As a result of routing the M5 to the West of Bristol it was necessary to cross over the tidal estuary of the River Avon, which has a tide range of some 40 feet, and the adjacent roads and a railway. The Consulting Engineers, Freeman Fox and Partners, who had already designed box girder structures elsewhere in the World, prepared the design using continuously welded twin box girders to achieve an overall width of 132.5 feet. The width catered for dual three lane carriageways and hard shoulders, a central reservation, a cycle track and footway. The design allowed for the conversion of the hard shoulders, at a later date, into carriageways and the cycle track and footpaths into new hard shoulders. The main span over the estuary is 570 feet long with a clearance of 100 feet over high water. The anchor spans are 370 feet long. Due to low bearing value of the ground on the flood plain adjacent to the bridge it was necessary to construct approach viaducts of ten spans on the north side of the bridge and seven on the south. The overall length of the total structure was 4550 feet. The contract for the bridge was awarded to Fairfield-Mabey of Chepstow for a tender sum of £4.2 million. and work commenced in 1969.
Fairfield-Mabey placed a domestic contract with Tarmac Civil Engineering (now Carrilion Construction) for the foundations, piers and deck concreting works. Work on the foundations began immediately and were substantially complete in early 1972. This was in line with the planned opening date in 1972, itself part of the overall programme of extending the M5 southwest
Prior to the letting of the contract for the bridge the Department of the Environment had become concerned about the failure of some continuously welded steel box bridges and appointed a committee chaired by Professor Merrison to examine the criteria for the design of such structures. In May 1971 the Department distributed the first interim rules prepared by the Committee. As a result it was necessary to re-appraise the whole Structure and make considerable changes to the design. Considerable stiffening and/or thickening of plates was required and the combined effect of the design and workmanship requirements of the rules doubled the work at site joints. The redesign led to a considerable delay to the Bridge completion and this was further aggravated by industrial action by the site welders who, because of the extra strengthening, recognised their strong bargaining position and demanded more money. The combined effect of these problems led to the granting of a two year extension of contract with the opening of the bridge in May 1974.
As stated earlier the M5 north of Avonmouth became a continuous dual carriageway Motorway to Birmingham in December 1971. Hence for a while traffic from the motorway heading on towards the Southwest had to travel through Bristol by way of the Portway and the Cumberland Basin complex and then into Somerset on A38. Meanwhile construction of the motorway further southwest between Avonmouth Bridge and Highbridge had commenced in a series of contracts before May 1971 and the completion was January 1973.
The consequences, in Bristol and North Somerset, of the delay of nearly eighteen months in the opening of the Avonmouth Bridge resulted in Motorway traffic flows discharging on to the local road network. The local County, City and District Councilors became very concerned about the potential effects and a conference chaired by a Minister, Mr. Keith Speed, was held to discuss how the situation could be alleviated. At this meeting the County Surveyor of Somerset's representative put forward proposals for a one way signing system from the interchange at Gordano, south of Avonmouth. The scheme, which needed the construction of minor highway improvements, required the signposting of northbound traffic from junction 19 via the A369 to the Cumberland basin and then the A4 Portway to Junction 18 at Avonmouth: the southbound traffic was signed onto the Portway at Avonmouth and then onto the Cumberland Basin leaving on the B3128 through Failand and then via a minor road through Portbury to junction 19. The Councils involved approved the proposal and despite the need for a Public Inquiry the works were completed and the signing installed in time for the opening of the motorway to Edithmead. Thus the worst of the congestion effects were contained for the period until the completion of the Avonmouth Bridge in May 1974.