Fire resistance

Eurocode 2, Part 1-2: Structural fire design5, gives a choice of advanced, simplified or tabular methods for determining the fire resistance. Using tables is the fastest method for determining the minimum dimensions and cover for beams. There are, however, some restrictions and if these apply further guidance on the advanced and simplified methods can be obtained from specialist literature6 Rather than giving a minimum cover, the tabular method is based on nominal axis distance, a (see Figure 1). This is the distance from the centre of the main reinforcing bar to the top or bottom surface of the

Continues page 17

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The Concrete Centreā„¢

Table 1

Beam 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 beam

2: Getting started

BS EN 1991 (10 parts) and National Annexes

3

Determine which combinations of actions apply

1: introduction to Eurocodes

NAto 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 'Fire resistance' section

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 CI 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 'Flexure' section

BS EN 1992-1-1 section 6.1

10

Check shear capacity

See 'Vertical shear' section

BS EN 1992-1-1 section 6.2

11

Check deflection

See 'Deflection' section

BS EN 1992-1-1 section 7.4

12

Check spacing of bars

2: Getting started

BS EN 1992-1-1 section 7.3

NA = National Annex

Table 2

Minimum dimensions and axis distances for beams made with reinforced concrete for fire resistance

Table 2

Minimum dimensions and axis distances for beams made with reinforced concrete for fire resistance

R60

bmin =

120

160

200

300

120

200

a =

40

35

30

25

25

12a

R90

bmin =

150

200

300

400

150

250

a =

55

45

40

35

35

25

R120

bmin =

200

240

300

500

200

300

450

500

a =

65

60

55

50

45

35

35

30

R240

bmin =

280

350

500

700

280

500

650

700

a =

90

80

75

70

75

60

60

50

1 This table is taken from BS EN 1992-1-2 Tables 5.5 and 5.6.

2 The axis distance, asd, from the side of the beam to the corner bar should be a +10 mm except where bmm is greater than the values in columns C and F.

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

4 For fire resistance of R90 and above, for a distance of 0.3/eff from the centre line of each intermediate support, the area of top reinforcement should not be less than the following:

x is the distance of the section being considered from the centre line of the support.

is the area of reinforcement required for normal temperature design.

is the minimum area of reinforcement required at the section being considered but not less than that required for normal temperature design. is the greater of the effective lengths of the two adjacent spans.

5 For fire resistances R120 - R240, the width of the beam at the first intermediate support should be at least that in column F, if both the following conditions exist:

a there is no fixity at the end support; and b the acting shear at normal temperature Vsd > 0.67 VRd,max.

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

As,req(x) leff

Figure 1 Figure 3

Section through structural member, Simplified rectangular stress block for showing nominal axis distances a and asd concrete up to class C50/60 from Eurocode 2

Figure 1 Figure 3

Section through structural member, Simplified rectangular stress block for showing nominal axis distances a and asd concrete up to class C50/60 from Eurocode 2

member. It is a nominal (not minimum) dimension, so the designer should ensure that:

Table 2 gives the minimum dimensions for beams to meet the standard fire periods.

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