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Bridge design

Design and Tenders

In the years before World War I, Bradfield himself submitted several proposals for a bridge across the harbour. In 1912 he proposed a suspension bridge. And in 1913, just one year the outbreak of WWI, the parliamentary committee responsible for recommending a bridge design finally chose Bradfield's design for a cantilever bridge, similar to the earlier cantilever design featured in Image 3. If it had not been for the outbreak of WWI in 1914, it is more than likely that this design would have been given the go-ahead. However, after the war when the bridge building plans were renewed, the cantilever design was eventually abandoned in favour of an arch bridge.

The Sydney Harbour Bridge was not originally thought of as an arch bridge. Dr Bradfield initially had a cantilever bridge in mind to span the harbour. However, on a trip to New York he was inspired by the Hell Gate Bridge and he realised the cantilever design was inferior to an arch for his proposed bridge.

Sydney Harbour Bridge: perspective elevation
‘Sydney Harbour Bridge: perspective elevation’ by Robert Charles Given Coulter, architect, Department of Public Works, 1921. Photograph on paper and linen, overdrawn with ink and wash, 57 x 196 cm. State Records NSW. In Bridging Sydney,

A cantilever bridge functions quite differently from an arch bridge. Being a beam bridge, a cantilever relies on the supports to provide only vertical reactions to vertical loads. The bridge transmits these loads to the supports and the supports react with vertical reactions. In an arch bridge, vertical loads create horizontal and vertical reactions because the loads try to flatten the arch and push out against the abutments. This means that an arch must have significantly stronger abutments than a beam bridge.

Cantilever bridge   Cantilever bridge
Comparison of reaction forces at supports on cantilever and arch bridges.

Dr Bradfield ‘… had come to the conclusion that there were no insuperable difficulties to the erection of an arch bridge of the span required, and that an arch bridge would cost £350 000 less than a cantilever …’. (Bradfield, 1932, p 2)

For Dr Bradfield the arch was the better system. It was a more efficient structure, which meant it would carry the same load as the cantilever but require less steel.

Sydney Harbour Bridge is not the longest span steel arch bridge in the world, but it is the widest and heaviest.

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