As I drive or ride over Commonwealth Avenue Bridge, I often think of Sir Robert Menzies—no, really. Menzies thought the Bridge was the ‘finest building in the National Capital’. Opened to traffic in November 1963 over the yet to be filled lake, the bridge was and is regarded as a lovely example of form and function working together. It remains the principal entry point to Canberra’s National Area. How did something we may now take for granted and not give a moment’s thought come about?

In 1957, British town planner Sir William Holford was invited by Menzies to review the Griffin plan, which had basically languished since Canberra was formally named capital on 12 March 1913. In his report, Observations on the Future Development of Canberra, Holford pushed Griffin’s idea of incorporating a lake to unite the two halves of Canberra. His view was that Canberra could either remain divided with the flood plain of the Molonglo as an ‘open wedge’ between the federal town on the south bank and a municipality on the north—or it could become a unified city. He didn’t shirk the issue—he was convinced the NCDC should opt for the latter option:

Real quality and imagination in the design of three features of the new city - the Commonwealth Avenue Bridge, the permanent Parliament House, and the Lakes, would lift Canberra at once into the ranks of the significant capital cities of the world.

The development of Lake Burley Griffin—one of the key features of Griffin’s plan for the capital—was perhaps the NCDC’s key undertaking during the 1960s. The Scrivener Dam was completed in September 1963 and began filling to form the lake. The decision to create the lake meant that new bridges had to be built to replace existing timber structures across the Molonglo River. Intriguingly, on some early maps showing Griffin’s plan, there are three bridges across the lake—Commonwealth and Kings Avenue bridges and an additional one spanning the lake from Black Mountain Peninsula to the area near the bottom of Novar Street in Yarralumla. Obviously, this was never built. So the four lane Kings Avenue Bridge was opened on 10 March 1962, with a motorcade led by Prime Minister Menzies.

The Commonwealth Avenue Bridge needed to accommodate six lanes of traffic on two structures, with pedestrian access cantilevered out from each. Approaches to each side needed to pass under Commonwealth Avenue. The design requirement was for a sleek, horizontal structure with a reasonably long span resting on slender piers—very modern. This low profile aesthetic would make sure there was visual continuity between the central and west basin of the proposed lake. The final design was submitted by G. Maunsell and Partners and William Holford and Partners, while Hornibrook Pty Ltd would be the construction company.

Each bridge was designed as a single geometrical arc formed by a continuous pre-stressed concrete box girder 2.7m thick and over 300 metres long. The roadway width on each bridge was over 11 metres and an asphalt/concrete surface for the road was designed to be placed on the top element of the box girder members.

Work commenced on the bridge in March 1961. At the peak of building, up to 500 men would gather on the flood plain to work—uprooting trees, removing soil, cottages and dismantling the timber truss bridge at Commonwealth Avenue. The construction required extremely close attention to detail and high quality materials for the pre-stressed concrete. After extensive testing it was discovered that all of the required materials were available locally: high quality aggregates, the right rock and various types of sand. Extensive falseworks were required for the initial support of the bridge superstructure and for the degree of accuracy required in placing the pre-cast concrete bridge segments. Critically, no floods were experienced during construction, which would have hampered these efforts with the timberwork.

The Bridge incorporated several design features that were new to major bridges in Australia. The superstructure comprised the first post-tensioned concrete multi-webbed torsion box made of jointed pre-cast segments. The one hundred segments were stressed together in one operation over the full length of deck by 320 metre long external tendons. To a non-engineer, this still seems like a remarkable feat. For each bridge 102 reinforced concrete box segments 3 metres long and weighing 45 tons were cast on site and after curing were carefully placed by gantry on the timber falseworks. The 3 inch wide gap between each segment was filled with fine concrete to make a continuous box girder over 300 metres long. This was post-tensioned by external high tensile steel cables. After final checking for stresses the cables were encased in concrete to prevent corrosion.

Following on from the successful example of Kings Avenue Bridge, lighting for the Commonwealth Avenue Bridge was achieved by fitting a line of fluorescent tubes in each outer handrail. This integrated lighting marked the edge of the carriageway and provided illumination for the footpath and an impressive night-time display of the bridge structure, seen in countless postcards since. The resulting absence of post-top lighting on the bridge further enhanced the sleek appearance of the structure and didn’t break the view of the lake and the Parliamentary Zone.

Commonwealth Avenue was more than just a link in the city’s road network: it also performed an important role in Canberra’s developing sewerage system. Who knew? The original intercepting sewer to the north had been built in an inverted siphon under the Molonglo River near Commonwealth Avenue. The proposed lake raised concerns that the tunnel might collapse during flood, so the decision was taken to incorporate sewer mains within the superstructure of the bridge itself: 500 millimetre inside the eastern bridge and 450 millimetre on the western bridge. A vent was incorporated in the south-east decorative pylon and a pumping station built next to the northern approach of the eastern bridge.

Ever noticed the old stones in the supporting embankments? When London’s historic Waterloo Bridge was demolished in 1936 some of the large, flat stones from the 119 year old structure were donated to the Commonwealth Government. The stones were stored in quarries around Canberra for decades and eventually used in the supporting embankment under Commonwealth Avenue Bridge.

The Bridge is a twin structure, with separate parallel carriageways. The bridge structures are of 5 spans totalling 310 metres, of fully continuous prestressed design. It has a higher camber than Kings Avenue Bridge, a design feature to allow for yachts to sail beneath. The end pylons give added importance to the bridge as the main route to Parliament House. To provide an exit and entry clover leaf layout, separate structures are built on the main bridge’s south approaches, totalling 49 metres each, in 4 approximately equal spans.

The Bridge is like an important piece in a puzzle. As we can see now, the Central Basin of the lake is the centrepiece of the National Area in the immediate foreground of the National Area, and unites the wider city-scape and public spaces on its foreshores in which are now set major national buildings. The Bridge brings the natural beauty of the mountains and adjacent hills into focus and visually unites them with this created setting for our public buildings. It provides marvellous aesthetic qualities and is a major feature of Canberra’s viewpoints at Black Mountain, Mount Ainslie and Red Hill lookouts.