Thin-Walled Structures 68 (2013) 135–140

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Thin-Walled Structures

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Experimental and theoretical investigation on torsional behaviour ofCFRP strengthened square hollow steel section

N. Abdollahi Chahkand a,n, M. Zamin Jumaat a, N.H. Ramli Sulong a, X.L. Zhao b, M.R. Mohammadizadeh c

a Department of Civil Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysiab Department of Civil Engineering, Monash University, Clayton, VIC 3800, Australiac Department of Civil Engineering, Hormozgan University, Bandar Abbas, Iran

a r t i c l e i n f o

Article history:Received 12 August 2012Received in revised form19 March 2013Accepted 19 March 2013Available online 24 April 2013

Keywords:TorsionSHS steel beamCFRPStrengthened

31/$ - see front matter & 2013 Elsevier Ltd. Ax.doi.org/10.1016/j.tws.2013.03.008

esponding author. Tel.: +60 142220488.ail addresses: newsha60@siswa.um.edu.my._abdollahi@yahoo.com (N. Abdollahi Chahkan

a b s t r a c t

Carbon fibre reinforced polymer (CFRP) has been used to strengthen steel members in bending andcompression. There is a lack of understanding on behaviour of CFRP strengthened steel beams subject totorsion. This paper presents an experimental study on the behaviour of CFRP strengthened square hollowsection (SHS) beams in pure torsion. A set of tests on CFRP strengthened steel specimens under torsionwas carried out in which several different strengthening configurations were used. CFRP sheet wrappingconsisted of different configurations including vertical, spiral, and reverse-spiral wrapping were used.The results showed that using CFRP could improve the elastic and plastic torsional strength of CFRPstrengthened steel beam specimens. The number of layers of CFRP and the strengthening configurationswere important factors for the improvement. Based on the measured values of the torsional moment atyielding and at ultimate, the corresponding twists, the torsional behavioural curves and the failuremodes of the strengthened beam specimens, useful concluding remarks are presented.

& 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Over the last decade, strengthening and retrofitting of existingsteel structures has become one of the important challenges forcivil engineers. Fibre reinforced polymer (FRP) has a great potentialto meet such challenges [1,2].

Several researchers have reported employing CFRP forstrengthened thin-walled steel structures such as flexuralstrengthening [3–5], tensile strengthening [6], enhancing sta-bility of steel members with CFRP [7,8], repair of fatiguedamage and enhancing fatigue life using CFRP [9–12], andstrengthened steel hollow section using CFRP [13–18]. Thin-walled steel structures could also be subjected to torsion loadwhen used in bridges and buildings. There seems to be nopublications on the behaviour of CFRP strengthened thin-walled steel structures in torsion. This paper attempts to fillthe knowledge gap in this area.

An experimental program was therefore set up in this study inorder to gain an understanding of the behaviour of square hollowsection (SHS) beams subject to torsion load. In this paper the

ll rights reserved.

d).

experimental procedure including detail of specimens, test setupand the results obtained are explained. Influence of the importantparameters that may affect the behaviour of the strengthenedspecimens are also elaborated.

2. Material properties

The type of used steel is mild steel, and its measured elasticitymodulus is 200,000 N/mm2. They are hot-formed SHS. Dimensionsof the section are presented in Fig. 1. The dimensions and materialproperties of SHS steel section are given in Table 1.

The type of CFRP sheet used in the tests is SikaWraps-200Cwith unidirectional woven carbon fibres fabric. The propertiesof the used CFRP sheet are shown in Table 2. The epoxy used inthis research was provided by the same supplier. This epoxy isused for a SikaWraps-200C which is called Sikadurs-330(Table 3).

3. Strengthening schemes

Six steel beam specimens with the SHS used in this study wereproduced in Malaysia. Four beam specimens of them (shownin Table 4) were strengthened using CFRP at the laboratory of

N. Abdollahi Chahkand et al. / Thin-Walled Structures 68 (2013) 135–140136

University of Malaya and were tested under pure torsion. Thelength of all the beam specimens was 1400 mm and the test regionlength was approximately 1160 mm.

Two specimens without strengthening scheme were designedas the control beam specimens. They were called Type 1. The restbeam specimens were strengthened by carbon fibre in differentconfigurations. Four types of strengthening configuration wereused in this study. Type 2 was wrapped with five layers of CFRPvertically with respect to the longitudinal axis of the specimen.Type 3 was wrapped with two layers of CFRP reverse-spirally withrespect to the longitudinal axis of specimen. The strengtheningconfiguration of Type 4 was a combination of three layers of CFRPreverse-spirally and one layer of CFRP spirally wrap with respectto the longitudinal axis of specimen. The strengthening configura-tion of Type 5 was a combination of two layers of CFRP spirally andtwo layers of CFRP reverse-spirally wrap with respect to thelongitudinal axis of the specimen. A schematic view of thedifferent types of strengthening configurations is shown in Fig. 2.

Table 2Properties of fibres.

CFRP Sheet: SikaWraps-200C

Fabric designthickness(mm)

Modulus ofelasticity(N/mm2)

Ultimatetensilestrength(N/mm2)

Ultimatetensileelongation(%)

Thickness(Impregnated withSikadurs-330)(mm)

0.111 230,000 3900 1.5 0.9 per layer

Table 3Properties of adhesive.

s

4. Specimen preparation

The first step was to have the surface preparation processwhich was completed before applying the CFRP wrap. The surfaceof the beam specimens was removed from the galvanised coatingand any oil, rust, paint, and impurities using grinding. The tubesurface was then cleaned using acetone before applying adhesive.For applying CFRP wrap on the surface of the specimens, the twocomponents of the epoxy were mixed according to the weightratio given by the manufacturer. The mix ratio was 4:1 by volumeof component A to component B. The epoxy was applied onto thesurface of the beam specimens using a brush. It was then spreadusing a paint brush. After applying the epoxy, the CFRP sheetsroller was used along the direction of the fibre to remove excessadhesive and air bubbles (Fig. 3). The specimens were kept atroom temperature for one week.

Fig. 1. Dimensions (cross-section and corner profile).

Table 1Dimensions and material properties of SHS.

Nominal dimensions(h�b� t) (mm)

Average measured dimensions Length,L (mm)

EffeLe (

h (mm) b (mm) t (mm)

50�50�3 50.21 50.21 2.7 1400 1160

5. Test set up and test procedures

The torsion testing rig comprised of a fixed grip and a pivotedrotating grip, betweenwhich a length of specimen could be causedto twist about its longitudinal axis (Fig. 4). The apparatus wasdesigned to function correctly and safely up to a torque of10,000 Nm. All tests were controlled and data were collected usingcomputer U60 software which displayed the result in the form of agraph plotting the torsion load vs. torsion angle under thespecified parameter conditions. This machine is used to carry outtorsion test steel or concrete specimen or other rod specimen anddesigned in accordance with client's requirements.

6. Experimental results and discussions

6.1. Brittle coatings of the control specimen

In this study in order to improve the visibility of the distribu-tion of stress, the control beam specimen was coated with a limewash (also a primitive form of brittle coating) so that the darkcoloured lines show up against the white background [19]. The

ct Length,mm)

Corner radius (mm) Stress (N/mm2)

Internal (r1) External (r0) Yielding (Fy) Ultimate (Fu)

2.7 4.2 382 431

Adhesive: Sikadur -330

Tensile strength(N/mm2)

Modulus of elasticity(N/mm2)

Elongation at break(%)

Tensile Flexural

30 4500 3800 0.9

Table 4The characteristics of the specimens.

Test type Specimen label Configuration No. of layers

Type 1 Ctrl 1 Control beam NoneType 1 Ctrl 2 Control beam NoneType 2 V5 Vertical wrap 5Type 3 RR Reverse-spiral wrap 2Type 4 RRRS Reverse-spiral and spiral wrap 4Type 5 SRSR Spiral and reverse-spiral wrap 4

Fig. 2. Types of CFRP strengthening configurations (not to scale). (a) Type 1.(b) Type 2. (c) Type 3. (d) Types 4 and 5.

Fig. 3. Application of CFRP sheets to the SHS.

Fig. 4. Torsion testing apparatus.

Fig. 5. Cracks in a brittle coating due to pure torsion on control beam.

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patterns of cracks that form in the coating are observed in order tomake deductions about the distribution of stress at the surface ofthe body [20]. Cracks in a brittle coating due to pure torsion oncontrol beam are shown in Fig. 5.

6.2. Strain gauge readings

6.2.1. Strain gauge readings of the control specimenUsing the rosette strain gauge mounted on one of surfaces of

the steel control beam specimen (at the middle of the specimenlength), the strains were monitored. Fig. 6 shows torque–straincurves for the control beam specimen. The curves in this figure areplotted based upon the strain in strain gauges no. 1, 2, 3, 4 and 5.Fig. 6 shows the yield and ultimate torque values for the controlbeam specimen. The yield and ultimate torques were 2643 Nmand 2782 Nm, respectively (Fig. 6a). Fig. 6b shows configuration ofthe strain gauges on surface of the control beam. It can beobserved in Fig. 6a and b that the values of strains correspondingto the strain gauges no. 3, 4 and 5 are close to zero. These valuescan be attributed to pure shear stresses. When a beam is subjectedto a pure torsional moment, normal stresses are negligible inlongitudinal and transverse directions of the beam. The shearstresses can be caused principal stresses in direction 7451 withrespect to the longitudinal axis of the beam. The strain gauges no.1 and 2 are in direction 7451 with respect to the longitudinal axisof the control beam.

6.2.2. Strain gauge readings of the strengthened beam specimensFig. 7 shows the experimental results in terms of the torque

versus strain of CFRP sheets. In this figure, typical torque–straincurves for the strengthened specimens with different strengthen-ing configurations are plotted. By qualitative study of Fig. 7, it canbe seen that for a given torque, strain levels of strengthenedspecimen Type 5 is less than those of specimens Type 3 and Type4 in the plastic region. For Type 2, negligible strain is recorded inthe elastic and plastic region.

6.3. Torque–rotation curves

Investigating the torque–twist angles throughout the tests forall specimens was another goal of this experimental study. Thevalues of twist angle versus the applied torque are presented inFig. 8 for all specimens. In Fig. 8, the difference observed in theinitial stiffness of the specimens can be attributed to a less-than-perfect fixed condition achieved in the setup. The authors believethat such difference does not substantially affect the result of thetorsional strengthening of the specimens.

In Fig. 8, three different zones can be seen on each curve. Thefirst zone represents the torsional stiffness of un-yielded speci-men, the second zone represents the stiffness of the yieldedspecimen and the last zone corresponds to the damaged cross-section with yielded torsional steel and ruptured CFRP sheets.

0

500

1000

1500

2000

2500

3000

-25 -20 -15 -10 -5 0 5 10 15 20 25

Tor

que

(N.m

)

Micro Strain (x 1000)

12345

Fig. 6. (a) The torque–strain curves. (b) Strain gauge pattern on the control beam at mid span.

V5RR

RRRSSRSR

0

1000

2000

3000

4000

5000

0 2 4 6 8 10 12

Torq

ue (N

m)

Micro strain ( 1000)

Type 2:V5Type 3: RRType 4: RRRSType 5: SRSR

Fig. 7. Strain values on CFRP for the strengthening beams.

0

1000

2000

3000

4000

5000

0 10 20 30 40 50 60 70 80

Torq

ue (N

m)

Angle of twist (Degree)

Type1 :Control Type 2: V5Type 3: RRType 4: RRRSType 5: SRSR

Fig. 8. Torque–twist curve for all specimens.

Fig. 9. Strengthened beam specimen during the test.

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As shown in Fig. 8, the torque–twist curves for all specimensare linear with a constant slope until yielding. After yielding, thetorsional stiffness decrease significantly while affected by volu-metric ratio and orientation angle of CFRP sheets.

The best orientation angle for fibres is in direction of theprincipal tensile stresses. Therefore, spirally CFRP wrap is the beststrengthening configuration. Fig. 8 also shows that the strength-ened specimens Type 4 (RRRS) and Type 5 (SRSR) provide muchhigher ultimate torque comparing to the specimens Type 2 (V5)and Type 3 (RR). The reason of deficiency of the strengthenedspecimen Type 3 in comparison with specimens Type 4 and Type5 is that the orientation angle of fibres is almost in direction ofthe principal compressive stresses. For specimen Type 2, since thefibres direction is perpendicular to the longitudinal axis of thespecimen, so it cannot sustain much higher load than specimenType 1 (the control beam).

A combination of spiral and reverse-spiral CFRP wraps could beuseful to resist cyclic torque caused by earthquake. In this study,specimens Type 4 and Type 5 were strengthened with combina-tions of spiral and reverse-spiral CFRP wrap around the specimens.For specimens Type 4 and Type 5, the gain in ultimate torsionalcapacity is 59.7% and 60.4% compared to the control specimen,respectively. Fig. 9 shows the strengthened beam specimen duringthe test. For all the strengthened beam specimens, the ultimatetorques along with their increase percentage comparing to that ofcontrol beam specimen and modes of failure are listed in Table 5.

6.4. Failure modes

Fig. 10 shows the failure modes for all the strengthened speci-mens. In specimen Type 2, splitting of the CFRP occurred in thedirection perpendicular to the longitudinal axis of the specimen,(Which is parallel to the CFRP direction). It is due to deficiency ofthe strengthening configuration. In this specimen, the CFRPscannot sustain the additional applied torque compared to the

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control specimen. For specimens Type 3, Type 4 and Type 5, CFRPrupture occurred and eventually failures followed by yielding inthe specimen body. CFRP rupture occurred in direction of theprincipal tensile stresses.

7. Torsional capacity prediction of CFRP strengthened SHS

In this section a proposed method for determining plastictorsional capacity of SHS are presented. The plastic torsionalcapacity (Tpl) for a box section can be determined by taking intoaccount the flow of uniform plastic shear around the cross-section,given as

Tpl ¼ 2tAhτy ð1Þ

where t is thickness of the SHS, τ is the shear stress which is equalto 0.6fy and fy is the yield stress of the SHS. Ah is the enclosed area(Fig. 11a) which is given by

Ah ¼ ðb−2rextÞðh−tÞ þ 2ðh−2rext Þrm þ πr2m ð2Þ

where rext is external corner radius, rint is internal corner radius

Table 5Ultimate torques obtained from experiments and corresponding percentageincrease for all the CFRP strengthened specimens.

Specimenlabel

No. oflayers

Torsionalcapacity (Nm)

Increasing torsionalcapacity (%)

Mode offailurea

Control – 2782 – –

V5 5 3186 14.6 SRR 2 3689 32.6 R–SRRRS 4 4442 59.7 R–S–DSRSR 4 4462 60.4 R–S

a S: Splitting, R: Rupture, D: delamination.

Fig. 10. Failure modes of all the CFRP strengthened specimens. (a) CFRP splitting (Tdelamination (Type 4). (d) CFRP rupture and splitting (Type 5).

and rm is the mid-corner radius given by

rm ¼ ðrext þ rintÞ=2 ð3ÞWhen CFRP is applied, the proposed method considers equiva-

lent thickness approach. The CFRP thickness can be equivalent tosteel thickness as given by

tes ¼ ðECFRP=EsteelÞntfiber ð4Þwhere ECFRP is Young's modulus of CFRP and Esteel is Young'smodulus of the steel, n is the number of layers and tfibre is thethickness of the fibre. Fig. 11b shows the dimensions of theequivalent SHS with CFRP, then the equivalent dimensions become

beq ¼ bþ 2tes ð5Þ

heq ¼ hþ 2tes ð6Þ

teq ¼ t þ tes ð7Þ

rext;eq ¼ rext þ tes ð8Þ

rm;eq ¼ ðrext;eq þ rintÞ=2 ð9Þ

Ah;eq ¼ ðb−2rextÞðheq−teqÞ þ 2ðh−2rextÞrm;eq þ πr2m;eq ð10Þ

Then, the torsional capacity, Tpl can be calculated by usingEq. (1), where t is replaced by teq (Eq. (7)) and Ah is replaced byAh,eq (Eq. (10)). The value τ should be increased because of thestrain hardening. The extreme case will be 0.6 fu where fu is theultimate tensile strength of steel SHS. Thus torsional capacity ofthe strengthened beam with CFRP is given by

Tpl;eq ¼ 2teqAh;eq0:6Fu ð11ÞThe calculated torsional capacity for Type 3, Type 4 and Type 5,

where fibre direction is along the shear direction are presented inTable 6. By comparing the experimental results and the calculatedvalues using the simplified expression, it was found that the values

ype 2). (b) CFRP rupture and splitting (Type 3). (c) CFRP rupture, splitting and

Fig. 11. Dimensions to calculate the enclosed area. (a) SHS without CFRP. (b) SHSwith CFRP.

Table 6Comparison of ultimate capacity of proposed method with test results.

Specimenlabel

No. oflayers

Experimentaltorsionalcapacity (Nm)

Proposed methodfor torsionalcapacity (Nm)

Experimentaltorsional capacity/Proposed method

Control – 2782 2766 1.00RR 2 3689 3571 1.03RRRS 4 4442 3920 1.13SRSR 4 4462 3920 1.14

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are in good agreement. It should be noted that the strengtheningscheme using vertical fibres alone was found not efficient. There-fore no predictions are included in the paper for this strengtheningscheme.

8. Conclusions

Based on the limited tests on the SHS steel specimensstrengthened using CFRP, the following conclusions can be drawn:

�

The ultimate torques of all strengthened steel specimens aregreater than that of the control specimens. The increase in

magnitude depends on CFRP's reinforcement ratio and thestrengthening configuration.

�

The best orientation angle for fibres is in the direction of theprincipal tensile stresses. This direction is related to spirallywrap configuration. Using this configuration, CFRP contributionto ultimate torque is greater than that of the strengthenedspecimens with vertical orientation of CFRP.

�

In order to resist cyclic torque, the best orientation angle fortorsional strengthening of steel structures is combination ofspiral and reverse-spiral CFRP wraps.

�

For strengthened specimens with the same volumetric ratios ofCFRP reinforcement (Type 4 and Type 5), the torsional resis-tance increases as the strengthening configuration is changedfrom spirally-reverse wrap to spirally wrap.

�

The proposed method of the torsional capacity prediction is ingood agreement with the experimental results.

Acknowledgement

The study presented herein was made possible by HIRG grantnumber D000036-16001 from the University of Malaya, KualaLumpur, Malaysia.

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