and Fire
5.2 S ERVICEABILITY L IMIT S TATES
5.3.1 Tabular Data
These are used when the fire resistance period is either specified according to standard requirements (e.g. Approved Document B) or determined from the time equivalence approach. This latter allows the effects of a real or parametric fire to EN 1991-1-2 to be related to the curve in the standard furnace test. It should be considered where the combustible fire load or the available ventilation is low as it may demonstrate that the equivalent time is such that no additional measures need be taken to give adequate fire performance. Time equivalence was originally conceived for protected steelwork, care therefore should be taken when applying it to concrete structures, especially columns as the maximum average concrete tempera- ture and the maximum reinforcement temperatures will not be coincident. Also time equivalence will give no indication of rate of heating as it only assesses the total heat input. It should also only be considered where the structure is of fixed usage throughout its design life.
Tabular data are for members within a structure, and it should not be assumed that if members are detailed for, say, 60 minutes standard exposure that the complete structure would last for 60 minutes in that exposure – if such a situation could be evaluated. Whole building behaviour whether observed in actual fires or the Cardington test (Bailey, 2002) would seem to suggest that whole building behaviour is better than that of individual elements as a structure can allow redistribution of internal thermal or mechanical forces generated during the fire. The exceptions to this are that failure of compartment walls caused by excessive deflection of the floors above would not be picked up in a standard furnace test, nor would lateral movement of the complete structure. Equally the efficiency of fire stopping between the edges of slabs and external curtain walls cannot be evaluated in furnace tests. The importance of such fire stopping was shown in the recent Madrid fire (Redfern, 2005) where fire spread between floors of a 32 storey concrete frame structure to such an extent that the structure needed demolition.
Unless otherwise indicated, tabular data imply that the load level is approximately 0,70 times that at ULS and thus are likely to be conservative at not all levels, but the 94 Chapter 5 / Durability, Serviceability and Fire
variable load may be present during a fire other than in the extremely early period during evacuation when any temperature rise within the concrete will be very low. This is recognized in EN 1990 where a factor is applied to variable loading in the calculation approach. EN 1992-1-2 does give a procedure whereby the critical temperature and resultant axis distance can be modified for lower load levels (cl 5.2). The critical temperatures crfor a load level of 0,7 are taken as 5008C for reinforcing
steel, 4008C for prestressing in the form of bars, and 3508C for strand or wire. This means that for bar prestressing, the tabulated values of the axis distance, a, should be increased by 10 mm, and for strand or wires by 15 mm.
The major departure from current practice is that EN 1992-1-2 uses axis distance not covered to reinforcement. This is actually more scientifically correct as it is observed in computer-based heat transfer calculations that the temperatures at the centre of a reinforcing bar are identical to that at the same position in plain concrete (Ehm, 1967, quoted in Hertz, 1981). Becker et al. (1974) indicate this is reasonable up to a reinforcement area of 4 per cent of the gross section. Where the reinforcement is in more than one layer, the weighted mean axis distance is used. The axis distance used for any bar is the least of that to any fire exposed face. Rather than cover each type of structural element, it is only proposed to cover those elements where interpretation of the Code is required. Note in this section, all Table and clause references are to EN 1992-1-2.
5.3.1.1 Columns
The Code provides two methods denoted A and B.
Method A:
This was developed from an empirical procedure originally proposed by Dortreppe and Franssen (undated). It can only be used under the following recommended limits:
Effective length l0,fi3 m, where l0,fi is taken as l0 (the effective length at
ambient) subject to the condition that if the standard exposure is greater than 30 minutes, then l0,fi is taken as 0,5l for intermediate floors and 0,5l l0,fi0,7l for the upper floor (l is the height of the column between
centre lines).
Eccentricity of load, e, defined as M0Ed,fi/N0Ed,fi, should be less than emaxwhich
has a recommended value of the lesser of 0,15h or b (although the code does suggest an upper limit of 04h or b). M0Ed,fiand N0Ed,fiare the first order moment
and axial force in the fire limit state.
Maximum reinforcement Asshould not exceed 0,4Ac.
NOTE: The design values of section size and cover given in Table 5.2a of EN 1992- 1-2 are for a value of ccof 1,00. Thus if a value of 0,85 is adopted (as is likely in
the UK), the formula attached to the Table MUST be used. This gives the value of Concrete Design 95
the fire resistance R as
R ¼ 120 Rfiþ Raþ Rlþ Rbþ Rn 120
1,8
ð5:40Þ
where the correction factor Rfifor load level fiand ccis given by
Rfi¼83 1,00 fi
1 þ ! ð0,85=ccÞ þ!
ð5:41Þ
where fiis defined as NEd.fi/NRd(i.e. the ratio of the axial load in the fire limit state to the design resistance at ambient) and ! is the mechanical reinforcement ratio (¼ Asfyd/Acfcd¼(Asfyk/s)/(Acfck/c),
the correction factor Rafor axis distance a ( 25 a 80 mm),
Ra¼1,6ða 30Þ ð5:42Þ
the correction factor Rlfor effective length l0,fi,
Rl¼9,60 5 l0,fi
ð5:43Þ the correction factor Rbfor a parametric width b0(200 b0450 mm),
Rb¼ 0,9b0 ð5:44Þ
where for a circular column b0is the diameter, and for a rectangular column b0is
given by
b0¼2 Ac
bþh ð5:45Þ
subject to the limit that h 1,5b.
Method B:
The background to Method B is given in Dortreppe, Franssen and Vanderzeypen (1999).
This is covered at a basic level in Table 5.2b with more detailed tables in Annex C of EN 1992-1-2.
The tables operate in terms of a load level n defined as
n ¼ NoEd,fi 0,7 AcfcdþAsfyd
ð5:46Þ
and the mechanical reinforcement ratio !.
Table 5.2b also has the restrictions that e/b 0,25 (subject to a maximum value of e of 100 mm) and the slenderness ratio in the fire limit state fi530.
5.3.1.2 Beams and Slabs
For beams, it should be noted there are requirements on section geometry for I beams (cl 5.6.1(5)) and that for other beams the width b is determined at the centroid of the reinforcing.
Where continuous beams (or slabs) have had elastic moments redistributed, then where the redistribution exceeds 15 per cent, the beam (or slab) must be treated as simply supported.
For situations where the standard fire resistance period is R90 or above, then for a distance of 0,3lefffrom any internal support, the reinforcement As,req(x) at a distance
x from the support should be not less than,
As,reqðxÞ¼ As,reqð0Þ 1 2,5
x leff
ð5:47Þ
where leffis the effective span and As,req(0) is the area of reinforcement required
at the support.
Where the minimum dimension for b is adopted and the reinforcement in a beam is in a single layer, then the axis distance to a corner bar must be increased by 10 mm. This also applies to ribbed slabs (without the restriction on single layered reinforcement).
5.3.1.3 High Strength Concrete
Tabulated data may be used but with an increase on cross-sectional dimensions of (k1)a for walls and slabs with single sided exposure and 2(k1)a for other structural elements where a is the axis distance determined from the tables. Additionally the axis distance actually required should be factored by k, where k is 1,1 for concrete C55/67 or 60/75 and 1,3 C70/85 or 80/95 given in cl 6.4.3.1(3). The potential problems due to spalling are dealt with in Section 5.3.3.1.