A A

Figure 4.7 Ribbed slab spanning between beams

4.1.3.1.1 Durability

For a dry environment, exposure class is 1. Minimum concrete strength grade is C25/30.

For cement content and w/c ratio, refer to ENV 206 Table 3.

Table 4.1 ENV 206 Table NA.1

Minimum cover to reinforcement

15 mm

Assume nominal aggregate size = 20 mm Assume maximum bar size = 20 mm

Nominal cover > 20 mm

Use nominal cover

25 mm

20 mm nominal cover is sufficient to meet the NAD requirements in all respects. Table 3

Check requirements for fire resistance to BS 8110: Part 2. NAD 6.1(a)

4.1.3.1.2 Materials

Type 2 deformed reinforcement, f = 460 N/mm2

C25/30 concrete with 20 mm maximum aggregate size

4.1.3.1.3 Analysis model

Span > 4 x slab depth 2.5.2.1(3) 6 m > 4 x 0.275 = 1.1 m OK

Flange depth = 100 mm 1

> — x clear spacing between ribs > 50 mm ... OK

Transverse ribs (at supports only)

Hence the ribbed slab may not be treated as a solid slab in the analysis under 2.5.2.1(5) the terms of this clause unless intermediate transverse ribs are incorporated. This is not always desirable.

The model adopted in this example uses gross concrete section properties of the T shape in sagging regions and a rectangular section, based on the rib width, in the hogging region.

EC2 Figure 2.3 has been used initially to define the extent of the hogging. This method can clearly be refined.

4.1.3.1.4 Effective span

Assume 300 mm wide supporting beams / = 5700 mm n a, at edge beam = a. taken as t = 150 mm a2 at central beam = a. = (y-) t =150 mm I „ = 6000 mm erf

For ratio of adjacent spans between 1 and 1.5 / = 0.85/, = 0.85 x 6000 = 5100 mm

4.1.3.1.5 Effective width of flanges

Effective flange width is assumed constant across the span for continuous beams in buildings.

For a symmetrical T beam b = b + (UI < b eff w v 5 ' o

= 125 + (-L) x 5100 < 600 mm Therefore b = 600 mm eff