Due to the increasing weight and volume of traffic, and also corrosion and deterioration of many bridges, the UK Department of Transport embarked upon an assessment program to check the strength of all the highway bridges in Britain by 1999. With around 150000 bridges, this was a major task.
This bridge in Wales suddenly collapsed without warning in 1985 as a result of corrosion of the prestressing steel
These assessments were almost exclusively performed using elastic analysis methods such as grillage, plate equations, or the finite-element method. However, experiments and full-scale load tests confirm that bridges are often able to carry loads well in excess of the theoretical capacity calculated using this approach. Plastic methods can more realistically model the actual strength of bridges and this results in more economical structures. The plastic approach has largely been neglected because, until now, no generalised solution technique has been available to perform the complex mathematical analysis required for realistic bridge assessment.
Research at Cambridge University has resulted in a new generalised plastic collapse analysis technique for concrete bridges. This has been implemented within the COBRAS computer program. This new approach uses computer graphics and solid modelling concepts to build a three dimensional model of a collapsing bridge within the computer. The 3D model provides all the required geometric information needed for the analysis, including the location of all the failure zones, the details of the fixity at the supports, and the deformed shape of the failed bridge. It also contains full details of the material components, bridge dimensions and the external loadinng.
Other types of mechanism problems, e.g. in soil foundations and building slab failures, could also be solved using the same collapse analysis method.
Dr Campbell Middleton