The Gladesville Bridge in New South Wales, the first 1,000-foot concrete span, built nearly 50 years ago

David Goodyear's excellent article on the spectacular Hoover Dam Bypass Bridge in the August issue of Structural Engineer served to remind me how few large concrete arches are built these days. In fact, there have only been six such spans in excess of 1,000 feet since the Gladesville Bridge in New South Wales, the first 1,000-foot concrete span, was built nearly 50 years ago.

The bridge spans the Parramatta River between Gladesville and Drumoyne, west of Sydney, New South Wales, and is the first crossing upstream of the famous Harbour Bridge.

The contract was awarded in October 1957 on the basis of an alternative design submitted by a bidder as allowed by the tender documents. This, and the ground-breaking nature of the design, served to prolong the detailed design and approvals process, and construction did not begin until some three years later. However, the bridge was substantially complete by 1962, although it was not opened to traffic for another two years, pending the completion of the adjacent Tarban Creek Bridge, a 300-foot span concrete portal frame/arch designed and built by the same team.

The Gladesville Bridge is comparatively little known in America, probably because it was designed in England and built in Australia, but at the time it was a noteworthy engineering achievement, setting several new standards and containing a number of innovations in bridge design and construction that have subsequently been widely adopted.

• It was the first 1,000-foot span concrete bridge. The recent development of cable-stayed concrete bridges has resulted in this being almost commonplace but the first big cable-stayed concrete bridge, the Ricardo Morandi-designed Maracaibo Bridge in Venezuela, with spans of 770 feet, was an almost exact contemporary of Gladesville.
• It held the distinction of being the longest span concrete arch bridge in the world for nearly 20 years, until the completion of the Krk Bridge in Croatia in 1980. After nearly 50 years, it still ranks seventh.
• It was the first major concrete arch bridge built using precast segments, albeit with (unreinforced) cast-in-place joints.
• It was the first major concrete arch bridge jacked at the quarter-points, rather than at the crown. Moreover, it was only jacked once, during construction, whereas the Krk Bridge, for example, was re-jacked twice more in subsequent years after completion.
• It was one of the earliest concrete bridges in which the deck was made structurally continuous for live load by the use of unstressed reinforcement contained in cast-in-place concrete between the precast girders over the piers.
• It was probably the first major bridge to rely entirely on the flexibility of concrete columns to accommodate longitudinal movements of the deck due to shrinkage, creep and temperature variation. The 2,000-foot-long deck contains only two expansion joints and is monolithic with the top of each pier. Concrete hinges were incorporated to increase the effective slenderness ratio of the columns, and hence their flexibility, and to prevent moments being transmitted into the deck by the flexure of the piers. The columns have a solid rectangular section only 2 feet wide so that the tallest ones, being approximately 110 feet high, have an effective slenderness (kl/r ) ratio in excess of 133.
• It was almost certainly the first bridge to use piers of this kind constructed from precast segments.
• It was also one of the first, if not the first, major bridge to use integral abutments, later to become a popular method of eliminating the problems surrounding bearings and expansion joints at conventional abutments.
• It was undoubtedly one of the first bridges for which the design utilized a suite of computer programs for analysis and detailed design. These programs were written for the purpose, as there was no such thing as proprietary engineering software on the market at that time.

The first program took equations of the parabolas of the top and bottom surfaces of the arch derived from preliminary calculations and determined the elevation of the centerline (the geometric profile) and the true depth, i.e., normal to the profile, of the arch at a large number of stations along its length.

The second program calculated the section properties at each of these stations using the true depth of the section. (The depth of the section was the only variable dimension.) The program then calculated the weight of each segment of the arch between these stations and derived the funicular profile.

The third program performed an elastic analysis of a fixed-ended arch of varying section. It produced influence lines from which the bending moments and thrusts due to the concentrated loads from the deck could be determined and it also calculated the bending moments due to unit thrust and bending moments and thrust due to unit strain so that the effects of rib-shortening, creep and shrinkage could be derived.

The fourth program combined the outputs of the two previous programs to obtain concrete stresses at all sections along the arch. This information was then used to go back and adjust the curves of the top and bottom surfaces of the arch as necessary, firstly to ensure that there was satisfactory concurrence between the geometric profile and the funicular profile so that it could be assumed that there were no bending stresses due to the self-weight of the arch and, secondly, that there was acceptable residual compressive stress at all sections along the arch under the combined effects of dead and live load from the deck, rib-shortening, shrinkage and creep. In the end, only one iteration was needed to achieve a maximum deviation of 3/8-inch between the geometric and funicular profiles over the 1,000-foot length of the arch.

Finally, the last program calculated the detailed dimensions for casting each of the tapering box-section arch segments, assuming them to be cast upright with their longer face at the bottom (there was no AutoCAD to do this for you in those days!).

Bearing in mind that this bridge was designed in the late 1950s, it can probably quite justifiably be claimed to have been somewhat ahead of its time in several respects. It is still in service and in excellent condition, partly due no doubt to Sydney's mild climate but also unquestionably because of some of the design innovations described above, aimed at reducing or eliminating future maintenance requirements. It could also be relevant that 99 percent of the concrete in the bridge, from the arch to the post-tensioned deck girders and the post-tensioned columns, is in a state of permanent compression.

The bridge was originally designed to carry six lanes of traffic and two generously wide footways. In the late 1970s traffic volumes decreed that one of the footways be converted into an additional lane and this was done without any structural modifications.

Anthony F. Gee, P.E. has been responsible for the design and construction of major steel and concrete bridges worldwide and is now Principal of Tony Gee International, based in Tallahassee, Fla. Contact him at tonygee@tonygee.net.