Advances in Steel Structures - part 43 pdf

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Experimental Study of High Strength Concrete Filled Circular Steel Columns
Y. C. Wang
Manchester School of Engineering, University of Manchester, Manchester M13 9PL UK
ABSTRACT
In places where usable floor space is at a premium, it is desirable to use the most
structurally efficient load bearing columns. In concrete filled steel tubes, the beneficial
interaction between the steel casing and the concrete core gives a load carrying system
that is highly efficient. When High Strength Concrete (HSC) is used, the column load
bearing performance is further improved.
This paper presents the results of a series of parametric experimental study on HSC filled
circular steel columns under axial compression. The parameters examined in these tests
are: concrete grade, steel grade, column slenderness and steel contribution factor.
The objectives of these tests are threefold:
1. To experimentally investigate the performance of HSC filled steel tubular columns;
2. To assess whether the design rules for normal strength concrete (NSC) filled columns
can be extrapolated to HSC filled columns, and
3. To examine the structural load bearing efficiency by changing different design
parameters.
From the results of this experimental study, the following main findings have been
obtained:
1. It is conservative to extrapolate the design method for NSC filled steel columns to
HSC filled ones;
2. The advantage of HSC in resisting compressive load can be effectively utilised in HSC
filled columns, even for slender columns where HSC does not offer much improved
rigidity to resist flexural buckling;
3. The improved column strength due to concrete confinement effect is noticeable only
for short columns;
4. The confinement effect may be appreciably reduced by a small eccentricity, and
5. The ductility of HSC filled columns is similar to that of NSC filled columns.
401

402
Y. C. Wang
1. Introduction
In places where usable floor space is at a premium, it is desirable to use the most structurally
efficient load beating columns. Concrete filled hollow steel columns are more structurally efficient
in resisting compressive loads than either bare steel columns or reinforced concrete columns. They
also have a number of other advantages including rapid construction, enhanced concrete strength
and ductility due to the confinement effect and inherent high fire resistance. Normal strength
concrete (NSC) filled columns are now being increasingly used in the construction of multi-storey
and high rise buildings and design recommendations for this type of construction are now firmly
established [1,2]. NSC is assumed to have the maximum cube strength of about 60N/mm 2.
Using high strength concrete (HSC) can further improve the structural load bearing efficiency of
concrete filled columns and improve their durabilit3'. However, before HSC filled columns can be
used with confidence and improved economy, their superior load carrying capacity should be
confirmed and suitable design guidelines developed.
HSC filled steel tubes have been investigated by a number of researchers. For example, O'Shea
and Bridge [3] concentrated on local buckling of thin walled tubes filled with HSC. Cai & Gu [4]
studied the confinement effect on HSC in short columns.
This paper reports the results of a series of tests on HSC (C100) and NSC (C40) filled circular
hollow section (CHS) steel columns. The objectives of these tests were threefold:
(1) To assess whether the design rules for NSC filled columns can be extrapolated to cater for
HSC filled columns;
(2) To experimentally study the performance of HSC filled columns, in particular, the confinement
effect on the column strength and ductility, and
(3) To examine the structural load bearing efficiency by changing different design parameters.
2. Test programme
2.1 Test parameters
This series of tests were carried out to examine the influence of a number of design parameters on
column performance. In total, 2 pairs of 12 columns were made and tested. Table 1 gives the
values of test parameters for each pair of columns.

2.2 Test set up
All columns were cast in December 1996 and tests were carried out about six months after casting.
For each column, three concrete cubes of 100 mm and two concrete prisms of 90 mm square and
300 mm in height were cast, to be tested on the column loading day. For each concrete mix, three
concrete cubes of 100 mm were cast. These cubes were tested after 28 days for quality control.
Four strain gauges were attached to the external surface of the steel tube at two opposing sides at
each column mid-height. Two strain gauges at each side measure the horizontal and longitudinal
strains in the steel respectively. For the shortest columns of 500 mm, a vibrating strain gauge was
cast in the column centre to measure the concrete axial main.
Study of High Strength Concrete Filled Circular Steel Columns
403
Column tests were carried out in the BRE axial test machine with the maximum capacity of 5000
kN. Each column was simply supported about one axis and rotationally restrained in the
perpendicular direction using roller supports. The end support increased the column length by
about 80 mm at each end. Therefore, the total column length (L) should be the column specimen
length (L0) plus 160 mm.
All columns were intended to be subjected to compression axial load and were checked to be so by
human eyes only. Also each column had some initial imperfections and the end supports had to be
adjusted for each test. Inevitably, the column could not be aligned perfectly nor in the central
position. This led to a small eccentricity, and bending moment in each column. The amount of this
equivalent eccentricity will be evaluated for calculating the column strength.
Each column was loaded incrementally until it reached its strength when it could not sustain the
applied load. The test was continued to study the column response during unloading at increasing
deformation until the column eventually found a stable position.
3. High strength concrete mechanical properties
For each column, three cube tests and two prism tests were carried out to determine various
properties of concrete. During each prism test, the concrete strain was measured and the complete
concrete stress-strain relationship up to the maximum stress was established. Results of the
compressive strength, the corresponding strain and the Young's modulus are given in Table 2. The
Young's modulus was obtained by using the proposed stress-strain relationship from Cla)~ton [5].

It is observed that the stiffness of HSC is only slightly higher (about 25%) than that of NSC, and
also that concrete strain at prism strength is almost independent of the concrete grade.
4. Test observation and results
When high strength concrete fails in compression, the failure mode is brittle, this was observed
during each prism test when HSC failure was accompanied by a noisy bang. In contrast, HSC
filled steel columns failed in a ductile manner, similar to NSC. This was demonstrated by the
ability of HSC filled columns to deform under decreasing loads and to find a stable position after
reaching the peak strength.
Different failure modes were observed for different columns. For short columns (Lo/D=3), the
failure mode was clearly local due to extensive concrete crushing and steel yielding. NSC filled
columns exhibited very ductile behaviour, with column failure due to splitting of the cold rolled
steel tube at the welding edge. HSC filled short columns also behaved in a ductile way.
Confinement effect was observed by the fact that the failure strain in HSC was several times higher
than the prism crush strain. Nevertheless, the extent of concrete confinement in HSC was lower
than in NSC filled columns and no steel tube splitting occurred.
Global buckling was the dominant failure mode for the longest columns (Lo/D=25). Due to
inevitable eccentricity induced bending effect, global buckling was not very clearly demonstrated in
most columns. However, for the two columns that had very little bending moment, column failure
was indicated by a sudden large lateral movement.
404
Y.C. Wang
All columns w~th the intermediate length (Lo/D= 15) failed in a mixed mode, both axial strain and
lateral deflection increased at steady but faster rates until peak applied load.
Table 3 presents results for all columns, including the column eccentricity e. To verify the
accuracy of the design method and to check the effectiveness of the confinement effect, it was
necessary to evaluate the column eccentricity. This value is calculated from the two axial strain
readings in the steel tube using the following equation:
A6.EI
e - [1]
N.D

where
Ae = the difference in longitudinal strain recorded by the two strain gauges
El =composite section flexural stiffiaess
N =applied load in the column
D =column cross-section diameter
Equation (1) is based on elastic analysis, therefore, the value of eccentricity was obtained from the
average of the few earlier load increments.
In table 3, all design strengths were calculated taking into account the eccentricity and by setting
the partial safety factors for steel and concrete to 1.0. Also, the short term concrete modulus of
elasticity in Table 3 was used for each column.
5. Analysis of test results
The test results have been analysed by a comparison against the predictions of various design
methods for concrete filled columns. From this comparison, a number of conclusions may be
drawn. This paper presents some of the more important ones.
5.1 Accuracy of current design rules for HSC filled columns
The current design rules for concrete filled columns have been derived from test results on NSC.
From the comparative results in Table 3, it may be concluded that these design rules give quite
accurate predictions for NSC filled columns (TI&T2, T5&T6, T9&TI0). Furthermore, it seems
that these design rules may be extended to HSC filled steel columns, as indicated by the overall
accuracy in Table 3. Indeed, the current design rules give conservative results for HSC filled
columns, thus they are acceptable for safety.
Nevertheless, for HSC filled steel cohmms, Table 3 suggests that the accuracy of the NSC-based
design rules depends on the column slenderness. While the code predictions are quite accurate for
short columns, discrepancy between predicted and test results increases at higher column
slenderness. Figure 1 presents the results in Table 3. It is clear that as the slenderness of HSC filled
steel columns increase, both BS 5400 Part 5 [2] and Eurocode 4 Part 1.1 [2] predict lower column
strength. Whilst this means that both design methods are safe to use for HSC filled steel columns,
it also suggests that it is possible to use a higher cohann buckling curve for HSC filled steel
columns for improved column efficiency. However, this can only be confirmed atter more extensive
experimental studies.

Study of High Strength Concrete Filled Circular Steel Columns
5.2 Effect of confinement on concrete
405
Strength
It is now well reax~gnised that when concrete is under tri-axial compression, both its load carrying
capacity and ductility are increased. Concrete confinement can be obtained through placing hoop
reinforcement or using steel casing. For concrete filled columns, although increase in the concrete
strength is at the expense of a reduction in the steel strength, the overall effect is a net increase in
the column strength.
This confinement effect diminishes for slender columns. Although BS 5400 Part 5 [1] gives a
limiting length of L/D=25, realistically, the confinement effect is noticeable only for columns of
m
L/D not greater than 5. In Eur~xxte 4 Part 1.1 [2], the limiting column slenderness is at 2 =0.5.
Nevertheless, in places where the column foot print is large, a L/D ratio of less than 5 is realistic.
Thus, it is beneficial to explore the enhancement due to concrete confinement.
However, the effect of confinement is greatly reduced by bending in the column. To illustrate the
effect of concrete confinement, only Eurocxxle 4 Part .1.1 [2] is used in this paper. Results are given
in Table 4 for L0/D=3. Without bending moment, the squash load of a column can be increased by
up to 20% due to enhancement. However, with an eccentricity to diameter (e/D) ratio of only 3%,
column strength increase due to the confinement effect is reduced by about 30%. For columns in
simple construction, BS 5950 Part 1 [6] gives a nominal eccentricit)" of 100mm plus D/2 for beam
reactions. For medium rise buildings, this end bending moment can give a significant eccentricity
to the overall column axial load, which may completely remove the enhancement due to
confinement. For example, for a 10 storey building with 300 mm diameter columns, the column
eccentricity (e/D) to the overall axial load of the bottom floor column is about 8%. Therefore, to
make use of the enhancement in design, an accurate assessment of the column ~tricity should
be carried out.
Duetili~ ~
One of the main concerns with using HSC is its lack of ductility and its brittle and explosive
failure. However, in the author's tests, no HSC filled steel column suffered from this failure mode

and all columns performed in a ductile manner.
The ductilit3, of a column is rather difficult to quantify. The unloading slope of the column may
give some indication. Figure 2 plots the load-axial strain relationship for tests T5-T8, two of which
used HSC and the other two NSC. In this figure, the applied load is norrnalised with regard to the
column test strength. The unloading slope seems to be comparable between NSC and HSC filled
columns. However, while the two NSC filled column curves are almost identical, there is a great
variability in the behaviour of the two HSC filled columns.
On the other hand, if the column ductility is measured by the maximum concrete strain reached at
the peak column strength, the enhanced strain due to the conflnernent effect may be predicted using
the equation obtained by Mander et al [7]:
8cc I Cr cc |1
- 1 + 5 - [2]
s \ 0"~
406
Y. C. Wang
Table 5 gives a comparison between test results and predictions using equation (4). Since the
confinement effect is negligible for slender columns, the comparison was carried out for short
columns (I.o/D=3) only. In addition, the theoretical value of the concrete strength enhancement
factor (t~/(rck) has been calculated using recommendations in Eurocode 4 Part 1.1 [2].
Table 5 only indicates a broad agreement between the predictions of equation (4) and test results.
Nevertheless, it suggests that the confinement effect can significantly increase the concrete ductility
and that equation (4) gives conservative results.
Table 5:
Test ID
T9
T10
TII
T12
T17
T18

T27
T28
Increased concrete strain due to confmement effect
t~cc/cck according to EC4 test
1.774 (1.544) 15.8
1.796 (1.56) 13.2
1.230 (1.162) 2.03
1.227 (1.213) 2.36
1.60(1.458) 4.0
1.571 (1.469) 6.12
1.442 (1.18) 5.07
1.412 (1.288) 4.5
2
0.17
0.17
0.21
0.21
0.20
0.21
0.18
0.19
NB: Values in brackets include the effect of eccentricity.
model [7]
4.87 (3.72)
4.98 (3.8)
2.15(1.81)
2.14 (2.07)
4.0 (3.29)
3.86 (3.35)
3.21 (1.9)

3.06(2.44)
5.3 Effect of high strength steel
One of the original objectives of this series of tests was to examine the effectiveness of using high
strength materials, including both high strength steel and high strength concrete. The effect of using
HSC has already been discussed in 5.1. It seems that despite only a modest increase in HSC
modulus of elasticity., column test strength increases in line with increase in the column squash load
regardless of the column slenderness.
However, unless the column is short, using high strength steel only gives a small increase in the
column strength. Tests T13-TI8 are directly comparable to Test T23-T28, the only difference
being that $355 steel was used in the former and S275 steel was used in the latter. Table 6 gives
increases in the column strength due to high strength steel. Clearly, the benefit of using high
strength steel diminishes
at
higher column slenderness.
Table 6: Comparison between results for different grades of steel
L/D=25 L/D=15 L/D=5
1.075 1.386 1.368
6. Conclusion
This paper has presented the results of a series of compression tests on NSC and HSC filled
circular steel columns. From an analysis of the test results, the following conclusions may be
drawn:
Study of High Strength Concrete Filled Circular Steel Columns
407
(1) Using HSC can significantly increase the strength of concrete filled columns. This conclusion
applies to a wide range of tested column slenderness (2, = 0.2-1.4 ).
(2) Since the modulus of elasticit3' of HSC is only slightly higher than that of NSC, it follows that
a higher column buckling curve may be used in design calculations for HSC filled steel
columns. However, a large number of tests should be carried out for confirmation. In the
meantime, the design rules for NSC filled steel columns may conservatively be used for HSC
filled columns.

(3) Using high strength steel is far less effective tlwu using HSC in increasing the column strength.
(4) The benefits of concrete confinement in increasing the concrete strength and ductility are
realised for short columns only. Furthermore, the increase in concrete strength can be reduced
by a small ~tricity. Therefore, in order to reliably use the beneficial effect of confining
concrete, accurate assessment of the column eccentricity should be made in design
calculations.
Acknowledgments
The tests reported in this paper were carried out by the author at the Building Research
Establishment and he acknowledges the technical support of various BRE staff members. He also
thanks Mr. Nigel Clayton of BRE for the concrete prism tests.
References
1. Design of composite bridges: use of BS 5400: Part 5:1979 for Department of Transport
structures, Department of Transport, London, December 1987
2. Eurocode 4: Design of composite steel and concrete structure, Part 1.1: General rules and rules
for buildings, British Standards Institution, London, 1994
3. O'Shea M D and Bridge R Q, "Circular thin walled concrete filled steel tubes", Proceedings of
the 4 th Pacific Structural Steel Conference, Vol. 3: Steel-concrete composite structures, pp. 53-
60, 1995
4. Cai, S H and Gu W P, "Behaviour and ultimate strength of steel tube confined high strength
concrete columns", Proceedings of 4 th International S3~posium on Utilization of high
strength/high performance concrete, pp. 827-833, Paris 1996
5. BS 5950: Structural use of steelwork in buildings, Part 1: Code of practice for design in simple
and continuous construction: hot rolled sections, British Standards Institution, London, 1990
6. Clayton N, "High grade concrete - stress-strain behaviour", BRE Client Report CR44/97,
Building Research Establishment, 1997
7. Mander J B, Priestley M J N and Park R, "Theoretical stress-strain model for confined
concrete", Journal of Structural Engineering, Vol. 114, No. 8, pp. 1804-1826, American
Society of Civil Engineering, 1988
Y.C. Wang
TI,T2 168.3 5.0

T3,T4 168.3 5.0
T5,T6 168.3 5.0
T7,T8 168.3 5.0
T9,TI0 168.3 5.0
TI1,T12 168.3 5.0
T13,T14 168.3 10.0
T15,T16 168.3 10.0
T17,T18 168.3 10.0
T23,T24 168.3 10.0
T25,T26 168.3 10.0
T27,T28 168.3 10.0
Table 2: Measured Material
Lo (mm)
Steel
$355
8rade
Test ID
4200
4200 S355 C100
2500 $355 C40
2500 $355 C100
500 $355 C40
500 S355 CI00
4200 $355 C100
2500 $355 C100
500 $355 C100
4200 S275 CI00
2500 S275 C100
500 $275 C100
Concrete 8rade

C40
TI
438
Steel yield
stress
(Nlmm 2)
52
Properties
Cube strength
(Nlmrn 2)
40.8
T2 438 51.8
T3 438 123.5
T4 438 121.2
T5 438 47.5
T6 438 47.8
T7 438 116.0
T8 438 115.3
T9 438 46
T10 438 44.7
Tll 438 115.3
T12 438 113.8
TI3 i480 120.7
Cylinder
strength
(N/mm 2)
41.3
106.3
91.0
39.0

39.0
97.8
408
Table 1: Test parameters
Test ID D(mm) t(mm)
Young's
modulus
(N/mm 2)
41000
6max
(mm/m)
2.3
45000 , 2.4
52500 12.83
53000 '2.05
39500 t2.2
42000 2.05
50500 2.45
101.0 54000 2.8
37.3 41000 2.05
36.5 41500 2.15
53500 2.48
52500 2.78
49000 2.4
50500 2.25
49500 2.48
99.5
100.0
92.7
T14 480 119.5 82.3

TI5 480 113.8 93.3
T16 480 114.2 90.5 52500 2.20
TI 7 480 126.0 87.8 52000 2.95
TI8 480 120.0 90.8 49000 2.23
50000 2.78
T23 330.5 116.8 , 98.8
T24 330.5 118.7 98.5
50500
2.78
T25 330.5 113.6 89.3 53500 2.23
T26 330.5 116.3 95.8 52500 I 2.48
T27 330.5 116.0 91.8 152000 ! 2.38
T28 330.5 114.2 97.0 ~ '
i 50500 2.58
Study of High Strength Concrete Filled Circular Steel Columns
409
Table 3: Comparison between design strength and test results
Test lD ~ e(mm) Test comparison between design calculations
and test results
load BS 5400 Part 5 [ 1] Euroexxte 4 Part 1.1
t21
(kN) (kN) _pred/test (kN~.__pred/test
T1 1.16 3 900 , 964 1.071 961 1.068
,_ _ __._._
T2 1.14 5 950 ! 963 0.993 932 0.981
T3 1 '~43 2.5 1550 ~ 0.754 1153 0.744
T4 1.35 5 1400 ~ 1124 0.803 ! 1053 0.752
T5 0.7 4 1300 i 1382 1.063 ~ 1448 1.114
T6 0.7 2 1445 1465 1.014 1513 1.047
T7 0.85 5 2330 1858 0.791 2007 0.854

T8 0.85 2.5 2450 2004 0.818 2197 0.897
T9 0.17 5 2360 2002 0.848 1891 0.801
T10 0.17 5 2360 1988 0.842 1879 0.796
Tll 0.21 5 3250 i 2784 0.857 2944 0.906
T12 0.21 1 3250 ~ 0.950 32/2 1.007
v~
T13 1.36 4.5 1900 ~ 0.827 1481 0.779
T14 1.33 1.5 2400 1661 0.692 1623 0.676
T15 0.83 2 3350 2855 0.852 2957 0.883
T16 ~ 0.2 3650 3032 0.831 3076 0.843
T17 ~ 4 4550 4326 0.951 3943 0.867
T18 0.21 3 4550 4386 0.964 4099 0.901
T23 1.25 6 1800 1429 0.794 ~ 0.738
T24 1.24 0.5 2200 1605 0.729 11627 ! 0.739
!T25 0.74 3 2600 2333 0.897 "2465 !
0.948
L T26 0.75 3.5 2450 2314 0.945 : 1.021
IT, 0.1 lO 2,0 2 60 10. 64
, T28 0.19 5 3400 3206 0.943 3-~ 10.952
Table 4: Effect of column squash load increase due to confinement effect
Test
iD
With bending moment Without bending moment
T9
1.145
1.207
T10 1.148 1.210

T1 i 1.066 1.094

TI2 1.087 1.092
,,
TI7 1.109 1.142
T18 1.113 1.137
T27 1.056 1.137
T28 1.091 1.130