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Assessment of existing structural strength

The Highways Agency document BD 44/95, The assessment of concrete highway bridges and structures, modified clauses in BS 5400 Part 4, Code of practice for design of concrete bridges, for use when determining the strength of existing bridges.

It uses the concept of ‘Worst credible strength’ for the concrete and the steel. These are defined as “The worst value of the strength which the Engineer based on his experience and knowledge of the material realistically believes could occur. The method of determining the worst credible strength should be agreed with the relevant Overseeing Authority”. Lower values of material partial safety factors may be adopted when the worst credible strength rather than the specified characteristic strength is used. For example, for concrete the partial safety factor reduces from 1.5 to 1.2 and for shear in concrete from 1.25 to 1.15.

Not all the design equations in BS 5400 specifically include the materials partial safety factor, so where appropriate BD 44 modifies the equations so that the materials partial safety factors are shown. The equations can then be used with the appropriate value for the partial safety factor (depending on whether the characteristic or worst credible strength is used) to determine the strength of the structure.

The same approach can be used to assess the strength of buildings, using the design approach in BS 8110, Structural use of concrete. It should be noted that in some cases the equations used differ slightly from those used in BS 5400 though the approaches are the same.

If details of the Standards to which the structure was designed and constructed are available, these can obviously be used to obtain an indication of the expected steel and concrete strengths. Further information on the development of specified steel and concrete strengths, design approaches etc over the years is given in Concrete Society Technical Report 70, Historical approaches to the design of concrete buildings and structures.

Note 1. From 1 April 2015, Highways Agency has changed name to Highways England

Note 2. BS 5400-4: 1990 has been withdrawn and replaced by BS EN 1992-2: 2005

Note 3. BS 8110: 1997 has been withdrawn and replaced with BS EN 1992-1-1: 2004

Acknowledgement: The Concrete Society

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TR70 Historical approaches to the design of concrete buildings and structures