No roof is more dramatic – or symbolic of a country, as well as a major city – than that of the Sydney Opera House in Australia. Yet it nearly did not happen and it is thanks to Jack Zunz, who has died aged 94, that it did.

The young Danish architect Jørn Utzon had won a competition in 1957 with a scheme resembling a Mayan temple topped by petal-like shell roofs, which could be enjoyed from any angle, since the opera house’s exposed promontory site has no back and is overlooked from the Sydney Harbour Bridge and the Rocks. He desperately needed an engineer to realise such a unique vision.

Ove Arup, raised in Denmark but practising in London, stepped in to offer the services of the brilliant team he had assembled since 1946 and which over the ensuing decade had become Britain’s leading specialists in constructing modern buildings.

Yet his righthand man, Ronald Jenkins, could not make the structural calculations for the opera house roof work. The answer eventually came from Utzon, when he took an orange and cut from it a series of triangles, each with one curved side, creating a series of thin shells (properly folding plates) that could be prefabricated off-site from a small number of components. The repetition made for some economy and gave the sail-like image we now admire.

Jack Zunz, centre, with Ove Arup, left, and Michael Lewis, on site at the Sydney Opera House in 1964. Photograph: Max Dupain

The resulting profile was very different from the loose forms shown in the competition-winning drawings, and Jenkins resigned. However, his colleague Zunz embraced the logic of Utzon’s new scheme wholeheartedly. With computer technology in its infancy, he relied on models to achieve a supremely logical solution and had the guts to demolish the foundations that had already been cast on site, but which were inadequate to carry the heavier sails now proposed.

Then he carried the job through, after Utzon failed to deliver detailed drawings for the interior and withdrew from the scheme in 1966. Earlier his delays had saved their lives, when they had to miss a flight from Sydney; the plane they should have boarded plummeted into the ocean a few minutes after take-off, killing all on board.

The relationship between Utzon and his engineers soured fatally, but Zunz provided the continuity that saw the project to its conclusion. The complex eventually opened in 1973 to international acclaim, the trials of the previous decade forgotten when faced by the poetry of those scintillating concrete shells.

Life after Sydney, said Zunz, was “dull”. Nevertheless he engineered other world-class buildings, working extensively with Norman Foster, with whom he became good friends. He was involved from an early stage in 1980 on the HSBC headquarters in Hong Kong, one of Foster’s major buildings, devising a system of suspended floors that allowed a public space beneath the building and flexibility within.

Jack Zunz said life after Sydney was ‘dull’. Nevertheless he engineered other world-class buildings, working extensively with Norman Foster. Photograph: Arup

The striking X-shaped struts that stabilised the structure again became a defining image, or what Zunz called “an artistic touch” when describing his collaboration with Foster in 1989; he concealed a critical horizontal member in the floors so that the X form would not be compromised. The two also worked together on Stansted airport, an unusually elegant terminal when first completed in 1991.

Son of Helene (nee Isenberg) and Wilhelm Zunz, Gerhard Jacob “Jack” Zunz was born in Mönchengladbach, Germany, but was raised in South Africa, where his civil engineering course at Witwatersrand University was interrupted by the second world war. He was determined to “do his bit” against fascism and served in Egypt and Italy with the South African Artillery.

In 1948 he married Babs Maisel, and they had three children, Marion, Laura and Leslie. Zunz had become increasingly disturbed by the growing apartheid movement following the fall of Jan Smuts, also in 1948. A friend had moved to London and recommended that he apply to join Arup’s office.

Zunz arrived in London in 1950 and duly met Arup. They chatted amicably for an hour, mainly about art, but there was no work available and Zunz departed, puzzled and disappointed. The next morning the telephone rang. Jenkins needed an expert on steel structures to work on Hunstanton school in Norfolk, for which his friends the young architects Alison and Peter Smithson had just won a competition. Zunz understood the latest theories of plastic design, or how a steel frame can operate as a single entity wherever a load is placed – a concept then little understood in Britain. Zunz stayed with Arup’s for the rest of his career.

In May 1954 he agreed to open an office in South Africa, where he and Michael Lewis developed a specialism in tall structures for telecommunications, culminating in the Albert Hertzog tower (now the Sentech tower) in Brixton, a suburb of Johannesburg, an influence on Arup’s later television mast at Emley Moor, Yorkshire (1969-71). However, the political environment worsened all the time and they returned to London in 1961.

Within two weeks, Zunz was working on the Sydney Opera House and quickly realised how to make the folding plates work. He was always eager to acknowledge that it was a team effort, in which he encouraged younger engineers such as Peter Rice and Tony Fitzpatrick, who themselves went on to enjoy brilliant careers with Arup.

Zunz became chair of Ove Arup and Partners in 1977 and was co-chair of the Arup Partnership from 1984 until 1989. He was knighted in 1989, the year of his retirement and a year after he had received the gold medal of the Institute of Structural Engineers.

Marion died in a skiing accident in 1992. Zunz is survived by Babs, Laura and Leslie.

• Gerhard Jacob (Jack) Zunz, civil engineer, born 25 December 1923; died 11 December 2018