behaviour of concrete ﬁlled steel tubular (cfst) short columns

Construction and Building Materials 47 (2013) 1362–1371

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Construction and Building Materials

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Behaviour of concrete filled steel tubular (CFST) short columnsexternally reinforced using CFRP strips composite

0950-0618/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.conbuildmat.2013.06.038

⇑ Corresponding author. Address: Dept. of Civil Engineering, Sethu Institute ofTechnology, Kariapatti, Virudhunagar 626 115, Tamilnadu, India. Tel.: +91989451881.

E-mail address: [email protected] (G. Ganesh Prabhu).

G. Ganesh Prabhu a,⇑, M.C. Sundarraja b

a Dept. of Civil Engineering, Sethu Institute of Technology, Tamilnadu, Indiab Dept. of Civil Engineering, Thiagarajar College of Engineering, Tamilnadu, India

h i g h l i g h t s

� The suitability of CFRP strips instrengthening of CFST columns undercompression were carried out.� Failure modes, axial stress–strain

behaviour, and enhancement inultimate strength were discussed.� Wrapping of CFRP strips delays the

local buckling by providing restraintagainst the deformation.� The strength and stiffness of the CFRP

confined columns increases as thenumber of layers increases.� Analytical model was proposed for

predicting the axial load capacity ofCFRP confined CFST columns.

g r a p h i c a l a b s t r a c t

a r t i c l e i n f o

Article history:Received 9 April 2013Received in revised form 2 June 2013Accepted 17 June 2013Available online 14 July 2013

Keywords:CFST membersCFRP fabricsStrengtheningCompressionExternally wrapped

a b s t r a c t

This paper focused on experimentally and analytically investigates the suitability of carbon fibre rein-forced polymer (CFRP) strips composites in strengthening of CFST members under compression. The sizeand height of the columns were 91.5 � 91.5 � 3.6 mm and 600 mm respectively. CFRP fabrics was used ashorizontal strips (lateral ties) with several other parameters such as the number of layers, width andspacing of strips. Among the 21 columns, eighteen columns were externally strengthened by CFRP stripshaving a constant width of 50 mm with the spacing of 30 mm and 40 mm, and the remaining three col-umns were reference column. Experimental results were revealed that external wrapping of CFRP stripsprovides restraint against the lateral deformation effectively and delays the local buckling of steel tube.Axial deformation control and load bearing capacity of the confined columns increases as the number oflayers increases in addition to that increases in the load bearing capacity mainly depends upon the properspacing between the CFRP strips. Analytical model was proposed herein for predicting the load bearingcapacity of CFRP confined CFST columns.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Over the past few decades, the infrastructure concerned withmetallic and concrete filled tubular structures become structural

unsatisfactory and ageing of those structures and its deteriorationsare often reported [1]. Therefore, actions like implementation ofnew materials and strengthening techniques become essential tocombat this problem. Even though the traditional strengtheningtechniques like section enlargement and external wrapping of steelplates are successful in practice, these techniques revealed seriousdifficulties. In contrast, rehabilitation methods using fibre rein-forced polymer (FRP) composites do not exhibit any of those draw-backs. The application of fibre materials for the external

G. Ganesh Prabhu, M.C. Sundarraja / Construction and Building Materials 47 (2013) 1362–1371 1363

strengthening of reinforced concrete (RC) structures has beenwidely carried out and reported in the past few decades. More re-search on strengthening of RC structures using FRP composites canbe found in Amir et al. [2], Xiao and Wu [3], Pantazopoulou et al.[4], Mukherjee et al. [5], Cem et al. [6], Ye et al. [7] and parket al. [8]. In recent years, there have been many investigationsemerged in strengthening of steel structures with FRP composites,especially in the area of thin-walled steel structures [9]. One of thefirst known studies on this topic involved that the use of CFRP lam-inates to repair steel structures conducted by Sen and Liby [10]. Sixcomposite beams were tested under four-point bending set-up. Anepoxy adhesive was used to wrap the CFRP laminates to the ten-sion flange of the steel beam in different configurations. Highstrength steel bolts were also used in an attempt to increase theload transfer to the CFRP laminates. Test results were indicatedthat more modest improvement in the elastic response was re-quired, even though significant ultimate strength was gained.

In another investigation, Jiao and Zhao [11] studied the perfor-mance of butt-welded very high strength (VHS) steel tubesstrengthened with CFRP fabrics under axial tension. Three typesof epoxy resins with different lap shear strengths were used. Threekinds of failure modes such as adhesive failure, fibre tear, andmixed failure were observed. The above investigation concludedthat a significant strength can be achieved using CFRP–epoxystrengthening technique and they also recommended a suitableepoxy adhesive for strengthening VHS steel tubes. Photiou et al.[1] investigated the effectiveness of an ultra-high modulus andhigh-modulus CFRP prepregs in strengthening the artificially de-graded steel beam of rectangular cross-section under four-pointloading using two different wrapping configurations. Seica andPacker [12] investigated the FRP materials for the rehabilitationof tubular steel structures for underwater applications. Six tubeswere wrapped with CFRP composites. Two specimens were pre-pared under in-air conditions and remaining four specimens wereunder seawater curing conditions. All specimens were tested underfour-point loading. From the test results it was observed that theultimate strength of the tubes wrapped under in-air and seawatercuring conditions have 16–27% and 8–21% more than that of baresteel beam, respectively. Tao et al. [13] presented the results of ax-ial compression and bending tests of fire-damaged concrete-filledsteel tubes (CFST) repaired using unidirectional CFRP composites.Both circular and square specimens were tested to investigatethe repair effects of CFRP composites on them. The test resultsshowed that the load-carrying capacity and the longitudinal stiff-ness of CFRP-repaired CFST stub columns increased while theirductility decreased with the increasing number of CFRP layers.And also it was recommended that appropriate repair measuresshould be taken in repairing severely fire-damaged CFST beamsor those members subjected to comparatively large bending mo-ments. In another study, Tao and Han [14] repaired the fire-ex-posed CFST beam columns by unidirectional CFRP composites.Choi and Xiao [15] presented a simplified analytical model of CFSTmembers confined by CFRP jackets with different parameters in or-der to strengthen the traditional CFST column system. RecentlyNarmashiri et al. [16] and Kadhim [17] experimentally and numer-ically investigate the structural behaviour of CFRP steel I-beamsflexurally strengthened by CFRP composites. Wu et al. [18], Al-Zub-aidy et al. [19] Pierluigi and Giulia [20] investigate the bond char-acteristics between CFRP laminates and steel under fatigue andimpact tensile loads.

From the past research, it was observed that there have beeninvestigations done with the use of CFRP as a strengtheningmaterial for metallic members and also external wrapping of FRPsignificantly enhanced the strength and stiffness of the steel tubu-lar members, besides investigation on strengthening of CFST mem-bers using fibre are not widespread. In addition, more tests are

required to derive an optimal combination of fibre orientation,number of layers and sequence in applying CFRP layers on repair-ing or strengthening of CFST members under compression. Withthe aim of this, an experimental study has been carried out toinvestigate the suitability of unidirectional carbon fibre reinforcedpolymer (CFRP) fabrics locally available in the market in strength-ening of CFST members under compression.

2. Materials

2.1. Concrete

The concrete mix proportion designed by IS method to achievethe strength of 30 N/mm2 and the mix ratio was 1:1.39:2.77 byweight. The designed water cement ratio was 0.35. Concrete cubespecimens were cast for each batching and tested at the age of28 days to determine the compressive strength of concrete. Theaverage compressive strength of the concrete was 38.5 N/mm2.

2.2. Carbon fibre

The unidirectional carbon fibre called MBrace 240 was used inthis study. It is a normal modulus CFRP fibre and their specifiedproperties are listed in Table 1. It is fabric type and can be tailoredinto any desired shape.

2.3. Adhesive

The MBrace saturant was used in this study to get sufficientwrapping between steel tube and carbon fibre. It is a two part sys-tems, a resin and a hardener and the mixing ratio was 100:40(B:H).

2.4. Steel tube

The square hollow steel tube confirming to IS 4923-1997 andhaving a dimension of 91.5 mm � 91.5 mm was used in this study.The thickness and height of the square hollow steel tube were3.6 mm and 600 mm respectively. The yield strength of the tubewas 258 MPa and chosen from the experimental values.

3. Experimental study

3.1. Specimen fabrication

The 600 mm height square hollow tubes were machined from6 m length hollow tubes. To get the flat surface, both ends of thesteel tube were surfaced by the surface grinding machine. Insideportion of the hollow steel tubes were thoroughly wire brushedto remove the rust and loose debris presented. Then the hollowsteel tube specimens were filled with concrete and the each layerof concrete fully compacted by a needle vibrator to ensure the con-crete free from flaws or air gaps. To eliminate the leakage of slurryduring compaction, a steel plate was placed at the bottom prior tofilling concrete. Then the concrete was allowed to cure for 28 days.Surface preparation of the metal substrate is very important toachieve good wrapping between steel tube and CFRP fabrics. Thestrength of the adhesive bond is directly proportional to the qualityof the surfaces to which is to be wrapped. So the exposed surface ofthe CFST specimen was blasted by the coarse sand to remove therust and also to make the surface rough one. The entire sandblasted surface was cleaned by using acetone to remove all con-taminant materials before retrofitting with the fibres. Prior to thecolumns strengthened by carbon fibre, the glass fibre fabric wasintroduced between the steel surface and CFRP composites to elim-

Table 1Properties of MBrace 240 specified by the manufacturer.

S.No Properties Value

1 Modulus of elasticity 240 kN/mm22 Tensile strength 3800 N/mm2

3 Weight of C fibre (main direction) 400 g/m2

4 Density 1.7 g/cm2

5 Thickness for static design weight/density 0.234 mm

1364 G. Ganesh Prabhu, M.C. Sundarraja / Construction and Building Materials 47 (2013) 1362–1371

inate the galvanic corrosion. Finally, the carbon fibres werewrapped to the exterior surface of the CFST members with the dif-ferent wrapping schemes and thicknesses is shown in Fig. 1. All thestrengthened columns were allowed to cure in room temperaturefor the duration of 7 days. During wrapping of CFRP fabrics, the re-sin and hardener are correctly proportioned and thoroughly mixedtogether and the excess epoxy and air were removed using a ribbedroller moving in the direction of the fibre.

Fig. 2. Experimental setup.

3.2. Experimental setup

The CFST columns were tested in compression testing machineof capacity 2000 kN. Each member was positioned on the supportsand taking care to ensure that its centerline was exactly in linewith the axis of the machine. The verticality of the specimenswas checked using plumb bob. The specimens were instrumentedto measure longitudinal axial compression is shown in Fig. 2. Theload was applied to the column by hydraulic jack and monitoredby using 1000 kN capacity load cell. Axial deformation of the col-umn was measured by using linear voltage displacement trans-ducer (LVDT) which was kept at top of the jack. The load cell andLVDT were connected with the 16-Channel Data Acquisition Sys-tem to store the respective data. At the beginning, a small load of20 kN was applied slowly, so that the columns settle properly onits supports. Then the load was removed after checking the properfunctioning of the instrumentation. Then the columns were testedto failure by applying the compressive load in small incrementsand the observations such as axial deformation and ultimate loadwere carefully recorded. The load at which the CFRP starts ruptur-ing and the nature of failure were also noted for each column.

600.

00

91.50

5030 50

40

91.50

HS-50-30 HS-50-40ALL DIMENSION ARE IN MM

Fig. 1. Wrapping schemes.

3.3. Description of specimens

Among twenty-one specimens, eighteen were externallywrapped by CFRP strips having a constant width of 50 mm withthe spacing of 30 mm and 40 mm and the remaining three werereference column. The size and length of the columns used were91.5 � 91.5 � 3.6 mm and 600 mm respectively. To identify thespecimen easily, the columns were designated with the namessuch as HS-50-30-T1, HS-50-30-T2 HS-50-30-T3, HS-50-40-T1,HS-50-40-T2 and HS-50-40-T3. For example, the specimen HS-50-30-T3 specifies that it was strengthened by three (3) layers of50 mm width horizontal strip (HS) of CFRP fabrics in transversedirection (T) with the spacing of 30 mm. The control columns arespecified as CC1, CC2 and CC3.

4. Results and discussion

4.1. Failure modes

All the columns were loaded up to failure to understand theinfluence of carbon fibre fabrics on the axial behaviour of CFSTmembers and the failure modes of the columns were summarizedin Table 2. The outward buckling of unbonded columns (CC1, CC2and CC3) was observed at the top on all four sides of the steel tubeand occurred at the load of 934 kN, 928 kN and 923 kN respectivelywhich is shown in Fig. 3. This is a result of the fact that, the uniformapplied concentric force expanded the concrete core laterally andthat dilation effect of the concrete caused outward buckling ofthe steel tube mainly located at the top/bottom/supports of thecolumn. The crushing of concrete was not occurred in order thatthe applied load decreased slowly after the failure load but favour-


able enhancement in ductility performance was noticed. In thecase of columns HS-50-30-T1(1), HS-50-30-T1(2) and HS-50-30-T1(3), snapping sound of fibre was observed at the load of900 kN, 930 kN and 933 kN respectively. Among these, the col-umns HS-50-30-T1(2) and HS-50-30-T1(3) failed by rupture of fi-bre which was observed at the bottom at the load of 1008 kNand 1027 kN respectively and is shown in Fig. 4. This is a resultof the fact that, when a uniform pressure applied in the top surfaceof the specimens, the concrete core will begin to expand laterallyas a result steel tube also start to expand laterally, in the mean-while CFRP lies in the outer limits provide restraint against the lat-eral deformation and they are subjected to tension in the hoopdirection. When the CFRP reached its ultimate strain, rupture ofCFRP will occurred followed by the buckling of steel tube occurredand the dilation effect of the concrete is shown in Fig. 5 caused out-ward buckling generally at the supports of the column. But thespecimen HS-50-30-T1(1) failed by local buckling of steel tube ob-served at the mid height of column at the load of 955 kN and inaddition no rupture of fibre was observed. The reason is attributedto poor wrapping of FRP composites to the column. The similarbehaviour as same as that of columns HS-50-30-T1(2) andHS-50-30-T1(3) was observed in the specimens HS-50-30-T2(1),HS-50-30-T2(2) and HS-50-30-T2(3) but the rupture of fibre wasoccurred at the bottom as shown in Fig. 6. In the case of specimensHS-50-30-T3(1), HS-50-30-T3(2) and HS-50-30-T3(3), as said ear-lier due to the dilation effect of the concrete, the local bucklingof steel tube followed by rupture of fibre was noticed at the bottomat the load of 1122 kN, 1200 kN and 1202 kN respectively. Finallythe fibres were delaminated which is shown in Fig. 7.

Table 2Experimental result of all specimens.

Designation ofcolumns

Failureload(kN)

Load at initialrupture of FRP(kN)

Maximumaxialdeformation(mm)

% of Reductiondeformation comto CC1

CC1 934 – 11.98 –

CC2 928 – 12.28 –

CC3 923 – 11.99 –

HS-50-30-T1(1) 965 823 9.94 22.11HS-50-30-T1(2) 991 820 8.79 19.58HS-50-30-T1(3) 1001 882 10.01 25.12

HS-50-30-T2(1) 1070 904 11.60 34.11HS-50-30-T2(2) 1022 934 11.89 42.12HS-50-30-T2(3) 1066 941 12.14 41.15HS-50-30-T3(1) 1122 928 11.23 50.01HS-50-30-T3(2) 1200 934 11.79 66.24HS-50-30-T3(3) 1105 918 12.12 50.12HS-50-40-T1(1) 956 836 9.73 5.88

HS-50-40-T1(2) 972 834 9.76 7.21

HS-50-40-T1(3) 989 846 9.98 13.08

HS-50-40-T2(1) 1033 912 10.87 50.16

HS-50-40-T2(2) 1032 927 11.12 31.22

HS-50-40-T2(3) 1022 951 10.76 39.63

HS-50-40-T3(1) 1084 962 11.18 50.15

HS-50-40-T3(2) 1112 976 11.07 35.90

HS-50-40-T3(3) 1099 933 11.23 49.23

The specimens confined by 50 mm width of CFRP strips withthe spacing of 40 mm, confined by one and two layers of CFRP fab-rics [HS-50-40-T1(1), HS-50-40-T1(2), HS-50-40-T1(2), HS-50-40-T2(1), HS-50-40-T2(2) and HS-50-40-T2(3)] were failed by buck-ling of steel tube which was observed at the unbonded region gen-erally located at the bottom of the column is shown in Figs. 8 and 9.The columns HS-50-40-T3(1) and HS-50-40-T3(3) exhibited localbuckling of steel tube at the bottom without any rupture of fibreand was occurred at 1033 kN and 1032 kN respectively as shownin Fig. 10. The local buckling of steel tube alone without any rup-ture of fibre is attributed to, when increasing the spacing of CFRPstrips, the unwrapped area will become more. Due to the absenceof confining pressure provided by the FRP composites in the un-wrapped area, they were subjected to more strain compared tothe wrapped area, and the buckling of steel tube was occurredwhen the steel reached its ultimate strain.

4.2. Axial stress–strain behaviour

Table 2 summarizes maximum axial deformation and its per-centage of control with respect to reference column. Up to850 kN on jack, a linear response was observed in all reference col-umns and thereafter non-linear response was observed. The CFSTmembers confined by CFRP fabrics sustained higher ultimate loadand lower axial deformation compared to control column and alsoreinforcement by CFRP strips significantly increases the stresscapacity of the specimens. In addition, a significant fall in curvewas observed at the peak stage due to sudden rupture of CFRP fab-rics. Compared to control column (CC1), the specimens HS-50-30-

in axialpared

% of Increase inaxial load carryingcapacity

Failure modes

– Buckling of steel tube observed on all four sideof the column at bottom support



3.32 Rupture of fibre observed at the bottom support6.10 Rupture of fibre observed at the bottom support7.17 Local buckling of steel tube alone observed at

the mid height of column14.56 Rupture of fibre observed at the bottom support

9.42 Rupture of fibre observed at the bottom support14.13 Rupture of fibre observed at the bottom support20.12 Rupture of fibre observed at the bottom support28.48 Rupture of fibre observed at the bottom support18.31 Rupture of fibre observed at the bottom support

2.43 Buckling of steel tube alone observed at bottomsupport without rupture of FRP









Fig. 3. Failure mode of control column.

N

Concrete

Steel Tube

Dilation of

Concrete

End Plate

Fig. 5. Load transferring mechanism.


T1(2), HS-50-30-T2(1) and HS-50-30-T3(2) showed significantcontrol in axial deformation and enhancement in stiffness andstress capacity, especially, the behaviour of HS-50-30-T3(2) wasoutperformed which is shown in Figs. 11 and 14.

Fig. 4. Failure mode of column HS-50-30-T1(1).

The columns HS-50-30-T1(2), HS-50-30-T2(1) and HS-50-30-T3(2) enhanced their axial deformation control by 19.58%, 34.11%and 66.24% respectively compared to the control column and theiraxial deformation at the respective failure load of control columnwas 7.66 mm, 6.83 mm and 5.51 mm respectively. When com-pared to columns HS-50-30-T2(1) and HS-50-30-T3(2), the axialdeformation control of column HS-50-30-T1(2) was very smallwhich is due to insufficient amount of confining pressure gener-ated by FRP fabrics. The axial stress strain behaviour of columnHS-50-30-T1(2) followed the same path of HS-50-30-T2(1) untilreach the load of 810kN, and thereafter, relaxation in axial





deformation control was observed and in addition, the columnHS-50-30-T2(2) sustained higher ultimate load and largeraxial deformation control which is shown in Fig. 12. The axialdeformation of column HS-50-30-T3(2) at the respective failureload of columns HS-50-30-T1(2) and HS-50-30-T2(1) was5.85 mm and 7.23 mm and the percentage of enhancement in axialdeformation control was 51.11% and 43.70% respectively as shownin Fig. 14.

Until reach the load of 350kN, there is similar axial stress–strainbehaviour was observed in the case of columns HS-50-40-T1(3),HS-50-40-T2(1) and HS-50-40-T3(2) and thereafter due to morenumber of FRP layers, the columns HS-50-40-T2(1) and HS-50-40-T3(2) showed better control in axial deformation compared tocolumns CC1 and HS-50-40-T1(3) which is shown in Fig. 12. Itwas also found that the specimens HS-50-40-T1(3), HS-50-40-T2(1) and HS-50-40-T3(2) enhanced their axial deformation con-trol by 13.08%, 50.16% and 35.90% respectively compared to controlspecimen and their mid-span deflection at the respective failureload of control column was 7.66 mm, 5.99 mm and 6.74 mmrespectively as shown in Fig. 14. Until reaching a failure load of510 kN, the column HS-50-40-T3(2) followed the same path of col-umn HS-50-40-T2(1), and thereafter meager relaxation in defor-mation control was observed. But better control in axialdeformation was observed only after the load of 993 kN. The col-umn HS-50-40-T3(2) enhanced their axial deformation control by28.74% and 14.90% when compared to columns HS-50-40-T1(3)and HS-50-40-T2(1) respectively as shown in Fig. 14. As expected,the columns confined by CFRP in both spacing, the axial deforma-tion control of the confined columns increases as the number oflayers increases but the enhancement in axial deformation controlwas also not proportional. The above nonlinearity in axial deforma-tion control when increasing the number of layers of fibre may beattributed to crushing of resin lying in between the fibres. When

the resin started to crush, a sudden drop in substantial load trans-fer was occurred. As a result, non linearity in axial deformationcontrol was observed. Furthermore, by increasing the number oflayers of fibre fabrics, the number of resin layers also increasedso that more nonlinearity in axial deformation control was ob-served. The axial stress–strain behaviour of columns having30 mm spacing of CFRP strips was outperformed when comparedwith that of columns strengthened by CFRP strips having spacingof 40 mm which is shown in Figs. 13 and 14. It can also be seen thatthe axial deformation control of the confined columns increases asthe spacing of the CFRP strips decreases. The column HS-50-40-T1(3) has higher axial deformation of 9.64 mm compared to col-umn HS-50-30-T1(2) which has a axial deformation of 9.29 mmas shown in Fig. 14. The column HS-50-30-T2(1) enhanced theirdeformation control by 9.37% compared to columns HS-50-40-T2(1). Fig. 14 also illustrates that the HS-50-40-T3(2) has more ax-ial deformation (10.9 mm) than that of column HS-50-30-T3(2)furthermore which is 39.7% higher.

4.3. Load bearing capacity

Table 2 summarizes the maximum load carrying capacity andpercentage increase in it of all CFRP strengthened columns com-pared with the control column. As expected, the external wrap-ping of CFRP strips considerably enhance the load carryingcapacity of the columns, especially the columns strengthened bythree layers of CFRP strips in all spacing were outperformed. Com-pared to control column, the specimens HS-50-30-T1(2), HS-50-30-T2(1) and HS-50-30-T3(2) enhanced their axial load carryingcapacity by 6.10%, 14.56%, 28.47% as shown in Fig. 15. In similar



0

20

40

60

80

100

120

140

Axial strain

Axi

al s

tres

s (N

/mm

2 )

CC1 HS-50-30-T1(2)

HS-50-30-T2(1) HS-50-30-T3(2)

0.000 0.005 0.010 0.015 0.020 0.025

Fig. 11. Axial stress–strain behaviour of columns HS-50-30 – comparison.


manner, the columns having 40 mm spacing of CFRP strips such asHS-50-40-T1(3), HS-50-40-T2(1) and HS-50-40-T3(2) having5.88%, 10.59% and 19.05% respectively more load carrying capacitythan that of control column as shown in Fig. 15. The above test re-sults were revealed that, the external wrapping of CFRP strips pro-vides external confinement pressure effectively and intended todelay the local buckling of steel tube and also enhance the loadcarrying capacity. From the Fig. 15, it was confirmed that the spec-imens strengthened by CFRP strips with smaller spacing havemore axial load carrying capacity and the increase in axial loadmainly depends upon proper designed spacing of CFRP strips.When compared to column HS-50-40-T1(3), the column HS-50-30-T1(2) has more load carrying capacity which is shown inFig. 15. In similar manner, the column enhanced its load carryingcapacity by 3.50% than that of HS-50-30-T2(1). Fig. 15 also illus-trates that the column HS-50-30-T3(2) has more axial load carry-ing capacity (1200 kN) than that of column HS-50-40-T3(2)(1112 kN) and furthermore which is 7.9% higher. From that itcan be understood that the increase in spacing between the CFRPstrips decreases the confining pressure exerted by the CFRP com-posites as a result decrease in load bearing capacity was observed.From the Fig. 15, it can be seen that the axial load carrying capac-ity of the confined columns increases as the number of CFRP lay-ers increases but the enhancement in axial load carrying capacitywas not proportional. The column HS-50-30-T3(2) enhanced itsaxial load carrying capacity by 21.12% and 12.64% more than thatof columns HS-50-30-T1(2) and HS-50-30-T2(1) respectively. Sim-ilarly, the column HS-50-40-T3(2) enhanced its load carryingcapacity by 12.44% and 7.64% when compared to columns HS-50-40-T1(3) and HS-50-40-T2(1) respectively. From the above

observations, it is suggested that CFRP strips having spacing of30 mm and 40 mm used in this research work are suitable forstrengthening of columns subjected to axial compression.

0

2

4

6

8

10

Axi

al d

efor

mat

ion

(mm

)

One Layer Two Layer Three LayerNumber of FRP layers

30mm Spacing 40mm Spacing

Fig. 14. Axial deformation with respect to number of CFRP layers – comparison.

0

20

40

60

80

100

120

140

Axial strain

Axi

al s

tres

s (N

/mm

2 )

CC1 HS-50-40-T1(3)

HS-50-40-T2(1) HS-50-40-T3(2)

0.000 0.005 0.010 0.015 0.020 0.025 0.030

Fig. 12. Axial stress–strain behaviour of columns HS-50-40 – comparison.

0

20

40

60

80

100

120

140

0.000 0.005 0.010 0.015 0.020 0.025 0.030Axial strain

Axi

al s

tres

s (N

/mm

2 )

CC1 HS-50-30-T1(2)

HS-50-30-T2(1) HS-50-30-T3(2)

HS-50-40-T1(3) HS-50-40-T2(1)

HS-50-40-T3(2)

Fig. 13. Axial stress–strain behaviour of all columns – comparison.


5. Analytical study

5.1. Prediction of the axial strength of FRP confined CFST column

5.1.1. Existing confinement modelsHajjar and Gourley [21] proposed the following equation for

predicting the load carrying capacity (Po) of the CFST column underaxial compression.

Po ¼ Asteel � fy þ Aconcrete � f 0c ð1Þ

where fy = yield stress of steel tube, f 0c = cylinder compressivestrength of concrete and Asteel and Aconcrete are cross sectional areaof steel tube and filled concrete.The axial load carrying capacity(Npl,Rd) of the CFT columns according to EC4 [22] (1994) can bedetermined by summing up the strengths of the steel tube andthe concrete core, as in Eq. (2). The application of EC4 [22] is re-stricted to composite columns with concrete cylinder strengthand steel yield stress not greater than 50 and 355 MPa, respectively.

Npl;Rd ¼ Asfy þ Acfck ð2Þ

k ¼

ffiffiffiffiffiffiffiffiffiffiffiNpl;Rd

Ne

sð3Þ

where Ne = elastic buckling load of the member (Euler critical load)

Ne ¼ p2 ðEIÞel2e

ð4Þ

where (EI)e = effective stiffness of the composite column

ðEIÞe ¼ EaIa þ 0:8EcdIc ð5Þ

where Ia and Ic are the moments of inertia of the cross sectional areaof the steel tube and the concrete respectively. Ea and Ecd are theyoung’s modulus of the steel tube and the concrete.

In the case of square columns, it is necessary to consider thecapacity reduction due to local buckling of the steel tube wall ofthe column with large (B/t) ratio rather than the confinement ef-fect of the steel tube. For predicting the axial load of CFT column(Nu), by taking into account the large (B/t) ratio, the modified equa-tion was given by Sakino et al. [23] as follows:

Nu ¼ Asrscr þ Accuf 0c ð6Þ

rcr ¼minðrsy; srsyÞ ð7Þ

1S¼ 0:698þ 0:128

Bt

� �2 rsy

Es� 4:00

6:97ð8Þ

where rsy = yield strength of steel tubeWhen a FRP confined concrete column is subjected to axial

compression, the concrete core expand laterally, thus this lateral

0

200

400

600

800

1000

1200

Ult

imat

e L

oad

(kN

)

One layer Two layer Three layerNumber of FRP layers

30mm Spacing 40mm Spacing

Fig. 15. Ultimate load for all columns – comparison.

f frp t

f l D


expansion is resisted by FRP lies in the outer limits as they are sub-jected to tension in lateral direction. Lam and Teng [24] proposedthe equation for calculating that confining pressure exerted bythe FRP.

fl ¼2f frptfrp

Dð9Þ

where D = diagonal length of square cross section, ffrp = tensilestrength of FRP in the hoop direction and tfrp = thickness of theFRP confinement.

f frp t

Fig. 16. confinement model for CFST square column.

Table 3Experimental and Analytical results – comparison.

Designation ofcolumns

Ultimate load (kN) Ptheo/Pexp Difference

D¼ Pexp�PtheoPexp

�100 (%)Experimental Pexp Theoretical Ptheo

CC1 934 – – –CC2 928 – – –CC3 923 – – –HS-50-30-T1(1) 965 980 1.015 �1.554HS-50-30-T1(2) 991 0.988 1.110HS-50-30-T1(3) 1001 0.979 2.098HS-50-30-T2(1) 1070 1047 0.979 2.150HS-50-30-T2(2) 1022 1.024 �2.446HS-50-30-T2(3) 1066 0.982 1.782HS-50-30-T3(1) 1122 1132 1.008 �0.891HS-50-30-T3(2) 1200 0.943 5.667HS-50-30-T3(3) 1105 1.024 �2.443HS-50-40-T1(1) 956 970 1.014 �1.464HS-50-40-T1(2) 972 0.997 0.206HS-50-40-T1(3) 989 0.988 1.921HS-50-40-T2(1) 1033 1023 0.990 0.968

5.1.2. Proposed approach modelBased on the above confinement models, new models are pro-

posed herein for predicting the axial load capacity of CFRP confinedCFST column. These models are simple one and in addition, addi-tional developments such as the effect of concrete strength, yieldstrength of steel tube and height of columns are required to takeinto account.

When a concentric load is applied to a CFRP wrapped CFST col-umn (assuming the load is applied uniformly across both materi-als), the steel tube lies in the outer limits and the concrete corewill both begin to expand laterally and, in the meanwhile, CFRP liesin the outer limits started to resist lateral expansion by providingconfinement pressure as they are subjected to tension in the hoopdirection. As the axial stress increases, the corresponding lateralstrain increases and the CFRP strips develops its maximum tensilehoop stress which is equal to the ultimate tensile strength of CFRP(ffrp) balanced by the lateral pressure (flcon) is shown in Fig. 16. Byconsidering the equilibrium, the following equation can be derivedto find the lateral confinement pressure (flcon) provided by the CFRPstrips.

flcon ¼3f frpntfrp

D½1þ 0:16½n� 1�� ð10Þ

HS-50-40-T2(2) 1032 0.991 0.872HS-50-40-T2(3) 1022 1.001 �0.098HS-50-40-T3(1) 1084 1092 1.007 �0.738HS-50-40-T3(2) 1112 0.982 1.799

flcon ¼4f frpnqfrp

5cCFRP½1þ 0:23½n� 1�� ð11Þ

HS-50-40-T3(3) 1099 0.993 0.637

qfrp ¼ACFRP

AGrossð12Þ

where flcon is lateral confinement pressure exerted by the CFRPstrips having spacing of 30 mm and 40 mm. n and cCFRP are thenumber of CFRP layers (n = 1, 2,3,4) and static design safety for CFRP(cCFRP = 1.2) respectively. ACFRP and AGross are the cross sectional areaof the CFRP and CFST column member.For predicting the compres-sive strength of CFRP confined CFST square column f 0ccon, the follow-ing formula were proposed.

f 0ccon ¼ 1þ kflcon

funcon

� �funcon ð13Þ

where funcon and k are unconfined compressive strength of CFST col-umn and effective confinement coefficient respectively. The pro-posed effective confinement coefficient (k) value for the columnconfined by CFRP strips having a spacing of 30 mm and 40 mmand are 2.5 and 2 respectively.The following equation was proposedto determine the axial load carrying capacity of unconfined CFSTmember.

funcon ¼ Asfy þ Acfck ð14Þ

The calculated axial load carrying capacity CFRP confined CFST col-umns are listed in Table 3 along with the failure load obtained fromthe experiments. The average percentage of difference between cal-culated and experimental value is ±5%.

6. Conclusions

Experimental and analytical investigations on the behaviour ofaxially loaded CFST columns externally strengthened by CFFRPstrips composites with two different spacing were presented inthis paper. From the experimental data obtained, the failuremodes, axial stress–stain behaviour and ultimate load carryingcapacity were discussed. Based on the compressive tests on eigh-teen specimens, the following conclusions recommendations aredrawn: CFST columns were strengthened by CFRP strips with30 mm spacing were shown CFRP rupture failure, however whenincreasing the spacing, the columns were failed by local bucklingof steel alone without any rupture of fibre. It was observed thatexternal wrapping of CFRP effectively delayed the local bucklingof the steel tube and also declines the axial shortening by providingconfinement/restraining effect against the elastic deformation. Col-umns HS-50-30-T1(2), HS-50-30-T2(1) and HS-50-30-T3(2) en-hanced their axial deformation control by 19.58%, 34.11% and66.24% respectively compared to the control column. Externalwrapping of CFRP strips provides external confinement pressureeffectively and intended to delay the local buckling of steel tubeand also enhance the load carrying capacity however the increasesin the axial load carrying capacity mainly depends upon the properspacing between the CFRP strips. It has been shown that the


application of CFRP strips may provide increases in axial capacityof up to 1.5 times than the capacity of the steel section alone. Itis suggested that external strengthening of CFST columns usingnormal modulus CFRP strips is a quite effective technique to in-crease the load carrying capacity and stiffness of the CFST section.

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