Severn Bridge

Severn Bridge

1966 - Severn Crossing

  • 988m main span suspension bridge
  • 88 bridge deck sections, each 40m long
  • Aerodynamic box girder deck

Large suspension bridges rank among the world's most sublime and heroic structures.

The Times

In the 1950s and 1960s, a UK-wide trunk road system was planned. To achieve this, bridges would be required across the estuaries of the Forth, Severn, Clyde and Humber.

The proposed crossings would take the form of suspension bridges, due to the long spans required. Freeman Fox & Partners, and Mott, Hay and Anderson were appointed as joint civil and consulting engineers to construct a bridge over the River Severn. The current Severn Suspension Bridge was completed in 1966 and bridges all over the world are descended from the design and engineering of the Severn Bridge.

Dr William Brown

Dr William Brown
Dr William Brown (centre)

Dr William (Bill) Brown is credited with designing the innovative aerodynamic box girder deck, which superceded the complex trusses used on earlier major suspension bridges in the UK (Forth Road Bridge) and USA. Under pressure to minimise cost and use less steel, Bill's studies showed that a shallow welded box construction would have good torsional stiffness and be lighter and easier to build.

An earlier truss deck design was accidentally destroyed during wind-tunnel testing and Brown and Roberts had the opportunity to test the new deck (above). It was found to have good aerodynamic performance that deflected the force of the winds.

Bridges all over the world are descended from the design and engineering of the Severn Bridge.

The roadway deck was constructed from 88 box sections, each 40 metres long. They were assembled on the riverbanks of the adjoining River Wye, floated into position and hoisted to road level.

The Severn Bridge replaced the Aust ferry, which had carried up to 19 cars at a time over the river. The long queues for the ferry that used to stretch two or three miles were now a thing of the past.

In 1966, it became an essential part of the new M4 motorway, reduced travelling time between London-Bristol-Cardiff and helped the economic development of South Wales.

The Severn Bridge proved very popular, and with up to 20,000 vehicles a day soon crossing it, a new second Severn Crossing was constructed in the 1990s.

Facts

Severn Bridge: Dr William Brown, Freeman Fix & Partners
988m main span

Effect

Improved transport links, increasing traffic from 450 (via ferry) to 20,000 vehicles per day

Challenge

Single span, huge tidal reach

Solution

Suspension bridge with box girder deck

Legacy

Box girder has been used on many bridges worldwide