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Figure 4.2 Section through short span support

The use of (2/3)Z at a direct support is an allowance for the transverse compression due to the support reaction.

Minimum area of reinforcement

As < -L < 0.0015£>td = 254 mm2/m 5.4.3.2.1(3)

Minimum area provided (T12 @ 400 mm crs.) near support

Maximum bar spacing =3 hp- 500 mm NAD

Table 3

Maximum spacing used = 400 mm near support OK

4.1.2.2 Design example of a continuous two-way spanning solid slab

Design a solid slab spanning between beams, as shown in Figure 4.3.

In addition to self-weight, the slab carries a characteristic dead load of 1.0 kN/m2 and an imposed load of 5.0 kN/m2.

Figure 4.3 Layout of slab

4.1.2.2.1 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.

Minimum cover to reinforcement = 15 mm

Assume nominal aggregate size = 20 mm

Assume maximum bar size = 12 mm

Nominal cover > 20 mm

Use nominal cover = 25 mm

Note:

20 mm nominal cover is sufficient to meet the NAD requirements in all respects. Check requirements for fire resistance to BS 8110: Part 2.

4.1.2.2.2 Materials

Type 2 deformed reinforcement, f. = 460 N/mm2

Table 4.1

ENV 206 Table NA.1

NAD Table 6

C25/30 concrete with 20 mm maximum aggregate size.

4.1.2.2.3 Loading

Assume 200 mm thick slab

For non-sensitive structures, a single design value for permanent actions may 2.3.2.3 be applied throughout the structure, i.e. yQ = 1.35 throughout.

Maximum ultimate load Minimum ultimate load

1.35 x 5.8 + 1.5 x 5.0 = 15.33 kN/m2 1.35 x 5.8 = 7.83 kN/m2

4.1.2.2.4 Load cases

For continuous beams and slabs in buildings without cantilevers subjected to 2.5.1.2(4) dominantly uniformly distributed loads, it will generally be sufficient to consider only the following load cases.

(a) Alternate spans carrying the design variable and permanent load (7Q0k + 7QGk), other spans carrying only the design permanent load, yQGk.

(b) Any two adjacent spans carrying the design variable and permanent load (7QOk + 7QGk)- All other spans carrying only the design permanent load, 7GGk-

4.1.2.2.5 Flexural design

Bending moment coefficients for two-way spanning slabs supported on four edges, with provision for torsion at the corners, have been calculated based on both elastic and yield line theory. The coefficients published in BS 8110: Part 1, Table 3.15, are based on yield line analysis and are used in this example.

For continuous slabs the effects of rotational restraint from the supports can be ignored.

Yield line methods can only be used for very ductile structural elements. Use high ductility steel Class H to prEN 10080(8).

No direct check on rotational capacity is required if high ductility steel is used. The area of steel should not exceed a value corresponding to x M

d bd2f.

BS 8110 Table 3.15

For the yield line (kinematic) method, a variety of possible mechanisms should 2.5.3.5.5(4) be considered. This is assumed in the use of the published bending moment coefficients.

The ratio of moments at a continuous edge to the span moment should be 2.5.3.5.5(5) between 0.5 and 2.0. This is true for the published coefficients.

Consider the design of the corner panel, D, in Figure 4.4. 2.5.1.2

Figure 4.4 Bending moment coefficients /// = 1.2

Figure 4.4 Bending moment coefficients /// = 1.2

Using the coefficients shown in Figure 4.4 and the method described in BS 8110 to adjust moments for adjacent panels with unequal conditions, the following moments and shears can be calculated for this panel:

0 0

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