THE world famous "coathanger", one of the icons of 20th Century design, is 75 years old on Monday. Tomorrow, 100,000 Aussies will celebrate by walking its length - 1,149 metres - and climbing its height - 139 metres.

And they'll be doing so on Teesside steel.

On January 24, 1924, the Middlesbrough steelmakers Dorman Long won a £4,217,721 11s 10d contract, in the face of global competition, to build the Sydney Harbour Bridge.

"I don't know what the 11s 10d really represents, unless its the profit," quipped the chairman Sir Arthur Dorman. "That's right," joshed Sir Hugh Bell, a director. "We knocked off the ¾d." Both men, octagenarians, were present in spring 1925 when the foundation stone was laid, but neither made it to see the bridge declared open on March 19, 1932.

Their firm rolled in Middlesbrough 79 per cent of the 52,800 tons of steel that built the bridge, but their involvement was controversial. Many in New South Wales thought the bridge should be made of 100 per cent Aussie steel. As a compromise, the NSW government promised that all the labour on the bridge - 1,500 men - would be Australian.

This was not quite the gift it sounded, as 16 constructors fell to their deaths and many more later suffered deafness, apparently caused by the noise of the six million rivets going in.

The method of construction owed much to Newcastle's Tyne Bridge. Dorman Long won the contract to span the Tyne a few months after it had won the Sydney contract, but had started work in Newcastle almost immediately. The Tyne Bridge is a scaled down version of the Sydney Harbour Bridge's "coathanger" design, which is itself based on New York's Hell Gate Bridge, and both Newcastle and New South Wales were built using two creeper cranes.

The cranes started on dry land on either shore. They winched a steel section into place in front of them, and then crawled onto it. In Sydney, they came together at the top of the arch in the middle of the harbour at 10pm on August 19, 1930.

"And now it stands, strong, stately and graceful, spanning with a single arch the beautiful Sydney Harbour, a marvel of British and Australian engineering skills and a permanent memorial to the ideals and principles of inter-Imperial collaboration and enterprise," said The Northern Echo's report of Sydney's opening.

The report contains two other fascinating lines. It tells how thousands gathered to witness the Premier of NSW, Jack Lang, make a speech, cut the ribbon and declare the bridge open.

He duly made his speech, but just as he stepped forward with his scissors, a man in a military uniform on horseback charged out of the crowd and slashed the ribbon in half with his sword "in the name of His Majesty the King".

Soldiers wrestled him to the ground. He said he was Captain Francis de Groot of The New Guard, a paramilitary organisation formed to stop Australia being "socialised". After a psychiatric test proved him to be sane, de Groot was convicted of offensive behaviour.

The ribbon was re-tied and Mr Lang, a Labour politician, formally cut it. As he stepped back, the massed male voice choirs burst into the National Anthem.

"There was great competition for the place of conductor of the choirs and the choice fell on Mr Robert William Dixon," said the Echo. "Durham County is interested in this honour."

From 1914 until he emigrated in 1922, Mr Dixon, 32, had been organist in the Primitive Methodist Chapel at Metal Bridge near Ferryhill. His parents, said the Echo, still lived there and were immensely proud.

And so the Sydney Harbour Bridge - 75 years old this weekend - was strung up by Teesside and sung open by Durham.