AP09_1.vp

1 Introduction

Filling hollow steel columns with concrete is an interesting
way to improve their fire resistance [7]. The temperature at
the surface of a hollow structural section without external pro-
tection increases quickly during the development of a fire.
However, if the steel tube is filled with concrete, while the steel
section gradually loses its resistance and rigidity, the load is
transferred to the concrete core. The concrete core heats up
more slowly, thus increasing the fire resistance of the column.
Besides its structural function, the steel tube acts as a radia-
tion shield to the concrete core. This, combined with a steam
layer in the steel-concrete boundary, leads to a lower tem-
perature rise in the concrete core when compared to exposed
reinforced concrete structures [7].

During a fire, the temperature distribution in the cross-
-section of a CFT column is not uniform: steel and concrete
have very different thermal conductivities, which generates a
behaviour characterized by noticeable heating transients and
high temperature differentials across the cross-section. Due to
these differentials, CFT columns can achieve high fire resis-
tance times without external fire protection [7]. However, it is
necessary to resort to numerical models in order to make an
accurate prediction of these temperature profiles along the
fire exposure time [8], [9].

In this work, the ABAQUS finite element analysis package
[1] was employed to model the behaviour of slender axially
loaded CFT columns exposed to fire. With this software, a
sequentially coupled nonlinear thermal-stress analysis was
conducted. The results of the simulations were compared with
a series of fire resistance tests available in the literature [11], as
well as with the predictions of the Eurocode 4 [6] simplified
calculation model.

2 Development of the numerical
model

2.1 Finite element mesh
A three-dimensional numerical model was developed in

ABAQUS [1], with variable parameters such as the length
of the column (L), the external diameter (D), the thickness of
the steel tube (t) and the thermal and mechanical material
properties. It consisted of two parts: the concrete core and the
steel tube. Due to symmetry on the geometry and boundary
conditions, only a quarter of the column was modelled.

The three-dimensional eight-node solid element
C3D8RT was used to mesh the model. It is an eight-node
thermally coupled brick, with trilinear displacement and tem-
perature, reduced integration and hourglass control. The
mesh density was controlled to have a maximum element size
of 2 cm, which proved to be sufficient to predict with enough
accuracy the thermal and mechanical behaviour of the CFT
columns under fire.

2.2 Material properties
The numerical model took into account the temperature

dependent thermal and mechanical properties of the materi-
als. For concrete, Lie’s model [12] was employed, as it proved
to be the one that best predicted the behaviour of the concrete
infill in CFT columns, according to Hong & Varma [9].
The mechanical model implemented in ABAQUS employed
the hyperbolic Drucker-Prager yield surface. The thermal
properties for concrete at elevated temperatures were ex-
tracted from EN 1992-1-2 [4]. For steel, the temperature
dependent thermal and mechanical properties recom-
mended in EN 1993-1-2 [5] were adopted. The isotropic

Acta Polytechnica Vol. 49 No. 1/2009

Fire Resistance of Axially Loaded
Slender Concrete Filled Steel Tubular

Columns
Development of a Three-Dimensional Numerical

Model and Comparison with Eurocode 4
A. Espinós, A. Hospitaler, M. L. Romero

In recent years, concrete filled tubular (CFT) columns have become popular among designers and structural engineers, due to a series of
highly appreciated advantages: high load-bearing capacity, high seismic resistance, attractive appearance, reduced column footing, fast
construction technology and high fire resistance without external protection. In a fire, the degradation of the material properties will cause
CFT columns to become highly nonlinear and inelastic, which makes it quite difficult to predict their failure. In fact, it is still not possible for
analytical methods to predict with enough accuracy the behaviour of columns of this kind when exposed to fire. Numerical models are there-
fore widely sought. Many numerical simulations have been carried out worldwide, without obtaining satisfactory results. This work proposes
a three-dimensional numerical model for studying the actual fire behaviour of columns of this kind. This model was validated by comparing
the simulation results with fire resistance tests carried out by other researchers, as well as with the predictions of the Eurocode 4 simplified
calculation model.

Keywords: Fire resistance, concrete filled steel tubular columns, finite element analysis.

multiaxial plasticity model with the Von Mises yield surface
was employed.

The values of the thermal expansion coefficient for con-
crete and steel recommended by Hong and Varma [9] were
employed: � s � �

�12 10 6 °C�1, �c � �
�6 10 6 °C�1. The mois-

ture content of the concrete infill was not modelled in this
research, which lies on the safe side.

2.3 Thermal analysis
For conducting the thermal analysis, the standard

ISO-834 [10] fire curve was applied to the exposed surface of
the CFT column model as a thermal load. The thermal
contact in the steel-concrete boundary was modelled by em-
ploying the “gap conductance” and “gap radiation” options.
For the governing parameters of the heat transfer problem,
the values recommended in EN 1991-1-2 [3] were adopted.

3 Validation of the numerical model
The three-dimensional numerical model was validated by

comparing the simulations with experimental fire resistance
tests [11] and with the EC4 simplified calculation model [6].

3.1 Comparison with experimental results
The numerical model was employed to predict the stan-

dard fire behaviour of a series of CFT column specimens
listed in Table 1. These specimens were tested at the NRCC,
and their results were published by Lie & Caron [11]. All the
specimens tested were circular, filled with siliceous aggregate
concrete and subjected to a concentric compression load.
Their total length was 3810 mm, although only the central
3048 mm were directly exposed to fire. Because of the loading
conditions, all the tests were assumed as fix-ended.

40 © Czech Technical University Publishing House http://ctn.cvut.cz/ap/

Acta Polytechnica Vol. 49 No. 1/2009

Column
specimen

D (mm) t (mm) fy (N/mm
2) fck (N/mm

2) N (kN) � � N Npl,Rd FRR (min)

1 141 6.5 401.93 28.62 131 8.90 % 57

2 168 4.8 346.98 28.62 218 15.37 % 56

3 219 4.8 322.06 24.34 492 26.19 % 80

4 219 4.8 322.06 24.34 384 20.44 % 102

5 219 8.2 367.43 24.34 525 18.88 % 82

6 273 5.6 412.79 26.34 574 17.08 % 112

7 273 5.6 412.79 26.34 525 15.63 % 133

8 273 5.6 412.79 26.34 1000 29.76 % 70

Table 1: List of CFT columns analyzed, from the NRCC research report [11]

Fig. 1: Comparison between calculated and measured axial displacement, for test no. 4

For each simulation, the axial displacement at the top of
the column versus the fire exposure time was registered, com-
paring this curve with the curve obtained in the fire resistance
test [11]. Fig. 1 shows an example of the comparison of the
two curves for one of the specimens studied.

From these curves, the fire resistance rating (FRR) was
obtained for each of the specimens under study. The failure
criteria from EN 1363-1 [2] were adopted. This standard
establishes that the failure time is given by the more restrictive
of the following two limits: maximum axial displacement, and
maximum axial displacement velocity. By applying these cri-
teria, the values in Table 2 were obtained. As shown in Fig. 2,
most of the values obtained lie in the region of 15 % error,
apart from two values, corresponding to column specimens
no. 1 and no. 2, which have the smallest diameters.

The maximum axial displacement (�max) was also ob-
tained for each of the column specimens studied here. Table 2
shows the calculated and measured values, which are plotted
in Fig. 3, where it can again be seen that most of the cases lie in
the region of 15 % error, apart from specimens no. 3 and
no. 8, corresponding to those with a higher loading level,
over 20 % of the maximum load-bearing capacity of the col-
umn at room temperature.

3.2 Comparison with the Eurocode 4 simplified
calculation model

In this section, the numerical model is compared with the
predictions of the EC4 simplified calculation model [6], ob-
taining the results shown in Table 3. It is seen in Fig. 4 that the
proposed numerical model gives a better prediction of the

Acta Polytechnica Vol. 49 No. 1/2009

Column
specimen

FRR (min) � FRR
test

FRR
FRR

� �max (mm) � �
�

�
max

max

max
�

, test

, NSTest Simulation Test Simulation

1 57 72 0.79 24.09 24.35 0.99

2 56 75 0.75 20.48 19.25 1.06

3 80 74 1.08 18.13 12.36 1.47

4 102 97 1.05 18.77 16.23 1.16

5 82 68 1.21 20.36 19.30 1.05

6 112 126 0.89 16.40 17.71 0.93

7 133 137 0.97 19.67 18.61 1.06

8 70 70 1.00 5.51 10.35 0.53

Average 0.97 Average 1.03

Standard deviation 0.15 Standard deviation 0.26

Table 2: Predicted and measured FRR and maximum axial displacement (�max)

100

120

140

160

0 20 40 60 80 100 120 140 160

Test results, FRR (min)

N
u

m
e
ri

c
a
l

p
re

d
ic

ti
o

n
s
,

F
R

R
(m

in
)

+15%

-15%

SAFE

Fig. 2: Comparison of the fire resistance ratings, calculated versus
test results

0 5 10 15 20 25

Test results, �max (mm)

N
u

m
e

r
ic

a
l
p

re
d

ic
ti

o
n

s
,

�
m

a
x

(m
m

)

+15%

-15%

Fig. 3: Comparison of the maximum axial displacement, calcu-
lated versus test results

fire resistance rating, showing a very accurate trend. However,
the EC4 simplified model turns out to be excessively con-
servative, as shown in the figure. We must note that the EC4
simplified model does not take into account the thermal ex-
pansion of the materials, nor the air gap at the steel-concrete
boundary, which lies on the safe side and gives a very con-
servative prediction. If we apply these simplifications to our
numerical model, smaller values of the fire resistance ratings
are obtained, very similar to those predicted by EC4, as shown
in Table 3. As can be seen in Fig. 5, our predicted values re-
produce quite well the results of EC4, except for those tests

with fire resistance ratings around 120 minutes, where our nu-
merical model provides more accurate results, producing a
trend that is closer to reality.

4 Summary and conclusions
A three-dimensional numerical model for axially loaded

slender CFT columns under fire has been presented. By
means of this model, a prediction was made of the behaviour
under standard fire conditions of eight column specimens
previously tested by the NRCC research group [11]. The pro-

Acta Polytechnica Vol. 49 No. 1/2009

Column
specimen

FRR (min) � FRR
test

calc

FRR
FRR

�

Test Simulation
Simulation

(no expansion)
EC4 Simulation

Simulation
(no expansion)

EC4

1 57 72 49 49 0.79 1.16 1.16

2 56 75 46 46 0.75 1.22 1.22

3 80 74 52 49 1.08 1.54 1.63

4 102 97 63 61 1.05 1.62 1.67

5 82 68 52 51 1.21 1.58 1.61

6 112 126 118 91 0.89 0.95 1.23

7 133 137 126 96 0.97 1.06 1.39

8 70 70 58 56 1.00 1.21 1.25

Average 0.97 1.29 1.39

Standard deviation 0.15 0.25 0.21

Table 3: Comparison of the numerical model and EC4 predictions with the tests

Fig. 5: Comparison of FRR, proposed model (without expan-
sion), and EC4 model

Fig. 4: Comparison of FRR, proposed numerical model, and EC4
model

posed numerical model showed better behaviour for columns
with low slenderness and loading levels under 20 %. Despite
these two aspects, the model showed an accurate response
when contrasted with the fire tests.

The study has also proved that the predictions of the EC4
simplified calculation model [6] can be reproduced with the
proposed numerical model by eliminating the thermal ex-
pansion of the materials, which lies on the safe side. However,
if the real behaviour of CFT columns under fire is to be pre-
dicted, this factor must be taken into account, extending the
failure time. The expansion of the steel tube produces an
opposed axial strain in the early stages of heating, as well as
an opening of the gap in the steel-concrete interface, which
delays the heating of the concrete core and thus increases the
fire resistance rating.

The proposed numerical model proved to give better pre-
dictions than the EC4 simplified model, which turned out to
be excessively conservative.

References
[1] ABAQUS. ABAQUS/Standard Version 6.6 User’s Manual:

Volumes I–III. Pawtucket, Rhode Island: Hibbit, Karlsson
& Sorenson, Inc., 2005.

[2] EN (European Committee for Standardisation): EN
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Brussels: CEN 1,999.

[3] CEN (European Committee for Standardisation). EN
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eral Actions – Actions on Structures Exposed to Fire. Brussels:
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[4] CEN (European Committee for Standardisation). EN
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[5] CEN (European Committee for Standardisation). EN
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[7] CIDECT. Twilt, L., Has,s R., Klingsch, E. W., Edwards,
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1996.

[8] Ding, J., Wang, Y. C.: Realistic Modelling of Ther-
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[9] Hong, S., Varma, A. H.: Analytical Modeling of the Stan-
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[10] ISO (International Standards Organization). ISO 834:
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[11] Lie, T. T., Caron, S. E.: Fire Resistance of Circular Hol-
low Steel Columns Filled with Siliceous Aggregate Con-
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Canada: Institute for Research in Construction, Na-
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[12] Lie, T. T.: Fire Resistance of Circular Steel Columns
Filled with Bar-Reinforced Concrete. Journal of Structural
Engineering-ASCE, Vol. 120 (1994), No. 5, p. 1489–1509.

Ana Espinós
e-mail: aespinos@mes.upv.es

Antonio Hospitaler

Manuel L. Romero

Instituto de Ciencia y Tecnología del Hormigón,
Universidad Politécnica de Valencia

Acta Polytechnica Vol. 49 No. 1/2009