The bridge from the eastern (Bitterne Manor) bank
|Carries||4 lanes (road)|
|Locale||Northam, Bitterne Manor (both in Southampton)|
|Maintained by||Southampton City Council|
|Total length||148 metres (485 ft 7 in)|
|Width||13.5 metres (44 ft 3 in)|
|Longest span||32 metres (105 ft 0 in)|
|Number of spans||5|
|Piers in water||4|
|Clearance below||9.2 meters (30 ft 2.2 in)|
|Construction begin||1796 (original);
|Construction end||1799 (original);
|Preceded by||St Denys Railway Bridge|
|Followed by||Itchen Bridge|
The Northam Bridge is a road bridge across the River Itchen in Southampton, England, linking the suburbs of Northam and Bitterne Manor. The current bridge was the first major prestressed concrete road bridge to be built in the United Kingdom. The bridge carries the A3024 road as a dual carriageway, with two lanes on each carriageway.
Prior to the construction of the Northam Bridge, the southernmost bridge across the River Itchen was at Mansbridge. Mansbridge was the lowest crossing point of the river until the early 18th century, when the Itchen Ferry began operating between Woolston and St Mary's, downriver of Northam.
The Northam Bridge was the idea of David Lance, who acquired land in Bitterne and built Chessel House there in 1796. Realising that access to his land was poor, he encouraged the building of a bridge linking Bitterne Manor to Northam, together with roads from the bridge to Botley and a further bridge over the River Hamble in Bursledon (and onwards to Portsmouth), with the fork between the Bursledon and Botley roads passing close to Chessel House. The Northam Bridge Company was formed in 1796, funded mainly by Portsmouth businessmen.
The new route between Portsmouth and Southampton would be four miles (6 km) shorter than travelling via Mansbridge, and as a result the proposal to improve transport between the two important port cities was keenly supported by the Admiralty, especially since this was the time of the Napoleonic Wars. Consequently when the Northam Bridge Company sought an Act of Parliament to build a bridge, the Act was passed quickly.
The new roads and bridges were built in 1799, and were originally operated as toll roads. The first Northam Bridge was of wooden construction. The wooden bridge was replaced in 1889 by an iron bridge at a cost of £9,000.
The bridge remained a toll bridge until 1929 when the ownership was transferred from the private sector to the Southampton Corporation. It was this change of ownership that allowed the first bus route across the River Itchen to be established in Southampton; Southampton Corporation decided against extending the existing tram lines across the bridge, opting instead to establish a double-decker bus service.
The iron bridge was replaced in 1954 with a third bridge, made of prestressed concrete, and it is this bridge that still stands today. The third Northam Bridge was the first major prestressed concrete road bridge to be built in the UK and cost £600,000. However this figure included the compulsory purchase of land and about 2,000 feet (610 m) of embankment construction as well as the bridge construction itself.
Construction and dimensions
The parapets of the first (wooden) bridge were 24 feet (7.3 m) apart, as were those of its wrought-iron successor.
The third bridge utilised the latest technology available at the time but the style of the bridge was of the pre-war era. The main deck structure has transverse diaphragms and narrowly spaced beams, which were pre-cast on site using deflected cables. Pre-cast, pre-stressed slabs, known as junction slabs or continuity slabs, were placed between the tops of the beams by transverse stressing over a length where the flanges of the tees were removed. These, together with in situ diaphragms between the ends of the beams, allowed the deck structure to be made continuous for live and superimposed loads. After the junction slabs were in place, the main beams were post-tensioned through the diaphragms.
The cement used to make the concrete in the bridge was Ordinary Portland Cement, which was both cheaper and resulted in less shrinkage than using rapid-hardening cement. It was used in a ratio of 1:11⁄2:3 - a mix which used more cement than German and British practice at the time - and a water-to-cement ratio of 0.3.
At mid-span, the bridge is 44 feet 4 inches (13.51 m) wide, 4.7 metres above mean high water springs and 9.2 metres above chart datum. The bridge is 148 metres long in total, and the supporting piers are up to 32 metres apart.
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