Southdown Road Skew Bridge
Encyclopedia
Southdown Road Skew Bridge is a ribbed skew arch
railway bridge, which carries the Midland Main Line
across Southdown Road in Harpenden
, Hertfordshire. Built of brick
by the Midland Railway
and opening in 1868, it is notable for its extreme skew angle of approximately 65°.
, the bridge was built to carry a double-track standard gauge
railway line across Southdown Road, which borders Harpenden Common in Hertfordshire
, as part of the Midland Railway's southern extension towards its London
terminus at St Pancras
, and opened to traffic in 1868. Owing to the highly acute angle at which the railway crosses the road the bridge was built as a ribbed arch, a variation of the "false" skew arch and a design championed by French civil engineer A. Boucher.
The bridge was widened in 1893 when the line was converted to quadruple track.
On 1 January 1923 ownership of the bridge, along with the rest of the line, passed to the London, Midland and Scottish Railway
and thence to the London Midland region of British Railways on nationalisation in 1948. It became a Grade II Listed structure in 1984.
Following privatisation
the bridge passed into the custodianship of Railtrack
and it is currently the responsibility of Network Rail
.
of bread
is sliced. The individual slices are then slid laterally with respect to one another in order to achieve the required oblique alignment. While the intrados of a "true" skew arch is smooth and cylindrical, the intrados of this type of "false" skew arch has a stepped appearance. Thus, the need to lay helical courses
of brick at such an extreme angle to the horizontal is avoided as the multitude of conjoined regular arches approximate the desired structure.
Because of the extreme skew angle the span of each "slice" (known as the span on the skew) is much greater than the perpendicular distance between the abutments (known as the span on the square), the latter being the usable span for road traffic passing under the bridge. Thus, despite the impressive looking span when viewed face on, the usable span along the axis of the barrel is less so, to the extent that the road is visibly narrower where it passes under the bridge. For a "true" skew arch the ratio of the span on the square to the span on the skew is equal to the cosine of the skew angle, but for a ribbed skew arch this ratio is made even less favourable due to the steps in the barrel wall.
Skew arch
A skew arch is a method of construction that enables an arch bridge to span an obstacle at some angle other than a right angle. This results in the faces of the arch not being perpendicular to its abutments and its plan view being a parallelogram, rather than the rectangle that is the plan view of...
railway bridge, which carries the Midland Main Line
Midland Main Line
The Midland Main Line is a major railway route in the United Kingdom, part of the British railway system.The present-day line links London St...
across Southdown Road in Harpenden
Harpenden
Harpenden is a town in Hertfordshire, England.The town's total population is just under 30,000.-Geography and administration:There are two civil parishes: Harpenden and Harpenden Rural....
, Hertfordshire. Built of brick
Brick
A brick is a block of ceramic material used in masonry construction, usually laid using various kinds of mortar. It has been regarded as one of the longest lasting and strongest building materials used throughout history.-History:...
by the Midland Railway
Midland Railway
The Midland Railway was a railway company in the United Kingdom from 1844 to 1922, when it became part of the London, Midland and Scottish Railway....
and opening in 1868, it is notable for its extreme skew angle of approximately 65°.
History
Located approximately 0.8 miles (1.3 km) south of Harpenden stationHarpenden railway station
Harpenden railway station serves the town of Harpenden, Hertfordshire, England. It is situated on the Midland Main Line. The station is managed by First Capital Connect and is served by its Thameslink route service....
, the bridge was built to carry a double-track standard gauge
Standard gauge
The standard gauge is a widely-used track gauge . Approximately 60% of the world's existing railway lines are built to this gauge...
railway line across Southdown Road, which borders Harpenden Common in Hertfordshire
Hertfordshire
Hertfordshire is a ceremonial and non-metropolitan county in the East region of England. The county town is Hertford.The county is one of the Home Counties and lies inland, bordered by Greater London , Buckinghamshire , Bedfordshire , Cambridgeshire and...
, as part of the Midland Railway's southern extension towards its London
London
London is the capital city of :England and the :United Kingdom, the largest metropolitan area in the United Kingdom, and the largest urban zone in the European Union by most measures. Located on the River Thames, London has been a major settlement for two millennia, its history going back to its...
terminus at St Pancras
St Pancras railway station
St Pancras railway station, also known as London St Pancras and since 2007 as St Pancras International, is a central London railway terminus celebrated for its Victorian architecture. The Grade I listed building stands on Euston Road in St Pancras, London Borough of Camden, between the...
, and opened to traffic in 1868. Owing to the highly acute angle at which the railway crosses the road the bridge was built as a ribbed arch, a variation of the "false" skew arch and a design championed by French civil engineer A. Boucher.
The bridge was widened in 1893 when the line was converted to quadruple track.
On 1 January 1923 ownership of the bridge, along with the rest of the line, passed to the London, Midland and Scottish Railway
London, Midland and Scottish Railway
The London Midland and Scottish Railway was a British railway company. It was formed on 1 January 1923 under the Railways Act of 1921, which required the grouping of over 120 separate railway companies into just four...
and thence to the London Midland region of British Railways on nationalisation in 1948. It became a Grade II Listed structure in 1984.
Following privatisation
Privatisation of British Rail
The privatisation of British Rail was set in motion when the Conservative government enacted, on 19 January 1993, the British Coal and British Rail Act 1993 . This enabled the relevant Secretary of State to issue directions to the relevant Board...
the bridge passed into the custodianship of Railtrack
Railtrack
Railtrack was a group of companies that owned the track, signalling, tunnels, bridges, level crossings and all but a handful of the stations of the British railway system from its formation in April 1994 until 2002...
and it is currently the responsibility of Network Rail
Network Rail
Network Rail is the government-created owner and operator of most of the rail infrastructure in Great Britain .; it is not responsible for railway infrastructure in Northern Ireland...
.
Design and construction
The easiest way to visualise Boucher's concept for the ribbed skew arch is to consider a regular arch bridge that carries the railway at right angles across the road and then to slice it vertically at regular intervals along the axis of its barrel, the planes all being parallel with the faces of the bridge, rather like the way a loafLoaf
A loaf is a shape, usually rounded or oblong, mass of food. It may refer to a bread, or meatloaf.The term "loaf" sometimes refers to "head" from the rhyming slang "loaf of bread" ....
of bread
Bread
Bread is a staple food prepared by cooking a dough of flour and water and often additional ingredients. Doughs are usually baked, but in some cuisines breads are steamed , fried , or baked on an unoiled frying pan . It may be leavened or unleavened...
is sliced. The individual slices are then slid laterally with respect to one another in order to achieve the required oblique alignment. While the intrados of a "true" skew arch is smooth and cylindrical, the intrados of this type of "false" skew arch has a stepped appearance. Thus, the need to lay helical courses
Course (architecture)
A course is a continuous horizontal layer of similarly-sized building material one unit high, usually in a wall. The term is almost always used in conjunction with unit masonry such as brick, cut stone, or concrete masonry units .-Styles:...
of brick at such an extreme angle to the horizontal is avoided as the multitude of conjoined regular arches approximate the desired structure.
Because of the extreme skew angle the span of each "slice" (known as the span on the skew) is much greater than the perpendicular distance between the abutments (known as the span on the square), the latter being the usable span for road traffic passing under the bridge. Thus, despite the impressive looking span when viewed face on, the usable span along the axis of the barrel is less so, to the extent that the road is visibly narrower where it passes under the bridge. For a "true" skew arch the ratio of the span on the square to the span on the skew is equal to the cosine of the skew angle, but for a ribbed skew arch this ratio is made even less favourable due to the steps in the barrel wall.
See also
- Hereford Road Skew BridgeHereford Road Skew BridgeHereford Road Skew Bridge is a disused railway bridge in Ledbury, Herefordshire. Built in 1881 to carry the Ledbury and Gloucester Railway across the Hereford Road at an angle of approximately 45°, it was built as a ribbed skew arch with stone spandrels and wing walls, and ribs of blue brick.The...
– a ribbed skew arch made from stone and brick. - Skew archSkew archA skew arch is a method of construction that enables an arch bridge to span an obstacle at some angle other than a right angle. This results in the faces of the arch not being perpendicular to its abutments and its plan view being a parallelogram, rather than the rectangle that is the plan view of...
– a discussion of ribbed and other forms of skew bridges.