Construction
The crossing was built by a business consortium under a public-private partnership. A company called Severn River Crossing plc, led by John Laing plc and GTM-Entrepose, was formed to build the new crossing. The company also took over the responsibility of managing and maintaining the old Severn Bridge crossing, as well as managing and maintaining the new crossing. The cost of constructing the new crossing was expected to be paid for by tolls collected from motorists using the two crossings. Work on the new crossing began in 1992 with completion in 1996.
Sub-assemblies for the bridge were constructed onshore and then shifted by a large tracked vehicle (similar to that used to move the Apollo and Space Shuttle at Cape Kennedy) onto a barge (the SAR3), prior to being floated out on the high tide to the appropriate site. The 37 bridge pier foundations on the approach viaducts are 107.3 yards (98.1 m) apart, and consist of open concrete caissons weighing up to 2,000 tonnes, which were founded on the rock of the estuary bed. The decking consists of 3.8 yards (3.5 m) post stressed match cast sections, weighing 200 tonnes each.
The cable stayed section of the crossing is over 984 yards (899.8 m) long, consisting of a 38 yards (35 m) wide deck made from steel plate girders with a composite reinforced concrete slab. These were prefabricated on shore and put in place using balanced cantilever methods. There are two 489 feet (149 m) high twin leg, reinforced and pre-stressed concrete pylons carrying 240 cables which support the bridge deck. Cable vibrations were experienced during construction and secondary cables were added to eliminate this. To avoid detracting from the aesthetics of the primary cables, the secondary cables are very slender and are not very noticeable.
Read more about this topic: Second Severn Crossing
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