## Ribbed or waffle slabs

Current practices for determining forces in ribbed and waffle slabs may also be used for designs to Eurocode 2.Where a waffle slab is treated as a two-way slab refer to previous section, but note that their torsional stiffness is significantly less than for a two-way slab and the bending moment coefficients may not be appiicabie.Where it is treated as a flat slab reference may be made to Chapter 7, originally published as Flat slabs4

The position of the neutral axis in the rib should be determined, and then the area of reinforcement can be calculated depending on whether it lies in the flange or web (see flow chart in Figure 6). The main differences compared with BS 8110 are that the assessment of the flange width is more sophisticated (see Figures 7 and 8).

Where a slab is formed with permanent blocks or a with a topping thickness less than 50 mm and one-tenth of the clear distance between ribs it is recommended that a longitudinal shear check is carried out to determine whether additional transverse reinforcement is required (see BS EN 1992-1-1, CI 6.2.4).

Table 8

Bending moment coefficients for two-way spanning rectangular slabs supported by beams

Table 8

Bending moment coefficients for two-way spanning rectangular slabs supported by beams

 Type or panel and moments considered Short span coefficients for values of ly/lx Long-span coefficients for all values of ly/lx 1.25 1.5 1.75 2.0 Interior panels Negative moment at continuous edge 0.031 0.044 0.053 0.059 0.063 0.032 Positive moment at midspan 0.024 0.034 0.040 0.044 0.048 0.024 One short edge discontinuous Negative moment at continuous edge 0.039 0.050 0.058 0.063 0.067 0.037 Positive moment at midspan 0.029 0.038 0.043 0.047 0.050 0.028 One long edge discontinuous Negative moment at continuous edge 0.039 0.059 0.073 0.083 0.089 0.037 Positive moment at midspan 0.030 0.045 0.055 0.062 0.067 0.028 Two adjacent edges discontinuous Negative moment at continuous edge 0.047 0.066 0.078 0.087 0.093 0.045 Positive moment at midspan 0.036 0.049 0.059 0.065 0.070 0.034

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### Responses

• Michael
How to design check continuous two way ribbed slab?
8 years ago
• lena k
How to determine the distance o f waffle in Bs?
4 years ago