Slab Design Details

Slab Design Details Minimum area of reinforcement (BS 8110 : Clauses 3.12.5.3 and 3.12.11.2.9) The area of reinforcement in each direction should not be less than: (a) 0.13% of bh for fy = 460 N/mm2; (b) 0.24% of bh for fy = 250 N/mm2. Where the control of shrinkage and temperature cracking is important, the minimum areas should be increased to: (c) 0.25% of bh for fy = 460 N/mm2; (d) 0.3% of bh for fy = 250 N/mm2. Concrete cover for durability (BS 8110 : Clause 3.3) Table 2.5-7 in Section 2.5(e) gives the nominal covers to meet the durability requirements for the slabs and other structural members. Note that in Table 2.5-7 the meaning of nominal cover is as defined in BS 8110 : Clause 3.3.1.1, namely: the nominal cover is the design depth of concrete to all reinforcement, including links. It is the dimension used in design and indicated on the drawings. Table 2.5-7 Durability requirements (BS 8110 : Clause 3.3.3)S

Exposure condition Mild Moderate Severe Very severe Maximum free w/c ratio Minimum cement content (kg/m3) Concrete fcu (N/mm2)

25 – – –

Nominal cover (mm) 20 20 35 30 40 – – 50

20 20 25 30

0.65

0.60

0.55

0.45

275 30

300 35

325 40

400 50

Fire resistance (BS 8110 : Clause 3.3.6) The fire resistance of a slab depends on its overall thickness and the concrete cover to the reinforcement, as shown in Table 8.8-1.

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Table 8.8-1 Fire resistance requirements for slabs (BS 8110 : Part 2 : Clause 4.3.1)

Fire rating (hours) 1 2 3 4

Minimum thickness (mm) Simply supported Continuous 95 95 125 125 150 150 170 170

Concrete cover to MAIN reinforcement (mm) Simply supported Continuous 20 20 35 25 45 35 55 45

Simplified rules for curtailment of bars (BS 8110 : Clause 3.12.10.3.1) The following simplified rules may be used if (i) the loading is pre-dominatly uniformly distributed, and (ii) for continuous slabs, the spans do not differ by more than 15% of the longest span. In these rules, l refers to the effective span length and Ø the bar size. (a) Simply supported slabs: All the tension bars should extend to within 0.1l of the centres of the supports. At least 40% of these bars should further extend for at least 12Ø (or its equivalent in hooks or bends) beyond the centres of the supports. (b) Cantilever slabs: All the tension bars at the support should extend a distance of l/2 or 45Ø, whichever is greater. At least 50% of these bars should extend to the end of the cantilever. (c) Continuous slabs: (1) All the tension bars at the support should extend 0.15l or 45Ø from the face of the support, whichever is greater. At least 50% of these bars should extend 0.3l from the face of the support. (2) All the tension bars at midspan should extend to within 0.2l of the centres of supports. At least 40% of these bars should extend to the centres of supports. (3) At a simply supported end, the detailing should be as given in (a) for a simply supported slab.

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