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IBRACON Structures and Materials Journal • 2012 • vol. 5 • nº 4
Short steel and concrete columns under high temperatures
also determined in the present study by computational analysis
using the program SuperTempcalc (ANDERBERG [2]).
As observed in Figure 2, for the case of a general composite col-
umn filled with concrete with the application of a constant stress
and temperature variations, at the instant when the curve of axial
displacement versus time reaches a maximum (part I), the steel
tube is absorbing almost the entire applied load. After this point
(part II), the steel tube begins to lose its strength and begins plastic
deformation, thereby transferring stresses to the concrete.
Thus, when using a load equal to 50% of the load bearing capacity
of the composite section at the temperature of a fire, this deforma-
tion peak in part I will not be reached, and the section will not suffer
plastic deformation. The results of tests at room temperature were
obtained by Simões [3], whose tested prototypes that had identical
dimensions to those used in the present study.
3. Characteristics of the short columns
As shown in Figure 3, 3/16’’-diameter holes were made in the steel
tubes before the concrete was added to allow for the insertion of
thermocouples at standardized internal points for all of the tests.
After curing, the plugs used to prevent the closure of the holes
during the curing process were removed to permit insertion of the
thermocouples.
Five thermocouples were placed in each model: two in the con-
crete core, two at the steel-concrete interface and one on the ex-
ternal surface of the steel tube.
4. Environment and equipment
The Structures Laboratory of the State University of Campinas
(Universidade Estadual de Campinas) was used for the tests. A
5,000-kN capacity press, a 600-kN capacity press and an oven
capable of achieving temperatures up to approximately 1,200ºC
were used for the tests.
The oven used for the tests has the capacity to produce tempera-
The main objective of the present study was to analyze the
loss of load bearing capacity of short columns after they are
subjected to a fire; the short columns used are those whose
normal compression strength is not affected by their slender-
ness. For this purpose, the behavior of short composite steel-
concrete columns was studied using physical experimental
tests, measuring the increase in temperature of the surface
and internal points of these columns for 30 and 60 minutes of
heating according to temperature elevation curves similar to
the ISO 834 [1] curve, as shown in Figure 1. Pre-heating to
200ºC was necessary so that the curve followed the ISO 834
[1] curve after reaching this temperature.
2. Experimental research methodology
The average compressive strength of the concrete was 28 MPa,
with a modulus of elasticity of 22.975 MPa. The height of the tubes,
30 cm, was the same for all of the models.
To describe the studied series, the models were differentiated
based on their load (L loaded and N not loaded – two columns with
load and one without load were made for each type of tube), the
thickness of the tube (6 mm, 8 mm or 6.3 mm) and the duration of
exposure to the high temperature (30 min or 60 min). The studied
models were grouped into series, as presented in Table 1.
During the heating process, axial loads were applied at 50% of the
intensity of the normal compression strength for the composite sec-
tion at high temperatures. This compression strength was obtained
for each model using the program SuperTempcalc (ANDERBERG
[2]). The models were loaded before heating with monotonic load-
ing, which was maintained until the models reached the set tem-
perature. The models were then cooled to room temperature and
subjected to static stress tests until failure. This procedure was
performed to evaluate the reduction in the columns’ normal com-
pression strength after being subjected to a fire.
The cross-sectional temperature distribution in the columns was
Figure 2 – Axial displacement as a function
of exposure duration (KODUR [4])
Figure 3 – Positioning of the thermocouples