Fire resistance

Eurocode 2, Part 1-2: Structural fire design5, gives a choice of advanced, simplified or tabular methods for determining fire resistance of columns. Using tables is the fastest method for determining the minimum dimensions and cover for columns. There are, however, some restrictions and if these apply further guidance can be obtained from specialist literature.6 The simplified method may give more economic columns, especially for small columns and/or high fire resistance periods.

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

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The Concrete Centreā„¢

Table 1

Column design procedure

Table 1

Column design procedure

Step

Task

Further guidance

Chapter in the publication

Standard

1

Determine design life

2: Getting started

UK NA to BS EN 1990 Table NA.2.1

2

Assess actions on the column

2: Getting started

BS EN 1991 (10 parts) and UK National Annexes

3

Determine which combinations of actions apply

1: Introduction to Eurocodes

UK NA to BS EN 1990 Tables NA.A1.1 and NA.A1.2 (B)

4

Assess durability requirements and determine concrete strength

2: Getting started

BS 8500: 2002

5

Check cover requirements for appropriate fire resistance period

2: Getting started and Table 2

Approved Document B. BS EN 1992-1-2

6

Calculate min. cover for durability, fire and bond requirements

2: Getting started

BS EN 1992-1-1 Cl. 4.4.1

7

Analyse structure to obtain critical moments and axial forces

2: Getting started and 'Structural analysis' section

BS EN 1992-1-1 section 5

8

Check slenderness

See Figures 2 and 3

BS EN 1992-1-1 section 5.8

9

Determine area of reinforcement required

See Figures 2 and 3

BS EN 1992-1-1 section 6.1

10

Check spacing of bars

'Rules for spacing' section

BS EN 1992-1-1 sections 8 and 9

NA = National Annex.

Table 2

Minimum column dimensions and axis distances for fire resistance

Figure 1

Section through structural member, showing nominal axis distance a

Table 2

Minimum column dimensions and axis distances for fire resistance

Standard fire resistance

Minimum dimensions (mm)

Column width 6min/axis distance, a, of the main bars

Column expose one side

d on more than

Column exposed on one side

f = 0.5

f = 0.7

( Hfi = 0.7)

R 60

200/36 300/31

250/46 350/40

155/25

R 90

300/45 400/38a

350/53 450/40a

155/25

R 120

350/45a 450/40a

350/57a 450/51a

175/35

R 240

450/75a

b

The table is taken from BS EN 1992-1-2 Table 5.2a (method A) and is valid under the following conditions:

Note

The table is taken from BS EN 1992-1-2 Table 5.2a (method A) and is valid under the following conditions:

1 The effective length of a braced column under fire conditions lof < 3m. The value of may be taken as 50% of the actual length for intermediate floors and between 50% and 70% of the actual length for the upper floor column.

2 The first order eccentricity under fire conditions should be < 0.15b (or h). Alternatively use method B (see Eurocode 2, Part 1-2, Table 5.2b). The eccentricity under fire conditions may be taken as that used in normal temperature design.

3 The reinforcement area outside lap locations does not exceed 4% of the concrete cross section.

4 ^fi is the ratio of the design axial load under fire conditions to the design resistance of the column at normal temperature conditions. may conservatively be taken as 0.7.

a Minimum 8 bars b Method B may be used which indicates 600/70 for R 240 and = 0.7. See BS EN 1992-1-2 Table 5.2b

Table 3

Minimum reinforced concrete wall dimensions and axis distances for load-bearing for fire resistance

Table 3

Minimum reinforced concrete wall dimensions and axis distances for load-bearing for fire resistance

Standard fire resistance

Minimum dimensions (mm)

Wall thickness/axis distance, a, of the main bars

Wall exposed on one side ( Hfi = 0.7)

Wall exposed on two sides ( pfi = 0.7)

REI 60

130/10a

140/10a

REI 90

140/25

170/25

REI 120

160/35

220/35

REI 240

270/60

350/60

Notes

1 The table is taken from BS EN 1992-1-2 Table 5.4.

2 See note 4 of Table 2.

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

a b reinforcing bar to the surface of the member. It is a nominal (not minimum) dimension, and the designer should ensure that:

For columns there are two tables given in Eurocode 2 Part 1-2 that Figure 2

Flow chart for braced column design

Flow Chart For Columns Eurocode

present methods A and B. Both are equally applicable, although method A has smaller limits on eccentricity than method B. Method A is slightly simpler and is presented in Table 2; limits of applicability are given in the notes. Similar data for load-bearing walls is given in Table 3.

For columns supporting the uppermost storey, the eccentricity will often exceed the limits for both methods A and B. In this situation Annex C of Eurocode 2, Part 1-2 may be used. Alternatively, consideration can be given to treating the column as a beam for determining the design fire resistance.

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