## Flexure

The design procedure for flexural design is given in Figure 1; this includes derived formulae based on the simplified rectangular stress block from Eurocode 2. Where appropriate Table 3 may be used to determine bending moments for flat slabs.

Table 1

Flat slab design procedure

 Step Task Further guidance Chapter in this publication Standard 1 Determine design life 2: Getting started NA to BS EN 1990 Table NA.2.1 2 Assess actions on the slab 2: Getting started BS EN 1991 (10 parts) and National Annexes 3 Determine which combinations of actions apply 1: Introduction to Eurocodes NA to BS EN 1990 Tables NA.A1.1 and NA.A1.2 (B) 4 Determine loading arrangements 2: Getting started NA to BS EN 1992-1-1 5 Assess durability requirements and determine concrete strength 2: Getting started BS 8500: 2002 6 Check cover requirements for appropriate fire resistance period 2: Getting started and Table 2 Approved Document B. BS EN 1992-1-1: Section 5 7 Calculate min. cover for durability, fire and bond requirements 2: Getting started BS EN 1992-1-1 Cl 4.4.1 8 Analyse structure to obtain critical moments and shear forces 2: Getting started and Table 3 BS EN 1992-1-1 Section 5 9 Design flexural reinforcement See Figure 1 BS EN 1992-1-1 Section 6.1 10 Check deflection See Figure 3 BS EN 1992-1-1 Section 7.4 11 Check punching shear capacity See Figure 6 BS EN 1992-1-1 Section 6.4 12 Check spacing of bars 2: Getting started BS EN 1992-1-1 Section 7.3 13 Check resistance to moment transfer from column to slab - BS EN 1992-1-1 Annex I 1.2(5) Note NA = National Annex

Table 2

Minimum dimensions and axis distances for reinforced concrete slabs

Standard fire resistance

Minimum dimensions (mm)

Slab thickness, hs

Axis distance, a

REI 60

180

15a

REI 90

200

25

REI 120

200

35

REI 240

200

50

Notes

1 This table is taken from BS EN 1992-1-2 Table 5.9.

2 The axis distance is to the centre of the outer layer of reinforcement.

3 The table is valid only if the detailing requirements (see note 4) are observed and, in the normal temperature design, redistribution of bending moments does not exceed 15%.

4 For fire resistance of R90 and above, at least 20% of the total top reinforcement in each direction over intermediate supports required by BS EN 1992-1-1 should be continuous over the full span. This reinforcement should be placed in the column strip.

5 There are three standard fire exposure conditions that may

need to be satisfied:

R Mechanical resistance for load bearing E Integrity of separation I Insulation

Key

a Normally the requirements of BS EN 1992-1-1 will determine the cover.

Procedure for determining flexural reinforcement

START

Carry out analysis of slab to determine design moments (M) (Where appropriate use coefficients from Table 3).

Carry out analysis of slab to determine design moments (M) (Where appropriate use coefficients from Table 3).

 M Determine K from: K = , 2 r bd2fck Determine K' from Table 4 or K = 0.606 - 0.1 S 62 - 0.21 where 6 s 1.0

Compression reinforcement required - not recommended for typical slabs

Obtain lever arm z from Table 5 or

Compression reinforcement required - not recommended for typical slabs

Obtain lever arm z from Table 5 or z = d [ 1 + V1 - 3 53 K ] < 0.95d z = d [ 1 + V1 - 3 53 K ] < 0.95d

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### Responses

• Sanni
How to find concrete slab thickness eurocode?
8 years ago