Research is currently being carried out at the Transport and Road Research Laboratory to refine methods used for the assessment of existing structures or develop new methods where none suitable exist. Techniques involve the testing of both large scale and full size structures and the development of computer programs. These enable the lessons learned about load redistribution through interlinking structural components to be utilised in assessment. The object of the work reported in the Research Report was to rationalise the knowledge of the behaviour of complex structural systems to allow some quantification of the gratuitous strength thought to exist in these structures. This would allow the assessed strength of many of the older structures to be increased. Currently all older bridges in the United Kingdom are being assessed to ensure that they are capable of carrying increased vehicular loading, and strengthening or replacement of understrength bridges will proceed over the next decade. More precise estimates of the strength of bridges would enable some of this strengthening to be deferred resulting in considerable savings of maintenance funding. The work described in this Report has shown that, under loading to failure, a simply supported concrete beam and slab deck did not behave in the way in which the design codes assume, in that the mode of failure was not predicted. The load at failure was close to that which was calculated using the design code once the implicit factors of safety were taken out and so extra strength could not be shown. However, as the mode of failure was not predicted, the agreement of failure load was fortuitous. (A)

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