Guidelines for the Supplementary Load Testing of Bridges

Book excerpt: Bridge authorities in the UK are currently facing a large programme of bridge assessment and strengthening. This has been caused, in part, by the necessity of ensuring that the European Union deadline for allowing 40-tonne lorries on to UK roads can be met. Many bridges have failed theoretical assessments and some bridge owners, frustrated by the fact that many failed structures are apparently in good condition and showing no signs of distress, have resorted to load testing their bridges to try to provide additional information. A National Steering Committee for the Load Testing of Bridges was set up to examine the role of bridge load testing as a tool for assisting the assessment process. The National Steering Committee consists of representatives from all major bridge owners including the Highways Agency, the County Surveyors Society, the London Bridges Engineering Group, Railtrack and the British Waterways Board. It also includes representatives from consulting engineers and universities and has the support of the Institution of Civil Engineers. The overall objective of the National Steering Committee was to produce authoritative guidance on load-testing techniques; which could be used by the practising engineer to determine capacities of existing bridges/structures that are safe, prudent and minimize levels of restriction to the transport infrastructure. In June 1995 the committee appointed Rendel Palmer & Tritton in association with Peter Lindsell & Associates and supported by Professors Bakht, Clark and Harding as consultants to carry out a preliminary study of all the available information on bridge load testing. They were to recommend a detailed methodology which would form the basis of a brief to consultants appointed to produce authoritative guidelines for the load testing of bridges. Their report concluded that there is a place for load testing in the evaluation of bridges and other structures and that load testing is a valid tool for bridge managers. They also concluded that there was enough information and experience available to permit safe and effective guidelines to be written. As a result of the preliminary study the National Steering Committee decided to divide the second stage work and restrict the scope of this document to guidelines for supplementary load testing. Work on proof and proving load testing is being carried out by others under the auspices of the Highways Agency. The guidelines contained in this document were not drafted in a prescriptive form, but seek to provide advice on the appropriate use of supplementary load testing as an aid to assessment by calculation. The guidelines have been written to enable engineers to determine: when it is appropriate to consider the use of supplementary load testing; the level of risk, both public safety and economic, associated with load testing; how to plan and carry out a load test including the level of expertise necessary, the appropriate loading methods and the type and quantity of instrumentation required. In addition, the document is intended to be a source of information on load testing, measuring equipment and specialist techniques that engineers can use for reference.