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The Humber Bridge
The Humber Bridge, designed by Mike Parsons and opened in 1981, broke new ground because of its dramatically long 1,410 metre main span. Photograph: Christopher Bailey/Alamy
The Humber Bridge, designed by Mike Parsons and opened in 1981, broke new ground because of its dramatically long 1,410 metre main span. Photograph: Christopher Bailey/Alamy

Michael Parsons obituary

This article is more than 2 years old

Structural engineer who had a big hand in the creation of the Severn, Forth and Humber road bridges

Michael Parsons, who has died aged 92, played a key role in the design of Britain’s three major suspension bridges, over the Forth, Severn and Humber estuaries – and was closely involved in the design of the first two suspension bridges over the Bosphorus in Turkey.

Working for the consultants Freeman Fox and Partners throughout his career, he was the originator of the streamlined box girder concept first adopted for the suspended deck of the Severn Bridge and taken up on most suspension bridges thereafter.

Michael Parsons. Among his colleagues, he was known for his collegiate approach and for his caution and rigour in design

The Forth Road Bridge, which opened in 1964 and on which Mike’s role was to carry out the detailed design of the superstructure, followed traditional US suspension bridge practice in using a substantial lattice structure, or truss, to stiffen the suspended deck. Early in his career, Mike was charged with calculating the loads in the individual truss members – a painstaking task with a slide rule in the days before suitable computer programmes had been developed. Alongside this he ran a test programme to determine the compression strengths of the deck truss members, as these were all-welded square tubes that were expected to behave differently from the riveted ones on American bridges.

For the Severn Bridge Mike was asked by his boss, Gilbert Roberts, to design a deck truss of reduced depth by integrating it with the steel plating supporting the roadway. He succeeded in demonstrating that the depth could be nearly halved, and a model was taken to the National Physical Laboratory in London for wind tunnel testing. During set up, however, the model broke free of its fixings and destroyed itself in the subsequent impact.

That setback led Mike to propose an alternative deck structure for wind tunnel testing – a box section. The consequent testing proved successful, leading to the adoption of a streamlined plated box section for the Severn suspended deck, which resulted in a considerable weight saving. There was also a dramatic reduction in wind loading on the deck from the side – to one-fifth of that for the Forth deck – which benefited the tower design.

Born in Bristol, Mike was the second of four children of Norris Parsons, a plumber and later a hospital maintenance engineer, and his wife, Kathleen (nee Holder), who ran an umbrella shop. From Cotham grammar school he went to Bristol University and gained a first in engineering.

Innovations led by Michael Parsons allowed for reductions in weight and wind loading on the Severn suspension bridge, which was opened in 1966. Photograph: Jim Dyson/Getty Images

Mike’s fascination with suspension bridges was inspired by Brunel’s suspension bridge in Clifton, and was developed at university by a research project testing a model bridge. This moved him to apply for a job at Freeman Fox, which had a distinguished track record in the design of long-span bridges. His familiarity with bridge deck vibration modes stood him in good stead at interview and his application was accepted.

Mike met Joan Wickett, a civil servant, during his first two years with Freeman Fox and they married in 1951. By then he had been called up for national service and was posted to Germany shortly afterwards. After his two years were up, he resumed his career with the firm.

Learning from the spectacular collapse in 1940 of the Tacoma Narrows suspension bridge in the US, which had been shaken to pieces by a 40 mph wind, Mike began to apply aircraft wing flutter theory to suspension bridge decks to understand how the structural properties of the deck could be adjusted to prevent any repeat of that disaster. The Tacoma failure had demonstrated that it was essential to provide sufficient torsional stiffness in suspension bridge decks, and this principle guided all subsequent Freeman Fox designs.

Michael Parsons, right, with his colleague Brian Smith at the top of the Severn Bridge during its construction

In 1961 Mike was appointed deputy resident engineer on the Severn Bridge site, initially supervising the construction of foundations on the English side and subsequently the erection of the superstructure, until its completion in 1966. During the opening of the bridge by the Queen, the Duke of Edinburgh noticed that a few of the hangers were vibrating in the wind – something that the site staff, Mike included, had missed. As a result, Mike designed and fitted damping devices similar to those used on overhead electricity cables. The streamlined box girder adopted for the Severn Bridge was a breakthrough because of its inherent high torsional stiffness, and earned Freeman Fox the MacRobert award, which Mike shared with four colleagues, in 1969. I joined Freeman Fox that year, and although I did not work directly with Mike, I became aware of his reputation for thoroughness.

On his return to the London office, his next assignment was the detailed design of the Bosphorus Bridge, which was an evolution of the Severn design, although with an increased main span of 1,074 metres compared with 988 metres. A three-metre depth was again chosen for the box girder, but its width was increased to carry six traffic lanes.

The design of the Humber Bridge, on which Mike was appointed project engineer for the superstructure in 1973, broke new ground because of the dramatically increased main span of 1,410 metres. However, the deck was the same width as at the Severn Bridge (for four lanes), so Mike increased the depth of the box from 3 metres to 4.5 metres to provide adequate torsional stiffness. At its opening in 1981 its main span was the longest in the world, and it held that record for 17 years.

Towards the end of his career, in 1984, Mike was put in charge of a team tasked by the UK government with examining bids for the construction of a link from the UK to France either above or below the Channel. There were ten bids to evaluate, including one for a carbon fibre suspension bridge with spans about three times those over the Humber. Mike went about his assessments in his characteristically quiet, confident manner, and drew around him a diverse team of advisers. His report in 1985 recommended the acceptance of one of the bids that proposed a tunnel – and led eventually to the go-ahead for Eurotunnel to start the work.

He retired in 1988, and in 2012 gave an oral history interview for the British Library on his life and career. Among his colleagues, Mike was known for his collegiate approach and for his caution and rigour in design. Spending hours on equations that are now solved by computers in milliseconds, he found that the process of checking and cross-checking actually helped him to get a feel for what the bridge would feel and act like in real life. “I felt the structure was almost a part of me,” he said.

He is survived by Joan and their two sons, Paul and Philip.

Michael Francis Parsons, structural engineer, born 16 October 1928; died 20 April 2021

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