The Oyster Channel Bridge near Yamba was constructed in 1937 and is being widened to provide an improved carriageway width and a pedestrian footpath. The project is funded by the Roads and Traffic Authority of NSW.

During the project conceptual design phase, a requirement to strengthen the existing bridge superstructure was identified. A proposal for strengthening using carbon fibre reinforced polymer (CFRP) laminates was proposed and included in the preliminary project cost estimates.

Maunsell Australia Pty Ltd was commissioned to undertake the detailed design phase of the bridge widening. It was recognized that the early estimates for deck strengthening requirements were based on an elastic analysis method. The elastic model does not account for the true post elastic behavior of materials at the ultimate limit state. The Austroads Bridge Design Code (or AS 5100) does allow plastic redistribution of moments, but this allowance is rarely used by designers of multi-beam bridges because of the computational difficulty.

SAM Integrated Bridge Software was used for the detailed design analysis, and effective use was made of the unique "plastic member limit" analysis feature. The analysis involved setting "plastic member limits" on the existing girders equal to their ultimate flexural strength. For this analysis, when the specified member limit is exceeded the load effect is forced to remain constant while the deflection increases by iteration until structure equilibrium is achieved for the applied load condition. By this method, the load was demonstrated to shed from the overloaded existing members under the traffic lanes to both the new stronger girders on the outside and to the lesser loaded existing girder under the new footpath. The moment curvature capacity of the deck components was checked to ensure that adequate ductility existed in the members to accommodate the plastic deflection with safety, and then serviceability limit state checks were carried out.

The SAM design code section analysis facilities were used for accurate calculation of the beam limiting capacities, and for determination of concrete and reinforcement stresses, and crack width calculations at the serviceability limit state. Service limit state conditions were checked using an elastic analysis taking advantage of cracked section properties where appropriate. The elastic analysis was also used for establishing the maximum shear forces (plastic deformations necessary for shear redistribution are less reliable and worst case of the plastic and elastic distribution of shear forces was used).

As a consequence of the above analysis significant CFRP strengthening to enhance flexural strength for the Oyster Channel Bridge was avoided. Shear strengthening was still considered necessary, and this was achieved using CFRP laminates bonded as shear stirrups in a zone adjacent to supports.

The method of analysis adopted for the Oyster Channel Bridge project demonstrates the significant advantages in adopting plastic methods of analysis for the assessment and widening of existing bridges.

Images courtesy of RTA and Maunsell Australia Pty Ltd