Changes to Bradfield's Design by Freeman.
The amended specifications for the Sydney Harbour Bridge issued on 31 January 1923 included both a cantilever and an arch option. The arch, at least at first glance, would seem to be the bridge that was eventually built. Both Bradfield and Ralph Freeman, the consulting engineer who went on to make the detail design, said that the winning tender was substantially the bridge as set out in the specification, but there are important differences introduced by Freeman.
One change, for which the people of Sydney should be thankful in terms of public open spaces, was to the approach spans. The specification showed these as being four lines of trusses, about 10m apart, supporting relatively small continuous joists to form the deck. Each line of trusses was supported by a granite-faced concrete pier, of unspecified dimensions, but they would have been about the same size as those eventually built – about four metres square. Some artist’s impressions of the bridge show these supports as a continuous wall pierced by openings. Generally, artists do not seem to have fully appreciated the great width of the bridge – 160 feet (49m). With the truss span at 174 feet (53m) the steel structure is nearly square rather than long and narrow as are most bridges.
The result at ground level for the public in Dawes Point Reserve would have been a series of walls, or sets of closely spaced piers, breaking the area up into uninviting spaces and from most viewing angles, a continuous wall of granite.
Freeman’s re-design used only two lines of trusses, spaced apart by the same dimension as the main arch trusses – 98ft 6in. (30m), with only two lines of piers which from no point of view, present an obstruction to the wide harbour vistas. The floor structure is formed by trusses as deep as the main trusses. Freeman stated that the two-truss design was amenable to accurate analysis as well as being cheaper by £150,000.
The second significant change which Freeman made was in the number of panels in the main arch trusses. Bradfield had made his design using a panel length of 50 feet and thus 33 panels in the 1650 feet span. This was probably based on the size of the holds of ships which would bring steel plates from overseas to Sydney. Freeman thought that a panel length of 60 feet was able to be economically supported by transport capacity and would be cheaper in that the number of members and joints would be fewer, even if sections were slightly larger. Thus, he arrived at 28 panels and quite deliberately chose an even number. Bradfield’s 33 panels would have required the centre panel to be counter-braced for symmetry. Freeman thought that that this was ugly. The Hell Gate bridge in New York, built in 1916, had a counter-braced centre panel and he didn’t like it. Bradfield was not too fussed about the centre panel.
However, the overwhelming argument for an even number of panels was erection. For a 27-panel bridge one half arch would have 13 panels, the other 14 as they reached out to meet. Each would have to be designed for its own load conditions, or the less loaded one built to the more loaded design and thus waste material and money. The difference would not have been trivial as the 14th panel at the extreme end of the cantilever would have loaded it significantly more than its unloaded mate. Freeman says the difference is 15%. The calculations for the jacking force required to close the top chord would also have been more difficult.
Bradfield was not convinced by this argument and suggested in discussion after the event that the two half arches could have been identical and lowered to meet at the intersection of the braces in the centre panel, with top and bottom chords then added and stressed. This would seem to require the centre panel braces to be very substantial with very strong joints.
The third change was more or less required by the changed number of panels. The deck passes through the lower chord in the fourth panel and in working out how an effective portal frame could be provided in the lower chord cross bracing system, the whole truss structure of the arch was lowered by 10ft 6in (3.2m) to RL 27.50; this had consequences in terms of the relationship to the abutment towers. The pylons were lowered from 310 feet to 285 feet, and other details changed.
