Detailed History of the M48 Severn 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.
Bridge General Dimensions
- Main Span 3,240 ft
- Distance Apart, c. to c. of cables 75 ft
- Side Spans, each 1,000 ft
- Level of tops of towers 445 ft O.D.
- Sag span ratio of cables 1/12
Two 24 ft wide carriageways, one 12 ft wide footway and on 12 ft wide cycle track.
- Live loading B.S.S. 153, Type HA and 45 units of type HB loading.
- Design wind speed 100 mph
- Design temperature range Maximum 98°f, minimum 8°f
A concrete monolith, 132 ft long, 40 ft thick and 63 ft high from its founding level of 18 ft O.D. oblong in shape with pointed cut-water ends. Founded on a limestone outcrop in the river bed known as Great Ulverstone. Construction was by means of precast blocks placed to form the outer skin and then "hearted" with in-situ concrete. Work was tidal during earlier stages. The pier is stressed down to the rock by 58 1 1/4 inch MacAlloy bars round its periphery, grouted 12 ft into the rock and stressed to a force of 45 tons each.
A concrete monolith similar to that of Aust Pier but resting on two 60 ft diameter concrete cylinders extending approximately 33 ft downwards from the river bed to a founding level at -50 ft O.D. Founded on hard mudstone of the Keuper Marl series.
Of the gravity type, these appear externally as roughly rectangular blocks of concrete, 155 ft long by 110 ft wide and extending up to a level of 132 ft O.D. The Aust anchorage is at a level of -8 ft O.D. 550 ft offshore of Aust Cliff. A three span viaduct carries the roadway between the Anchorage and the cliff. The Beachley anchorage is found on limestone approximately -20 ft O.D.
The towers are constructed of high tensile steel with a yield stress of 22 1/2 tons per square inch. The thickness of the steel in the legs varies from 1 inch to 9/16th of an inch. Each leg is a single hollow box, 17 ft by 12 ft at the base, tapering to 17 ft by 9 ft 6 in at the top. The sides of the box are bolted together at the corners on inwardly turned seams so that no bolt or nut is exposed on the outer side. The bolts are mostly 7/8 of an inch in diameter. The transverse portal members are also hollow box sections. The towers were erected in sections of 55 ft 6 inches long by climbing cranes. The bottom of the legs are held down to piers by 1 1/8 inch diameter pr-stressed strands, penetrating 50 ft into the concrete and post tensioned. The legs are capped by saddles which carry the main cables. The saddles are 443 feet above mean water level. Each tower carries a vertical dead load of 13,200 tons and each weighs 1,300 tons.
Approximate Material Quantities
- Steel in towers 2,700 tons
- Steel in cables and suspenders 4,600 tons
- Steel in deck 11,200 tons
- Concrete in two piers 28,000 c.u. yards
- Concrete in two anchorages 98,000 c.u. yards
Contractors and Cost
- Consulting Engineers Messrs Mott, Hay & Anderson and Freeman Fox & Partners
- Substructure Contractors John Howard and Co Ltd
- Superstructure Contractors Associated Bridge Builders Ltd
- Total Cost 8 million pounds (1966)