The M48 Wye Bridge and Viaduct

M48 Wye Bridge
M48 Wye Bridge

The Severn Bridge was opened by the Queen on the 8th September 1966 to replace the ferry service crossing from Aust Cliff to Beachley Peninsula, 8 miles upstream from Avonmouth. At this point the River Severn is almost exactly 1 mile wide at high water and the bridge creates a direct link for the M4 motorway into Wales.

The Severn Bridge is in fact two bridges. The main section spans the Severn Estuary, whilst the second smaller section spans the River Wye.

The Wye Bridge and Viaduct

The Wye Bridge is of a stayed girder construction and is located between Beachley in Gloucestershire and Chepstow in Monmouthshire. The bridge is a continuation of the Severn crossing and forms part of the M4 London to South Wales motorway.

Statistics

  • Main Span: 770 ft
  • Side Spans: 285 ft

Approach Viaducts

  • West Side: 420 ft long consisting of two 210 ft spans
  • East Side: 2,023 ft 9 inches long consisting of 10 spans of up to 210 ft.
  • Overall Length: 3,783 ft 9 in.
  • Clearance: 52 ft above high water level

Construction

River Piers: Reinforced concrete pier supported on twin 18 ft diameter caissons sunk to the bed rock.

Viaduct piers: Twin 10 ft 6 in square columns with 14 ft 6 in square footings founded on Marl or bed rock at depths of up to 35 ft below ground level. The two columns are tied together with a flexible prestressed beam 3 ft below ground level.

Trestles: Splayed steel portal frames with hollow tapered legs made of welded stiffened plates varying in thickness from 1/2 in to 1 1/4 inches. Hinged bearing at top and bottom accomodate the temperature movements of the deck structure.

Deck: Continuous box girder of welded steel 10 ft 6 in deep and 49 ft wide with inclined web plates and a 25 ft 9 in cantilevered deck on each side making the overall width 100 ft 6 in. Diaphragm plates at 14 ft intervals keep the box rigid. The top plates of the box and the inner portions of the cantilevered decks form the roadway while the outer portions of the cantilever decks form the cycle track and footway. This deck is the same for the bridge and the viaduct approaches.

Towers: Each tower consists of a single welded steel box 94 ft 5 inches long made up of welded stiffened plates 1 1/4 in thick and positioned on the centre line of the bridge in the central reserve. The towers are supported on the top surface of the deck by a hinged bearing, their thrust being transmitted through the deck to the trestle beneath by a diaphragm plate.

Cable Stays: One cable passes over each tower top and is anchored at each end inside the deck box to a system of diaphragms. The cable consists of 20 single spiral strands 2 7/16 in diameter built up into a triangular form, each end of each strand being socketed. A threaded rod is screwed into the end of the sockets to provide adjustment.

Capacity of Bridge: Four 12 foot lanes forming a dual carriageway with marginal strips, one 12 ft cycle track and one 12 ft footway.

Loading: Full British Standard loading including 180 tons heavy vehicle.

Design Wind Speed: 100 m.p.h at 10 metres.

Approximate Material Quantities

  • Concrete: 12,600 c.u. yards
  • Steel in towers and trestles: 500 tons
  • Steel in deck: 8,000 tons
  • Steel in cables: 130 tons

Contractors and Cost

Consulting Engineers: Messrs Mott, Hay & Anderson and Freeman Fox & Partners

Contractor: Cleveland Bridge and Engineering Co. Ltd. with the Fairfield Shipbuilding and Engineering Co Ltd as main contractors.

Total Cost: 1.8 Million