The Avon River bridge has a very interesting history.
Originally the bridge consisted of 162 timber openings - each 15 foot - followed by six 30 foot iron girders. The timber section of the bridge started further towards Melbourne, in fact it must have started close to the current Princes Hwy underbridge. The curve in the bridge was entirely on the final section of timber openings, and the curve had a radius of 1320 feet (20 chains). The iron girder section was (and is) entirely straight, and the river ran under the centre two openings. The iron girders were erected on timber piers, each consisting of eight vertical piles which were originally (and still are today) closely covered with timber to prevent problems with debris.
This was all very rational, unfortunately, the river had other ideas. The river bed is shingle and it erodes easily. The river moved towards the Melbourne side, and by 1891 had eroded a new channel largely under the end of the timber viaduct, leaving the original iron girders over a shingle bank. The timber bents were braced, but flooding of the river caused major problems. The eroding river banks upstream would bring down whole trees, which got caught up in the 15 foot openings (which were never intended to pass the main channel). The opening would dam up and bring down the bents.
It was decided around 1896 to replace the final 300 feet of the timber openings with three new steel openings, which would provide wider openings. The openings between piers of the new section were 93 feet, 105 feet, and 94 feet - note the wider opening in the middle over the then river channel. To reduce the length of the steel span over the wider centre opening, the centre girders were cantilevered from the outside spans. The end girders were 109 feet long, with the final 15 feet projecting beyond the pier. This supported the centre span - the girders of which were 76 feet 2 inches and 74 feet 9 inches long. This new section was entirely on a 1320 foot curve. To accommodate this the end girders were bent over the piers and the final 15 feet was offset by 7 1/2 inches at the end. The two centre girders were also of different length. (A discussion of the new section of the bridge can be read athttps://digitised-collections.unimelb.edu.au/bitstream/handle/11343/24249/305667_UDS2013255-27-0018.pdf
The river continued to move towards Melbourne, and around 1900 the steel section was extended by a further 400 feet in four 100 foot spans. This time the design was more conventional. Each girder spanned the gap between a pair of piers, and in each span the outer girder was slightly longer than the inner girder. This gave a much better curve. One curious feature was the actual curve - the first three spans are on a curve of 1635 feet radius, and the last is the 1320 foot radius to match the 1900 section.
More work appears to have been done in 1920 (almost certainly related to strengthening the bridge for heavier axle loads). The timber piers under the original iron girders were strengthened with RSJs. In addition, lengthy sections of the original timber viaduct were filled in (spans 1 to 26, and 55 to 71). The final two openings on the Stratford side were filled in in 1923.
So the bridge today has four portions. From the Melbourne end they are:
* The remnants of the original timber approach viaduct over the flood plain, which is about half the curve
* The c1900 extension, consisting of four steel spans, each nominally 100 feet long
* The c1896 extension, consisting of three steel spans with a nominal total length of 300 feet. The centre span is longer than the other two, and the centre girders are suspended from the outer girders.
* The original iron girders, six spans each of 60 feet.