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Fire resistance of concrete-filled steel columns
Aulik, A.; National Research Council of Canada. Division of Building
Research
CANADA l NSTITUTE
FOR SCIENTIFIC AND TECH IVICAL
INFORMATION
INSTITUT CANADIEN
DE L'INFORMATIOIV SClElVTlFlQUE
ET TECHNIQUE
N R C / C N R TT-
1806 TECHNICAL TRANSLATION TRADUCTION TECHNIQUEFIRE RESISTANCE OF CONCRETE-FILLED STEEL COLUMNS
T R A N S L A T E D BY
/
T R A D U C T l O N DED. A. SlNCLAlR
THIS IS THE TWO HUNDRED AND EIGHTEENTH OF THE SERIES OF TRANSLATIONS PREPARED FOR THE DIVISION OF BUILDING RESEARCH
TRADUCTION N U M ~ R O 218 DE LA S ~ R I E P R ~ P A R E E POUR LA D I V I S I O N DES RECHERCHES EN BATIMENT
OTTAWA 1975
National Research
Conseil national
PREFACE
The performance o f s t e e l columns i n f i r e i s one o f t h e problems b e i n g s t u d i e d by t h e D i v i s i o n o f R u i l d i n c R e s e a r c h . Two methods a r e known f o r i n c r e a s i n g t h e f i r e r e s i s t a n c e o f s t e e l columns: one i s b a s e d on i n s u l a t i o n o f t h e s t e e l , t h e o t h e r on i n c r e a s i n g t h e t h e r m a l c a p a c i t y o f t h e column.
S t u d i e s on i n s u l a t e d s t e e l columns, c a r r i e d o u t i n Canada by t h e D i v i s i o n and by f i r e r e s e a r c h o r g a n i z a t i o n s o v e r s e a s , have r e a c h e d t h e s t a g e where many problems t h a t n o t l o n g ago had t o b e s o l v e d by e x p e r i m e n t , can now be s o l v e d by c a l c u l a t i o n . R e l a t i v e l y l i t t l e i s
known, however, o f t h e e f f e c t on f i r e r e s i s t a n c e o f i n c r e a s i n g t h e t h e r m a l c a p a c i t y o f t h e column.
I n t h e p r e s e n t p a p e r , a method i s d i s c u s s e d whereby t h i s e f f e c t can be c a l c u l a t e d f o r c o n c r e t e f i l l e d columns. It i s hoped t h a t u s e o f t h i s method w i l l l e a d t o a more a c c u r a t e a s s e s s m e n t o f t h e performance o f such columns, n o t o n l y f o r s t a n d a r d f i r e c o n d i t i o n s , b u t a l s o f o r
t h e v a r i o u s f i r e c o n d i t i o n s met w i t h i n a c t u a l p r a c t i c e .
The D i v i s i o n w i s h e s t o t h a n k M r . D . A . S i n c l a i r f o r p r e p a r i n g t h i s t r a n s l a t i o n and D r . T.T. L i e o f t h e D i v i s i o n who checked t h e t r a n s l a t i o n .
Ottawa J u l y 197
5
C .B
.
Crawford D i r e c t o rNATIONAL RESEARCH COUNCIL OF CANADA CONSEIL NATIONAL DE RECHERCHES DU CANADA
TECHNICAL TRANSLATION 1806 TRADUCTION TECHNIQUE
Fire resistance of concrete-filled steel columns (Betongfyllda stalpelares brandmotstand)
Author/Auteur : A. Aulik
Translator/Traducteur: D .A. Sinclair
Canada Institute for Institut canadien de
Scientific and Technical l'information scientifique et
Information technique
Ottawa, Canada KlA OS2
FIRE RESISTANCE OF CONCRETE-FILLED STEEL COLUMNS
Summary
An alternative to the use of insulation for limiting temperatures in a steel structure exposed to fire is to increase its thermal capacity. Where there are steel columns with hollow profiles, this can be realized by
filling the profiles with concrete, which, when joined to its relatively high mass and its moisture content, increases the thermal capacity of the structure. Civil engineer Andres Aulik in the Steel Construction Department of the Royal Technological Institute, has studied the temperature variations of uninsulated concrete steel columns, with the aid of theoretical data calculation, for various imagined cases of fire stress.
The dimensioning of bearing constructions from the standpoint of fire engineering has hitherto often been carried out in a very stereotyped manner, which was a great disadvantage for builders of steel structures.
As an alternative to the stereotyped dimensioning, however, a more
theoretical dimensioning is permitted under SBN 67. According to this, a more
graduated assessment of the different factors which determine the true fire resistance of a structural part has become possible.
The theoretical dimensioning has often resulted in a reduction of the
costs for retaining the required fire resistance on account of a reduced need for fire insulation.
As a result of this theoretical dimensioning, uninsulated structural steel
girders can be used, even in multistorey buildings, for sufficiently small (1)
Eire loads
.
The bearing capacity of a steel structure under fire stress depends on Iiow high the temperature rises in the steel. In buildings where it is
essential that the hearing skeleton does not collapse during a fire, therefore,
one is often forced to attempt to limit the temperatures in the steel.
The usual way of doing this is to provide steel structures with a fire insulation which retards the increase of temperature in the steel. The latter
depends not only on the insulation capacity of the fire protection, of course,
but also on the intensity and duration of the fire. Hence the temperature
variation in the structure depends on its thermal capacity.
A
structure witha high thermal capacity will experience lower temperatures than one with small thermal capacity, all other things being equal.
An alternative to limiting temperatures in a steel structure exposed to fire by the use of insulation would therefore be to increase its thermal
capacity. In the case of hollow-profiled steel columns, this can be achieved
hy f i l l i n g t h e profiles with concrete. Owing to its relatively large mass and its m o i s t u r e content, the concrete gives the structure a greater thermal capacity.
The temperature variations in such uninsulated concrete-filled steel
columns was studied by a theoretical calculation of the data"). The
temperature curves in steel columns were derived for various imagined cases of fire load.
Initially the temperature curves of the columns were calculated from the standard fire curve used as a basis for standard fire testing and which also furnishes the basis for the schematic fire-engineering dimensioning method.
The greater part of the investigation, however, is founded on more
realistic fire developments which are applied by means of a more graduated -
(3)
dimensioning from the fire-engineering standpoint
.
Schematic Fire-Engineering Dimensioning
by category symbols of the type
A
15, B 15,A
30, B 30,A
60, B 60, etc.The letter
A
implfes that the part consists almost entirely of incom-bustiblc~ mntc.ria1, while R suggests that the part contains combustible
material that cannot be disregarded for fire-engineering purposes,
'rhc numerical symbol gives the time in minutes during which the part in
question is able to withstand a standard fire test, bearing in mind the demands to be made on the structural part with respect to bearing, separation, or
bearing and separation functions.
Since it is costly and tedious to conduct fire tests on a steel structure under load, fire tests are normally limited to the measurement of the
structure's temperature change under fire stress according to the standardized
(4
temperature curve, the so-called standard fire curve ,
According to earlier practice, it has been assumed in Sweden that the
bearing capacity of a steel structure is exhausted at 4 5 0 ' ~ .
The calculations carried out on the steel temperature variations in c-oncretc-filled steel columns on the basis of the standard curve show that
t l t c , steel tempcLrntures exceed 4 5 0 " ~ quite rapldly, despite the concrete filling.
It does not seem realistic, therefore, to classify uninsulated concrete-
filled columns - in any case not those with normal material thicknesses
-
inany of the above fire-engineering classes. If the increase of fire resistance
produced by the concrete filling is to be used in fire-engineering dimensioning, a more graduated fire-engineering dimensioning is therefore indicated,
Graduated Fire-Engineering Dimensioning
SRN pcrmits exceptions to the requirements made of the fire-engineering
c1:lssificntions of buildings parts (A 60, R 60, etc) in t h e following cases:
on the basis of the fire load statistics for the type of building or premises
made on them with regard to bearing, separating, or bearing and separating functions, for the f i r , . duration and course (including the vapor phase) pertaining to the fire load used in the dimensioning,
It can be shown that a bearing structure will satisfy the demands made on it as far as bearing function is concerrled by carrying out a diversified dimensioning for fire-engineering purposes.
( 5 )
Such dimensioning will include the following calculation steps
.
1. Choosing the fire load for dimensioning purposes;2. A calculation of the temperature-time curve for fire cells, depending on the size of the fire load, among other factors;
3. Calculation of the temperature-time curve for the steel structure;
4.
Calculation of the bearing capacity of the structure for the actual tempcratures.In Sweden, statistical inventories and estimates of sizes of fire load have been carried out for apartment blocks ( 6 ) , off ice buildings (') and schools
(8) andhotels
.
Results of calculations for temperature-time curves for fire cells under different loads and ventilation conditions, defined by the so-called aperture
( 3 )
factors, are also reported in the literature
.
In the investigation(2) the temperature-time curve of concrete-filled columns is calculated with the aid of heat mass balance equations, using the
temperature-time curves for fire cells as reported in Reference
3
as initial data.If the temperature-time curve for a steel-bearing structure is known, its bearing capacity can be determined by taking into account the dependence of
( 9 )
Model for Calculating the Temperature Variations in Concrete-Filled Steel Columns with Square Cross Section
Consider a unit length of a concrete-filled hollow column. On account of symmetry, the cross section can be divided into four equivalent "pieces of cake" with equal mass (Figure
1).
Thus there is no heat transport between wedges, and the flow of heat can be regarded as one-dimensional. Under fire stress, points in the concrete at equal distancesfrom the steel profile will have the same temperature.The concrete filling in the wedge is divided into a number of trapezoidal elements (Figure 1). The number of elements will depend on the accuracy
desired. In the investigation reported in Reference 2, ten elements were chosen.
or
each element, including the steel itself, a heat mass balance equation can beconstructed by putting the net quantity of heat furnished during a short interval of time equal to the amount of heat given off for the heating of respective elements. The temperature state is calculated for each 18th second
w i t h the data in each such element.
In the investigation, the influence of the initial moisture content of
the concrete was taken into account. The standard equilibrium moisture quotient for buildings which have not been subjected to special wetting is ahout 1.5 - 29. However, in concrete-filled steel columns the moisture
content of the concrete can be taken as higher, since the steel profile around the concrete impedes and retards drying. References are lacking in the
literature on the moisture content in steel columns. In the investigation, 0 and 107 were taken as limiting values for the moisture quotient.
Many oL' the magnitudes used in the heat mass balance calculations vary with the temperature.
The thermal conductivity coefficient of the concrete varies with the temperature according to Figure 2"'). The steel profile was not divided into strips since, owing to its good heat conductivity, it can be assumed that the entire steel profile at any time will have the same temperature. The specific
h e a t of the steel varies with its temperature in accordance with Table I (11)
In the calculatio~:~ we interpo1,lted linearly between the values of the table.
The coefficient or heat transfer between the fire cell and the steel
profile also dc.pcnds on the. temperature
(I2).
The heat transfer coefficientis made up of a consti~nt convection component and a radiation component that
depends on the temperature, determined by Stefan-~olzman's law.
Calculation Model for Other Shaues of Cross Section
Like the square cross section, circular cross sections can also be
divided into equal "pieces of cake". Again, the flow of heat is one-dimensional. However, in the circular cross section one may use the results from the square cross section, as studied exhaustively in the investigation. In
circular cross sections one may thus calculate on the "safe side" by inscribing it in a square cross section. In this way we obtain an unfavourable temperature
curve owing to a smaller thermal capacity. A circular profile (radius R ) can
therefore be calculated as a square profile with the same material thickness
1 and with sides
~ n .
If we divide a rectangular cross section into four "pieces of cake" the latter are of different shape and thus have different masses. The two smaller wedges heat up more quickly than the larger ones. The flow of heat, therefore, comes about through and between the wedges, The heat flux in this case is two-dimensional. The analytical solution of two-dimensional heat flow is complicated.
I
Alternately, a rectangular cross section can be calculated (cf. circular cross section) as a square one with the same material thickness and with sides
equal to the shorter side of the rectangle. With this procedure we calculate
on the "safe side", since the square cross section gives a less favourable temperature curve than the rectangular one,
The overestimation of temperatures resulting from the above approximations is small, however, because the column width has a comparatively small effect on the temperature-time curve, owing to the steep temperature gradient in the concrete.
Results
In the investigation(2) the effect of the concrete filling on the maximum
steel temperature was studied for various square profiles influenced by a
number of given fire curves according to Reference 3 (including the standard
fire curve). These fire curves (time-temperature curves) depend on such
magnitudes as the aperture factor of the fire cell and the fire load. Examples of calculation results are given in Figure 3, where the maximum steel
temperature for moisture contents of O and 10% for concrete is given as a
function o f the fire load for an aperture factor equal to 0.12. The so-called
r~>sultant coefficient of emission used in the calculation of the steel
temperatures was assumed to be 0.7, as recommended in Reference 5 for steel
columns in fire cells. The curves show that the concrete filling considerably diminishes the maximum steel temperatures. The decreasing effect of the
concrete filling on the maximum steel temperatures increases with increasing aperture factor. At an aperture factor, according to Figure 3 (0,12), the
difference between an unfilled and filled column is as much as 2 0 0 " ~ under
certain fire loads. The greater the moisture content of the concrete, the
greater the reduction in steel temperature.
An interesting observation which was made in the investigation was that
for a low aperture factor, of the kind resulting in a slowly developing fire,
the concrcte filling has its greatest effect on large profiles. Small profiles m:in;igc to remn in "uni forrnly I~ot" over the entire cross section during the slow
clcvelopment of the i l rc. For the large aperture factor, on the other hand, the
owing to the rapid development of the fire the concrete filling does not at all succeed in becoming "uniformly warm" in its small profile.
Steel has a high thermal capacity. This means that a large material
thickness reduces the m~~\rimum steel temperature more than if a small one were 5nhjccted to t h e same fire curve. The eflect of the material thickness on the maximum steel temperature in concrete-filled hollow profiles at an apc>rture
factor
o r
0.12 is shown for different fire loads in Figure4.
The crosssvction is constant and square with sides 200 mm. For a slow fire development.
c 3 . g . , for aperture factors of 0.04, an increase of material thickness, e.g.,
10 - 20 mm, reduces maximum steel temperatures by approximately 70'~.
A
more rapid course of the fire (an aperture factor of 0.12) results in a greater decrease, about 100 - 200'~.A
large cross section means that more concrete can be accommodated in the hollow profile. This increases the thermal capacity of the column. The effect of the column width for an aperture factor of 0.12 and various fire loads is shown in Figure 5. The material thickness is constant at 10 mm. The curves show that the width of a column has comparatively little effect on themaximum steel temperature. This depends on the steep temperature gradient in the concrete. 'rile investigation showed that as a rule it is only the outer layers of concrete which managed to store any considerable amount of heat in the course of the fire.
Example of Application
Conditions: A six-storey hotel building with a vertical bearing
structure of concrete-filled steel columns; column dimensions
304.8
x 304.8 x16 mm; true compressive stress in the column 1,300 kp/cm2 (only the steel
profile is considered load bearing); slenderness coefficient of the column =
25;
4aperturv factor assumed to be 0.12 m ; resultant coefficient of emmission
t- 0.7.
Problem: Can the concrete-filled columns perform their bearing function under fire stress?
Solution: The bearing capacity of the columns is checked with the aid
o f a divcrsificd dimensioning for fire-engineering purposes. The calculation
~ ~ r o c ~ c ~ t l ~ t r c ~ l ol lows tit(. c;rlculnt Ion stcps dcsrr ibcd nbovc,
Ic[rc load: According to Reference 8 , thc fire load for dimensioning for
hotel buildings is 19.5 ~cal/rn' if a claim for 807, occupancy is assumed. The
supplement for lining material on walls and roofs is estimated at 3.5 ~cal/m'.
The total fire load for dimensioning purposes = 23 ~ c a l / m ~ .
Temperature curve of the fire cell
A fire curve does not have to be determined explicitly, because this is
taken into account by calculating the maximum steel temperature.
Maximum steel temperature
1
With an aperture factor = 0.12 rnZ and fire load = 23 ~cal/m', Figure 3
gives a maximum steel temperature = 460°C for the columns (304.8 x 304.8 x 16).
(The moisture content of the columns is assumed to be zero. Thus the calculation
i s on the "safe sideff.)
Rearing capacity of the column
F Lgurc h (1 3 ) gives the critical buckling stress as a function of the
sl~~ndcrncss coefficient A and the steel temperature 8
.
S
460°c Figure 6 gives a critical buckling stress of about 1,400 kp/cm3. Tile true con;pressi~:e stress = 1,300 kp/cm2.
Thus the concrete-filled steel columns perform their bearing function under fire stress.
A corresponding calculation for similar columns without concrete filling gives a maximum steel temperature of about 6 0 0 ' ~ . (The curve is shown in Figure 3. ) The critical buckling stress in this case is about 650 kp/cm2. A s t e e l (olumn without concrete filling, therefore, does not perform its bearing ftinc t i o n under fire stress.
7 ' 1 1 ~ ~ investigation shows that the maximum steel temperatures in a fire can
considerably reduced by filling the steel columns with concrete. Tests have
a l s o been carried out in England, among other places ( I 4
'
15),
and in Germany (16: 9w l i i c h conrirm this. In the British investigations the percentage reduction of fire insulation that is permissible with concrete-filled steel columns, compared with unfilled ones, is given. In the investigation reported in
Reference 2, hare uninsulated concrete-filled steel columns were studied. The data program is thus written in such a way that it can easily be converted for application to insulated concrete steel columns as well.
In the case of concrete steel columns it is important that the foot of the column be furnished with drain holes. The purpose of the holes is to provide an escape for the liberated water collected during the pouring of the concrete, as w e l l as to allow the water vapor to escape the concrete when this column is exposed to fire stress.
Thc3 function of thc holes in allowing the water vapor to escape from the
11c.n ted conrrc,tc is very important. German tests (I6) have shown that sometimes
References
I 1 l Thor. J Flervdntngs parkeringshus m e d st$/- slomme ulan brend~solering. Stblbyggnads- tnstitutet, publ. 21.
121 Aultk. A: Betongtyllda stdlpelares brend- motsldnd. Exernensarbete i Stalbyggnad 1971172. SlBlbyggnads~~st~tulel. rapport 52.1. 131 Maqnusson. S E och Thelandersson. S . Tem- peraluro-trme curves 01 complete process of
lire development lnst Ibr byggnadsstetik LTH. Lund 1970.
141 SBN 67. Ststens planverks publ. nr 1. Sthlm 1967.
151 Maynusson. S E och Pottorsson, 0. Kveltfl-
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161 N~lsson. L - Brandbelaslntng I bostadsl8gen- heter. Inst. IOr byggnadsstallk. LTH. Lund
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In Berggren. K och Erlksson. U: Brandbelestnlng I konlorshus. Exarnensarbete I byggnads- leknlk. K T H 1969.
I81 Forsberg. U och Thor. J: Brandbelastnlng l o r skolor och hotell. SIBlbyggnadslnstitutet. 1971. 191 W~lteveen I Brendvetlrghed Slealconstruc- ltos Centrum Beuwen In Staal. Rotterdam
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1101 Odren K Berllknlng av lckestatIcn8r. tvddl- mens~onell vdrmelodn~ng t brendutsatte be-
lonqkonslrukl~oner Sldicns Provn~ngsanslall. b r ~ n d t e k n ~ s k e laborator~ot. Stockholm 1964 11 11 Pottorsson 0 Slruclural I ~ r o engtneermg re-
search lodoy and tomorrow Acta Polytech- Iilca Scandlnav~a. Sthlm 1965
1121 Peltrrsson 0 Pr~nclpnr for en kvaltllcerad brondteknlsk d~rnons~onermg av sldlb(lrverk Korilrrenskornpendtum Sl~lbyggnedsinstl- W e t . Slalbyggnadsdagen 1968.
1131 Mngnusson. S E o ~ t i Petlersson. 0: Brandtek-
nisk dlmenstonortng ev lsolored stdlkonsf- ruklion I bdrande ollor avskll/ande tunkflon. V8g- och vatleribyggeren 1969. nr 4. 1141 Concrete llllod hollow section steel columns.
CIDECT. London 1970.
1151 0esrgn manual l o r RHS concrete Nlled columns. Brltlsh Steel Corp.. Glasgow 1970. 1161 Bout?. P Der Feuorschufr Irn Stahlhochbau
lnsbosondere von StahlslOlzen Deulscher Ausschuss IUr Stehlbeu, Kbln 1959.
T a b l e I
The relattonsh~p between the temperature end the speclflc heat for steel
Temperature CP*
OC Kcrllkg "C
Fig. 1
A square sectton divlded into "p~eces of cake". One of the pieces has been subdivided into ten trapezoi- dal elements.
T h e lornporalure parametre for the Itiermal cnridue- livlty of concrete
0 0, I i n i n ~ u l a t e d c o n c r e t e - f i l l e d columns
,
,,, ," for v a r i o u s f i r e l o a d s q A IIO,I
F i g . 3 I oonT h e nfloclc of t h ~ concrete lllllng on the rnsxlrnurn steel lemperature
A, .,
,,
~n a fire loaded unlnsulated concrete Illled steel column withvarytng l ~ r e load q
_--- . . - I
__-- . I
, ,, - I
- -
.
- :rriflIled steel column-- ioricrttlcj l~lled steel column, rno~sture content 0%
10 I S 20 Mate ial thiciness mm 4 0 0 300 zoo
..
Fig. 4 -. .-The effocts ol the steel thtckness on the maximum steel temperature
As ,,, ~n a flrn loadod, untnsulated concrete Illled steel column wlth vurytng lire load q A square cross sectlon w ~ l h a constant slde length (200 mm).
0% molsture content In the concrete
o 100 200 300 400 Column
5 width mm
F i g . 5
The effects of the w~dth of the column on the maxlmum steel tempe- raturelvs mex in a f ~ r e loaded, un~nsulated steel column with varylng fire load q. Square cross secllon w~th constant steel thickness (10 mm)
0% molsture content in the concrete
F i g . 6
The varlatlon w ~ t h the sleol temperature 8 , of the retal~onshlp bel- ween buckllng stress uk and slenderness ratlo A
for columns m a d e o f sleol h a v ~ n g a yield pOlnl stress at room temperature us-2.600
