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Sydney Harbour Bridge

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Rivets

The use of rivets in steel construction is now an oddity, but at the time it was acceptable practice. Riveting was done on this bridge on a much greater scale than in any previous iron or steel structure in Australia.

The structural rivets used in the bridge utilised a mild steel with a UTS of 413-482 MPa and high shear strength. Like the approach span steel, the microstructure was primary ferrite with approximately 20% pearlite.

Forming the rivets – forging

More than 6,000,000 rivets were used to assemble the various plates on the bridge. The rivets were inserted into the plates red hot with a head on one side and the other end headless, something like an unthreaded round head bolt. The headless end facilitated insertion into the hole to join parts together.

A rivet gun and the shot.
A rivet gun and the shot. The workman is about to drop a red-hot rivet into a rivet gun. He presses the step, and the rivet shoots along the flexible metal tubing attached to the gun.
Source: Parables of the Sydney Harbour Bridge by Frank Cash, 1930, p 335.
The pneumatic riveter
The pneumatic riveter is operated by hand, and is used both inside and outside the workshops.
Source: Parables of the Sydney Harbour Bridge by Frank Cash, 1930, p 317.

Once the rivet was inserted into the plate, the headless end was rounded over while still hot, using a pneumatic riveting gun which prevented its removal. By forming the rivet head in this way the rivet was hot-forged into place.

This method results in favourable grain flow which eliminates weak spots in the rivet. It is analogous to drop-forging objects in obtaining a favourable grain flow.

rivetrivet

Grain Flow in a rivet (right) machined from hot rolled bar. Note the sharp change in grain flow under the rivet head. Favourable grain flow in rivet (left) after inserting and rounding while hot, analogous to hot forging.

The Supply of Rivets

Over the years there has been some controversy as to where the rivets for the bridge came from, that is, who was the supplier?

A search of the available reference material provides the answer – the rivets were made by McPhersons P/L of Melbourne. They turned out some 5 million rivets, some as long as 15 inches (380 mm) long and noted in their advertising that “… this huge quantity of rivets … was delivered to the entire satisfaction of the contractors.”

Why wasn’t welding used?

Riveting of steel structures was a well-understood and proven method of construction for large civil engineering structures in the late 19th and early 20th centuries. It is not hard to find examples still existing in cities and country towns.

Welding was known in the late 19th century but structural welding had not been adequately developed to be used on the Bridge construction.

‘Each rivet was heated red hot before being inserted into the drilled holes and the joint closed by hydraulic or pneumatic hammer which compressed the tip.’ (Bridging Sydney, 2007, p 194.)

Problem 1

What would be the macrostructure of a typical hot set steel rivet?

Solution 1

Hot riveting is a typical example of hot forging where the grain structure of the steel is forced to follow the shape of the forged part. This makes the part relatively stronger than a part machined from rolled bar where the grain flow is axial.

Hot forged steel gear blank.
Hot forged steel gear blank.
Source: John Gibson
The horseshoe hydraulic riveter
The horseshoe hydraulic riveter weighs 12 tons, and exerts a pressure of 95 tons on each rivet.
Source: Parables of the Sydney Harbour Bridge by Frank Cash, 1930, p 317.
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