Berri Bridge Arrives

The town of Berri, some two hundred kilometres North-East of Adelaide, will never be the same again following the completion of a new bridge across the Murray River. "It's about time! We have been waiting for more than 30 years for this bridge", said a middle aged gentleman as he watched the bridge being launched.

The bridge now provides a convenient crossing, and replaces a ferry service which had been in operation for over 70 years. Gone are the frustrating days when the crossing could take up to 45 minutes during peak traffic times.

Hence, construction of the launched composite steel bridge was of great interest to the townspeople and provided a conversation piece both during its inception and construction period. The climax came at its opening which was marked by a day of festivities and fanfare with locals and politicians enjoying the walk across the bridge to the sound of an orchestra with the spectacle of parachutists descending overhead and fireworks at dusk.

The realisation of this bridge is an example of what can be achieved with the close co-operation of enterprising private industry and government coupled with innovative engineering. Built Environs and Connell Wagner prepared the proposal in close consultation with the Department of Road Transport, local townspeople and the Aboriginal community who owned some of the land on the Loxton side of the river. The contractor financed the work and then transferred ownership to the State of South Australia on completion.

The bridge was completed on budget and ahead of time, despite delays due to high floods. It also won the coveted Institution of Engineers Excellence Award (South Australia), with Built Environs and Connell Wagner, the design consultant, being joint recipients of the award.

Concept and Design Details

The Berri Bridge is an incrementally launched, composite steel girder and concrete deck bridge of 330m length with 11 spans. The spans vary from 20m at each abutment to 40m at the navigation span, with the typical span being 33m. The bridge is straight in plan, but has a vertical circular curve of 2485m radius.

The abutments are spread footings founded behind gravity retaining walls. The Berri abutment provides the longitudinal fixed support. All piers are supported on piled footings. These include bulb-base cast-insitu driven piles for land piers and driven steel tube piles for river piers.

Design was carried out in accordance with the AUSTROADS '92 Bridge Design Code, except for the superstructure and all steelwork, which were designed to the NAASRA Bridge Design Specification (1976). This approach was necessary since the steel and composite sections of AUSTROADS had not been published at the time of design.

lan Ide, director of Connell Wagner, said: "The initial intention was that the bridge be a concrete box structure with integral deck, although the option of adopting steel girders with composite concrete deck was also examined. BlueScope Steel was helpful in ensuring that a design involving steel girders could be a viable option. For the remainder of the concept design stage both the steel and the concrete options were considered. The decision to adopt the steel option was made shortly before the final agreement was signed. This proved to be a winner ".

Incremental Launching

After consideration of alternatives, an early decision was made to adopt the incremental launching technique as the minimum cost solution for this bridge.

In addition, it was judged that this approach would reduce construction time and risks, principally due to fewer construction activities being required over water.

Ship Impact and Pier Redundancy

The navigation span was required to be 40m to allow adequate clearance for shipping. Discussions at the concept stage regarding ship impact indicated the design vessel to be a 1760 tonne vessel travelling at 4 knots, with a river current velocity of about one metre per second. All river piers were to accommodate this loading.

Connell Wagner recognised that the cost to construct the five river piers for this loading was prohibitive and proposed the concept of 'pier redundancy' for the steel girder option. This option was adopted such that if any pier is removed the bridge would not collapse and continue to provide a limited service. "The steel option is the only one suited to this concept because of its lightness and ductility", said Vic Nechvoglod, designer for Connell Wagner. "... the superstructure has the overload capacity to carry limited one-way traffic on the centreline of the bridge with any one of the river piers removed. I believe this is the first bridge in the world to be designed in this way".

Superstructure

A four girder option was the minimum cost solution, given the construction method and the weight reduction achieved by using a minimum thickness deck. Typically, the girders are a constant 1395mm deep with a 23mm web, 395 x 20mm top flange and 550 x 40mm bottom flange. The bottom flange was increased to 550 x 50mm at the main navigation span. All steel is Grade 350 L15 XLERPLATE®*.

A compact girder design was adopted to ensure ultimate failure by yielding of the girder flanges. This was considered a prerequisite for the redundant pier philosophy since the navigation span would increase from 40m to a maximum of 73m if a pier were removed.

The girder spacing was chosen to ensure essentially even loads on the bearings and piers under dead load. This was important during launching and for pier design under the pier removed load case.

Bracing was selected to ensure that the girders can develop their full flange yield moment capacity. The girder web was designed to ensure that web buckling did not occur during launching and included allowances for construction tolerances for bearing installation, girder soffit fabrication and the casting bed assembly.

In keeping with the principle of repetition, all girders and bracings were identical and the girder web thickness and top flange were selected to be constant for the full length of the bridge. This allowed the use of 'telescoping' deck forms, necessary to achieve a weekly launch cycle and reduction in time and costs.

Bearings

The pot bearings were designed to meet service and launch loads. A number of configurations and types of bearings were considered in the concept stages and trials carried out on some innovative concepts. Launching over the permanent bearings using teflon pads was finally adopted as a proven economical option for the bridge.

Aesthetics

A decision was taken during the early part of the design and documentation phase to engage an architect, Greenway International Architects, to comment on the bridge elements - such as the hand rails, lighting, pile cap, pier and cross head configuration. "I believe that the end result more than justified this approach" said David O'Sullivan, director of Built Environs. "... I think the bridge is an attractive addition to the landscape of the town".

Design for Construction

The principle of repetition was fundamental for economy in the design of nearly all items including the piles, girders and pedestrian hand rails. The design and detailing of all steelwork included close involvement of the designers with the builder and fabricator.

Fabrication of the girders and detailing for minimum cost was discussed with the fabricator, Ahrens Engineering, at several meetings prior to final design. Initiatives included:

• Trial welding of the flange to the web
• Flange butt weld, bracing cleat and bearing stiffener welding
• A full scale mockup of a girder splice site weld
• Trial welding of sections by Ahrens to indicate practical tolerances
• Choosing the web thickness as 23mm so web plate stiffeners were not required for launching
• Beam camber allowance for welding.

In establishing the segment lengths, trucking load limits and costs were reviewed, and the girders split into 17 equal lengths of just under 20m. Any longer would have resulted in police escorts at substantial extra cost. To reduce fabrication splicing costs, BlueScope Steel provided plates rolled to the required length, except for the 40mm and 50mm bottom flange plates which would have been too heavy to handle. The segment length was incorporated into the final design after consideration of costs by the fabricator, construction staff and designer.

To reduce site activity, time and costs, the girders were assembled in pairs with the bracing installed in the painter's yard after painting. This girder pair assembly was then transported to site. Girder bracing was bolted to enable adjustment on site to meet launch tolerance requirements. Bracing was provided in the outer two bays of the girders leaving the central bay completely clear. This clearance was specifically designed to serve two main functions:

• To allow a clear travel space for the launching bracket and jacking system to enable a continuous launch of approximately 39m, at a launch rate of about 12 m/hr.
• To allow space for guide frames on each pier to engage a drop down concrete panel in the deck soffit. This provided guidance and lateral restraint during construction and launch.

The design was aimed at achieving a one week launching cycle. This was achieved for seven of the eight launches. Each launch cycle included a field splicing of two segments to each of four girders (eight splices per launch), erecting formwork, fixing reinforcement, pouring 39m of deck, and installing 39m of traffic barrier and hand railing.

The concrete deck was poured on all but the first 30m of girders. This 30m of undecked girders was designed to double as the 'launching nose', thus reducing construction time and costs.

Novel and innovative design for temporary works included a launching bracket for fixing the jacking system to the girders, and a lifting mechanism for the launching nose to raise it onto the bearings. Both were jointly developed by Build Environs and the designers, this showing the benefit of a co-operative approach in adapting design to meet construction requirements. In this case the need for a special 'launch nose' was avoided and construction costs minimised by achieving 39m continuous launch operations on a weekly cycle.

Conclusion

The inception and execution of this difficult project was simply exemplary. The innovations used in its design and construction are world class and provide a benchmark for bridge designers and constructors.

The spirit of co-operation achieved on this project is exemplified by the positive involvement of the local people and the Aboriginal community who celebrated by painting a huge mural under the Berri abutment. The painting is set to become a historic landmark and a popular tourist attraction.

* From 1 August 2002, BlueScope Steel Plate Products are known as XLERPLATE®