The Sydney Harbour Bridge is a steel structure, with the steel arch carrying the live and dead loads out to the concrete abutments (skewbacks). The steel used by Dorman, Long & Co. Ltd. was not mild steel or plain-carbon steel. They chose to use silicon steel, which was a precursor to modern day structural steels. (Factor of safety)
The structural rivets used in the bridge were made of a mild steel which had a UTS of 413-482 MPa. This was strong enough in both tension and shear for it to be used for rivets in the construction of the Bridge.
The Sydney Harbour Bridge’s use of ferrous alloy presents a significant challenge in protecting the structure from the ravages of corrosion. Unfortunately steel oxidises to form rust, a porous corrosion product that exposes more metal to oxidation.
Casting and forging
The thrust from each arch truss is transferred from the lower chord to the concrete skewbacks through the steel pedestals.
The steel upper saddle is fixed to the end of the lower chord of the arch and transmits the thrust to the bearing pin. The pin transmits the thrust to the lower saddle which is attached to the two forged steel webs 9½inches (241.3 mm) thick. The two webs are held together with 10 cast steel diaphragms. The webs and diaphragms transfer thrust to six steel castings which, when bolted together, form a base which rests on the face of the concrete skewback. The bearing weighs a total of 296 tons (301 tonnes).
Further detail on the processes of casting can be found at NSW HSC online website.
Further detail on the processes of forging can be found at NSW HSC online website.
About 14% of the steel used in the Bridge was produced at the Newcastle works of BHP, with most of it being supplied by Dorman, Long & Co. Ltd., the bridge builders, from their plant in the UK. Beams, plate, rod and other sections were hot rolled to shape in the bloom mill, and transported to Australia by steamship. Beams and sections were unloaded at Milson’s Point, sorted and transported to the erecting shops which were where Luna Park now stands. Here the steel was marked out, cut, drilled, planed and reamed to accurately produce each component for the bridge structure.
Substructures were then assembled ready for transfer to the Bridge site for erection.
More information on Rolling can be found at NSW HSC online website.
Very large planing machines were used in the workshops to true-up the edges and ends of beams to ensure accuracy in dimensions. In this operation the steel beam is fixed to the horizontal bed of the planer. A moving head with a tool steel cutter mounted on it traverses the length of the work, to machine the edge straight and true.
Composition of steel
The microstructure of low carbon plain carbon steels such as mild steel (below) shows areas of white, and areas of dark grey.
The white areas are a solid solution known as Ferrite, a normally low strength, soft and very ductile component of low carbon steels. Ferrite is almost pure iron with a minute amount of carbon dissolved in it. The microstructure of mild steel of 0·2% C in the annealed state shows about 75% Ferrite and about 25% Pearlite.
The dark grey areas are actually a composite called Pearlite. A microstructure of Pearlite is shown below, highly magnified.
Here we can see that Pearlite is made up of two components clearly laminated on one another, not unlike plywood — although we need to remember that these components are microscopic in size! The light areas are the low strength, soft, ductile Ferrite, and the dark areas are a hard iron–carbon compound called Cementite.
The properties of Pearlite are good strength, hardness and toughness.
The 0.38% C silicon steel used in the Sydney Harbour Bridge contains more carbon than mild steel. It would therefore show about 47% Pearlite and 52% Ferrite in the microstructure. Because the steel has more Pearlite in its microstructure, it would reflect more of the properties of the Pearlite, which is stronger, harder and tougher than the Ferrite.
As well as having a higher proportion of carbon than mild steel, the bridge steel also contains 0.2% Silicon, and 0.9% Manganese. The microstructure does not show any Silicon or Manganese because these metals dissolve in the Ferrite causing its strength to increase through a process known as solid solution hardening.
Through the cooperation of the Australian Key Centre for Microscopy and Microanalysis at The University of Sydney, and the Powerhouse Museum, some samples of the Dorman, Long & Co. silicon steel have been prepared for microanalysis and the following optical micrographs and electron micrograph taken.
The steel used by Dorman, Long & Co. to construct the bridge was therefore higher in carbon content than mild steel making it stronger and tougher by having a greater proportion of Pearlite present. Added to that, their steel had relatively high quantities of Silicon and Manganese that increased the strength of the Ferrite in the microstructure. These facts account for the fact that their silicon steel has an average yield strength 1.3 times that of the average yield strength of mild steel.
The Silicon steel was cast into ingots and then hot rolled into sections of the required shape and thickness. Beams and chords for the bridge were then fabricated from these rolled sections and plates by riveting. These fabricated sections were pre-assembled in the workshops, and then taken on-site to be riveted in place on the bridge.
The Bridge steel girders were manufactured by hot rolling, and some horizontal alignment of the Pearlite can be seen in the microstructure caused by this process. Hot rolling normalises the steel and refines the microstructure.