Journal of Constructional Steel Research 75 (2012) 131–141

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Journal of Constructional Steel Research

Seismic behavior of special shaped column composed of concrete filled steel tubes

Ting Zhou a, Zhihua Chen a,b,⁎, Hongbo Liu a

a Department of Civil Engineering, Tianjin University, Tianjin, 300072, Chinab Tianjin Key Laboratory of Civil Engineering Structure & New Materials, Tianjin University, Tianjin, 300072, China

⁎ Corresponding author at: Department of Civil EnTianjin, 300072, China. Tel.: +86 13820589038; fax: +

E-mail address: zhchen@tju.edu.cn (Z. Chen).

0143-974X/$ – see front matter © 2012 Elsevier Ltd. Aldoi:10.1016/j.jcsr.2012.03.015

a b s t r a c t

a r t i c l e i n f o

Article history:Received 28 May 2011Accepted 19 March 2012Available online 13 April 2012

Keywords:Special shaped columnConcrete-filled steel tubeComposite columnsSeismic behaviorSeismic behavior testFinite element analysis

This paper experimentally investigated the behavior of a special shaped column composed of concrete-filledsteel tubes (SCFST column) subjected to a constant axial load and a cyclically varying flexural load. The effectsof both the axial compression ratio and the length to width ratio on the behavior (stiffness, strength, ductility,and energy dissipation) of SCFST columns were studied. The asymmetry character of the column was studied.The connection plate was an important member transferring shear force. The mono columns of SCFST col-umns worked together well and the seismic behavior of SCFST columns was good. An increase of the axialcompression ratio increased the stiffness with a decrease of energy dissipation ability, ductility, and bearingcapacity. On the other hand, an increase of the length to width ratio led to an increase of energy dissipationability and ductility and a decrease of stiffness and bearing capacity. Furthermore, a finite element analysiswas also carried out to simulate the behavior of SCFST columns and the test results agreed with the resultsof finite element analysis.

© 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Special shaped column is a column with L-shaped, T-shaped orcrisscross-shaped sections, which is located in the corner of structures.The special shaped columns are widely employed in the field of struc-tural engineering because of the advantage of saving indoor space andconvenient arrangement for furniture. In past decades, the reinforcedconcrete special shaped columns have been extensively studied. Mostwork focused mainly on the static behavior of T-shaped and L-shapedstub columns subjected to the bi-axial eccentric load and some corre-lation curves of resistance [1–5]. For the reinforced concrete specialshaped columns, the cracking resistance and seismic ability are criticalissues [6–8]. In recent years, concrete-filled steel tube (CFST) columnshave attracted much attention because of their high stiffness, ductilityand earthquake resistance [8–10]. Chen [11,12] proposed a kind of anew type of column, namely, special-shaped column composed ofconcrete-filled steel tubes (SCFST column).

The SCFST columns are composed of several concrete-filled squaretubular columns connected by connection plates as shown in Fig. 1.The SCFST columns are suitable for the construction of residentialbuildings by utilizing the advantages of both special-shaped sectionsand concrete-filled square tubular columns. In order to reduce struc-tural self-weight, the L-shaped section is a prior option for designers.

Several researches have been conducted to investigate the behav-iors of SCFST columns. Chen investigated six SCFST columns under

gineering, Tianjin University,86 022 27409933.

l rights reserved.

axial load and developed a superposition method to evaluate theultimate capacity of the SCFST columns [12]. The design approach ofSCFST column structures has been developed considering manydetails of this kind of structure. A SCFST column structure is shownin Fig. 2. Although there are some investigations reported on thestatic behavior of SCFST columns, its seismic behavior is less experi-mentally studied.

This paper investigated the seismic behavior of L-shaped SCFSTcolumns. Three specimens were tested under constant axial compres-sion and cyclic horizontal load with incrementally increasing lateraldeformation. The geometrical and material characters of specimen-1(S-1) were set according to the structure shown in Fig. 2, thus itcould meet the project requirement. The axial compression ratio ofS-1 was 0.4, which is commonly seen in the project. In order to inves-tigate the effect of axial compression ratio and length to width ratio,the axial compression ratio of specimen-2 (S-2) increased to 0.6,compared to S-1, and the length to width ratio of mono column ofspecimen-3 (S-3) was reduced to 10 while that of S-1 was 15. Thetest results were compared with those of a finite element analysis.According to these results a mathematic model was proposed inorder to develop an analytical approach.

2. Experimental program

2.1. Introduction

An experimental study including three specimens was performedto assess the response of L-shaped SCFST columns under axial com-pression and cyclic horizontal load. S-1 and S-2 had different axial

a) L-shaped section b) T-shaped section c) Crisscross section

Fig. 1. Schematic of SCFST column.

132 T. Zhou et al. / Journal of Constructional Steel Research 75 (2012) 131–141

compression ratios (α=N/N0) and S-1 and S-3 had different length towidth ratios of mono column (λ). The main objective of the testwas to study the deformation process and failure modes of SCFST

a) SCFST column structure

b) SCFST columns swathed in the corner

Fig. 2. A photo of SCFST column structure.

columns. The effects of the test parameters were examined on thebehavior. According to the test results, a finite element model wasdeveloped.

2.2. Specimens

The details of the SCFST column specimens are shown in Fig. 3.Table 1 gives a summary of the specimens. The test parameterswere axial compression ratio (α) and length to width ratio of monocolumn (λ). The compressive strength of concrete was 25 MPa forall the specimens. The material properties of steel and concretewere obtained by material tests, which are listed in Tables 2 and 3.Based on the results of material tests, N0 and N of each specimencalculated by Eqs. (1) and (2) are shown in Table 1.

N0 ¼ 3� fy Ay þ f cAc

� �ð1Þ

N ¼ α � N0: ð2Þ

2.3. Test setup and procedure

The picture of the test is shown in Fig. 4. Both the details of the testsetup and instrumentation layout for the L-shaped SCFST columnspecimens are shown in Fig. 5. The specimens were fixed at thebase and subjected to a constant axial load and cyclically varyinglateral loading at the top. The axial load was applied and maintained

Fig. 3. Details of specimens.

Table 1Summary of specimens.

Specimen b(mm)

a(mm)

t1(mm)

t2(mm)

Ay

(mm2)Ac

(mm2)fy(MPa)

fc(MPa)

l(mm)

N0

(kN)α N

(kN)

S-1 100 150 4 3 1536 8464 269 17.336 1500 1680 0.4 672S-2 100 150 4 3 1536 8464 269 17.336 1500 1680 0.6 1008S-3 150 150 4 3 2336 20,164 290 17.336 1500 3081 0.4 1232

Table 2Properties of steel tube and connection plate.

Steel t(mm)

fy(MPa)

fu(MPa)

Es(MPa)

εy(μ)

Connection plate 3.00 312 426 148,261 2104Steel tube(D=100 mm)

3.75 269 445 208,305 1291

Steel tube(D=150 mm)

4.00 290 401 176,520 1643

Table 3Properties of concrete.

Concrete grade fcu(MPa)

fc(MPa)

Ec(MPa)

C25 23.2 17.336 19,373

133T. Zhou et al. / Journal of Constructional Steel Research 75 (2012) 131–141

constantly by an axial loading arrangement, which consisted of a3000 kN capacity hollow core hydraulic jack, a reaction frame, andseveral rollers. The rollers were set between the reaction frame andthe hydraulic jack. Because of the tiny friction between the rollerand reaction frame, the roller device could transfer axial loading butcouldn't limit lateral displacements. The column bases of the reactionframe were fixed, so the roller device couldn't rotate, and it couldtransfer the moment. The lateral load was applied by imposing cycli-cally lateral varying displacements at the top of the specimen by a

RollerJack

Specimen

Reaction frame

Jack

Tie down beam

Fig. 4. Picture of

1000 kN capacity bi-directional hydraulic jack. As shown in Fig. 5(b),the lateral force was applied to the centroid O to avoid torsionalrotation.

As shown in Fig. 5(a), the lateral displacement applied to the topof the specimen was measured using a linear variable displacementtransducer. As shown in Fig. 5(b), there were 5 dial meters to mea-sure the displacements at the middle of each mono column (Section3–3) and one dial meter was placed near the steel plate at the bottomto measure the slide of the whole specimen. The purpose of the first 5dial meters are as follows: dial meters 1 and 2 were used to obtainout-plane relative displacement of mono-column A and B; dial meters3 and 4 were used to obtain out-plane relative displacement of mono-columns B and C. In order to study the load-transferring ability of con-nection plate, dial meters 2 and 5 were used to obtain in-plane rela-tive displacement of mono-columns B and C. As shown in Fig. 5(b),because of the lateral load applied along the vertical direction of A–B,the in-plane relative displacement of mono-columns A and B was tinysmall so that no dial meters were set to measure it.

12 strain gauges and 4 strain rosettes were bonded to the two con-nection plate to study the direction of main strain. Additionally, 40strain gauges and 15 strain rosettes were also bonded to the steeltube of each mono column at Sections 1–1, 2–2, 3–3, 4–4, and 5–5to measure the strains in the tubes at different locations along thetest length. Fig. 5(c) shows the strain gauge arrangement of Section1–1, which is the same as the other 4 sections. Fig. 5(a) shows thestrain rosette arrangement of steel at the front side view. Fig. 5(d)shows the strain gauge arrangement of steel at the right side view,

Roller

test set up.

a) Test set up (front view)

b) Dial meter layout

c) Strain gauges layout (section 1-1)

d) Strain gauges layout (right side view)

e) Strain gauges layout (back view)

Fig. 5. Test set up and instrumentation layout.

134 T. Zhou et al. / Journal of Constructional Steel Research 75 (2012) 131–141

which is the same as that at the left side view. Fig. 5(e) shows thestrain rosette and gauge arrangements of steel at the back side view.

After the instrumentation arrangement was finished, the axial com-pressive load of 20% of design axial load N was applied to the top inorder to examine the test setup and instrumentations. The preloadingwas held for 10 min and then released to zero. The axial compressiveload of N was varied gradually with a holding of 10 min between eachstep as shown in Fig. 6(a). Then, the lateral displacement applied tothe top of the specimen was varied as shown in Fig. 6(b). The loadinghistory included elastic cycles and inelastic cycles. The elastic cycleswere conducted under load control at load levels of 0.3 py, 0.7 py, and1.0 py. One cycle was imposed at each of the lateral load levels of 0.3py, 0.7 py. The inelastic levels were taken to lateral displacement levelsofΔy, 1.0Δy, 2.0Δy, 3.0Δy, 4Δy,…. Three cycleswere imposed at each dis-placement level. The testwas completed until the lateral load resistance

decreased below 85% of themaximummeasured lateral load capacity ofthe specimen.

3. Test results

3.1. Deformation and failure modes

The failure modes of the specimens included the local buckling ofthe steel tube at the bottom of the column, the large distortion of theconnection plate, and the slope of the whole column as shown inFig. 7. The local buckling occurred at the bottom of each mono column,the concrete crushed at the bottom and the steel tube bulged out. Thelocal buckling of the left side of the steel tube occurred when the nega-tive direction lateral displacementwas applied and the local buckling ofthe right side occurredwhen the positive direction lateral displacement

a) Axial loading history

b) lateral displacement history

Fig. 6. Loading history.

135T. Zhou et al. / Journal of Constructional Steel Research 75 (2012) 131–141

was applied. The residual deformation of the steel tube was formedwhen the lateral force was released and the residual deformationbecame larger with the increase of lateral displacement. The deforma-tion of mono column C was more serious than those of the other twomono columns. This was because there were no out-plane braces formono column C, while the other two mono columns braced eachother. As the lateral force increased, the circular hole of the connectionplate became ellipse and the edge was ripped. The bulge distortion ofconnection plate between two stiffeners was located along the 45°diagonal.

The degrees of deformation of the three specimens were different.S-2 had the most serious local buckling deformation of steel tube,which was because the axial compression ratio of S-2 was the largest.While S-1 had the most serious distortion of the connection plate andthe slope of the whole column, which indicated that the lateral stiff-ness of S-1 is smaller than that of the other two specimens.

3.2. Hysteretic curve

The experimental lateral load–displacement curve under cyclicloading, which is called hysteretic curve, is an important factor repre-senting the seismic behavior of a structure. The area surrounded bythis curve represents the energy dissipation ability of a structure.

The hysteretic curves of the specimens are given in Fig. 8. The curveshapes of S-1 and S-2 were smooth and full and these shapes belongedto shuttle-shaped, which indicated they had a better seismic ability. Thecomparison between S-1 and S-2 indicated that an increase of axialcompression ratio from 0.4 to 0.6 decreased the ultimate bearing capac-ity, yield displacement, and the area surrounded by hysteretic curves.But the hysteretic curve of S-3 was very asymmetrical. In order toreduce steel consumption, the thickness of the steel tube and connec-tion plates was chosen as thinly as possible. The thicknesses of S-1and S-2 are the same as that of the structure shown in Fig. 2. Thewidth of the steel tube is 100 mm, and the thickness of the connectionplate is 3 mm, respectively, which could meet the need of the project.

In order to make sure the effect of length to width ratio of the monocolumn, the thickness of the connection plate of S-3 didn't changewhile the width of the steel tube increased to 150 mm. Consequently,the thickness of connection plate is relatively thinner, thus collaborativeability of connection plate became weaker. That's why the hystereticcurve of S-3 was very asymmetrical. The positive direction curvebelonged to the shuttle-shaped curve whereas the negative directioncurve belonged to the S-shaped curve, which represented an obviousshear sliding phenomenon.

3.3. Energy dissipation ability

In order to evaluate the accurate energydissipation ability of the spec-imens, energy dissipation ratio E and equivalent viscous damping ratio he[13] were calculated by Eqs. (3) and (4). S(ABC+CDA) and S(OBE+ODF) arethe areas of the shadow as shown in Fig. 9. The curve was one of thecycles of hysteretic curve. The cycle corresponding to the ultimatebearing capacity was chosen in this test. The values for energy dissipa-tion ratio E and equivalent viscous damping ratio he shown in Table 4indicated that an increase of the axial compression ratio and length towidth ratio decreased energy dissipation ability. Furthermore, thevalues of the equivalent viscous damping ratio were compared withfive related references [10,14–17] as shown in Table 5. It can be seenfrom the comparison that the equivalent viscous damping ratio ofSCFST is larger than that of the L-shaped reinforced concrete (RC) col-umn. The same result occurs when compared with the L-shaped steel-reinforce concrete (SRC) column. So the SCFST owns good energy dissi-pation ability.

E ¼ S ABCþCDAð ÞS OBEþODFð Þ

ð3Þ

he ¼ E=2π: ð4Þ

3.4. Envelope of hysteretic curve

Fig. 10 shows the envelopes of the hysteretic curves. The enve-lopes of the hysteretic curves were used to determine the stiffness,ductility, and the bearing capacity. The envelopes of hysteretic curveof S-1 and S-2 had gently descending stages, which represented agood ductility. The increasing and descending stages of the envelopecurve of S-1 were gentler and the peak was larger than that of S-2.From the comparison of envelopes of the two specimens, S-1 had asmaller stiffness and larger ultimate bearing capacity than those ofS-2. The comparison of envelopes between S-1 and S-3 showed thatS-3 had a larger stiffness and ultimate bearing capacity, but the enve-lope curve of S-1 was much smoother. This indicated that S-1 had abetter ductility. In conclusion, the stiffness became larger with an in-crease of axial compression ratio and a decrease of length to widthratio whereas the ductility became larger with a decrease of axialcompression ratio and an increase of length to width ratio.

3.5. Ductility

In order to evaluate accurate information on the ductility of speci-mens, displacement ductility ratio μ was calculated by Eq. (5). Becausethere was not an apparent yield point on the envelopes of hystereticcurves, δu and δy cannot be directly obtained from the above curves.Generally speaking, there are twoways to determine δywhen the enve-lope curve has no apparent yield point: general yield moment methodand equivalent energy method [13]. The former one is given byFig. 11(a), the tangent line of the curve went through Origin O, thetangent line of the curve at the peak Point G. These two lines intersectedat the Point H. A vertical line through Point H intersectedwith the curveat Point I. The line between Point O and Point I intersected with the

a) Local buckling of S-1 b) Distortion of connection plate c) Slope of S-1

d) Local buckling of S-2 e) Distortion of connectionplateof S-2

of S-3

of S-1

f) Slope of S-2

g) Local buckling of S-3 h) Distortion of connection plate i) Slope of S-3

Fig. 7. Failure modes.

136 T. Zhou et al. / Journal of Constructional Steel Research 75 (2012) 131–141

tangent of the curve through peak point G at Point H′. The vertical linethrough point H′ intersects with the curve at Point B, which is the ap-proximate yield point. Generally speaking, the order of general yieldmoment method is: tangent line OH→tangent line GH→vertical lineHI→ line OI→Point of intersection H′→vertical line H′B→yield pointB. As shown in Fig. 11(a), the lines intersecting a process need to repeattwice. The latter one is shown in Fig. 11(b), the approximate yield pointB is achieved when the area of shadow ① equals to that of shadow ②.The disadvantage of the lattermethod is that there aremany approaches,so this test has chosen the former one to ensure δy. δu was set to designultimate lateral displacement, which is the displacement correspondingto the 0.85 Pu. The ultimate lateral displacement δu, the yield lateral dis-placement δy, and the displacement ductility ratio μ are given in Table 6.Table 6 shows that the ductility became larger with a decrease of axial

compression ratio and an increase of length to width ratio. Additionally,the ductility ratios of two direction forces of S-3 were greatly different,which represented an obvious asymmetry of S-3.

μ ¼ δuδy

ð5Þ

where, δu is ultimate lateral displacement and δy is yield lateraldisplacement.

3.6. Bearing capacity of specimens

Table 7 shows the yield strength Py and ultimate strength Pu of thethree specimens under different directional lateral forces. The results

-300-250-200-150-100-50

050

100150200250300350

-80 -60 -40 -20 0 20 40 60 80

a) Hysteretic curve of the S-1

-300-250-200-150-100-50

050

100150200250300350

-80 -60 -40 -20 0 20 40 60 80

b) Hysteretic curve of the S-2

-300-250-200-150-100-50

050

100150200250300350

-80 -60 -40 -20 0 20 40 60 80

c) Hysteretic curve of the S-3

Fig. 8. Hysteretic curve.

B

EC

P

X

D

AF o

Fig. 9. Energy dissipation ratio calculation diagram.

Table 4Energy dissipation ratio and equivalent viscous damping ratio.

S(ABC+CDA) S(OBE+ODF) E he

S-1 15135.4 9453.7 1.60 0.255S-2 9643.5 6571.8 1.47 0.234S-3 8040.8 8899.2 0.904 0.144

137T. Zhou et al. / Journal of Constructional Steel Research 75 (2012) 131–141

showed that the behavior of the specimen under two different direc-tion forces was asymmetrical. The strength under positive directionlateral displacement was a little larger than that under negative direc-tion. That was because of the asymmetry geometry of the columnsection shape. When the positive direction lateral displacement wasapplied, two mono columns (B and C) were compressed and bracedeach other. However, when the negative direction lateral displace-ment was imposed, only mono column A was compressed and therewas no out-plane braces. So the positive bearing capacity was largerthan the negative bearing capacity. From the comparison shown inFig. 12, it could be concluded that the strength decreased with anincrease of compression ratio and length to width ratio.

3.7. Coordination of mono column of SCFST columns

In order to study the coordination of mono column, the dial meterswere set up as shown in Fig. 5(b) to study the relative displacement ofeach mono column. The responses of three specimens were similar toeach other. Take S-1 for example, the results of dial meters are shownin Fig. 13. The value of dial meter 1 agreed well with that of dial meter2, the value of dial meter 3 agreed well with dial meter 4. The littledifference indicated that the coordination of mono SCFST columnswas good.

3.8. Strain distribution

Many strain gauges and strain rosettes were bonded to the steeltubes and connection plates as shown in Fig. 5. Fig. 14 shows thevertical strain distribution on the right side view of mono column Aof S-1. Fig. 14 shows that the strains at the top of column and thebottom of column were similar but in opposite directions and thestrains at the middle of column were much lower. The shape of thecurves indicated that the boundaries of the two sides of the columnwere fixed and plastic hinges were located at the middle.

Strain distribution on the connection plate was studied using thevalues of the strain rosettes bonded to it as shown in Fig. 15(a). The

Table 5Comparison of equivalent viscous damping ratios.

Specimen Type Geometrical size(mm) Cycle number(Δ/Δy)

he

S-1 in thispaper

L-shapedSCFST

Section size 350×350×100(length×width×thickness)Height: 1500 mm

3 0.255

Reference[10]

Circular CFTcolumn

Diameter: 330Tube thickness: 6Height: 1500

3 0.3

Reference[14]

Square CFTcolumn

Width: 200Tube thickness: 1.48Height: 1200

3 0.17

Reference[15]

Square SRCcolumn

Width: 200Height: 1500

3 0.25

Reference[16]

L-shaped RCcolumn

Section size 350×350×125(length×width×thickness)Height: 1050

3 0.21

Reference[17]

L-shaped SRCcolumn

Section size 280×280×80(length×width×thickness)Height: 560

3 0.18

a) General yield moment method

b) Equivalent energy method

Fig. 11. Method for searching approximate yield point.

-400-300-200-100

0100200

300400

1 2 3

Specimen number

Py(+)

Py(-)

Pu(+)

Pu(-)

Bea

ring

cap

acity

(kN

)

Fig. 12. Bearing capacity of specimens.

Table 7Bearing capacity.

Specimen α λ Py (kN) Pu (kN)

S-1 0.4 15 178.5 −168 204 −193S-2 0.6 15 161 −144.2 180 −177.5S-3 0.4 10 266.2 −224 303 −288.5

30

40Dial meter 1Dial meter 2Dial meter 3

)

-350

-250

-150

-50

50

150

250

350

-80 -60 -40 -20 0 20 40 60 80L

oad

(kN

)

Displacement (mm)

S-1

S-2

S-3

Fig. 10. Envelopes of hysteretic curves.

138 T. Zhou et al. / Journal of Constructional Steel Research 75 (2012) 131–141

reference axis of gauge rosette is shown in Fig. 14. The strain rosetteconsisted of three strain gauges with the degrees of 0°, 45°, and 90°(Fig. 16). The 90°strain gauge paralleled with the length of the steeltube. The principal stress and principal strain could be evaluated byEqs. (6)–(8). The θp of all strain rosettes ranged from 35° to 45°. Thebulge distortion of the connection plate between two stiffeners waslocated along the 45° diagonal as shown in Fig. 7(b). The connection

Table 6Displacement ductility ratio of specimens.

Specimen α λ δy (mm) δu (mm) μ(+) μ(−)

S-1 0.4 15 27.0 −24.8 73.8 −67.2 2.73 2.71S-2 0.6 15 19.7 −17.4 50.1 −44.4 2.54 2.55S-3 0.4 10 17.8 −25.3 45 −40.2 2.53 1.59

plate could be reduced to lacing bar with a degree of 45°and the stiff-ener could be reduced to lateral lacing bar as shown in Fig. 15(b).

εx ¼ ε0εy ¼ ε90

γxy ¼ 2ε45−ε0−ε90

8<: ð6Þ

ε1ε2

¼ εx þ εy2

�ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiεx−εy

2

� �2þ γxy

2

� �2s

ð7Þ

tg2θp ¼ γxy

εx−εy: ð8Þ

4. Finite element analysis

A finite element model was further established. The behavior ofthe SCFST columns was rather complicated due to non-linearity ofinner concrete and the interaction between concrete and steel tube.Therefore, a finite element model could provide a powerful way tosimulate the behavior of the SCFST columns subjected to combinedconstant axial load and cyclic flexural loading. The specimens weremodeled and analyzed in this paper using the commercial finite elementsoftware ANSYS.

4.1. Modeling of SCFST columns

There were four main components that need to be modeled inorder to simulate the behavior of the SCFST columns. These werethe steel tubes, connection plate, the inner-filled concrete, and theinterface between the concrete and the steel tube. In addition, the

-30

-20

-10

0

10

20

1 11 21 31 41 51 61 71

Load step

Dial meter 4

Dis

plac

emen

t (m

m

Fig. 13. Information of dial meters of specimen-I.

-3000-2500-2000-1500-1000-500

0500

10001500

2 4 6 8 10

+5Δ-5Δ+4Δ-4Δ+3Δ-3Δ-2Δ-2Δ+Δ-Δ

Strain gauge number

top of column bottom of column

stra

in

Fig. 14. Vertical strain distribution.

a) Strain gauges bonded to connection plate

b) Reduced graph

Fig. 15. Connection plate between two stiffeners.

a) Model of steel tube and connection plate

b) Model of concrete

Fig. 17. Model of CFST columns.

139T. Zhou et al. / Journal of Constructional Steel Research 75 (2012) 131–141

selection of the element type, the mesh size, the initial geometric de-formation, the boundary conditions (fixed at the top and the bottomof column), and the load application were also important in simulat-ing the SCFST columns. The modeling method was reported in a

Fig. 16. Reference axis.

previous literature [12]. The model of SCFST columns are shown inFig. 17.

4.2. Comparison of results

The failure mode of the specimens obtained by the finite elementanalysis is shown in Fig. 18. It was observed that the deformationagreed well with that of the test specimens given in Fig. 7. The mostserious local buckling appeared at the bottom of the mono columnC. The circular hole of the connection plate became ellipse and thebulge distortion of connection plate between two stiffeners was locatedalong the 45° diagonal. The slope of the whole column varied with thelateral displacement. The envelopes of hysteric curves were obtainedby experiments and finite element analysis, which are given in Fig. 19and Table 8. There were some differences between the results of test

a) Local buckling of steel tube

b) Distortion of the connection plate

c) Slope

Fig. 18. Failure mode.

a) S-1

b) S-2

c) S-3

Fig. 19. Comparison of envelopes of specimens.

140 T. Zhou et al. / Journal of Constructional Steel Research 75 (2012) 131–141

and finite element method. The yield displacement obtained by test waslarger than that obtained by FEM, this was because the gaps between thetest set up and the specimens were compacted. The reason that theultimate bearing capacity obtained from test was a little larger thanthat obtained from FEMwas that the constitutive curve of steel and con-crete has a difference with the real. In conclusion, considering the aboveresidual, the finite element analysis agreed with the experiment.

Table 8Bearing capacity.

Specimen Method +Δu (mm) +Pu (kN) −Δu (mm) −Pu (kN)

S-1 Test 40 204 −38.5 −193FEM 20 159 −20 −158

S-2 Test 30.1 180 −37.2 −178FEM 15 148 −15 −145

S-3 Test 32.2 303 −40.2 −289FEM 15 351 −15 −347

5. Conclusions

The behavior of the SCFST columns subjected to combined constantaxial load and cyclic flexural loading was experimentally investigated.The effects of axial compression ratio and length to width ratio ofthe SCFST columns were studied. Three SCFST column specimenswere tested. The experimental results were compared with the resultsfrom finite element analysis.

These experimental results and the comparisons indicated asfollows.

(1) When subjected to lateral load, the mono-columns of SCFSTworked together with the connection plate. The load wastransferred from one mono-column to another mono-columnthrough the connection plate which was one of the mostimportant shear resisting members. Additionally, the resultsof dial meters indicated that the mono columns of SCFSTcould work together well.

(2) SCFST columns had a characteristic of asymmetry because twoof the mono-columns braced each other when subjected tolateral load. Whereas, the other one had no bracing in the out-plane direction. Additionally, because the connection plate thick-ness was relatively too thinner than the width of steel tube, S-3had an obvious asymmetry property.

(3) The shape of hysteric curves and the envelopes of the SCFSTcolumns exhibited a good seismic ability.

(4) The results of dial gauges represented that the boundaries ofthe two sides of column were fixed. Plastic hinges were locatedat the middle. The connection plate and stiffeners could bereduced to lacing bars.

(5) The comparisons of the test results of the three specimenswere as follows: an increase of the axial compression ratioincreased stiffness and decreased energy dissipation ability,ductility, and bearing capacity. An increase of the length towidth ratio increased energy dissipation ability and ductilityand decreased stiffness and bearing capacity.

(6) The failure mode and bearing capacity obtained by finiteelement analysis agreedwith those obtained by test. So the finiteelement analysis was an effective way to simulate the behaviorof the SCFST columns.

Nomenclatureα axial compression ratio, α=N/N0

λ length to width ratio of mono columna distance between mono columnsb width of mono columnt1 thickness of steel tubet2 thickness of connection platet thickness of steel in material testsAy steel area of mono columnAc concrete area of mono columnl length of columnN0 axial compressive strength of short SCFST columnsN axial compression applied to specimenEs Young's modulus of steelEc Young's modulus of concretefy yield strengthfu ultimate strengthεy yield strainfcu axial compressive strength of concrete cube with the width

of 150 mmfc axial compressive strength of concrete cuboids with the

size of 150 mm×150 mm×300 mmpy estimated lateral yield load capacity of specimens according

to FEM analysisΔy yield displacement corresponding to py

141T. Zhou et al. / Journal of Constructional Steel Research 75 (2012) 131–141

E energy dissipation ratiohe equivalent viscous damping ratioμ displacement ductility ratioδy yield displacement calculated according to envelopes of

hysteretic curvesδu ultimate displacement calculated corresponding to the

0.85PuPy yield strength correspond to δyPu ultimate strength (extreme value of envelopes of hysteretic

curves)Δu lateral displacement corresponding to Puε0 the value of lateral strain gauge;ε90 the value of vertical strain gaugeε45 the value of strain gauge with degree of 45°εx, εy normal strainsγxy shearing strainε1, ε2 principal strainθp the angle between the first principal and X axis

Acknowledgments

This work is sponsored by the key project in the Tianjin Science &Technology Pillar Program (Grant No: 11ZCGYSF01000) and theConstruction Office of Hebei Province (Grant No: 2010-153).

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