RCC Design Userguid

RCC92 Ribbed Slabs (Tables).xls This spreadsheet designs simple single-, two-span and multiple-span ribbed slabs to BS 8110: Part 1 using the moment and shear factors in, or in the case of single and two spans, consistent with Table 3.12 of BS 8110. The use of factors from Table 3.12 is governed by Clause 3.7.2.7 as follows

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A single load case is assumed and the

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Conditions of 3.5.2.3 are met bays > 30 m 2, qk

> / 1.25gk,

qk

> / 5.0

kN/m2 and

at least three bays are of approximately equal span

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The corresponding factors for beams also restrict use of the factors to where spans differ by no more than 15% of the maximum span.

The factors used for two-span slabs should be considered subject to these same conditions. They were derived by modelling the appropriate number of spans with a single load case of 4 kN/m 2 dead and 5 kN/m2 imposed, and allowing any one span to be 15% less than the input length (strictly according to BS 8110 this is applicable to beams only). The factors used are based on continuous end supports. Where the relevant conditions are not met, users are directed towards RCC32.xls where continuous beam type analysis overcomes many of these caveats.

MAIN! This single sheet consists of the input and main output. In itself it should prove adequate for the simplest ribbed slab designs. Inputs are underlined and most should be self-explanatory. The option to have top steel in spans or not has bearings on whether shear links can be accommodated and on deflection calculations. The option to have links, minimal (or nominal) links or  no links is a matter of choice for the designer. Most contractors prefer to prefabricate reinforcement for  ribbed slabs on the ground or off-site: this means at least nominal links and nominal top steel are usually required.

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Designed links are taken to be those provided where (vc + 0.4) < v < 0.8 f cu0.5;

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Minimal links are taken to be those that are required to provide shear resistance for  vc < v < (vc + 0.4)

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Nominal links are those used if required for temporary support only in areas where v < v c

Under  Bending , the Width of solid from CL refers to the distance between centre line of support and the rib/solid intersection. It determines where shear and, at internal supports, hogging moment in ribs are checked. The user inputs preferred diameters of reinforcement in the rib. At supports, these bars usually need to be supplemented by intermediate bars to comply with either spacing rules or with hogging moments in the solid section of slab. In spans, the area of steel required, A s, may be automatically increased to reduce service stress, f s, and to increase modification factors to satisfy deflection criteria. An approximate reinforcement density is given. It excludes mesh, supporting beams, trimming to holes etc. Please note that the bending moment diagrams are indicative only. The factors from Table 3.12 give rise to a single load case that has been subject to 20% redistribution: a bending moment envelope is inappropriate. The factors used are given in the table below.

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