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1. Introduction
Structures lose their load bearing capacity when they are exposed
to high temperatures. The strength and rigidity of steel are reduced
with increasing temperature, and in addition to losing these proper-
ties, concrete suffers a reduction in area due to the “spalling” phenom-
enon.The composite system of concrete-filled steel tubes, beyond in-
creasing the structure’s load bearing capacity at room temperature,
allows for greater exposure times to high temperatures compared to
the same materials evaluated separately. Because the concrete is
confined, it does not suffer from spalling, as the wall of the steel tube
prevents the displacement of the concrete by not allowing a reduction
in area. The presence of microcracks in the concrete helps to slow the
deformation and internal heating of the filled steel tube. This type of
composite structure collapses when the steel and concrete lose their
load bearing capacity due to reductions in strength and rigidity, caus-
ing them to be incapable of supporting the applied load.
Studies conducted in Europe and the United States concluded that
the reduction in the strength of composite columns at high tem-
peratures depends on the following factors: the duration of the ex-
posure, temperature, column diameter, type of concrete, thickness
of the steel tube and strengths of the concrete and steel.
531
IBRACON Structures and Materials Journal • 2012 • vol. 5 • nº 4
A. E. P. G. A. JACINTHO | V. P. SILVA | J. A. V. REQUENA | R. C. C. LINTZ
| L. A. G. BARBOSA | L. L. PIMENTEL
Figure 1 – Temperature elevation
curve with pre-heating
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90
0
200
400
600
800
1000
Temperature (ºC)
Time (min)
Pré heating
ISO 834
Table 1 – Characteristics of the models tested and the load applied during the tests
Series
D
(mm)
e
(mm)
D/e
f
y
(MPa)
t
(min)
N
fi,plRd
(kN)
Applied Force
0.5·N
fi,plRd
(kN)
C 6-30-1
114.3
6.0
19.05
305.1
30
233.74
116.87
C 6-30-2
114.3
6.0
19.05
305.1
30
233.74
116.87
C 6-60-1
114.3
6.0
19.05
305.1
60
78.20
39.50
C 6-60-2
114.3
6.0
19.05
305.1
60
78.20
39.50
C 8-30-1
114.3
8.6
13.29
331.5
30
295.98
150.92
C 8-30-2
114.3
8.6
13.29
331.5
30
295.98
150.92
C 8-60-1
114.3
8.6
13.29
331.5
60
87.51
45.48
C 8-60-2
114.3
8.6
13.29
331.5
60
87.51
45.48
C 63-30-1
141.3
6.3
21.41
324.8
30
389.67
194.84
C 63-30-2
141.3
6.3
21.41
324.8
30
389.67
194.84
C 63-60-1
141.3
6.3
21.41
324.8
60
159.25
79.63
C 63-60-2
141.3
6.3
21.41
324.8
60
159.25
79.63
S 6-30
114.3
6.0
19.05
305.1
30
233.74
-
S 6-60
114.3
6.0
19.05
305.1
60
78.20
-
S 8-30
114.3
8.6
13.29
331.5
30
295.98
-
S 8-60
114.3
8.6
13.29
331.5
60
87.51
-
S 63-30
141.3
6.3
21.41
324.8
30
389.67
-
S 63-60
141.3
6.3
21.41
324.8
60
159.25
-
D – external diameter (mm)
e thickness of the tube (mm)
f characteristic strength of the steel (MPa)
y
t time
N value for the calculated normal compression strength in the situation of a fire, as obtained by the computational
fi,plRd
program SuperTempcalc (ANDERBERG [2])
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–
–
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