In 1910 ferries transported 13 million passengers across Sydney harbour, and peak periods saw up to 75 ferries docking at Circular Quay every hour. It was clear from the outset that the proposed Sydney Harbour Bridge would need to accommodate a massive volume of traffic.
Cheap iron and well-made steel were making possible the design of stronger bridges with even greater spans. Cable suspension bridge designs, like the Brooklyn Bridge in New York, use wire ropes anchored to tall towers to support the roadway. Cantilevered bridge designs use two beams stretching out from each shore, supporting a short middle section to cross the gap between them. For an arch bridge design, the carriageway is held up by vertical ties suspended from the arch.
The particulars of the Sydney Harbour Bridge design were the subject of two Royal Commissions, various parliamentary inquiries, and heated debate among engineers, politicians and the public. As Chief Engineer for the Metropolitan Railway Construction and Sydney Harbour Bridge, JJC Bradfield campaigned for an arch bridge design. The entire load would be transmitted through the arch to the base at each end, formed by the four giant hinges. The towers at each end of the Bridge would serve no structural purpose.
In March 1924 the British firm of Dorman Long & Co won the contract for a vast twin-hinged steel arch design, at a contract price of precisely £4,217,721/11 and 10 pence.