Railway Structures
Pre-stressed Concrete
| The major advance that greatly expanded the utility of concrete,
was the development of pre-stressed concrete. This has steel reinforcement in
the form of wires that are stressed, so they are in tension as the concrete
hardens. The resulting force keeps the concrete in compression, overcoming its
tensile weakness. The technique was developed between the wars, principally by
the French engineer, Freyssinet, and enabled significantly longer and stronger
concrete beams to be used. Following experiments at Southall in 1938, the Pre
Stressed Concrete Company started manufacture of beams at Tallington,
Lincolnshire. This was the first production of pre-stressed concrete beams in
Britain and the Tallington plant, now owned by Tarmac Concrete Products Ltd,
remains in production to this day. Since 1942 its output has included
pre-stressed concrete railway sleepers.
Following the outbreak of war a significant number of pre-stressed concrete bridge beams were made at Tallington for use in repairing bomb-damaged structures. Exactly how many beams were manufactured and where they were used is not recorded. The majority appear to have been used for post-war reconstruction. The first pre-stressed concrete bridge to carry a railway, was the Adam Viaduct, near Wigan Wallgate, built by the LMS and the Pre Stressed Concrete Co in 1946.
When the Kingston Bypass was built between 1924 and 1927 the reinforced concrete bridge over the railway at Hinchley Wood required two spans. Columns between the tracks support the concrete beams. When the bridge was widened using pre-stressed concrete beams, only a single span was needed. This was possible because of the greater strength of pre-stressed concrete and particularly its ability to resist the tension in the lower edge of the beams.
Elmers End Road bridge (left) is a typical example of a bridge reconstructed with a deck of pre-stressed concrete beams. The parapet is of concrete, in order to comply with modern containment standards, but has been faced in brick. The underside of Station Road bridge at Otford (right) clearly shows the pre-stressed concrete beams.
Skew Bridge, between Salisbury and Wilton, was widened in 2004, by adding pre-stressed concrete beams either side of the original brick arch. Pre-stressed concrete underbridges are less usual than overbridges.
Trinity Road bridge takes the Reading line over a dual carriageway at Wandsworth Town. It was constructed by jacking a series of rectangular cross-section concrete tubes through the railway embankment. These formed the abutments and the centre pier, each comprising three separately jacked sections. The bridge deck could be built off those, after which the remaining embankment material was removed. The span over each carriageway is 15 metres wide and comprises 33 pre-stressed concrete beams, plus the parapet beams. |
All photographs are copyright
return to the Railway Structures menu
return to picture gallery page
This page was created 9 January 2010
