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Comparison between predicted and test results
Study of High Strength Concrete Filled Circular Steel Columns 411
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STRENGTH AND DUCTILITY OF HOLLOW CIRCULAR STEEL
COLUMNS FILLED WITH FIBRE REINFORCED CONCRETE
G. Campione, N. Scibilia, G. Zingone
Dipartimento di Ingegneria Strutturale e Geotecnica
Universitb, di Palermo, 1-90128, ITALY
ABSTRACT
The focus of the present investigation is the study of the behaviour of hollow circular steel cross-
sections filled with fibre reinforced concrete (FRC), subjected to monotonic loads. Using the same
volume percentages of fibres, the influence of different types of fibres (steel, polyolefin) on the
behaviour of the columns was investigated. Results of fibre reinforced composite columns were
compared with those of columns filled with plain concrete, showing the advantages of using FRC
compared to plain concrete, in terms of both strength and ductility. A simplified analytical model to
predict load-deformation curves for composite members in compression is proposed, and the
comparison between experimental and analytical results has shown good agreement. Finally, a
comparison is made between the bearing capacity of circular hollow steel columns filled with plain
concrete evaluated according to recent European and International codes and that evaluated using the
proposed model.
KEYWORDS
Circular steel columns, fibre reinforced concrete, composite members, strength, ductility, active
confinement.
INTRODUCTION
In the design of tubular structures, after determining the shell plate thickness satisfying tensile stress, the
stability of the shell should be checked for compressive stresses against buckling. A thin-walled cylinder
shell subjected to compression may fail either due to the instability of the shell, involving bending of the
axis, or due to local instability, as shown in Figure. 1, and also depends on the ratio of the thickness to
the radius of the shell wall and on the length of the columns. Failure of this type of structure is due to
the formation of characteristic wrinkles or bulges, circular or lobed in shape. To mitigate or prevent this
type of failure it is common to encase or fill steel profiles with concrete. The coupling of concrete and
steel shapes makes it possible to obtain structural elements which, compared to the single constituent

elements, ensure high performance in terms of both resistance and ductility, Cosenza & Pecce (1993).
413
414 G. Campione et al.
Modern codes like the Eurocode relating to composite steel-concrete structures contemplate the use of
composite steel-concrete columns made using W shapes or thin-walled tubular profiles with a circular,
rectangular or square section, as shown in Figure 2. Concrete, inside or outside the profile, exerts
beneficial confinement action against phenomena of local or overall instability and markedly increases
the resisting and dissipative capacity of steel columns, Schneider (1998). A further advantage is the
increase in fire resistance, Lie (1994), Frassen et al. (1998).
Figure 1: Overall and local instability
Dwelling on the case of tubular steel columns, we can observe that, filled with concrete, they present
deeply different behaviour from hollow ones and much better performances. There is not only a higher
bearing capacity, but also greater safety against flexural instability, which makes it possible to construct
particularly slender columns. The concrete is highly confined and hence is more ductile and has a
greater bearing capacity, Shakir et al. (1994). The coupling of several tubular sections makes it possible
to face very high stresses, greatly increasing the critical load values, both against local and overall
buckling of composite columns.
Figure 2: Typical composite members
Recent applications of the structural elements concern arch road bridges in which the deck is supported
by hollow steel columns filled with concrete. These very slender elements, thanks to the combined use
of the two materials, can face the high stresses induced by external loads, with the evident advantage of
constituting carpentry during the construction of the structure.
Recent theoretical and experimental studies have shown that if traditional reinforcement made up of
bars and stirrups is added inside tubes, or these are filled with fibre reinforced concrete (FRC), their
Strength and Ductility of Hollow Circular Steel Columns 415
bearing capacity and ductility increase, Campione et. al (1999). In the last few decades interest in the
field of composite materials and especially in fibre reinforced concrete has led to the development of
new types of fibres (carbon, polyolefin, kevlar, steel) with different shapes (hooked, crimped,
deformed). Due to the bridging capacity of the fibres across the cracks, FRC behaves better than either
plain concrete or plain concrete confined with traditional reinforcement in terms of energy absorption

capacity, and sometimes strength, especially when a high volume fraction of fibres (1-2 %) is used,
Campione et al. (1999).
EXPERIMENTAL INVESTIGATION
In the present section there are briefly mentioned experimental results discussed in detail in a previous
investigation by the authors, Campione et al. (1998). The experimental research involved the casting of
different types of composite members: steel columns, steel columns filled with normal strength concrete
(NSC) and steel columns filled with fibre reinforced concrete (FRC). Different types of fibres
(polyolefin straight, hooked and crimped steel fibres) were added to the fresh concrete at a dosage of
2% by volume. The fibres had the characteristics shown in Table 1.
TABLE 1
CHARACTERISTICS OF THE FIBRES
Type of
fibres
Shape Diameter
equiv.
, (ram)
Length.
Lf
(mm)
Tensile
strength
f't (MPa)
Modulus of Weight
elasticity density
Ef (MPa) (kg/ m 3)
Poyolefin 0.80 25 375 12000
Hooked steel ~ "-" 0.50 35 1115 207000
Crimped steel ~ 1.00 50 1037 207000
900
7860

7860
The columns had a circular cross-section welded along their length; the yielding stress was 206
MPa
and the ultimate stress 324 MPa; the internal diameter was 120 mm and the thickness 3.5 mm, the
length of the entire columns was 1000
mm.
Figure 3: Composite columns tested in compression
The columns were tested in uniaxial compression utilising a universal testing machine operating in
displacement control (Figure 3). A load cell and several LVDT's connected to a data acquisition system
were used to record the load P and the vertical deformation 8 of the columns.
Monotonic tests were carried out on 100x200
mm cylinders in concrete and FRC both in indirect split
tension and in compression to characterise the materials as shown in Figure 4. It is interesting to
observe that adding fibres to the matrices the behaviour of the latter changes significantly, especially in
416 G. Campione et al.
the sottening branch, in terms of both energy absorption capacity and residual strength. More details on
the strength and strain values and cyclic response of the materials are given in a previous paper,
Campione et al. (1998). Table 2 gives the most rapresentative experimental results of the compression
and indirect split tension tests, particularly the maximum compressive strength f c, and corresponding
strain e0, and maximum tensile strength ft.
TABLE 2
EXPERIMENTAL RESULTS FOR COMPRESSION AND INDIRECT TENSION TESTS
Types of fibres
Matrix
E;o
0.0032
f~
(A/IPa)
25.20
(*)f,

(MPa)
1.64
Hooked steel 0.0061 27.45 3.56
Polyolefin 0.0034 29.34 2.42
Crimped steel
35.40
0.0064
2.72
(*) ft=2P/0td h)
Figure 4: Monotonic tests in compression of FRC with 2% fibres
Figure 5 gives load-deformation (P-8) curves in compression for steel pipes and composite members in
the case of montonic loads. Experimental results have shown that columns filled with FRC exhibit
higher strength than those filled with plain concrete. The maximum strength of composite members
filled with FRC is 20 % higher than that recorded for steel pipes filled with FRC. After the peak load
was reached, failure was due to the crushing of concrete and to local and global buckling of the steel
pipes. At this point, the peak load and also the stiffness decreased. By contrast, the addition of fibres
ensured better softening behaviour and more available ductility.
Figure 5" Load vs. deformation curves for composite columns with 2% fibres
Strength and Ductility of Hollow Circular Steel Columns
STRENGTH OF COMPOSITE COLUMNS SUBJECTED TO COMPRESSIVE LOADS
417
Several European and international codes give design rules and simplified formulae to predict the
bearing capacity of steel columns filled with plain concrete. These are able to take into account the
strength of the materials, and the buckling problems of the composite members. When columns are
subjected to axial forces the properties which must be taken into account in the design of members
include strength of the constituent materials, local instability, and the capacity to transfer internal
stresses between the steel pipe and the concrete core.
For composite members having a transverse circular cross-section, EC4 allows one to neglect local
buckling problems when the ratio between the diameter d and the thickness t obeys the relationship
d/t<90 e 2, where e 2=235/fy, and fy is the yield stress of the steel. According to EC4, the axial plastic

force of the transverse cross-section of a steel column filled with plain concrete can be obtained, as a
first approximation, as the sum of the contributions of the concrete core and the steel section, also
taking into account the increase in strength due to the confinement in the concrete core and the
reduction in steel stress due to the biaxial state of stress in the steel pipes; this is done by introducing
the coefficients rl, which depend on the slenderness of the composite members. The ultimate axial load
capacity of the columns is obtained by multiplying the axial plastic force by the reduction coefficient ~,
which depends on the slenderness ,i.
~, ___ _._.~
TTT
~~- ~" t ~'II,"L'"ll
T
l~s ,t
Figure 6: Distribution of tensions in the steel pipes
It is interesting to observe that EC4 and LRFD give very conservative values of the maximum beating
capacity for short columns. In fact, in composite members loaded concentrically the concrete core and
steel tube are subjected to a combined state of stresses (Figure 6). For this reason many researchers,
like in Pecce (1993), have modelled the behaviour of composite members subjected to compression
considering a triaxial state of stresses in the concrete core and a biaxial state in the steel pipes and
imposing the compatibility for each loading step in terms of longitudinal and lateral strains.
Unfortunately, few experimental data are available for plain concrete and fibre reinforced concrete
subjected to triaxial stresses, and so it is difficult to calibrate the parameters of an analytical model of
concrete core subjected to a multiply stresses.
In the present investigation the authors refer to a monoaxial state of stresses for concrete, in which the
confinement effect due to the lateral pressure f'~ of the steel pipes, varying at each loading step, is taken
into account. For steel pipes a biaxial state of stresses is assumed and the relationship between
longitudinal and circumferential stresses is that proposed by Von Mises up to failure condition:
2 2
418 G. Campione et al.
For steel in uniaxial tension an elasto-plastic stress-strain relationship was assumed, having conventional
yielding stress of 300

MPa.
When the yielding condition occured in steel pipe due to biaxial stresses a
linear reduction of longitudinal stress ~s,I up to 2 % strain was assumed, in accordance with
experimental data (Figure 7-b). It was also assumed that circumferential stress ~s,t in steel pipes,
evaluated using Eqn. 1, increased up to the yielding value and consequently that in the concrete core the
confinement effect was maximum (Figure 7-a). The longitudinal stress in the concrete core was
obtained utilising the Mander et al. (1988) model in which the lateral pressure fl is assumed variable.
The Mander et al. (1988) stress-strain curve for concrete adopted is:
~c
t3~ r
~ cc
c~r = (2)
r
r-I+ ~
The r coefficient is related with the initial tangent modulus Er and with the secant modulus Eso~=o~dec~
as follow:c
E~
r = ~ (3)
The maximum strength in the concrete core ~cr is:
C~cc = f~"( 1.254 + 2.254"11+~ ~
7.94.f/ 2 - ~ '1
(4)
L' L)
The lateral pressure ft on the concrete core is:
t
ft' = 2.o,., d
(5)
The maximum longitudinal strain is:
E - 11]
e~=eo" I+ tf~'

(4)
Figure 7 shows the longitudinal stress-strain relationship for plain concrete (f'c=25.2
MPa)
and steel
obtained assuming different values of maximum longitudinal stresses.
To determine the load-deformation curves (P-8) of the columns the compatibility of the lateral and
longitudinal deformations was assumed and an equilibrium equation was utilised in which the stresses
were calculated through constitutive relationships given before.
The vertical loads of the composite members were obtained by considering of the steel pipes and
concrete core to strength, without any reduction in the transverse cross-section of the concrete core,
because of the effective confinement reached in concrete inside steel pipes.
Strength and Ductility of Hollow Circular Steel Columns
419
Figure 7: Analytical longitudinal stress-strain curves : a) concrete; b) steel
Figure 8 shows experimental and analytical results obtained with the model proposed. It is interesting to
observe that the model permits one to obtain a good level of approximation in terms of maximum
bearing capacity and corresponding deformation, but overestimates initial stiffness. This is due perhaps
to the fact that initial deformations are affected by boundary test effects that the model does not take
into account.
Figure 8: Comparison between analytical and experimental results for composite members
In the case of FRC the buckling effects are reduced by the presence of fibres and failure is due
essentially to plasticization of materials ensuring a good prevision of experimental results with the
analytical model based on these concepts. In the case of steel pipes filled with plain concrete, the
buckling effects reduce the bearing capacity of the columns in the softening branch and the analytical
model is not able to predict these effects.
TABLE
3
ULTIMATE LOAD OF COMPOSITE MEMBERS: EXPERIMENTAL AND ANALYTICAL VALUES
Columns filled with
Plain concrete

FRC with hooked steel fibres
FRC filled with polyolefin fibres
FRC filled with crimped fibres
g g NpI, R kc Per g me~ee Nsp
0.303 0.977 678 0.640 552 876 889
0.308 0.976 700 0.650 568 966 976
0.311 0.975 718 0.660 581 1030 980
0.322 0.972 778 0.670 623 1250 1055