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Hammersmith Flyover
Great West Road (A4), Hammersmith, London, UK
associated engineer
G Maunsell & Partners
date  1959 - 1962
era  Modern  |  category  Road Viaduct  |  reference  TQ233784
ICE reference number  HEW 2263
Hammersmith Flyover carries the A4 trunk road and was built to cope with increasing traffic in central London and travelling to Heathrow Airport. It was the first UK structure to use precast segmental post-tensioned concrete, and because London County Council saved money over a conventional structure they paid Maunsell & Partners an ex-gratia fee for the design.
The need for an additional traffic route through Hammersmith was recognised in the late 19th century but its alignment was not finalised until 1935. In 1948 there was further discussion over the relative merits of a tunnel or viaduct.
The Ministry of Transport decided to award a grant towards the cost of a flyover in 1956, perhaps influenced by the constraints of maintaining traffic flow and the cost of demolishing property for road widening. New dual carriageways at the west end of the proposed viaduct (known as the Cromwell Road extension, completed 1956) had doubled the number of vehicles using the route between July 1956 and July 1960.
Hammersmith Flyover overflies part of the Great West Road, Fulham Palace Road, London Underground (District and Piccadilly lines), Talgarth Road and three other roads. The contract also included widening the existing railway overbridge, modifying existing services and constructing a pedestrian subway at Fulham Palace Road.
The engineering designers Guy Anson Maunsell and Charles Peter Wroth chose to use precast reinforced concrete elements wherever possible, combined with the then unusual technique of post-tensioning with steel tendons. Being able to precast most of the units offsite (at Heston 14km away) was a huge advantage in a site so congested and cramped, especially as traffic had to flow unimpeded throughout the construction period.
Construction began at the west end and proceeded eastwards. Erection of precast units and post-tensioning were carried out during off-peak hours, but all other operations continued in live traffic. On site working space was limited to just an 8.5m wide strip. The beam and cantilever units were placed by a steel gantry (designed by F.S. Jackson for the contractor) driven by an electric motor that ran on rails over the beam units already in position. One span could be completed every three weeks.
The flyover is supported on a single central row of columns. It has 16 spans — 11 at 42.7m long, two of 36.6m, two of 28.8m and one of 22.6m, giving a suspended length of 622.7m. The approach ramps at either end increase the total length to 862.9m. There is only one expansion joint, 64m west of the flyover's halfway point, which can accommodate movement of up to 360mm.
The deck is 18.6m wide overall, with a 7.3m wide carriageway and a service footway on either side of the 1.5m wide median strip. The carriageways have a crossfall of 1 in 40 towards the median strip, which carries the lamp standards and access manholes. The deck has a minimum clearance of 5m over all the road crossings.
The main structural element is a continuous hollow spine beam 7.9m wide, which varies in depth from 2m at midspan to 2.75m at the supports. It has curved undersides and was precast in units 2.6m long alternating with 300mm thick transverse cantilever units, with fine concrete joints 76mm thick between. There are 202 units each of beams and cantilevers.
These units are post-tensioned together longitudinally with 29mm diameter steel cables, using the Gifford-Udall system and post-tensioning each span individually on completion. There are four clusters of 16 tendons, one cluster on either side of each of the two inner spines — passing through 250mm diameter ducts within the lower flange midspan. The central compartment of the beam carries water mains, drainage pipes and electric cables for street lighting and road heating.
The 18m wide heavily reinforced concrete cantilever units act as diaphragms for the spine beams as well as supporting the 200mm thick precast reinforced concrete slabs 4.4m wide and 2.9m long that form the outlying parts of the carriageways. The slabs are joined with in situ concrete and the roadway was waterproofed with a 19mm layer of mastic asphalt. Continuous in situ reinforced concrete edge beams 1.1m wide and 0.45m deep, flanked by safety fencing, form the footways.
The tapering concrete columns were cast in situ in a single pour and are connected to the spine beam by vertical prestressing bars. They have a hollow rectangular section that varies from 3m by 2.6m at the top to 2.6m by 2.1m at the bottom with walls 600mm thick and 4.9m tall. Each column has 24 Lee-McCall high tensile steel tendons 32mm in diameter, prestressed from the top and passing through ducts inside the column walls with anchorages cast into the column bases.
A pair of steel roller bearings each 200mm in diameter and 800mm long at the base of every column accommodates any longitudinal movement in the flyover. The columns are founded on reinforced concrete footings bearing on the Thames ballast (sand and gravel) underlying the site. Foundation depth varies, with those adjacent to the railway cutting being taken to the same level as its mass concrete retaining walls.
The approach ramps comprise compacted fill between reinforced concrete retaining walls, with reinforced concrete roadway slabs laid over the fill. The ramps have a gradient of 1 in 20 and join the abutments some 4.6m above ground level. The abutments are cellular reinforced concrete structures that anchor the ends of the flyover and prevent longitudinal movement.
The flyover was completed in 1962 and cost about £1.2m. It is now under the care of Transport for London. The original waterproofing was replaced in August 2003.
Architect: Hubert Bennett, London County Council
London County Council engineer: Joseph Rawlinson
Contractor: Marples, Ridgeway & Partners Ltd
Roller bearings: Fritz Kreutz of Dusseldorf
Research: ECPK
"Hammersmith Flyover" in The Engineer,
Vol.207, pp.230, London, February 1959
"The Hammersmith Flyover", London County Council,
Cement & Concrete Association, London, October 1960
"This Flyover Has Structural Elegance" in Engineering,
Vol.191, pp.246-247, London, February 1961
"Obituary: Guy Anson Maunsell” in ICE Proceedings,
Vol.22, Issue 3, pp.347-348, London, July 1962
reference sources   CEH Lond

Hammersmith Flyover