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11.12 “slab specification” with regard to negative reinforcement calculation

This manual includes three distinct “slab specifications” to allow the designer to provide designs in accordance with the British or Australian minimum reinforcement requirements. The background to this is detailed in Section 3.0 “Secondary Reinforcement” and Section 10.0 “Shrinkage”. A major difference between the two codes is their allowance of non ductile mesh used in Negative Moment Calculations. The British Code ( slab specification “B” ) strictly prohibits is use in both continuity and fire resistance calculations. The Australian Codes on the other hand ( Slab Specification “A” and “R” ) do allow non ductile secondary reinforcement for both.

A review of section 11.10 and its basis, HERA document R4-82, indicates that substantial redistribution occurs in negative regions under fire loading conditions. Non-ductile mesh is not used in this manual, under fire loading, as the possibility of premature failure could occur:

A consequence of these limitations within the negative reinforcing tables include :-
• Slab Specification “B” results in a greater level of negative reinforcing compared to “A” and “R” slab specifications for the same span. This is due to the mesh being able to contribute to normal continuity calculations.
• Because of the higher levels of reinforcement associated with Slab Specification “B”, it appears that for some spans, greater FRR are achievable. The designer should consider all facets of the design before using this as a basis of selection, since other design considerations may be dominant.

Slab Specifications “A” and “R” should provide better overall control of cracking, at a minimal cost.