Thin-Walled Structures 100 (2016) 213–223

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Full length article

Pitch spacing effect on the axial compressive behaviour of spirallyreinforced concrete-filled steel tube (SRCFT)

Mohammad Reza Hamidian n, Mohd Zamin Jumaat, U. Johnson Alengaram,N.H. Ramli Sulong, Payam ShafighDepartment of Civil Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia

a r t i c l e i n f o

Article history:Received 22 April 2015Received in revised form9 December 2015Accepted 11 December 2015Available online 5 January 2016

Keywords:Composite columnSpiral columnSpiral shear reinforcementPost-yield behaviourConfinementSteel tube

x.doi.org/10.1016/j.tws.2015.12.01131/& 2015 Elsevier Ltd. All rights reserved.

esponding author.ail addresses: hamidian.m.r@gmail.com (M.R.um.edu.my (M.Z. Jumaat).

a b s t r a c t

Concrete-filled steel tubes (CFTs) exhibit superior performance under static and dynamic loads, due tocomposite action. Factors, such as the thickness of the steel tube and the concrete core condition, have asignificant effect on the structural behaviour of CFT columns, particularly in the post-yield behaviour.Studies show that if the concrete core is reinforced with steel bars, the new composite member hasbetter characteristics compared to CFT columns. In this study, the axial compressive behaviour of re-inforced concrete-filled steel tube (RCFT) columns using spirally reinforced concrete (SRCFT) was in-vestigated and compared with CFTs. The main variation was the pitch spacing of the spiral shear re-inforcement. Fifteen specimens, including three CFTs and twelve SRCFTs, were tested in five groups. Thetest results indicated that a SRCFT column has much better post-yield behaviour than a CFT column. Areduction in the pitch spacing rate further improves the post-yield behaviour of the SRCFTs. A com-parison of the measured strength of the specimens with corresponding values predicted by two inter-national codes (ACI 318-11 and EC4-1994) shows a good prediction of EC4 and a conservative estimationof ACI.

& 2015 Elsevier Ltd. All rights reserved.

1. Introduction

The use of concrete-filled steel tube (CFT) columns has beenincreasingly developed in structural applications, due to the greatadvantages of their behaviour. Various analytical and experimentalstudies have been conducted on CFT columns. In 1957, Klöppel andGoder carried out collapse load tests on CFT columns and proposeda design formula for CFTs [1]. Gardner and Jacobson [2] attemptedto predict the ultimate load of CFT columns. Furlong [3,4] in-vestigated the behaviour of CFT columns in detail and pointed outthe properties of CFTs. He presented design graphs and formulasfor CFT columns. In 1970, Knowles and Park [5,6] investigatedaxially loaded CFT columns over a wide range of slenderness ratios.They presented the design equations to compute the ultimatecompression strength of CFT columns. In recent years, researchershave paid more attention to the advantages of the usage of CFTcolumns in the construction industry. Investigations show that CFTcolumns provide substantial energy dissipation under seismic loadand as a composite column in composite frames they can showconsiderable seismic resistance behaviour [7,8]. The characteristics

Hamidian),

of CFT columns form a practicable structural system, especially forhigh-rise buildings because of the higher construction efficiency [9].Uchikoshi et al. [10] through a trial design of unbraced frames showedthat the total steel consumption of the CFT system is 10% less than thesteel system for the entire building. Due to the specification of CFTcolumns, recently, in the United States, CFT columns have been usedas super columns for primary load bearing members in high-risestructures. They performed both gravity and lateral resistance systemsfor the buildings [8]. Kitada [11] studied the properties of differenttypes of bridge pier in Japan. The results showed that, in general, thestrength and ductility of composite columns are larger than that ofother types, such as reinforced concrete and steel columns, while, thecircular CFT columns have better performance compared to othertypes of composite column. CFT piles can be constructed withoutformwork and shoring, which results in faster and more economicalconstruction [12]. In addition, the inherent advantages of CFTcolumns were proven in the Hansin-Awaji earthquake in Japan. Inthis earthquake, in 1995, most of the reinforced concrete and steelstructures were heavily damaged due to the shear failure and localbuckling while the CFT structures avoided collapse [13].

The main reason for the improvement in the structural prop-erties of CFT columns is the composite action between the steeltube and the concrete. The confined concrete created by the steeltube enhances the concrete core properties by placing the con-crete in triaxial stress, while the local buckling of the steel tube is

Nomenclature

Ac Cross-section area of concrete coreAg Cross-sectional area of composite columnAsb Cross-sectional area of longitudinal steel barsAsp Cross-sectional area of spiral steel barsAst Cross-section area of steel tubedb Diameter of longitudinal steel bardp Diameter of spiral steel barD Diameter of steel tubeDp Diameter of spiral core concreteEa Modulus of elasticity of steel tubeEs Modulus of elasticity of steel barEcm Secant modulus of elasticity of concretefck Characteristic concrete strength ( =f f0. 67ck cu)fcm Mean value of concrete cylinder compressive strengthfcu Concrete cube strengthfyb Yield stress of longitudinal steel barfyp Yield stress of spiral steel barfyt Yield stress of steel tube′fc Compressive strength of standard cylinder

Ia Second moment of area of the steel tubeIc Second moment of area of the un-cracked concrete

sectionIs Second moment of area of the steel barL Length of specimenn Number of longitudinal steel barsN1 Maximum axial strength of specimenN2 Minimum axial strength of specimen after load dropNexp Experimental ultimate strengthPn max, Maximum allowable value of nominal axial strength of

cross sections Center to center spacing of spiralt Wall thickness of steel tubeU1 Displacement corresponding to maximum axial load

(N1)U2 Displacement corresponding to minimum axial load

after load drop (N2)ρsb Ratio of Asb to Ag

ρsp Ratio of volume of spiral reinforcement to total vo-lume of core confined by the spiral

ρst Ratio of Ast to Ag

M.R. Hamidian et al. / Thin-Walled Structures 100 (2016) 213–223214

delayed by the lateral pressure of the concrete core; hence, thestrength and ductility of the composite column is increased sig-nificantly [14]. In addition, cost savings for formwork and fasterconstruction are other advantages of this kind of column.

The concrete in this type of composite column can be plain(CFT) or reinforced with steel bar (RCFT). Fig. 1 shows a schematicmodel of CFT and RCFT columns.

The use of RCFT columns in structures has more advantagesthan those for CFT columns. Recent studies [13,15–17] show thatRCFT columns have better load bearing capacity and more tough-ness and ductility compared to CFT columns. In addition, a struc-ture constructed with RCFT columns shows better anti-seismic

Fig. 1. CFT and RCFT columns: 1a) Concrete-filled steel tube (CFT) c

performance than a similar CFT structure. In fact, it can be said thatRCFT columns were developed principally for the purpose ofcombining the abilities of reinforced concrete (RC) columns andCFT columns.

One of the most important parameters in the study of the be-haviour of CFT columns is the steel contribution factor. Thisparameter can be defined by ξ [18,19] in Eq. (1).

ξ=( )

A f

A f 1

st y

c ck

Fig. 2 shows the σ ε− curves for CFT columns, according to thedifferent values of ξ. As can be seen in this figure, the curve

olumn, 1b) reinforced concrete-filled steel tube (RCFT) column.

Fig. 2. Typical axial stress (s) versus axial strain (ε) curves in CFT columns [19].

Fig. 4. Model of SRCFT Column.

M.R. Hamidian et al. / Thin-Walled Structures 100 (2016) 213–223 215

behaviour of different types of CFT column is the same up to thepoint A(σ ε )o o, with the main difference occurring after this point. Ithas been shown [18,19] that if ξ is 1.1 (defined by ξ0¼1.1), the σ ε−curve shows elastic-perfectly plastic behaviour. In addition, thestrain hardening behaviour (for ξ ξ> 0) and strain softening beha-viour (for ξ ξ< 0) are other possible behaviours of σ ε− curves. It hasbeen proven that the load drop percentage in strain softeningbehaviour can be more than 50% [19].

Xiamuxi et al. [13] proposed Eq. (2) to evaluate the σ ε− be-haviour of RCFT columns. They showed that if γ40.5 the beha-viour of RCFT columns is the same as ξ41.1 in Fig. 2, and, also,when γo0.3 the strength drops more than 50%.

γ =( + + ′) ( )

A f

A f A f A f 2

st yt

st yt sb yb c c

Eqs. (1) and (2) show that the post-yield behaviour of thecolumn can change from strain-softening to strain-hardening byan increase in the Ast . In fact, a higher value of Ast causes an in-crease in the confinement effect on the concrete core and en-hances the mechanical properties of concrete, such as the strengthand ductility. In RCFT columns, longitudinal steel bars, tied as steelcage, can improve the effect of confinement. It is clear from formerresearches [20–24] that the lateral reinforcement has an essentialeffect in the increase of the shear strength of the concrete core andhigher shear strength results in a higher confinement effect [25].

Based on the type of lateral reinforcement, RC columns aredivided into two main groups; namely, tied and spiral RC columns.As can be seen in Fig. 3, the most important difference between

Fig. 3. Behaviour of reinforced concrete columns with spiral and tied [26].

the two kinds of columns is the ductile behaviour [26]. Greaterductility of the spiral columns, compared to the columns withnominal ties, was observed in buildings after the San Fernandoearthquake in 1971. In Olive View hospital, the tied columns weredamaged, while the spiral columns were unscathed and able tobear their loads, even though the cover of the concrete core hadcollapsed. Normally, after spalling, the concrete area and the loadcapacity of the column are reduced, while the load-carrying ca-pacity of the concrete core is increased because of the radialcompressive stress of the spiral [27].

Tests have shown that in concrete columns, closely spaced tiesalso enhance the strength and ductility of the confined concrete,although not as effectively as circular spirals [23,28]. Based on thedifferent behaviour of spiral and tied columns, the AmericanConcrete Institute (ACI 318-11) [29] uses different limitations andformulas to calculate the strength of spirally and tied columns. Theupper limit for spirally reinforced columns is taken as 0.85 timesthe design strength, and for tied columns as 0.8 times the designstrength, as defined in Eqs. (3) and (4). Fig. 4

φ φ= [ ′( − ) + ] ( )P f A A f A0. 85 0. 85 3n max c g st yt st,

φ φ= [ ′( − ) + ] ( )P f A A f A0. 80 0. 85 4n max c g st yt st,

Different strength reduction factors (φ) are also applied to thespiral and tied columns. This factor is taken as 0.75 and 0.65 forspirally and tied columns, respectively. The higher upper limit andreduction factor for the spiral column emphasizes the better be-haviour of this type of column compared to the tied columns.

Considering the significant advantages of CFT columns and thesuperior features of spiral reinforced concrete columns, the mainaim of this study is to investigate the influence of lateral steel barsin spirally reinforced concrete-filled steel tube (SRCFT), under axialcompression. Different amounts of spiral are used to investigatethe effect of the spiral pitch spacing on the axial compressivebehaviour of SRCFT.

2. Experimental programme

2.1. Materials properties

2.1.1. ConcreteNormal aggregate was used for making normal weight con-

crete. The maximum nominal size of the coarse aggregate was10 mm. Superplasticiser was also used in the concrete mixture toimprove the workability and for easy placement inside the steeltubes. The standard cylinder specimens (150 mm�300 mm) were

Table 1Concrete mix proportions.

Concrete type Mix proportions (kg/m3)

Cement Coarseaggregate

Fineaggregate

Water SuperPlasticizer

(kg) (kg) (kg) (kg) (%)

Normalconcrete

425 900 800 175 0.8

Fig. 6. Spiral steel cores with different pith (15, 25, 35, and 45 mm).

M.R. Hamidian et al. / Thin-Walled Structures 100 (2016) 213–223216

prepared and tested according to the British Standard BS EN12390. Based on the compression test, the average compressivestrength of the concrete was 52 MPa. The mix proportions of theconcrete used are shown in Table 1.

2.1.2. SteelAll the steels used in this study were mild steel. The steel tube

samples were prepared from 6 m lengths of hollow steel tube. Theoutside diameter of the steel tubes was 140 mm with 2.8 mmthickness. The coupons were prepared and tested according to theBritish Standard BS EN 10002. Based on the coupon tests in a100 kN Instron tensile testing machine, the yield strength obtainedfor the steel tube was 355 MPa. The longitudinal steel bars wereplain bar with 6 mm diameter and 340 MPa yield strength. Steelbar with 3.2 mm diameter and 330 MPa yield strength was used asthe spiral steel bar.

2.2. Specimens

2.2.1. Fabrication of spiral steel coreFifteen specimens were prepared in five groups with different

conditions. For each condition, three similar specimens were fab-ricated. The specimens included 12 samples of SRCFT with fourdifferent pitches and three samples of CFT without reinforced steelbar. The clear spacing (see Fig. 5b) between the spirals in thedifferent SRCFT samples was 15, 25, 35, and 45 mm. Six

Fig. 5. Formation of SRCFT specimens: 5a)

longitudinal steel bars of 6 mm diameter, and a spiral of steel barof 3.2 mm diameter were used to make the reinforcement cage ofsamples. Fig. 5 shows the illustration of formation of the SRCFTspecimens and Fig. 6 shows the complete spiral steel cores ofdifferent pitch.

All the specimens were 140 mm outside diameter and 400 mmlong (Fig. 7). The core diameter of the samples, as determined bythe centreline of the perimeter spiral, was 85 mm.

2.2.2. Fabrication the specimensAfter installing the spiral steel core into the steel tube, the

SRCFT specimens were cast and manually compacted. After 28days of curing, all samples were capped with high strength mortarto a smooth surface before the test. Fig. 8 illustrates the fabricationprocess of samples. Concrete standard cylinders (150 mm�300 mm) were prepared from the concrete batch and tested on thesame day of the test for the specimens.

SRCFT column, 5b) reinforcement cage.

Fig. 7. Spiral core and steel tube: 7a) Before installation, 7b) After installation.

M.R. Hamidian et al. / Thin-Walled Structures 100 (2016) 213–223 217

2.2.3. Test setup and instrumentationThe samples were tested under a 2000 kN universal testing

machine in the construction research institute of Malaysia(CREAM). Two linear variable displacement transducers (LVDTs)measured the axial deformation of the specimen by measuring therelative displacement between the loading plates on the top andbottom of the test machine. Fig. 9 shows the instrumentation andtest setup. In this study, a mix of displacement and loading patternwere used for performing the test. The specimens were subjectedto monotonic axial loading at an initial rate of 75 kN/min, until theapplied load reached 70% of the predicted maximum load. Fromthis point, the rate was changed to a displacement pattern of0.6 mm/min until the end of the test. All the specimens werepreloaded with 30 kN before the final loading to reduce the effectof unevenness and looseness at both ends and to avoid the stressconcentration of the specimens.

Fig. 8. Fabrication process of specimens: 8a) Before c

3. Test results and discussions

Three circular CFT and 12 SRCFT columns, with details as shownin Table 2, were tested. The numbers in the labels of SRCFT spe-cimens represent the clear spacing of the pitch.

3.1. Load–displacement curve of specimens

Fig. 10 shows the axial load–displacement curves of five groupsof specimens. As mentioned in Section 2.2.1, each group containsthree similar specimens. From these figures, it can be observedthat the curves of the specimens for each group show almost thesame behaviour. Therefore, a mean value from three specimenscan be plotted for each group. Fig. 11 shows the mean values for allfive groups. It can be seen that the ascending part of all types ofspecimen are similar. However, the main difference between the

asting, 8b) after casting, 8c) curing, 8d) capping.

Fig. 9. Test setup and instrumentation: 9a) Test setup, 9b) instrumentation.

M.R. Hamidian et al. / Thin-Walled Structures 100 (2016) 213–223218

specimens is in the descending part. It is clear from Fig. 11 that theuse of a spiral reinforcement core in the CFT columns significantlyinfluences the post-yield behaviour of the column. In addition, thepitch spacing of the spiral steel core has a significant effect on thepost-yield behaviour of the SRCFT columns.

3.1.1. Load drop of specimensFig. 12 illustrates the load drop value based on N1 andN2. Table 3

shows a summary of the results of the experimental tests for thefive groups, and Fig. 13 shows the value of N1 and N2 for thespecimens in different groups. The CFT columns constructedwithout spiral steel core exhibit the maximum load drop and S15columns with a minimum pitch spacing (15 mm) show the mini-mum load drop. For a better study of the behaviour, the load droppercentage is computed using Eq. (5) and the results are

Table 2Detail of specimens and materials.

Group Specimen label Steel tube properties Longitudinal st

D t D/t L/D fyt db

(mm) (mm) (MPa) (mm)

1 CFT-1CFT-2 140 2.8 50 2.85 355 –

CFT-3

2 S45-1S45-2 140 2.8 50 2.85 355 6S45-3

3 S35-1S35-2 140 2.8 50 2.85 355 6S35-3

4 S25-1S25-2 140 2.8 50 2.85 355 6S25-3

5 S15-1S15-2 140 2.8 50 2.85 355 6S15-3

summarized in Table 3 and Fig. 14.

=( − )

*( )

N NN

Ld 1005

1 2

1

According to Fig. 14, the maximum percentage of load dropoccurs in the CFT columns with 43.6%. The load dropped was ap-proximately half of the maximum axial strength. By contrast, 11.5%is the minimum load drop that occurs in the S15 specimens. Thisdifference shows the significant effect of the spiral steel core andpitch spacing on the behaviour of the SRCFT columns. The resultsshows that the load drop in CFT is approximately 4 times morethan S15, 2 times more than S25, 1.8 times more than S35 and1.6 times more than S45.

eel bar properties Spiral steel bar properties ′f c (MPa)

n fyb dp fypClear pitch spacing

(MPa) (mm) (MPa) (mm)

– – – – – 52

6 340 3.2 330 45 52

6 340 3.2 330 35 52

6 340 3.2 330 25 52

6 340 3.2 330 15 52

Fig. 10. Load–displacement curves: 10a) CFT, 10b) SRCFT-S45, 10c) SRCFT-S35, 10d) SRCFT-S25, 10e) SRCFT- S15.

Fig. 11. Mean curves of load–displacement of groups.

Fig. 12. Load drop.

M.R. Hamidian et al. / Thin-Walled Structures 100 (2016) 213–223 219

Table 3Results of experimental tests for five groups.

ρsb ρsp N1 U1 N2 U2 ΔN¼ N1� N2 ΔN/N1

% % (kN) (mm) (kN) (mm) (%)

CFT 0 0 1425 4.0 804 12.0 621 43.6S45 1.10 0.79 1435 4.5 1039 17.0 396 27.6S35 1.10 1 1461 5.5 1123 16.0 338 23.1S25 1.10 1.34 1454 6.0 1169 18.0 285 19.6S15 1.10 2.08 1478 4.5 1308 17.0 170 11.5

Fig. 13. Comparison of the load drop in different groups.

Fig. 14. Percentages of load drop in mean curves.

Fig. 15. Load drop percentage versus ρsp.

M.R. Hamidian et al. / Thin-Walled Structures 100 (2016) 213–223220

3.1.2. Relationship between ρsp and percentage of load dropFig. 15 shows the relationship between ρsp and the load drop

percentage. It can be seen that there is a linear relationship, with astrong correlation between these two factors. Based on the equa-tion obtained in Fig. 15, it is possible to predict the load droppercentage of SRCFT columns for the other values of ρsp. In fact, the

load drop percentage of SRCFT specimens is a function of ρ .sp Forinstance, if ρsp is equal to 3%, the load drop percentage will be zero(see Fig. 15). On the other hand, the longitudinal steel bars andsteel tube sections are the same in all SRCFT specimens. Therefore,γ has the same value for the SRCFT specimens and based on Eq. (2)is equal to 0.35 (γ¼0.35). Thus, for a SRCFT column with γ¼0.35and ρ =0. 03sp , the load drop is zero, which means an elastic-per-fectly plastic behaviour of specimen. This behaviour is the same asthe behaviour of the RCFT specimen with γ¼0.5 [13] and the CFTspecimen with ξ0¼1.1 [19].

Based on the results, it can be said that to improve the structuralbehaviour of SRCFT columns, increasing the amount of spiral steelbar can be chosen instead of increasing the thickness of the steeltube. In this way, the total steel section used in a SRCFT column willbe reduced. For example, in CFT columns, to reach ξ0¼1.1 it is ne-cessary to increase the thickness of the steel tube from 2.8 mm to4.2 mm [see Eq. (1)]. This means that ρst increases from 0.078 to0.116. On the other hand, in SRCFT (with ρ =0. 03sp and γ¼0.35) thecross-sectional area of the longitudinal steel bar including the effectof the spiral steel bar can be computed from Eq. (6).

= + ( )A A A 6sl sb sp

Asp is the equivalent cross-sectional value of the spiral steel barand can be calculated from Eq. (7) [30].

π π=

( )A

d D

s

f

f4 7sp

p p yp

yt

2

The ratio of steel sections (including steel tube, longitudinaland spiral steel bar) to the cross-sectional area of SRCFT column,based on Eq. (8), is equal to 0.1 ( ρtotal¼0.1).

ρ = ( + + ) ( )A A A A/ 8total st sb sp g

Eq. (8) shows that the ratio of the cross-sectional area of thesteel sections to the cross-sectional area of column decreases from0.116 in the CFT columns to 0.1 in the SRCFT columns, which meansa significant decline in the steel used in the SRCFT columns com-pared to the CFT columns. In addition to saving steel, this columnshows better behaviour against seismic loads [13], and, therefore,SRCFT columns can provide a superior section for structures.

3.1.3. Post-yield behaviour of specimensFig. 10 indicates that there is a clear relation between the spiral

pitch spacing and the post-yield behaviour of the specimens. Thepost-yield behaviour has a particular importance in the behaviourof structures, especially on the energy absorption of members. Inthis study, the behaviour of all specimens can be classified asstrain-softening post-yield behaviour. However, there is a sig-nificant difference in the behaviour of the specimens after theultimate strength in the different groups. The strength of the CFTcolumns dropped sharply after maximum strength and reachedminimum strength after 12 mm (U2 ¼12 mm) (see Table 3). While,from Fig. 10a and b, it can be observed that even by using a smallamount of spiral steel in the concrete core (S45 specimens) thepost-yield behaviour significantly improved, which results in abetter energy absorption in the SRCFT compared to the CFTcolumns.

In the SRCFT columns, the decrease in the spiral pitch spacing(or increasing the percentage of the spiral steel bar) results in abetter enhancement of the specimens after ultimate strength.Accordingly, the S15 specimens exhibited the nearest behaviour toelastic-plastic post-yield behaviour with maximum capacity forenergy absorption. The better behaviour in S15 could be due to theminimum spiral spacing and the best confinement effect on theconcrete core.

Fig. 16. Failure modes of specimens: 16a) CFT, 16b) SRCFT-S45, 16c) SRCFT-S35, 16d) SRCFT-S25, 16e) SRCFT- S15.

M.R. Hamidian et al. / Thin-Walled Structures 100 (2016) 213–223 221

3.2. Failure mode of specimens

Fig. 16 shows the failure mode of specimens in the five groups.The failure mode in three specimens of each group, as explained inSection 2.2, exhibited a similar pattern. In the CFT columns, amajor diagonal crack formed in the concrete core that was asso-ciated with the load drop in the columns. The confinement effectdue to the steel tube increased the friction between the crackedsurfaces that slid against each other as the applied compressionload was increased. The interaction between the steel tube and theconcrete core after the diagonal crack is believed to be the mainmechanism for resistance of the CFT columns against the appliedaxial compression load [25,31]. In addition, SRCFT specimens ex-hibited a different mode of failure from CFT specimens. Two di-agonal cracks in different directions were formed along the col-umn height in the SRCFT specimens. In these specimens, thedouble confinement effect due to the steel tube and spiral steelcage on the inner core of the concrete, and, consequently, higherfriction between the cracked surfaces, caused different behaviourin the failure mode. The different mechanism in the shear failuremode can explain the different post-yield behaviour of the CFT andSRCFT specimens. Although the SRCFT samples with different pitchspacing exhibited the same behaviour in the failure mode (withtwo diagonal cracks), the difference in the friction forces caused adifferent post-yield behaviour. In fact, the reduced pitch spacing inthe SRCFT specimens caused a greater confinement effect in theconcrete core, which resulted in more friction between the sur-faces after cracking. Therefore, the higher friction forces resultedin greater resistance against the axial compressive load, and,consequently, the specimens show better performance in post-

Fig. 17. Failure modes in concrete core: 17a)

yield behaviour. Fig. 17 (a) and (b) shows the inside of the CFT andSRCFT specimens, respectively, and the types of failure mode in theconcrete cores.

3.3. Experimental test results compared to predicted values fromstandards

The experimental test results from this study are comparedwith the results obtained from two international standards, EC4-1994 [32] and ACI 318-11 [29].

3.3.1. EC4 standardEC4 presents a formula to determine the axial capacity of the

concrete-filled steel tube with longitudinal steel bars, as follows:

η η= + ′( +′

) +( )

N A f A ftd

f

fA f1

9EC a st yt c c c

yt

csb yb4

For members with pure axial loading the values ηa¼ ηa0 andηc¼ ηc0 are given by (9a) and (9b), respectively:

η = ( + λ̅) ( )0. 25 3 2 9aa0

η = − λ̅+ λ̅ ( )4. 9 18. 5 17 9bc02

λ̅ =( )

N

N 9cpl Rk

cr

,

CFT specimens, 17b) SRCFT specimens.

Table 4Comparison of test results with EC4 and ACI.

Group No. label Nexp (kN) Nexp/NEC4 Nexp/NACI

1 CFT-1 1441 0.97 1.311 2 CFT-2 1409 0.95 1.28

3 CFT-3 1442 0.97 1.314 S45-1 1460 0.98 1.32

2 5 S45-2 1445 0.97 1.316 S45-3 1418 0.95 1.297 S35-1 1452 0.98 1.32

3 8 S35-2 1454 0.98 1.329 S35-3 1489 1.00 1.3510 S25-1 1476 0.99 1.34

4 11 S25-2 1431 0.96 1.3012 S25-3 1464 0.98 1.3313 S15-1 1446 0.97 1.31

5 14 S15-2 1514 1.02 1.3715 S15-3 1484 1.00 1.35

Mean 0.98 1.32

M.R. Hamidian et al. / Thin-Walled Structures 100 (2016) 213–223222

=ϒ

+′

ϒ+

ϒ ( )N A

fA

fA

f

9dpl Rk styt

Mac

c

csb

yb

s,

π=

( )( )

NEI

l 9ecreff

2

2

( ) = + + ( )EI E I E I K E I 9feff a a s s e cm c

where

= ( )K 0. 6 9ge

= [( )

] ( )Ef

2210 9hcmcm 0.3

= + ( ) ( )f f 8 Mpa 9icm ck

3.3.2. ACI standardIn this code, the ultimate axial capacity has the same equation

for composite columns and also reinforced concrete columns. Inother words, the interaction between the steel tube and concreteand confinement effect due to steel tube on concrete core is notconsidered. The equation for a column with steel tube and long-itudinal steel bars is given by:

= ′ + + ( )N A f A f A f0.85 10total c c st yt sb yb

Table 4 summarizes the results for the performed study and thetwo codes (EC4-1994 and ACI 318-11).

The results show a good prediction of the EC4 and a con-servative prediction of the ACI compared to the experimental re-sults. Based on Table 4, the experimental results, on average, are2% less than the predicted results of EC4 and 32% more than ACI.The difference between the two codes in the prediction of strengthcan be due to the confinement effect. EC4 code considers theconfinement effect of the steel tube on the concrete core, whereasthis effect is not considered in the ACI code. The reliable predictionof the EC4 and the conservative estimation of the ACI standardshave also been reported by other researchers [33,34].

4. Conclusions

This study investigated the axial compressive behaviours ofspirally reinforced concrete-filled steel tube (SRCFT) columns with

different spiral pitch spacing. According to the experimental re-sults, the following conclusions can be drawn:

1. Using reinforced concrete with spiral shear reinforcement in-stead of plain concrete in a CFT column significantly improvesthe post-yield behaviour of the column.

2. In a SRCFT column, the pitch spacing rate has a significant effecton the post-yield behaviour of the reinforced concrete-filledsteel tube.

3. The effectiveness of the pitch spacing rate on the post-yieldbehaviour of a SRCFT column is more pronounced than thethickness rate of the steel tube.

4. The load drop percentage in SRCFT columns is a function of ρsp(the ratio of the volume of the spiral reinforcement to the totalvolume of the core confined by the spiral) and a linear equationshows this relationship.

5. For the same axial compressive behaviour, the SRCFT columnsneed a smaller percentage of steel section (including the steeltube, longitudinal steel bar and spiral steel bar) compared to theCFT columns.

6. Due to the confinement effect of the spiral shear reinforcement,the concrete core in the SRCFT specimens showed differentbehaviour in shear failure compared to the CFT specimens. TheSRCFT columns exhibited two diagonal shear cracks comparedto the CFT columns with one diagonal shear crack.

7. The estimated axial strength of the composite columns fromEC4-1994 is reliable and from ACI 318-11 is conservative. Onaverage, the test results were 2% lower and 32% higher than theestimated axial strength, respectively.

AcknowledgementsThe authors would like to acknowledge the financial assistance

from the University of Malaya, High Impact Research Grant (HIRG)No. UM.C/625/1/HIR/MOHE/ENG/36 (16001-00-D000036) –

“Strengthening Structural Elements for Load and Fatigue”

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