 3.7m to 8.1m, and backfilled with concrete. Adjacent shafts touch at base level (below river bed) and are connected with reinforced concrete beam structures above river bed level.
The piers and south abutment are supported on a row of four adjacent shafts. However, a pair of London Underground Northern Line tunnels run within 4.6m of the north abutment foundations, preventing shaft construction on the east side. Consequently, the north abutment is founded on only two shafts, joined by a full width deep beam and raft arrangement. The presence of the train tunnels, and a second pair of tunnels upstream, restricted the methods permitted for pile driving, shaft sinking and explosives use.
The bridge’s two slender piers are supported on the cross beams capping the shaft groups. They are of mass concrete faced in granite with recessed joints. The abutment walls are similarly faced above their foundations.
The spanning elements comprise four twin-celled parallel box beams made up of precast reinforced concrete segments. Precasting was used to minimise the amount of on-site work. The casting was done 6.4km downstream at Russia Dock in Surrey Commercial Docks, Rotherhithe, and the segments brought to site by barge.
Gaps between the units, lifted and positioned by the gantry, were filled with in situ concrete. The completed box girder ribs were then prestressed progressively. To do this, steel strands known as tendons were threaded through and along the precast box segments and attached to anchorages at each end, where the tensioning was applied. Internal (flange) and external (web) tendons were used. Pipework and cabling for essential services crosses the River Thames inside the hollow ribs.
The four ribs were also stitched together using in situ concrete, longitudinally and by a series of transverse diaphragms. These were intended to carry concentrated loads from an overhead walkway (not built). Circular holes through the diaphragms may have been meant to carry additional services.
Though the bridge appears continuous, expansion joints indicate it is not. There are two expansion joints in the central span, two more at the south abutment and one at the north abutment. The bridge is actually two large double cantilevers supporting a short central suspended span.
Settlement was measured during construction and found to be less than anticipated, with virtually no movement at the abutments. The south pier moved a little more than the north pier. In 1976, it was calculated the "ultimate settlement is likely to be of the order of 90-130mm".
The bridge is 262.1m long between abutments. Its decoratively sand-blasted spandrels are clad in polished granite panels topped by granite block parapets with stainless steel handrails. The 32m wide deck carries six lanes of traffic (now two vehicle lanes and one bus lane in each direction). The two walkways, totalling 12.5m of its width, were constructed with integral heating elements to prevent ice formation (no longer functioning).
Centre to centre, the north and south spans measure 79.2m each and the middle span 182.9m, with maximum clearance above high water of 9m in the centre. The roadway over the three spans follows a 3.8km radius vertical curve, over the north abutment it is a 1.6km radius vertical curve and over the south abutment, a 1 in 28.9 gradient.
Modern London Bridge was completed in 1972, and opened officially on 16th March 1973 by HM Queen Elizabeth II. A plaque on the bridge commemorates the occasion.
In 2007, the quality of the bridge’s construction was praised and the prestressing system judged to be "performing well". It was noted that there was "no sign of distress at the stitch joints, which is something of an achievement for in-situ work". Some years earlier, it also passed the loading assessment for carrying 44 tonne vehicles "with flying colours".
And what of the bridge it replaced? In 1967, Rennie’s London Bridge (1831) was sold to American entrepreneur Robert Paxton McCulloch (1911-77) of McCulloch Properties, Inc. for $2,460,000. The pieces were shipped to Long Beach, California. The re-assembled bridge, supported by a reinforced concrete skeleton, was rededicated on 10th October 1971.
Architectural advisor: William Holford & Partners
Contractor: John Mowlem & Co
Research: ECPK
bibliography
"London Bridge: Demolition and Construction, 1967-73" by P.F. Mead, Proceedings of the Institution of Civil Engineers, Vol.54, pp.47-69, London, February 1973
"Foundation settlements at London Bridge" by A.G. Simpson, Ground Engineering, pp.17-19, London, October 1976
http://oldlondonbridge.com
http://onthethames.net
www.bristol.ac.uk
www.buildingtalk.com
www.cityoflondon.gov.uk
www.ice.org
www.londonremembers.com
reference sources   CEH Lond
Location

London Bridge (1972)