Bridge Title
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Name Severn Bridge, UK
Who  
Owner The UK Severn Bridges Act of 1992 enabled the concession for both Severn Bridges which is now managed by Severn Crossing plc.
Design Mott, Hay & Anderson, Freeman Fox & Partners
Superstructure Contractor Associated Bridge Builders Ltd which was a consortium of Sir William Arrol & Co Ltd, The Cleveland Bridge & Engineering Co Ltd and Dorman Long (Bridge & Engineering) Ltd
Substructure Contractor John Howard & Co Ltd
Where Over River Severn just north of Bristol, England, UK
Latitude N 51 36' 30"
Longitude W 02 38' 12"
Why Links England and Wales with the M48 motorway
When 1966
What Read more.....
How to read a bridge Read more about suspension bridges.....

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Pictures from David Blockley, UK







What  
Overall type Hanging - suspension bridge
Width 22.9 m
Length From west bank of the Wye to east bank of the Severn (so including the Wye cable stayed bridge) is 2,992 m.
Spans Main span: 987.6 m. with 2 side spans of 304.8 m. The east Aust end approach viaduct is 3 spans.
Towers 123.2 m. tall from top of piers. Each leg is a simple rectangular tube formed from 4 stiffened plates - the corner joints were arranged so no exterior staging was needed. The tower saddles at the very top were made from mild steel plate and bolted to the to the tower.
Cables Cross section about 0.5 m. consisting of 19 strands of 438 wires of 5 mm. diameter arranged in a hexagon (8322 wires in all). Hangers strands have 178 wires and are inclined to increase the damping of vibrations. Dampers were installed soon after erection to reduce oscillations. The reduction in weight reduced the overall stiffness of the bridge and so the sag to span ratio was reduced to 1/12 (compared to 1//11 for the Forth Road Bridge). Each cable is anchored into solid mass concrete block. The slope of the force from the cable is 11 - 52' above the horizontal. The two blocks at each end are connected by reinforced concrete walls so that they appear as one structure.
Deck
The first design was for a truss girder similar to but shallower than the Forth Road Bridge. A model was being tested in a wind tunnel when it broke loose and was destroyed.

As it was to take a long time to make a replacement some simple alternatives were tested in the mean time. It was from these that the final aerodynamic box girder design was developed.

The streamlined deck shape (see the cross section on the right) was the first of its kind and resulted in a bridge that was lighter and easier to paint than the more traditional design of the trussed girder of the Forth Road Bridge.

The roadway deck is high strength steel just under 12 mm. thick stiffened by 6 mm. high strength steel troughs 23 mm deep.

Soild web diaphragms or cross girders are spaced at 4.6 m. centres inside the box deck (see the cut out diagram to the left).

The Aust approach spans are two steel box girders 3 m. deep by 2.13 m. wide at 14.3 m centres and with I-section cross girders every 3 m. The deck is 0.2 m. thick reinforced concrete slab cast in situ.
Materials Steel - both mild steel and high strength steel for the towers, deck and cables. Mass and reinforced concrete anchorages
Foundations The east side called the Aust pier is founded on a limestone outcrop. On the west side (called Beachley) the pier is founded on Keuper marl. The main deck span is free at both ends and expansion joints in the deck are provided at each tower. The side spans are effectively hinged at the anchorages and free to move longitudinally at the towers.
Erection Stiffened plates 2.4 m. wide were transported to site and the 18.3 m. lengths of the deck were fabricated and assembled on a slipway to get correct alignments and camber. They were then floated out on pontoons and lifted into place.
References Roberts G "Severn Bridge: Design and contract arrangements", Proc Inst Civ Engrs Paper 7138, Vol 41, Issue 1 Sept. 1968, 1-48
Gowring G I B, Hardie A "Severn Bridge: Foundations and substructure", Proc Inst Civ Engrs Paper 7119, Vol 41, Issue 1 Sept. 1968, 49-67
Hyatt K E "Severn Bridge: Fabrication and Erection", Proc Inst Civ Engrs Paper 7084, Vol 41, Issue 1 Sept. 1968, 69-104

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