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