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Standard deductions are applied to the dimensions of bent bars between concrete faces to improve the confidence in achieving minimum covers and to improve fit.
Traditionally, the deductions in Table 3.24 of BS 8110-1 Structural use of concrete (withdrawn and replaced by BS EN 1992-1-1 Eurocode 2: Design of concrete structures) have been applied. These values are the basis of Table 5.5 in Standard Method of Detailing Structural Concrete (3rd edition, Institution of Structural Engineeers, 2006) which are applied by detailers to the nominal dimensions of bars that are intended to be used between two concrete faces.
BS EN 13670:2010 Execution of structural concrete introduced new sets of permitted deviations in covers and element thicknesses. This allows much greater (and perhaps more realistic) permitted deviations in dimensions than the 5mm that is the basis of BS 8110 Table 3.24. Greater deductions than those in the BS 8110 Table 3.24 should be applied, but the continued use of Table 3.24 deductions with the new permitted deviations may lead to the potential of low covers. This has led to the need to reconsider these deductions.
A proposal has been published in The Structural Engineer, Bar schedule dimensions: deductions for permitted deviations, Institution of Structural Engineers, Dec 2013 p45-51 [Goodchild CH and Toplis P]