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Assessment of existing structural strength

The Highways England document CS 455 - The assessment of concrete highway bridges and structures (formerly BD 44/15), modified clauses in BS 5400 Part 4, Code of practice for design of concrete bridges, for use when determining the strength of existing bridges. BS 5400-4:1990 has been withdrawn and replaced by BS EN 1992-2:2005 however the Eurocodes do not currently contain provision for assessment.

CS 455 uses the concept of ‘Worst credible strength’ for the concrete and the steel. These are defined as “The worst value of that strength which the assessor, based on experience and knowledge of the material, realistically considers could be obtained in the structural element under consideration. The method of determining the worst credible strength should be agreed with the relevant Overseeing Authority.

Worst credible strengths for concrete should generally be derived from tests on concrete cores as assessed by BS EN 13791:2019 Assessment of in-situ compressive strength in structures and precast concrete components.

Note that the strength from cores is from the actual structure and not from standard water cured cubes. The materials safety factor applied in design can be reduced (worst credible strength) as the unknowns (batching, placing, compaction, curing) are part of the in-situ strength. For example, for concrete the materials partial materials factor can be reduced from 1.5 to 1.3 – see BS EN 1991-1-1 Annex A.2.3 and PD CEN/TR 17086 Clause 4.3.

Not all the design equations in BS 5400 specifically include the materials partial safety factor, so where appropriate CS 455 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can be used to assess the strength of buildings using the design approach in BS 8110 Structural use of concrete (withdrawn and superseded by BS EN 1992-1-1:2004). 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.

Further information on partial factor reduction can be found in fib Bulletin 80, Partial factor methods for existing concrete structures, 2016.

Acknowledgement: The Concrete Society

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TR70 Historical approaches to the design of concrete buildings and structures