Design of Concrete-filled Steel Tubular Members According to the Australian Standard as 5100 Model...

9

© Institution of Engineers Australia, 2008 Australian Journal of Structural Engineering Online

* Paper S08-977 submitted 29/02/08; accepted for publication after review and revision 6/05/08.

† Corresponding author Prof Brian Uy can be contacted at [email protected].

Design of concrete-filled steel tubular members according to the Australian

Standard AS 5100 model and calibration *

Z Tao University of Western Sydney, Sydney, NSW, Australia

Fuzhou University, Fuzhou, Fujian Province, China

Brian Uy † University of Western Sydney, Sydney, NSW, Australia

L-H Han and S-H He Tsinghua University, Beijing, China

SUMMARY: Procedures given in the Australian bridge design standard AS 5100 (Standards Australia, 2004) for the design of concrete-filled steel tubular (CFST) columns, beams and beam-columns are presented and described briefly in this paper. A wide range of experimental data from two currently available test databases (2194 test results altogether) is used to evaluate the applicability of AS 5100 in calculating the strength of CFST members. Some other existing design codes, such as the Japanese code AIJ (1997), American code AISC (2005), British bridge code BS 5400 (2005), Chinese code DBJ 13-51-2003 (2003) and Eurocode 4 (2004), are also compared with the test results in this paper. From the comparisons, useful information is provided for future possible revision of AS 5100 and for the suggestion that this model be used for building construction.

1 INTRODUCTION

In recent times, concrete-filled steel tubular (CFST)

members have been widely used in civil engineering

in Australia and other countries (Uy, 2000; Han,

2007). Several examples of such engineering practice

in Australia include: the Latitude building and

Market City in Sydney; the Casselden Place and the

Commonwealth Centre in Melbourne; Riverside

Office and Myer Centre in Adelaide; and the Forrest

Centre, Exchange Plaza and Westralia Square in Perth

(Uy & Patil, 2006).

Figure 1 shows the Latitude building in Sydney

during construction. This building was completed

in 2005, and is on George Street on the World Square

Site, directly adjacent to Sydney’s Chinatown at

Haymarket. It is a landmark building, which was

designed by Hyder Consulting and constructed by

Multiplex. The building has a total height of 222 m

over 45 floors and has some very innovative features

in its design. The building uses twin composite

columns on the perimeter frame, using 508 mm

diameter steel tubes filled with 80 MPa concrete.

The building has required the design of 7 m deep

transfer trusses using large diameter steel tubes

filled concrete and large high strength steel boxes

filled with concrete (Chaseling, 2004; Australian Steel

Institute, 2004).

The practical application of CFST construction

is now supported by many well-known national

standards or recommendations, such as the Japanese

code AIJ (1997), American code AISC (American

Institute of Steel Construction, 2005), British bridge

code BS5400 (British Standards Institution, 2005),

Chinese code DBJ 13-51-2003 (2003) and Eurocode 4

(2004). Research and practice of CFST members and

structures has also led to the development of these

design codes.

In 2004, a new version of the Australian bridge

design standard AS 5100 (Standards Australia,

2004) for bridge design was issued, where design

10

Australian Journal of Structural Engineering Online

“Design of concrete-filled steel tubular members according to the Australian ... ” – Tao, Uy, Han & He

guidance for composite columns (including CFST

columns) was incorporated. The aim of this paper is

to provide useful information for a future possible

revision of AS 5100 and for the suggestion that this

model be used for building construction. To fulfil this

task, procedures given in AS 5100 for the design of

CFST columns, beams and beam-columns are firstly

presented and described briefly. In order to evaluate

the applicability of AS 5100 in calculating the strength

of CFST members, a wide range of experimental data

from two currently available test databases (2194 test

results altogether) are used for comparison. Effects of

different parameters such as steel strength, concrete

strength and section slenderness on the accuracy of

the strength predictions are discussed. This is to check

the possibility of relaxing the limitations specified in

AS 5100. The above-mentioned existing design codes

are also compared with the test results in this paper.

For simplicity, these codes are to be referred to as

“AIJ”, “AISC”, “BS 5400”, “DBJ 13-51-2003” and

“EC4” in the following.

2 AS 5100 PROVISIONS

2.1 General specifications

The Australian Standard AS 5100 was prepared by

the Standards Australia Committee BD-090, Bridge

Design, to supersede HB 77.6-1996, Australian Bridge

Figure 1: Latitude, Sydney (2005).

Design Code. Seven parts are included in AS 5100

with objectives to provide nationally acceptable

requirements for the design of road, rail, pedestrian

and bicycle-path bridges; the specific application of

concrete, steel and composite construction; and the

assessment of the load capacity of existing bridges.

Part 6 of the standard AS 5100 is concerned with the

design of steel and composite construction, in which

procedures are given for the design of concrete-filled

circular and rectangular hollow steel members,

which take account of the composite action between

the various components forming the cross-section.

The specifications in AS 5100 related to the design of

CFST members are described briefly as follows.

In AS 5100, it is specified that the steel tube should

be fabricated from steel with a maximum yield stress

of 350 MPa. The elastic modulus of steel (E) is given

as 200,000 MPa by AS 5100. The selection of wall

thickness (t) should ensure that the plate element

slenderness (λe) is less than the yield slenderness limit

(λey). The value of λe for rectangular hollow sections

(RHS) is calculated as ( / ) /235yh t f , where h is the

overall height of a RHS, fy is the characteristic yield

strength of the steel. For circular hollow sections

(CHS), the slenderness (λe) is given as (do/t)(fy/235),

where do is the outside diameter of a CHS. The yield

slenderness limit (λey) for CHSs is equal to 82, while

slightly different values of λey (35, 40 and 45) are

specified for RHSs with different fabrication process.

The larger the residual stress remaining in the section,

the smaller the λey resulting. For lightly welded

(longitudinally) tubes or cold formed sections, a

moderate value of 40 is used for λey. It should be

noted that the yield slenderness limit specified in

AS 5100 for CFST members is virtually the same as

those for hollow steel sections, that is, the beneficial

effects from concrete restraint is neglected.

Concrete with normal density and strength is

recommended in AS 5100 to fill the steel tubes. The

characteristic compressive cylinder strengths (fc’) of the standard strength grades of concrete are 25,

32, 40, 50 and 65 MPa, respectively. The maximum

aggregate size is 20 mm. As far as the concrete

modulus of elasticity (Ec) is concerned, a similar

formula presented in AISC is recommended in AS

5100 as follows:

1.5 0.043c cE fρ= × ′ (1)

where ρ is the concrete density taken as not less than

2400 kg/m3 for normal weight concrete.

A steel contribution factor αs is specified in AS 5100

with an allowed range from 0.2 to 0.9, where αs is

defined as the ratio of the contribution of the steel

section (φAsfy) to the total axial capacity (Nus). The

above notation of φ and As, as well as the calculation

method of Nus, will be given in the following

section.

11“Design of concrete-filled steel tubular members according to the Australian ... ” – Tao, Uy, Han & He


In AS 5100, it is suggested that reinforcement is not normally required in CFST compression members. Also, almost all currently available tests were carried out without steel reinforcement used. Therefore, the contribution from reinforcement is omitted in the following review of design methods.

2.2 Members subjected to axial compression

2.2.1 Ultimate section capacity

To calculate the section capacity under axial compression, an assumption was used that the steel yields before the concrete reaches its ultimate stress state. Thus, the ultimate section capacity (Nus) for rectangular CFST members can be calculated by summing up the axial load capacities of the tube and the concrete. This leads to:

Nus = φAsfy + φcAc fc’ (2)

where As and Ac are the areas of the steel tube and the core concrete, respectively; and φ and φc are the capacity factors for steel and concrete respectively, given as 0.9 and 0.6 in AS 5100 for section capacity.

For a circular CFST member, the benefits of the increase in concrete strength due to confinement may be taken into account if the relative slenderness of the member (λr) is not greater than 0.5, and the load eccentricity (e) under the greatest design bending moment is not greater than do/10. Otherwise, Nus should be calculated using equation (2). If the benefits of confinement are taken into account, Nus may be calculated as follows:

1

2 ' 1'

yus s y c c c

o c

tfN A f A f

d f

ηφ η φ

⎛ ⎞= + +⎜ ⎟

⎝ ⎠ (3)

in which, η1 and η

2 are coefficients used to reflect

the confinement benefit that are dependent on the relative slenderness (λr) and load eccentricity (e). The coefficient of η

2 is used to account for the strength

reduction of the steel because of the circumferential tensile strains in the steel induced by confining the concrete, while the coefficient of η

1 is used for the

concrete to reflect the strength increase from the tube confinement. The calculation formulae for η

1 and η

2

are given in Clause 10.6.2.2 of AS 5100: Part 6.

From the above introduction, it can be seen that the formula for calculating the ultimate section capacities is virtually the same as those suggested in EC4, except different values have been used for the capacity factors.

2.2.2 Ultimate member capacity

Like many other codes, a slenderness reduction factor αc is introduced in AS 5100 to reflect the basic relationship between strength and stability for an axially loaded column, as follows:

290

1 1cα ξξλ

⎡ ⎤⎛ ⎞⎢ ⎥= − −⎜ ⎟⎢ ⎥⎝ ⎠⎣ ⎦

(4)

where ξ and λ are coefficients related to the relative

slenderness (λr). λr is defined herein as /s cN N r , in

which Ns is determined according to equation (2) or (3),

with φ and φc taken as 1.0, and Ncr is the elastic critical

load. The expression for Ncr is given as equation (5),

where (EI)e is the effective flexural stiffness determined

according to equation (6), and Le is the effective length

of a composite compression member.

2

2

( )ecr

e

EIN

Lπ

=

(5)

(EI)e = φEIs + φcEcIc (6)

In equation (6), Is and Ic are the second moment of

areas of the steel section and the uncracked concrete

section, respectively, and φ and φc herein are also

taken as 1.0.

After the slenderness reduction factor αc is determined

from equation (4), the member capacity of a composite

column can be expressed as:

Nuc = αcNus ≤ Nus (7)

2.3 Members under combined compression and bending

In AS 5100, strengths of CFST beams and beam-

columns should be calculated on the basis of

rectangular stress blocks, assuming that the maximum

concrete compressive stress is (φc fc’) and the maximum

steel stress is (φfy). An interaction curve based on the

plastic resistance analysis can be obtained as shown

in figure 2(a). It should be noted that both φ and φc

are taken as 0.9 for a composite member subjected

to combined axial and flexural actions.

To verify the resistance of a beam-column subjected

to compression and uniaxial bending, the following

criterion should be satisfied:

Mx* ≤ 0.9Mrx (8a)

My* ≤ 0.9Mry (8b)

where Mx* and My* are the design bending moments

about the principal major x-axis and minor y-axis,

respectively; Mrx and Mry are the section moment

capacities reduced by the effect of axial compression,

slenderness and imperfection (see figure 2(a)).

In figure 2(a), Mdx and Mdy are the total moment

capacities of the section when the design axial force

N* is acting on the section; αn is a factor for the

interaction curve, given by αc(1 + βm)/4; βm is the ratio

of the smaller to the larger end bending moments

taken as positive when the member is bent in reverse

curvature.

12



In order to simplify the design process, the full interaction curve shown in figure 2(a) may be approximated by the polygon joining the five points A, B, C, D and E, as shown in figure 2(b). These points are determined as follows:

1. Point A is defined by the nominal axial capacity (Nus) of the member without bending.

2. Point B is defined by the nominal section moment capacity (Msx or Msy) of the member.

3. Point C is determined by moving the neutral axis determined for point B to a new position equidistant from the centroid, but on the other side of the centroid, and parallel with its previous position. Therefore, the stresses in the section with the neutral axis in this position will create a moment equal to that derived from point B, ie. Msx or Msy, but with a compression load equal to the axial load in that part of the section between the neutral axis positions for points B and C.

4. Point D is determined by placing the neutral axis at the centroid of the section. At this location, the axial load in the section is half that for point C, and the moment is a maximum.

5. Point E is any point approximately mid-way between points A and C, determined with the neutral axis approximately mid-way between its location for point C and the edge of the section, which is in tension when determining point C.

In determining the value of Mrx or Mry, the second order moment Mp due to imperfections (imperfection moment) of the column can be determined using the simplified interaction curve shown in figure 2(b). By reading off the horizontal distance representing the imperfection moment as shown in figure 2(a), the moment resistance of the composite column under combined compression and bending may then be evaluated.

It should be noted that the benefits of the confining stresses on the concrete may be considered to

determine the plastic compressive stress for circular

members in accordance with section 2.2.1. It should

also be noted that the above methodology generally

follows that presented in the last version of Eurocode

4 (1994). To simplify the design process, point E in

figure 2(b) has been removed in the new version of

Eurocode 4 (2004).

Due to page limitations, the design procedures

given in other standards is not presented herein.

More details and limitation provisions for them can

be found in Chung & Matsui (2005) and Zhang et

al (2007).

3 COMPARISONS BETWEEN TEST AND PREDICTED CAPACITIES

3.1 Brief introduction to test databases

Over the last few decades, numerous tests have

been carried out on CFST members. A database was

established by Goode (2006) recently, in which 1792

test results from 92 references were included. These

test results can be accessed via the website http://

web.ukonline.co.uk/asccs2 (ASCCS, 2007). In this

paper, 1575 test results from Goode’s database,

including 918 for circular specimens and 657 for

rectangular specimens (square sections mainly), are

used to perform the code comparisons. The other test

results in this database have been discarded because

they are not relevant to this study.

Apart from the test results in Goode’s database,

another database developed by Wu (2006) contains

1514 experimental results from 104 references, where

some of them have not been included in Goode’s

database, especially for 81 tests on beams. No test

results on beams are available in Goode’s database.

After merging the two databases, 1232 and 962 test

results (2194 altogether) from 130 references on

circular and rectangular specimens, respectively, are

used in this paper. The ranges of the test properties

are given in table 1. It should be noted that in some

Msx (or Msy)o

Interaction curve for the cross-section

N

M

Nus A

E

C

D

B

Msx (or Msy)

o

Interaction curve for the cross-section

N

M

Nus

Mdx (or Mdy)

Nuc(= cNus)

N*

nNus

Mrx or Mry

Figure 2: Interaction curve for CFST members subjected to combined compression and bending.

(a) (b)



references no concrete cylinder strength (fc’) was available. Instead, a compressive strength (fcu) of 150 mm cubes was reported. In general, fc’ can be taken as 0.8fcu for normal strength concrete, but this relationship is not quite applicable for high-strength concrete (Chen et al, 1996; Mansur & Islam, 2002). Therefore, equivalent cylinder strengths (fc’) were determined according to Chen et al (1996), where a table demonstrating the approximate relationship of two types of concrete strengths can be found in Yu et al (2008). This relationship is quite close to that given by Mansur & Islam (2002).

3.2 Strength comparison

When comparing design calculations with the tests, the material partial safety factors specified in all design codes have been taken as unity. At the same time, all code limitations are ignored with a purpose to check the feasibility of those design codes in predicting the load-carrying capacities of the test specimens. In the following sections, “stub column” is defined as a short member (Le/do or Le/b ≤ 4; b is the section width of a rectangular tube) under axial compression to determine section capacity, while “column” is defined as a long one (Le/do or Le/b > 4) under axial compression, where the slenderness

effect is expected to be considered. “Beam-column”

is defined as a member subjected to the combined

action of compression and flexure. It is worth noting

that, the classification standards for short and long

columns in different codes are quite different. Some

of them are very complex to follow and, in some

codes, a slenderness reduction factor is applicable

even for a very short column. Therefore, the above

rather simple criterion used by Goode (2006) is also

adopted in this paper.

In order to better reflect the deviations of code

predictions from the experimental results, the –15%

and +15% error bounds are depicted in figures

presented in the following sub-sections. It is worth

noting that this is not a criterion used to assess the

acceptability of prediction accuracy. Generally, a

reliability analysis should be performed based on a

regional reliability standard to accomplish this task

(Han, 2007).

3.2.1 Section capacity under axial compression

Currently available test results are 484 and 445 for

circular and rectangular stub columns, respectively.

Figure 3 shows the comparison between experimental

ultimate strength Nue and predicted strength Nuc

Section type Member typedo(h)

(mm)λe

fy (MPa)

fc’ (MPa)

No. of tests

No. of references

Circular

Stub column 48-1020 13-237 186-853 10-110 484 39

Column 25-500 9-192 178-682 10-96 420 28

Beam-column 76-200 17-198 186-433 18-114 304 19

Beam 34-300 23-196 262-436 23-82 24 4

Rectangular

Stub column 60-400 13-181 192-835 12-103 445 34

Column 60-360 19-163 217-550 10-94 234 25

Beam-column 76-323 18-94 205-761 18-103 226 20

Beam 100-306 22-115 194-750 19-88 57 7

Table 1: Summary of test properties.

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0.5

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0 1000 2000 3000 4000 5000Test strength N ue (kN)

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Figure 3: Comparison between test results and predictions using AS 5100 (circular stub column).

(a) (b)

14



using AS 5100 for circular stub columns. Table

2 also shows both the mean value (μ) and the

standard deviation (σ) of the ratio of Nue/Nuc for all

the strength predictions. As can be seen, generally

good agreement is achieved with an average value

(μ) of 1.037 and a standard deviation (σ) of 0.139. In

the test databases, some tests were performed on

specimens with a rather large diameter. In order to

illustrate more clearly, those tests are compared in

figure 3(b), which demonstrates that AS 5100 is also

applicable in this circumstances without apparent

size effect observed.

Comparison results from other design codes are

given in figure 4 and table 2, which clearly show

that AISC is quite conservative in its prediction

and BS 5400 gives slightly (3.4%) higher capacities

on average than the test results. For clarity, the test

results for those specimens with large diameters are

not given in figure 4. The agreement of the measured

and predicted strengths is generally good.

Figure 5 compares the test strength (Nue) with the

calculated strength (Nuc) using different design

codes for rectangular stub columns. All codes give

accurate predictions except that AISC and BS 5400

underestimates the strength by as much as 15% on

average (table 2). At the same time, all predictions

have smaller variations compared to those for

circular stub columns. This may be attributed to the

fact that a lot of circular specimens did not show

strain softening behaviour, thus different definitions

of ultimate strength have been applied by different

authors.

3.2.2 Column member capacity

Figures 6 and 7 show the comparisons between

test results (Nue) and code predictions (Nuc) for

circular and rectangular columns, respectively.

It appears that AISC and BS 5400 give the most

conservative prediction results. As far as AS 5100 is concerned, its predictions are generally accurate, but it underestimates the load-bearing capacity of circular columns (16% lower on average). The same trend is found for the predicted results from EC4. The reason is attributed to the fact that no confinement effect is considered for columns with a relative slenderness λr greater than 0.5. In fact, the apparent concrete confinement can still be expected even for a very slender circular column (Han, 2000).

Figure 8 compares the calculated strength of column members based on different code provisions. Test results reported by Matsui et al (1995) are shown as dots in this figure. Parameters for the circular specimens are as follows: do =165.2 mm, t = 4.08 mm, fy = 353 MPa, fc’ = 40.9 MPa; while those for the square ones are: b = 149.8 mm, t = 4.27 mm, fy = 412 MPa, fc’ =31.9 MPa. From the comparisons, it seems that all curves are close and generally agree with the test results, except that BS 5400 gives an obvious conservative prediction for square columns. Also, there are apparent discrepancies amongst predictions for circular columns when the relative slenderness λr is less than 0.5.

3.2.3 Beam-column member capacity

The comparisons between predicted load-bearing capacities (Nuc) and test results (Nue) are illustrated in figures 9 and 10 for circular and rectangular beam-columns, respectively. It has been demonstrated that AISC gives the most conservative results for circular beam-columns (μ = 1.385, σ = 0.467), and BS 5400 does that for rectangular beam-columns (μ = 1.300, σ = 0.278). All codes except BS 5400 give less conservative predictions for rectangular members than for circular ones. The predictions from AS 5100 are quite close to those from EC4, which demonstrates that they are quite accurate in predicting load-bearing capacities for circular

Section type

Member type

AS 5100 AIJ AISC BS 5400DBJ

13-51-2003EC4

μ σ μ σ μ σ μ σ μ σ μ σ

Circular

Stub column

1.037 0.139 1.097 0.154 1.275 0.199 0.966 0.150 1.155 0.149 1.048 0.139

Column 1.163 0.170 1.115 0.177 1.195 0.172 1.130 0.302 1.080 0.189 1.133 0.162

Beam-column

0.997 0.146 1.138 0.211 1.385 0.467 1.185 0.234 1.078 0.161 1.004 0.160

Beam 1.194 0.151 1.422 0.323 1.422 0.323 1.236 0.281 1.204 0.213 1.194 0.151

Rectangular

Stub column

1.062 0.123 1.061 0.123 1.150 0.138 1.169 0.147 1.037 0.125 1.061 0.123

Column 1.049 0.120 1.036 0.127 1.130 0.133 1.195 0.161 1.092 0.161 1.030 0.124

Beam-column

0.952 0.134 1.041 0.138 1.278 0.305 1.300 0.278 1.076 0.192 0.966 0.138

Beam 1.141 0.146 1.306 0.196 1.306 0.196 1.188 0.158 1.164 0.160 1.141 0.146

Table 2: Comparison results of code predictions with test results.



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Figure 4: Comparison between test results and predictions using other codes (circular stub columns) – (a) AIJ, (b) AISC, (c) BS 5400, (d) DBJ 13-51-2003, and (e) EC4.

beam-columns, but overestimate those of rectangular

beam-columns (4-5% on average). It seems that the

assumed rectangular stress blocks are less valid for

rectangular beam-columns.

To illustrate the differences among the code

predictions more clearly, the predicted axial load

(N) versus moment (M) interaction curves using

different methods are compared in figure 11, with the

test results of circular members obtained by Matsui

et al (1995). The section properties are the same as

those of the columns given before (figure 8 (a)). As

can be seen from figure 11, there are considerable

discrepancies among predictions from different

codes. For shorter members, AS 5100 and EC4 give

accurate predictions. As the slenderness increases,

they tend to overestimate the strength compared

(a) (b)

(c) (d)

(e)

16



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Figure 5: Comparison between test results and code predictions (rectangular stub columns) – (a) AS 5100, (b) AIJ (EC4), (c) AISC, (d) BS 5400, and (e) DBJ 13-51-2003.

to the test results reported by Matsui et al (1995).

Overall, DBJ 13-51-2003 gives the best prediction in

this comparison.

3.2.4 Beam moment capacity

The moment capacities (Muc) predicted using the

six design codes are compared with test results

(Mue) shown in figures 12 and 13 for circular and

rectangular beams, respectively. The ratios of Mue/Muc

for all codes are presented in table 2. As can be seen,

all predicted results are conservative overall. AIJ and

AISC give the most conservative predictions due to

ignoring the concrete contribution. AS 5100, EC4 and

DBJ give the best predictions for both circular and

rectangular beams.

(a) (b)

(c) (d)

(e)



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Figure 6: Comparison between test results and code predictions (circular columns) – (a) AS 5100, (b) AIJ, (c) AISC, (d) BS 5400, (e) DBJ 13-51-2003, and (f) EC4.

3.3 Discussion

For design purposes, all codes have provided

some limitations on material strengths and section

slenderness. However, many tests have been

conducted to date beyond those limitations, which

makes it possible to check the possibility of relaxing

those limitations. The following sections discuss this

topic for AS 5100.

3.3.1 Effect of steel strength

Figure 14 shows the effect of steel strength (fy) on

the prediction accuracy of AS 5100. Table 3 provides

the mean values (μ) and the standard deviations (σ)

of measured to calculated strength ratios for test

specimens with fy larger than 350 MPa. From the

comparisons, it can be seen that there is a decrease

of 2-3% in μ except circular beam-columns with a

(a) (b)

(c) (d)

(e) (f)

18



0

0.5

1

1.5

2


Nor

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Nue

/Nuc

0

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2


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15%

15%

15%

15%

0

0.5

1

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2


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15%

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15%

15%

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2


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1

1.5

2


Nor

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cal

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Nue

/Nuc

15%

15%

15%

15%

Figure 7: Comparison between test results and code predictions (rectangular columns) – (a) AS 5100, (b) AIJ, (c) AISC, (d) BS 5400, (e) DBJ 13-51-2003, and (f) EC4.

0

500

1000

1500

2000

2500

0 0.5 1 1.5 2 2.5

Relative slenderness r

Ulti

mat

e st

reng

th (k

N)

Test resultsAS 5100AIJAISCBS 5400DBJ 13-51-2003EC4

0

400

800

1200

1600

2000

0 0.5 1 1.5 2 2.5

Relative slenderness r

Ulti

mat

e st

reng

th (k

N)


Figure 8: Column strength based on different code provisions – (a) circular section, and (b) square section.

(a) (b)

(c) (d)

(e) (f)

(a) (b)



0

0.5

1

1.5

2


Nor

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0

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2


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15%

15%

15%

15%

0

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1

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2


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15%

15%

0

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2


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0

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1

1.5

2


Nor

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cal

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stren

gth

Nue

/Nuc

15%

15%

15%

15%

Figure 9: Comparison between test results and code predictions (circular beam-columns) – (a) AS 5100, (b) AIJ, (c) AISC, (d) BS 5400, (e) DBJ 13-51-2003, and (f) EC4.

(a) (b)

(c) (d)

(e) (f)

decrease of 5% and circular beams with an increase

trend. But all mean values are still above unity except

those for beam-columns.

3.3.2 Effect of concrete strength

The effect of concrete strength (fc’) on the prediction

accuracy of AS 5100 is shown in figure 15. For test

specimens with fc’ larger than 65 MPa, the mean

values (μ) and the standard deviations (σ) of

measured to calculated strength ratios are presented

in table 3. From figure 15 and table 3, it seems that

the effect of concrete strength on circular specimens

is different from that on rectangular specimens. For

circular specimens, a decrease of 3-6% in the mean

value μ is found. In the case of rectangular specimens,

only a decrease of 4% is found for stub columns while

an increase of 3-4% is found for columns, beam-

20



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15%

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15%

15%

Figure 10: Comparisons between test results and code predictions (rectangular beam-columns) – (a) AS 5100, (b) AIJ, (c) AISC, (d) BS 5400, (e) DBJ 13-51-2003, and (f) EC4.

(a) (b)

(c) (d)

(e) (f)

columns or beams. Once again, all mean values of

measured to calculated strength ratios are above or

near unity except those for beam-columns.

3.3.3 Effect of section slenderness

Figure 16 illustrates the effect of section slenderness

(λe) on the prediction accuracy of AS 5100. Table

3 provides the mean values (μ) and the standard

deviations (σ) of measured to calculated strength

ratios for test specimens with section slenderness

beyond the allowed values given in AS 5100. Though

different yield slenderness limits have been specified

for RHSs, only the middle value of 40 is used herein

to analyse the data for simplicity considerations. It

can be seen from figure 16 that there is a declining

trend of the measured to calculated strength ratios

as λe increases. For circular specimens, a decrease of



0

500

1000

1500

2000

2500

0 15 30 45 60

Moment M (kN m)

Axi

al lo

ad N

(kN

)Test resultsAS 5100AIJAISCBS 5400DBJ 13-51-2003EC4

0

500

1000

1500

2000

2500

0 15 30 45 60

Moment M (kN m)

Axi

al lo

ad N

(kN

)


0

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1500

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2500

0 15 30 45 60

Moment M (kN m)

Axi

al lo

ad N

(kN

)


0

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1000

1500

2000

2500

0 15 30 45 60

Moment M (kN m)

Axi

al lo

ad N

(kN

)


0

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600

900

1200

1500

0 15 30 45 60

Moment M (kN m)

Axi

al lo

ad N

(kN

)


0

200

400

600

800

1000

0 15 30 45 60

Moment M (kN m)

Axi

al lo

ad N

(kN

)


Figure 11: Comparison of predicted interaction curves with test results by Matsui et al (1995) – (a) λr = 0.18, (b) λr = 0.35, (c) λr = 0.51, (d) λr = 0.80, (e) λr = 1.22, and (f) λr = 1.82.

(a) (b)

(c) (d)

(e) (f)

about 5% in the mean value μ is found when λe is

larger than 82. However, only 1-2% in the mean value

μ is found for rectangular members when λe is larger

than 40. It can also be seen from table 3 that all mean

values of measured to calculated strength ratios are

above or near unity except those for beam-columns.

This demonstrates the fact that there is a tendency to

relax the limitation of section slenderness.

4 CONCLUSIONS

In this paper, the AS 5100 approach to the design of

CFST members has been described briefly. 2194 test

results from two currently available test databases

are used to evaluate the design approach of AS 5100.

Some other existing design codes are also compared

with the test results. The following conclusions may

be made within the present scope of investigation:

Moment M (kN m)

22



0

0.5

1

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2

0 100 200 300 400Test strength M ue (kN)

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/Muc

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15%

15%

Figure 12: Comparison between test results and code predictions (circular beams) – (a) AS 5100 (EC4), (b) AIJ (AISC), (c) BS 5400, and (d) DBJ 13-51-2003.

(a) (b)

(c) (d)

0

0.5

1

1.5

2

0 60 120 180 240 300Test strength M ue (kN)

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15%

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15%

15%

Figure 13: Comparison between test results and code predictions (rectangular beams) – (a) AS 5100 (EC4), (b) AIJ (AISC), (c) BS 5400, and (d) DBJ 13-51-2003.

(a) (b)

(c) (d)



0

0.5

1

1.5

2

180 360 540 720 900Steel yield strength f y (MPa)

Nor

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cal

cula

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stre

ngth

Stub column ColumnBeam-column Beam

0

0.5

1

1.5

2

180 360 540 720 900Steel yield strength f y (MPa)

Nor

mal

ized

cal

cula

ted

stre

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15%

15%

fy=350 MPa

15%

15%

fy=350 MPa

Figure 14: Effect of steel strength on the prediction accuracy of AS 5100 – (a) circular section, and (b) square section.

0

0.5

1

1.5

2

0 30 60 90 120Concrete cylinder strength f c (MPa)

Nor

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1.5

2

0 30 60 90 120Concrete cylinder strength f c (MPa)

Nor

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cal

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stre

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15%

15%

fc =65 MPa

15%

15%

fc =65 MPa

Figure 15: Effect of concrete strength on the prediction accuracy of AS 5100 – (a) circular section, and (b) square section.

0

0.5

1

1.5

2

0 44 88 132 176 220Section slenderness e

Nor

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stre

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0

0.5

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1.5

2

0 40 80 120 160 200Section slenderness

Nor

mal

ized

cal

cula

ted

stre

ngth


15%

15%

e=40

15%

15%

e=82

Figure 16: Effect of section slenderness on the prediction accuracy of AS 5100 – (a) circular section, and (b) square section.

(a) (b)

(a) (b)

(a) (b)

1. The approach in AS 5100 gives generally

accurate predictions, thus it is also possible to

be used for building construction. However,

it should be noted that it overestimates the

strength of rectangular beam-columns by 5% on

average.

2. After ignoring all code limitations and taking

material partial safety factors as unity, there are

considerable differences among different code

predictions. The predicted results using AS 5100

are quite close to those from EC4.

3. All three factors of steel strength, concrete

strength and section slenderness slightly affect

the prediction accuracy, but the comparisons

still indicate a tendency to relax the limitations

24



Section type

Member type

fy > 350 MPa fc’ > 65 MPaλe > 82 (circular)

or λe > 40 (rectangular)

No. of tests

μ σ No. of tests

μ σ No. of tests

μ σ

Circular

Stub column

213 1.024 0.130 153 0.994 0.115 161 0.996 0.105

Column 141 1.137 0.135 13 1.102 0.081 58 1.085 0.151

Beam-column

45 0.947 0.100 55 0.944 0.112 31 0.957 0.072

Beam 2 1.320 0.058 4 1.383 0.088 10 1.322 0.083

Rectangular

Stub column

166 1.038 0.104 64 1.020 0.088 237 1.044 0.124

Column 65 1.011 0.118 36 1.093 0.146 141 1.039 0.125

Beam-column

82 0.929 0.140 55 0.988 0.115 113 0.951 0.129

Beam 28 1.121 0.088 4 1.172 0.092 2 1.021 0.045

Table 3: Comparison results for tests beyond the limitations of AS 5100.

of AS 5100. This relaxation will allow a designer

to use higher strength materials and to design

composite members with larger section

slenderness.

4. Additional concrete confinement at higher

slenderness ratios can be expected for circular

columns. This beneficial effect may be

considered in a column design.

ACKNOWLEDGEMENTS

This research work has been partially supported

by the International Research Initiatives Scheme

provided by the University of Western Sydney.

REFERENCES

American Institute of Steel Construction, 2005, ANSI/AISC 360-05 Specification for structural steel buildings,

Chicago, Illinois, USA.

Architectural Institute of Japan (AIJ), 1997,

Recommendations for design and construction of concrete filled steel tubular structures, Japan (in Japanese).

ASCCS, 2007, Database of Concrete-Filled Steel Tube Columns, http://web.ukonline.co.uk/asccs2.

Australian Steel Institute, 2004, “Latitude reaches

skyward in steel: Construction technology at its best”,

Steel Australia, Vol. 17, No. 1, pp. 10-12.

British Standards Institution, 2005, BS 5400 Steel, concrete and composite bridges, Part 5, Code of practice for the design of composite bridges, London, UK.

Chaseling, C. 2004, “Star attraction”, Modern Steel Construction, Vol. 44, No.12, pp. 36-42.

Chen, Z. Y., Zhu, J. Q. & Wu, P. G. 1996, High strength concrete and its application, Tsinghua University Press,

Beijing, China (in Chinese).

Chung, J. & Matsui, C. 2005, “SRC standards in

Japan and comparison of various standards for CFT

columns”, International Journal of Steel Structures, Vol.

5, No.4, pp. 315-323.

DBJ 13-51-2003, 2003, Technical specification for concrete-filled steel tubular structures, The Construction

Department of Fujian Province, Fuzhou, China (in

Chinese).

Eurocode 4, 2004, Design of composite steel and concrete structures, Part1.1, General rules and rules for Building,

BS EN 1994-1-1: 2004, British Standards Institution,

London, UK.

Eurocode 4, 1994, Design of composite steel and concrete structures, Part1.1, General rules and rules for Building,

DD ENV 1994-1-1: 1996, British Standards Institution,

London, UK.

Goode, C. D. 2006, “A review and analysis of over

one thousand tests on concrete filled steel tube

columns”, Proceedings of 8th International Conference on Steel-Concrete Composite and Hybrid Structures, Harbin,

China, pp. 17-23.

Han, L. H. 2000, “Tests on concrete filled steel tubular

columns with high slenderness ratio”, Advances in Structural Engineering – An International Journal, Vol.

3, No. 4, pp. 337-344.



Han, L. H. 2007, Concrete filled steel tubular structures – theory and practice, China Science Press, Beijing, China (in Chinese)

Mansur, M. A. & Islam, M. M. 2002, “Interpretation of concrete strength for non-standard specimens”, Journal of Materials in Civil Engineering, ASCE, Vol. 14, No. 2, pp. 151-155.

Matsui, C., Tsuda, K. & Ishibashi, Y. 1995, “Slender concrete filled steel tubular columns under combined compression and bending”, Proceedings of the 4th Pacific Structural Steel Conference, Vol. 3, Steel-Concrete Composite Structures, Singapore, pp. 29-36.

Standards Australia, 2004, AS5100.6-2004 Bridge design, Part 6: Steel and composite construction, Sydney, Australia.

Uy, B. 2000, “Strength of concrete filled steel box columns incorporating local buckling”, Journal of Structural Engineering, ASCE, Vol. 126, No. 3, pp. 341-352.

Uy, B. & Patil, S. B. 2006, “Concrete filled high strength

steel box columns for tall buildings: Behaviour and

design”, The Structural Design of Tall Buildings, Vol.

5, No. 2, pp. 75-94.

Wu, F. Y. 2006, “Compressive behaviour of recycled

concrete-filled steel tubes”, Masters thesis, College

of Civil Engineering, Fuzhou University, China (in

Chinese).

Yu, Q., Tao, Z. & Wu, Y. X. 2008, “Experimental

behaviour of high performance concrete-filled steel

tubular columns”, Thin-Walled Structures, Vol. 46,

No. 4, pp. 362-370.

Zhang, S. M., Ma, X. B. & Goode, C. D. 2007,

“Comparison between Chinese and three worldwide

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633-638.

26



ZHONG TAO

Zhong Tao is Professor of Structural Engineering at Fuzhou University, China, and is currently visiting the University of Western Sydney. He received his MS and PhD degrees from the Harbin Institute of Technology, China. His main research interests are steel-concrete composite structures and FRP-confined concrete. Zhong has published more than 80 journal papers, and recently published a Chinese book about innovative composite columns. He has played an important role in drafting five Chinese design codes on steel-concrete composite structures, including DBJ13-61-2004, DBJ13-51-2003 and GJB4142-2000. He was awarded three patents by the Chinese National Bureau of Knowledge Property Rights since 2005. He is currently on the Editorial Board of the international journal of Steel & Composite Structures.

BRIAN UY

Brian Uy is Professor of Structural Engineering and Head of the School of Engineering at The University of Western Sydney. He has been involved in research in steel and steel-concrete composite structures for over 15 years and published more than 300 articles. Much of his research has been underpinned by competitive grant research funding from the Australian Research Council (ARC) and industry. He is currently a member of the ARC College of Experts, providing advice to the federal government on peer reviewed research in civil and structural engineering. Brian currently serves on the Australian Standards Committee for Composite Structures (BD 32), the AISC Task Committee 5 on Composite Construction, ASCE Technical Committee on Composite Construction, and the IABSE Working Commission 2 for Steel, Timber and Composite Structures. He also holds roles on the Editorial Board for seven major international journals in steel and composite structures, including being the Chief Editorial Asia-Pacific of Steel & Composite Structures. Brian is a chartered civil and structural engineer in Australia, the UK and the USA.

LIN-HAI HAN

Lin-Hai Han is Professor of Structural Engineering at Tsinghua University, China. He has published 60 refereed international journal papers and 40 refereed international conference papers. He has received several excellence awards from the Chinese government since 1995 in recognition of his contributions to the research and application of steel-concrete composite construction. He is one of the outstanding young researchers awarded by the National Natural Science Foundation of China. He holds roles on the Editorial Board for several international and national journals. Lin-Hai has consulted industry and government authorities on a wide range of structural engineering projects. He has played an important role in drafting several design codes on steel-concrete composite structures in China. His current research interests include steel-concrete composite and hybrid structures under different kinds of loadings, such as static, dynamic and fire.

SHAN-HU HE

Shan-Hu He is a PhD candidate at Tsinghua University and her supervisor is Professor Lin-Hai Han. She received her bachelor’s degree from Tsinghua University in 2007 and is currently working in the field of concrete-filled steel tubular columns.

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