The Sydney Harbour Bridge was not originally designed to be an arch bridge. 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’s Gate Bridge and he realised the cantilever bridge was inferior to an arch bridge for his proposed bridge.
The cantilever bridge functions quite differently to an arch bridge. Being a beam bridge, a cantilever relies on the supports to carry only vertical reactions to vertical loads. The bridge transmits these loads to the supports and the supports react with vertical reactions. This is different to an arch where vertical loads will create horizontal reactions, because an arch bridge will try to flatten the arch and push out horizontally against the abutments. This means that an arch must have significantly stronger abutments than a beam bridge.
For 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.
SHB is not the longest span steel arch bridge, but it is the widest and heaviest.
At the time the bridge was designed and built, riveting was the normal method of joining steel members as electric arc welding of steel was still a developing technology and its use on load-bearing structures was rare. Consequently, the components of the Bridge were joined by riveting and because of the Bridge's size, riveting was on a much greater scale than any previous structure. The use of rivets is now an oddity.