han2015_2

8172019 han2015_2

httpslidepdfcomreaderfullhan20152 116

Performance of concrete-encased CFST box members under bending

Lin-Hai Han a Yu-Feng An a Charles Roeder b Qing-Xin Ren c

a Department of Civil Engineering Tsinghua University Beijing 100084 PR Chinab Department of Civil Engineering University of Washington Seattle WA 98195-2700 USAc School of Civil Engineering Shenyang Jianzhu University Shenyang 110168 PR China

a b s t r a c ta r t i c l e i n f o

Article history

Received 16 September 2014

Accepted 9 December 2014Available online xxxx

Keywords

Concrete-1047297lled steel tube (CFST)

Concrete-encased

Box

Bending

Finite element analysis (FEA)

Flexural capacity

Thispaperdescribeda seriesof tests and1047297nite element analysis (FEA) on large-scale concrete-encased concrete-

1047297lled steel tube (CFST) box members under bending Eight full-scale specimens including six concrete-encased

CFST box and two corresponding reinforced concrete (RC) box members were testedto investigate the in1047298uence

of variations in diameter of steel tube (from 749 mm to 1022 mm) and sectional height (from 840 mm to

1260 mm) on the performance of concrete-encased CFST box members under bending The failure modes and

performance of concrete-encased CFST box members under bending were investigated and compared with

those of corresponding RC box members A 1047297nite element analysis (FEA) model was developed to analyse

the 1047298exural performance of concrete-encased CFST box members including full-range analysis of momentndash

curvature relation loading transfer and effect of shear-span-to-depth ratio Finally a parametric study was

carried outbased on theFEA modellingand a simpli1047297ed model was proposed for predicting the1047298exural capacity

of concrete-encased CFST box members

copy 2014 Elsevier Ltd All rights reserved

1 Introduction

Concrete-encased concrete-1047297lled steel tubes (CFST) are gainingincreasing usage in China due to their good mechanical performance

(Han and An [1] An and Han [2]) Reinforced concrete (RC) box mem-

bers have been used extensively in bridges due to their high torsional

and bending strength and stiffness as well as low self-weight (Rasmus-

sen andBaker [3]) Recently concrete-encased CFST and RC box member

concepts have been combined and utilized in bridge construction in

China which is called concrete-encased CFST box member as shown in

Fig 1 where concrete-encased CFST box members are mainly used as

arches or piers because of their high strength and stiffness better ductil-

ity and durability and small sectional size (An et al [4] An et al [5])

In practice a concrete-encased CFST box member is mainly subjected

to axial compression or combined axial compression and bendingwhen it

is used as an arch or pier While a concrete-encased CFST box member is

seldom subjected to pure bending they can be used as bridge girders due

to their high bending strength and stiffness but there are no known

examples of this used in practice at present Pure bending is the extreme

case of beam-column behaviour in which there is no axial load As a result

1047298exural capacity is an important reference point in the compression load-

bending moment interaction diagram and 1047298exural stiffness is a critical

component of the behaviour of these composite members under

combined axial compression and bending Therefore it is important to

understand the behaviour of these composite members under bending

Some previous research has been carried out on concrete-encased

CFST members and RC box beams under bending (An et al [6] Wang

[7]) and (Galal and Yang [8] Rasmussen and Baker [3] Yuan et al [9])An et al [4] and An et al [5] performed experimental and theoretical

investigation of eccentrically loaded concrete-encased CFST box

columns respectively The failure modes full-range loadndashde1047298ection

response the parameters in1047298uencing member behaviour and strength

calculation of the concrete-encased CFST box column were investigated

in these studies It is expected that the failure modes and loading trans-

fer mechanism of the composite member under bending are different

from those of the composite eccentrically-loaded column

This paper thus describes a series of testsand1047297nite element analysis

on the behaviour of concrete-encased CFST box members under

bending The main objectives of the research work are threefold

(1) to present test results of concrete-encased CFST box members

under bending including the failure mode and load versus deformation

relations (2) to study the behaviour of composite members under

bending using the veri1047297ed numerical model and (3) to provide a para-

metric study of the 1047298exural capacity and stiffness and develop a simpli-

1047297ed model to predict the 1047298exural capacity of these composite members

2 Experimental investigation

21 Experimental programme

Eight large-scale specimens including six concrete-encased

CFST box members and two corresponding RC box members were

Journal of Constructional Steel Research 106 (2015) 138ndash153

Corresponding author Telfax +86 10 62797067

E-mail address lhhantsinghuaeducn (L-H Han)

httpdxdoiorg101016jjcsr201412011

0143-974Xcopy 2014 Elsevier Ltd All rights reserved

Contents lists available at ScienceDirect

Journal of Constructional Steel Research

8172019 han2015_2


tested The test parameters for the composite member specimens

included

(a) Diameter of steel tube (D) from 749 mm to 1022 mm

(b) Sectional height (H ) from 840 mm to 1260 mm

Table 1 summarizes the specimen dimensions and properties The

specimen labels listed in Table 1 are de1047297ned as follows (1) The initial

character ldquoBrdquo indicates the box section (2) The second character is an

ldquoHrdquo (if any) for specimens without inner CFST (3) The 1047297rst Arabic nu-

meral stood for the different group with the same section type and

(4) The second Arabic numeral (if any) meant the different specimen

in the same group All specimens had the same 1047298ange and web thick-

ness of 87 mm and 66 mm respectively All specimens had longitudinal

reinforcement of 64 mm diameter and stirrups with 64 mm diameter

and 65 mm spacing Detailed information for size geometry and rebar

placement in cross section is shown in Fig 2 All the specimens were6 m in length The 1047297rst and second groups of specimens with the label

ldquoB1rdquo and ldquoB2rdquo had the same dimension of cross section (B times H ) but

different steel tubes The second and fourth groups of specimens with

the label ldquoB2rdquo and ldquoB4rdquo had the same inner CFST and width of cross

section but the sectional height (H ) was different

Standard tensile coupon tests were conducted to measure the mate-

rial properties of the steel tubes and rebar The measured average yield

strength ( f y) the ultimate strength ( f u) the modulus of elasticity (E s)

and Poissons ratio are listed in Table 2 Two different self-consolidating

concrete (SCC) mixes were used with the higher strength concrete

used as concrete 1047297lled inside the CFST components The maximum size

of coarse aggregate was 25 mm Six 150 mm cubes were cast for each

batch of concrete and cured in conditions similar to that of the related

specimens and tested to measure the compressive strength of the

concrete The physical properties of the concrete are given in Table 3

To construct the specimen the cold-formed steel tubes of inner CFST

initially were cut and machined to the required length 1047297rst and the

outside surfaces of the tubes were brushed to remove any rust and

loose debris Then the four tubes were welded to two 10 mm thick

steel end plates to achieve the design geometry Four holes were

drilled into one of the end plates for casting concrete The steel tubes

were sloped at about 30deg to the horizontal during concrete placement

and self-consolidating concrete (SCC) was poured into the inner steel

tubes As soon as the inner CFST was prepared the rebarswere installed

and outer concrete waspoured Theouter RC componentwas fabricated

by two steps ie (1) the bottom 1047298ange and parts of the webs under

the middle 1047297rst and (2) then the top 1047298ange and remainder of the

webs

A general view of the test setup is given in Fig 3 The load wasapplied by a 5000 kN capacity hydraulic jack through a rigid steel

beam with four-point loading In-plane displacements were measured

at locations along the specimen by three displacement transducers

as shown in the 1047297gure Strain gauges were used for each specimen to

measure strains in the steel tube and longitudinal bar at the mid-span

of each specimen as shown in Fig 2 A load increment of 120th of the

estimated ultimate load was used for each load step and each step

was held constant for about 2 min The strain load and the de1047298ection

measurements were automatically recorded

Nomenclature

A Cross-sectional area of the whole section

Acore Cross-sectional area of core concrete in CFST

Ahol Cross-sectional area of the hollow part

Al Cross-sectional area of longitudinal bar

As Cross-sectional area of steel tube of CFST

Asc Cross-sectional area of CFST (= Acore + As)

Aout Cross-sectional area of outer concreteav Length of shear span

B Sectional width

D Outer diameter of steel tube

Di Diameter of core concrete

E c Elastic modulus of concrete

E s Elastic modulus of steel

f cu Concrete cube strength

f c Concrete cylinder compressive strength

f yl Yield strength of longitudinal bar

f ys Yield strength of steel tube

f u Ultimate strength of steel

H Sectional height

L Length of the specimen

M Moment

M cr Moment when cracks occur

M rc Moment of the outer box RC component

M cfst Moment of the inner CFST component

M ser Moment at the serviceability limit state

M u The ultimate moment

M y Yield moment

N cfst Load of the inner CFST component

N rc Load of the outer box RC component

SI Strength index given by M ue minus cecfst M ue minus rc

α l Longitudinal bar ratio (= Al Aout + Al)

α s Steel ratio of CFST (= As Acore)

w Width of concrete crack

um Mid-span de1047298ection

ε Strain

ξ Con1047297nement factor frac14 As f ys Acore f ck

of the inner CFST component

Fig 1 Practical applications of concrete-encased CFST box members

139L-H Han et al Journal of Constructional Steel Research 106 (2015) 138ndash153

8172019 han2015_2


22 Experimental results and discussions

221 Failure mode

Fig 4 shows the concrete crack patterns and indirectly the yield

mechanism and ultimate failure mode of all tested specimens Speci-

mens B1-1 B2-1 and B2-2 with H of 1260 mm showed a 1047298exural-

shear failure mode where uniformly distributed vertical cracks in the

pure bending segment and diagonal cracks along the line connecting

load and support points occurred in the shear span The vertical cracks

developed 1047297rst in the pure bending segment and then diagonal cracks

occurred in shear span which led to a reduction in load capacity The

longitudinal bars and steel tubes had tensile yielding in the pure

bending segment Specimen B1-2 was the same as Specimen B1-1 and

had uniformly distributed vertical cracking in the pure bending

segment but the test was stopped due to loosening of the ground

anchor during the experiment before Specimen B1-2 failed There was

no obvious difference of the failure modes of Specimen B1 and B2

with different D and thesame H SpecimenB4-1 had a 840 mmsectional

height and it developed a 1047298exural failure mode with uniformly distrib-

uted vertical cracks in the pure bending segment and very small

diagonal cracks in the shear span The longitudinal bar and steel

tube yielded in the tension and the concrete crushed in compression

in the middle segment The shear-span-to-depth ratio of B1-1 wassmaller than that of B4-1 and shear dominated the behaviour of

specimens with small shear-span-to-depth Therefore the failure

modes of specimens with different H were different Specimen B4-2

failed unexpectedly due to the crushed concrete in the concrete

construction joint in the shear span The RC box specimens BH3 and

BH5 both had a 1047298exural failure mode where obvious vertical cracks

occurred in the pure bending segment but no obvious diagonal cracks

were found in the shear span Diagonal cracks occurred in the shear

span in the composite specimens with H of 1260 mm while the corre-

sponding RC specimen had no diagonal cracking The reason was that

the 1047298exural capacity of the composite specimens increased signi1047297cantly

due to the contribution of inner CFST component in the corners

compared with that of corresponding RC specimens While the shear

capacity of the composite specimens where the webs provide the

main contributions to the shear had no corresponding increase due to

the CFST components in the corners The webs in the shear span of

the composite specimens failed due to shear which led to a decrease

in the load

Fig 5 shows the typical 1047297nal condition of the exposed steel tube and

the core concrete of the inner CFST for Specimen B4-1 Due to the

constraint provided by the core concrete and the encasement provided

by theouter concrete no local buckling wasobserved in any of theinner

steel tubes as shown in Fig 5(a) Fig 5(b) shows that the core concrete

of the inner CFST remained intact due to the con1047297nement of the steel

tube A few tension cracks were distributed uniformly in the tensile

zone of the core concrete

222 Behaviour analysis

The measured moment in the mid-span section (M ) versus mid-

span de1047298ection (um) curves are shown in Fig 6 The um of B1-2 was

smaller than that of B1-1 with the same parameters because the test

Table 1

Specimen information and test results

No Specimen

label

Section

dimension

B times H (mm)

Inner tube

D times t (mm)

M cr

(kN m)

M ser (kN m) M y(kN m)

M ue

(kN m)

M serM ue

SI Simpli1047297ed method

M uc M uc M ue

1 B1-1 956 times 1260 749 times 26 370 580 870 1370 0423 1343 1238 0904

2 B1-2 956 times 1260 749 times 26 365 560 860 1347 0416 1321 1238 0919

3 B2-1 956 times 1260 1022 times 29 372 798 1233 1637 0487 1605 1380 0843

4 B2-2 956 times 1260 1022 times 29 366 798 1190 1609 0496 1577 1380 0858

5 BH3 956 times 1260 ndash 311 508 580 1020 0498 ndash ndash ndash6 B4-1 956 times 840 1022 times 29 179 362 670 924 0392 2174 795 0860

7 B4-2 956 times 840 1022 times 29 180 362 650 894 0405 2104 795 0889

8 BH5 956 times 840 ndash 160 217 260 425 0511 ndash ndash ndash

Mean 0879

Standard deviation 0030

Fig 2 Dimension of box section (unit mm)

140 L-H Han et al Journal of Constructional Steel Research 106 (2015) 138ndash153

8172019 han2015_2


for B1-2 was stopped due to loosening of the ground anchor during theexperiment Specimen B4-2 had less ductility than Specimen B4-1 with

the same parameters because the concrete crushed in the concrete

construction joint in the shear span due to a problem of the concrete

construction

For concrete-encased CFST box members with the 1047298exural-shear

failure mode (H = 1260 mm) the M ndashum curve could be generally divid-

ed into four stages as shown in Fig 7(a) Four characteristic points were

de1047297ned for the convenience of comparison and analysis ie Point A

indicates initial cracking of the outer concrete Point B indicates initia-

tion of tensile yielding of the bottom steel tube Point C indicates the

ultimate moment and Point D indicates the initiation of the unloading

due to the decrease in moment arising from diagonal cracks in the

shear span As shown in Fig 7(a)PointsA and B alsoare pointsof reduc-

ing stiffness and increasing de1047298ection For concrete-encased CFST box

members with 1047298exural failure mode (H = 840 mm) M ndashum curve

could be generally divided into three stages as shown in Fig 7(b)

Three characteristic pointswere de1047297nedfor theconvenience of compar-

ison and analysis ie Point A indicates initial cracking of the outer

concrete Point B indicates tensile yielding of the bottom steel tube

and Point C was the start of the unloading due to specimen damage

As before Points A and B correspond to reduced stiffnessand increasing

de1047298ection The moment did not decrease in the whole loading process

of these two specimens

The comparison of in1047298uence of D on M ndashum relationships is shown in

Fig 7(a) The in1047298uence of D on the stiffness at the 1047297rst two stages (from

Point O to B) was not signi1047297cant Increased D increased the moments at

Point B and PointC and thetangent modulus stiffness in this third zone

was somewhat larger The maximum vertical crack width (w) also is

shown in Fig 7 and w decreased with increased D at the same loadFig 7(b) shows the in1047298uence of H on M ndashum relationships As H

increased the stiffness and moments at Point B and Point C increased

The ductility of the members decreased signi1047297cantly with increasing H

as can be seen by comparing the curves for Specimens B2-1 and B4-1

As H increased the maximum vertical crack width of concrete crack

decreased at the same load

Fig 8 gives typical moment (M ) versus strain (ε s) of the extreme

1047297bre of the bottom tensile steel tube at mid-span When vertical cracks

occurred in the middle zone the strain began to increase rapidly

because of the reduced stiffness due to concrete cracking as noted

earlier When the bottom tensile steel tube achieved tensile yielding at

Point B stiffness decreased and deformation increased appreciably

The gradual development in yield of tensile steel tubes and longitudinal

bars in the web led to an increase in the moment after the bottomtensile steel tube yielded

223 Comparison between composite and RC members

Fig 9 compares M ndashum curves of the concrete-encased CFST box and

corresponding RC members There was no signi1047297cant difference in the

M ndashum relations of thecomposite and correspondingRC members before

Point A but the stiffnessof thecomposite members was largerthan that

of the corresponding RC members after Point A Further the um corre-

sponding to the ultimate moment of the composite member with H

equal to 1260 mm was smaller than that of corresponding RC memberbecause shear cracking limited the deformation capacity Diagonal

shear cracking was not noted in the corresponding RC member The

um corresponding to the ultimate moment of composite member with

H equal to 840 mm was larger than that of the corresponding RC mem-

ber No obvious diagonal crack occurred in shear span in both composite

and corresponding RC members for the 840 mm depth This indicates

that the strength and ductility of the composite members were larger

than those of the corresponding RC members if 1047298exural failure modes

occurred The 1047298exural capacity of the concrete-encased CFST increased

signi1047297cantly due to the contribution of inner CFST compared to that of

the corresponding RC members To illustrate this observation compar-

ison of the maximum resistance of B4-1 was very similar to BH3 even

though BH3 was50 deeper than B4-1 The CFST signi1047297cantly increased

thearea of the tensile reinforcement andthe concretewithinthe core of the CFST had better con1047297nement and increased compressive capacity

The shear capacity of composite members did not increase correspond-

ingly because the web area and shear reinforcement were identical for

all compositeand RC specimens Forthe composite members withsmall

shear-span-to-depth ratio where shear capacity and shear cracks

dominated the behaviour the shear capacity must be increased if a

ductile 1047298exural failure mode is to be achieved

224 Ultimate strength

The moments at Point A and Point B were de1047297ned as M cr and M y

respectively The moment at serviceability limit state and the ultimate

moment were de1047297ned as M ser and M ue respectively In this paper

the serviceability limit state for RC was also used for the composite

members under bending The serviceability limit state was de1047297ned asthe maximum um smaller than 1200 of the member length (L) and

with the maximum crack width smaller than 02 mm according to

GB50010-2010 [10] In the test the moment when the maximum

width of cracks was 02 mm was smaller than that when the mid-span

de1047298ection was L200 as shown in Fig 7 While the moment did not fall

in the whole loading process for Specimen B4-1 M ue was de1047297ned as

Table 2

Material properties of steel

Steel type (mm) f y (MPa) f u (MPa) E s (MPa) γ

Circle tube (D times t = 749 times 26) 325 410 221 times 105 0302


Rebar (d = 64) 290 462 220 times 105ndash

Table 3

Material properties of concrete

Concrete type Cement

(kgm3)

Coarse aggregate

(kgm3)

Fine aggregate

(kgm3)

Fly ash

(kgm3)

Breeze

(kgm3)

Water

(kgm3)

Water reducer

(kgm3)

f cu 28day

(Nmm2)

f cu

(Nmm2)

C50 290 1090 740 65 85 170 63 523 617

C70 400 960 720 172 ndash 144 69 584 742

Fig 3 Arrangement of test specimens (unit mm)


8172019 han2015_2


a)

b)

c)

d)

e)

f)

g)

h)

Fig 4 Failure modes of specimens


8172019 han2015_2


the moment at the extreme 1047297bre strain of the steel tube at tension

of 001 because the moment remained nearly stable after that pointThe de1047297nition of the ultimate moment of concrete-encased CSFT

box member under bending was the same with those of CFST and

concrete-encased CFST described in Lu et al [11] and An et al [6]

respectively Though the test of Specimen B1-2 was stopped due to

loosening of the ground anchor the peak moment of Specimen B1-2

was almost the same with that of Specimen B1-1 Thus Specimen B1-

2 is included in the strength discussion The characteristic moments

are shown in Table 1

M cr was about 20ndash30 of M ue for the concrete-encased CFST box

members An increase in H led to an increase in M cr while there was

no signi1047297cant in1047298uence of diameter of steel tube on M cr M cr of the

concrete-encased CFST box members was almost the same with that

of the corresponding RC members M ser was about 40ndash50 of M ue for

the concrete-encased CFST box members An increase in D and H led

to an increase in the M ser M ue ratio M ser M ue of the concrete-encased

CFST box members was a little smaller than that of the corresponding

RC members but M ue was signi1047297cantly larger for the concrete-encased

CFST M y was about 65ndash75 of M ue for the concrete-encased CFST

box members An increase in D and H led to an increase in M y M ue

For convenience of analysis a strength index (SI ) which represents

the in1047298uence of inner CFST component on the ultimate moment is

de1047297ned as follows

SI frac14 M ue‐cecfst

M ue‐rc

eth1THORN

where M ue-cecfst and M ue-rc were the ultimate moments of the concrete-

encased CFST box and corresponding RC members

The values of SI are shown in Table 1 The in1047298uence of inner CFST on

the ultimate momentwas signi1047297

cant Theminimum value of SI was132for test specimen B1-2 and the maximum value was 217 for test

specimen B4-1 The moment capacity of the concrete-encased CFST

box members increased at least 30 compared with the corresponding

RC members due to the inner CFST component with the parameter

limits of this research SI increased as D increased while SI decreased

as H increased

3 Finite element analysis (FEA) modelling

31 General description and veri 1047297cation

The above tests on concrete-encased CFST box members under

bending enhance the understanding of failure modes loading and

deformation However other characteristics of composite membersincluding the stress distributions of steel and concrete interactions

between steel tube and concrete loading transfer mechanism and

other parameters affecting the 1047298exural capacity and stiffness need to

be analysed by 1047297nite element analysis (FEA) modelling Thus a FEA

model on concrete-encased CFST box members under bending

(shown in Fig 10) was developed with the ABAQUSStandard module

(Hibbitt et al [12]) This model is based on the work presented by An

et al [6] and Han and An [1]

The material models of the concrete and the steel are the same with

those provided in An et al [6] Three different concrete models are used

to simulate the different con1047297nement conditions in concrete-encased

CFST box member ie outer un-con1047297ned concrete outer con1047297ned

concrete in the corner of the box and core concrete in the steel tube as

shown in Fig 10(a) The un-con1047297ned concrete includes the concrete

Fig 5 Typical failure mode of the inner steel tubes and core concrete


8172019 han2015_2


outside of the stirrup in the corner and the concrete in the wall of box

section The con1047297nement provided by stirrup to the concrete in the

wall is not considered in the model because the width to the thickness

of the wall is larger than 2 and Hoshikuma et al [13] recommended

thecon1047297nement effects of this concrete be excluded The uniaxial com-

pressive stressndashstrain relations of outer un-con1047297ned concrete outer

con1047297ned concrete in the corner and core concrete in the steel tube are

proposed by Attard and Setunge [14] Han and An [1] and Han et al

[15] respectively The steel tube rebar and concrete are modelled by

the four-node conventional shell 2-node truss and 8-node 3-D solid

elements respectively ldquoHard contactrdquo in the normal direction and a

MohrndashCoulomb friction model in the tangential direction are used at

the contact between concrete and steel tube surfaces (An et al [6])

Different grid sizes are evaluated to determine an appropriate mesh as

shown in Fig 10 The composite members are loaded under four-point

loading method Due to the symmetry of loading and geometry only

(a) (b)

(c) (d)

Fig 6 Moment (M ) versus mid-span de1047298ection (um) relationship

a) b)

Fig 7 Comparisons of M ndash

um relationships (unit mm)


8172019 han2015_2


one fourth of the composite members are modelled in the analysis

Boundary conditions of a model are shown in Fig 10(b) Load is simulat-

ed by applying displacement in the loadingplate as shown in Fig 10(b)

The loading plate is assumed to be a rigid block with a stiffness that is

large enough that a deformation can be neglected

Comparison between the predicted and observed failure modes of

the specimens are shown in Fig 4 In general good agreementis obtain-

ed in the concrete crack distributions in the predicted and measured

specimens including 1047298exural-shear failure (Specimen B1-1 B2-1 and

B2-2) and 1047298exural failure (Specimen B4-1 BH3 and BH5) Fig 6 shows

the comparisons of predicted and measured M ndashum relations It can be

found that in general good agreementis obtained between the predict-

ed and measured results for the specimens with 1047298exural-shear failure

and 1047298exural failure modes

32 Analytical behaviour

A typical concrete-encased CFST box member under bending with

the cross-section shown in Fig 10(a) is analysed The dimensions of

the square cross-section are B = 2000 mm thickness of wall thicknesst c = 350 mm and wall-thickness-to-sectional-width ratio (2t c B) is

035 Diameter of steel tube D is 400 mm thickness t is 93 mm and

steel ratio α s (de1047297ned as As Acore where As and Acore are cross-

sectional areas of steel tube and core concrete in the inner CFST) is

01 diameter of longitudinal bar is 22 mm and longitudinal bar ratio

α l (de1047297ned as Al( Aout + Al) where Al is the total area of longitudinal

bar and Aout is the area of the out concrete which is all the concrete

except the core concrete in the steel tubes) is 0011 diameter of stirrup

is 12 mm and spacing s = 150 mm The cube strengths of outer

concrete and core concrete are f cuout = 40 Nmm2 and f cucore =

60 Nmm2 respectively The yield stress of the steel tube longitudi-

nal bar and stirrup are f ys = 345 Nmm2 f yl = 335 Nmm2 and f yh =

335 Nmm2 respectively The shear span av and the length of the

pure segment are both 6000 mm and the shear-span-to-depth

ratio (av H ) is 3

321 Complete loadndashdeformation curves

The failure mode of the above typical member is 1047298exural failure

which is the same with that of Specimen B4-1 shown in Figs 4 and 5

The typical moment (M ) versus curvature (ϕ) response of the

concrete-encased CFST box member under bending is shown in

Fig 11 Three points A B and C represent the indicated initial cracking

of outer concrete initiation of tensile yielding of bottom steel tube and

M u respectively M u isde1047297ned as the moment when the maximum ten-

sile strain at the extreme 1047297bre of the steel tube is 001 Points A and B

correspond to reduced stiffness and increasing de1047298ection

Fig 12 gives thedistribution of longitudinal stress of thelongitudinal

bar and steel tube at mid-section The stress in the longitudinal bars is

approximately a linear function of depth at Point A as shown in

Fig 12(a) The longitudinal bars from approximately mid-height of the

section to the bottom of the section have yielded in tension at Point B

leading to a steady progression toward nonlinear distribution of stress

Almost all the longitudinal bars in the web yield and longitudinal bars

at the top compressive 1047298ange in compression yield at Point C The

longitudinal stress of steel tubes is also linear with the sectional depth

at Point A suggesting linear elastic behaviour with plane-sections re-

maining plane The dashed lines of Fig 12(b) clearly show an increas-

ingly nonlinear distribution of the stress between the top and bottom

steel tubes at Point B and C Some part of top steel tube is in tension

when bottom steel tube yields at Point B At Point C the whole section

of the bottom steel tube yields Some part of top steel tube remains in

tension but does not yield

Fig 13 shows the longitudinal stresses of concrete at mid-section of

the member The neutral axis is in the middle of the member at Point Ain the elastic stage After Point A the neutral axial moves up due to the

cracking of concrete The neutral axial is near the compression 1047298ange

at Point B After Point B themovement of theneutral axial begins slow-

ly At Point C the extreme compressive concrete is crushed The neutral

axial is through thetop core concrete andsome part of top core concrete

is in tension

Fig 8 M ndashε s at mid-span relationship

a) b)

Fig 9 Comparison of M ndash

um relationships between the composite and RC member (unit mm)


8172019 han2015_2


Fig 14 shows thecontact stresses between steel tube and concreteat

mid-section where P 1 is the contact stress between steel tube and core

concrete and P 2 is the contact stress between steel tube and outer

concrete It can be seen that P 1 in the tension side is much larger than

that in the compressive side It can be explained that the volume strain

of concreteat thetensile side is higher than that at compressive side and

the cracked concrete still effectively resists the ovalization of steel tube

(Lu et al [11]) However P 2 is generally less than 05 Nmm2

322 Effect of shear-span-to-depth ratio

The shear-span-to-depth ratio (av H ) is an important parameter

affecting the behaviour of RC members under bending (Nawy [16])

The analysis shows that concrete-encased CFST members have signi1047297-

cantly greater shear resistance under bending that comparableRC members due to the bene1047297cial reinforcement provided by the

steel tube (An et al [6]) Four concrete-encased CFST box members

with av H from 1 to 4 are compared to discuss the effect of shear-

span-to-depth ratio Fig 15 gives the computed in1047298uence of av H on

M u and the corresponding curvature ϕu in the middle of the member

For the member with av H of 4 and 3 deterioration in resistance due

to shear cracksis not signi1047297cant and M u is the momentwhen the tensile

strain at the extreme 1047297bre of the steel tube is 001 While for the

member with av H of 2 and 1 M u is controlled by the ultimate shear

load V u and equal to V u times av Fig 15(a) shows that M u for the members

with av H of4 and 3 are almostthe same (M u forthe member with av H

of 4 is larger 3 than that with av H of 3) If av H le 3 a decrease in M uoccurs because of the shear effect M u for the member with av H of 2

and 1 are 10 and 24 smaller than that with av H of 3 respectively

Fig 15(b) shows that ϕu of the member is relatively constant with

av H of 4 and 3 (ϕu for the member with av B of 4 is larger 3 than

that with av H of 3) However a decrease in av H leads to a decrease

in ϕu when av H le 3 ϕu for the member with av H of 2 and 1 are 39

and 67 smaller than that with av H of 3 respectively A decrease in

av H leads to a decrease M u and ϕu when av H le 3 because the in1047298u-

ences of shear crack and deformation in the shear span become more

signi1047297cant with smaller av H For the members with av H = 2 and 1

the obvious shear cracks occur in the shear span leading to a smaller

M u larger shear deformation in the shear span and smaller ϕu in themiddle However when av H = 3 or larger the members are expected

to have 1047298exural failure mode and M u andϕu are thesame The extreme

1047297bre strain of the steel tube at tension at M u for av H b 3 is smaller than

001

323 Load transfer mechanism

Fig 16 shows the stress distribution along the lengthof the member

The maximum compressive stress in the concrete occurs at the top

of the member in the longitudinal direction in the pure bending

segment and another zone of large compressive stress occurs on diago-

nal along the line connecting theloading point and support point in the

shear span as shown in Fig 16(a) The stirrups yield along the line

connecting the loading and support points in the shear span and they

do not yield in the pure bending segment as shown in Fig 16(b) Mostof the longitudinal bars in the pure bending segment yield in tension

side or compression while few longitudinal bars in the shear span

yield as shown in Fig 16(c) The bottom steel tube in tension along

the longitudinal direction yield while the top one in compression

does not yield as shown in Fig 16(d)

The well-known strut-and-tie model was proposed to analyse

the load transfer mechanism of RC members subjected to bending

(ACI 38-11 [17] EC2 [18]) Lu et al [11] and An et al [6] used strut-

and-tie model to analyse the loading transfer mechanism of CFST and

concrete-encased CFST members under bending Fig 17 illustrates the

proposed load transfer mechanism of concrete-encased CFST box

member under bending by this method The compressive strut AE

in the pure bending segment consists of compressive concrete longitu-

dinal bars and steel tubes and the tensile tie CF includes tensile

a) b)

Fig 10 Finite element model of concrete-encased CFST box member subjected to bending

Fig 11 Typical M ndashϕ response


8172019 han2015_2


longitudinal bars and steel tubes In the shear span the diagonal

compressive struts AC and BD consist of diagonal compressive concrete

and the tensile tie BC includes stirrups

Although the contribution of steel tubes to the shear resistance of

the composite member is not directly re1047298ected in this strut-and-tie

model the contribution of the steel tubes to the shear resistance exists

especially forthe members with small av H Fig 18 shows the computed

V s as a function of av H where V s represents the contribution of the steel

tubes to the shear resistance V s is determined by (V cecfstminus V rc) where

V cecfst and V rc are the shear capacity of concrete-encased CFST and the

corresponding RC members under bending In the analytical model

the arrangements of rebars are α l = 3 f yl = 400 Nmm2 diameter

of stirrup is 8 mm s = 300 mm The above arrangements assure that

the composite and corresponding RC members with av H less than 3

fail due to shear in the shear span and the ultimate load V u represents

the shear capacity The other parameters are the same with those of

a) b)

Fig 12 Longitudinal stress of steel at mid-section

a) b) c)

Fig 13 Development of longitudinal stress of concrete at mid-span (unit Nmm2)

a) b)

Fig 14 Contact stresses between steel tube and concrete at mid-span


8172019 han2015_2


the typical one in Part 321 It can be seen that V s decreases as av H

increases The contribution of the steel tube to the shear resistance

can be divided into three parts 1) the shear resistance of itself 2) the

dowel action as the tube acts as a longitudinal component which is

similar with longitudinal bar (Chana [19]) and 3) concrete crack

control provided by the longitudinal as described in Part 2 and

the increased aggregate interlock associated with restrained crack

development

4 Parametric analysis and prediction of 1047298exural capacity

41 Parametric analysis

The parameters affecting M ndashϕ relations of concrete-encased CFST

box member under bending in the analysis can be summarized as

1) for the outer RC component f cuout = 30ndash50 Nmm2 α l = 06ndash

16 f yl = 235ndash400 Nmm2 2t c B = 015ndash055 2) for the inner CFST

component f cucore = 40ndash80 Nmm2 α s = 005ndash015 f ys = 235ndash

420 Nmm

2

and DB = 01ndash

025 These parameters were evaluatedand Fig 19 shows the in1047298uence of different parameters on M ndashϕrelations These analyses show no signi1047297cant in1047298uence of f cuout 2t c B

and f cucore on M u but an increase in α l f yl α s f ys and DB leads to an in-

crease in M u

From the typical M ndashϕ relation of the concrete-encased CFST mem-

ber the 1047297rst two stages (OA ad AB) can be simpli1047297ed as two straight

line as shown in Fig 9 The slope of the 1047297rst line (OA) is de1047297ned as K i

and that of the second line (AB) is de1047297ned as K s as shown in Fig 11 K ican be used to calculate the Euler elastic buckling strength of the

concrete-encased CFST box member because the compressive stresses

are high and the section is likely to have only limited concrete cracking

K s can be used to calculate the deformation of concrete-encased CFST

box column at the service state because the moment at Point B ranges

from 07 to 08 of M u and the load at the service state is in this range

Fig 20 shows the effects of the above parameters on K i and K s The

vertical axis of this 1047297gure shows stiffness values determined for a

given analytical parameter and the parameter is identi1047297ed at the base

of each column of data At the top of each column the average increase

ratio Ra (= K ieth THORNmaxminus K ieth THORNmin

2 K ieth THORNstandard the samefor K s) is provided to show a measure

of the variation caused by that parameter The Ra value is normalized by

the standard specimen stiffness as de1047297

ned by the typical compositemember in Part 3 It can be seen that the effect of f cuout and 2t c B on K iis the most signi1047297cant while the effects of the other parameters are

Fig 15 Effect of av H on M u and ϕu

a) b)

c)d)

Fig 16 Stress distribution along the longitudinal direction


8172019 han2015_2


smaller than 10 The effects of α s α l and DB on K s are the most

signi1047297cant while the effects of the other parameters are smaller than

10 The simple formulas based on the parametric analysis to predict

K i and K s are proposed as follows

K i frac14 07 E coutI cout thorn E ccoreI ccore

thorn E s I s thorn I leth THORN eth2THORN

K s frac14 71 Al thorn 4 As

Aminus

Ahol

thorn 0002 K i eth3THORN

where I cout I ccore I s and I l are the inertia moments of outer concrete

core concrete steel tubes and longitudinal bars respectively A and

Ahol are the areas of the whole section and the hollow part E cout and

E ccore are elastic modulus of outer and core concrete and calculated as 4

730 ffiffiffiffiffi f 0c

p accordingto ACI 318-11 [17] E s areelastic modulus of steeland

equal to 206000 Nmm2

The mean value and the standard deviation of K i by Eq (2) to K i by

FEA modelling are 0938 and 0044 respectively The mean value and

the standard deviation of K s by Eq (3) to K s by FEA modelling are

0908 and 0084 respectively The above simple formulas are suitable

for predicting K i and K s

42 Prediction on 1047298exural capacity

An et al [4] provided a simpli1047297ed strain-compatibility method to

predict sectional capacity of concrete-encased CFST box member

under combined compression and bending as shown in Fig 21 This

method employed the basic assumptions that the cross section remains

plane the contribution of tensile concrete is ignored the strain of the

extreme compressive outer concrete 1047297bre at the ultimate load ε cu is

3300με and the stressndashstrain relationship of longitudinal bar and steel

tube is idealized by an elastic-perfectly relation In addition it assumes

that the neutral axis of the member does not cross the core concrete

of the CFSTas shown in Fig 22(a)and the core concrete in compression

can be treated as a single 1047297bre acting at the centre of the CFST and the

stress in the concrete is based upon the strain at that location

This method effectively separates the strain compatibility evaluation

of the RC portion of the member from the concrete-encased CFST

elements and

N rc thorn N cfst frac14 0 for flexure without axial loadeth THORN eth4THORN

M rc thorn M cfst frac14 M u eth5THORN

where N rc and N cfst are the loads of the outer RC component and inner

CFST component respectively M rc and M cfst are the moments with

respect to the sectional centroid of the outer RC component and inner

CFST component respectively

Flexural capacity is the extreme case in which there is no axial

compression present and therefore this method may also be used to

evaluate the bending capacity of concrete-encased CFST box member

under bending if the neutral axis does not pass though any of the

encased CFST elements as shown in Fig 22(a) However1047298exural behav-

iour results in large movements of the neutral axis and the neutral axis

may be very near the top of the top of the box member and through

encased CFST components as shown in Fig 22(b) where c ai and Di

are the distance from neutral axis to the extreme compressive outer

concrete1047297bre thedistance from theextreme compressive core concrete

1047297bre to the extreme compressive outer concrete 1047297bre and the diameter

of core concrete of the CFST respectively This Case 2 condition requires

a modi1047297

ed evaluation procedureWith the Case 2 condition theRC box and the steel tube components

are the sameas usedfor the Case1 Howeverthe calculations ofthe core

concrete in the two cases are different In Case 1 the strength of thecore

concrete at the top can be calculated according to the stress in the

centroid of the core concrete and the concrete area as described in An

et al [4] But the strength of the core concrete needs special consider-

ation in Case 2 because the neutral axis crosses the core concrete and

the area below the neutral axis is not considered into the strength

contribution In order to calculate the strength contribution of the core

concrete the discretization method where the section is divided into

individual 1047297bres the cross-section remains plane and the con1047297nement

provided by steel tube to core concrete may be used as follows

N core frac14 Xσ corei Acorei eth6THORN

M core frac14X

σ corei Acorei 05H minus xcoreieth THORN eth7THORN

where N core and M core are the load and moment of the core concrete in

the inner CFST component and Acorei andσ corei are the area and stress of

each 1047297bre on compression in the core concrete xcorei is the distance

from the extreme compressive outer concrete 1047297bre to the centroid of

each 1047297bre σ corei and the corresponding ε corei can be calculated by

Eqs (8) and (9) where the stressndashstrain of the core concrete provided

by Han et al [20] is used

y frac14 2 xminus x2

xle1eth THORN eth8 1THORN

y frac141 thorn q x

01ξ

minus1

ξge112eth THORN x

β xminus1eth THORN2 thorn x ξb112eth THORN

8gtltgt xN1eth THORN eth8 2THORN

ε corei frac14 ε cu c minus xcoreieth THORN=c eth9THORN

where x frac14 ε corei

ε 0 y frac14 σ corei

σ 0 and σ 0 and ε 0 are the peak stress and the corre-

sponding strain the other parameters can be found in Han et al [20]

The above discretization method is complicated and not convenient

in the practical design It needs to be simpli1047297ed A simpler method to

Fig 17 Load transfer mechanism

Fig 18 Effect of av H on V s


8172019 han2015_2


Fig 19 Effects of different parameters on M ndashϕ relations


8172019 han2015_2


calculate the load and moment of core concrete is proposed in Eqs (10)

and (11)

N core frac14 γ Accoreσ ecore eth10THORN

M core frac14 γ Accoreσ ecore 05H minus

xecore

eth11THORN

An equivalent Point A is de1047297ned and shown in Fig 22(b) which

is the centroid point of the load N core The stress(σ ecore) at Point A

represents the average stress of the compressive core concrete xecore

is the distance from the extreme compressive outer concrete 1047297bre to

Point A Accore is the area of compressive core concrete and γ is the

equivalent area factorσ ecore is calculated by Eq (8) according to xecore

An analytical study is needed to 1047297nd the value of xecore and γ The possible parameters affecting the value of xecore and γ are c Di B

α s f y and f cucore and the parameters in the parametric analysis are

Di B = 01-025 α s = 005ndash015 f ys = 235ndash420 Nmm2 and f cucore =

40ndash80 kNm2 ai b c b ai + Di The discretization 1047297bre analysis method

in Eqs (6) and (7) isused to1047297nd the value of xecore andγ that provided

accurate results with the equivalent method of Eqs (10) and (11) The

results show thatγ = 1 and c and Di B have the most signi1047297cant effect

on xecore Eq (12) is an empirical equation for determining xecore

based on the analytical work as shown in Fig 23 If c is smaller than ai

it indicates that the whole core concrete is in tension and the load

contribution is ignored

xecore frac14 046c thorn 004Di thorn 054ai eth12THORN

The predicted 1047298exural capacities (M uc) by the above method arecompared with the current test results (M ue) in Table 1 It can be seen

that M uc is smaller than M ue Although the specimens with H of

1260 mm failed due to diagonal cracking in the shear span M ue is still

larger than M uc This might attribute to the facts that (a) the vertical

cracks in pure bending segment of these specimens fully developed

prior to shear cracking (b) the strain hardening of the steel also leads

to an increase in measured moment while the yield strength is used

in the predicted method The mean value and standard deviation of

M uc M ue are 0879 and 0030 respectively The simpli1047297ed strain-

compatibility method is reasonable for predicting the moment capacity

of box concrete-encased CFST members in the parameter limits of the

test

5 Conclusions

The following conclusions can be drawn based on the current test

research

(1) The tested concrete-encased CFST box member under bending

showed two typical failure modes in the test one was 1047298exural-

shear failure mode for specimens with H of 1260 mm another

was 1047298exural failure mode for specimens with H of 840 mm The

inner steel tubes had no apparent local buckling for CFST in the

tension or compression section The core concrete of CFST in

compression zone remained intact and a few uniformly distrib-

uted cracks were found in core concrete of CFST in the tension

zone M ue increased with an increase in H and D The stiffness

increased with an increase in H while the in1047298uence of D on

stiffness in the 1047297rst two stages was not signi1047297cant(2) The CFST component can increase the tensile reinforcement of

the of concrete-encased CFST box member and the CFST also

provides a well con1047297ned high strength compressive element to

enhance the 1047298exural performance of the composite member As

a result this concept may be attractive for use in applications

where member is huge and member weight must be limited

(3) The FEA model provides a good prediction for the concrete-

encased CFST box member under bending A decrease in av B

leads to a decrease in M u and ϕu A strut-and-tie model is

proposed based on FEA model to analyse the load transfer

mechanism of concrete-encased CFST box member subjected to

bending but this methods tends to underestimate the shear

resistance of concrete-encased CFST box members particularly

for small av H ratios

a)

b)

Fig 20 Effects of different parameters on K i and K s

a) b)

Fig 21 Strain-compatibility method


8172019 han2015_2


(4) The parametric analysis shows that there is no signi1047297cant in1047298u-

ence of f cuout 2t c B and f cucore on Mu but an increase in α l f yl

α s f ys and DB leads to an increase in M u f cuout and 2t c B have

the most signi1047297

cant effect on the increase of K i α s α l and DBhave the most signi1047297cant effect on the increase of K s A simpli1047297ed

strain-compatibility method is presented and it is reasonable for

predicting the 1047298exural capacity of concrete-encased CFST box

members

Acknowledgements

Theresearch reported in thepaper is part of Projectsupported by the

National Natural Science Foundation of China (no 51378290) as well as

the Tsinghua University Initiative Scienti1047297c Research Programme (no

2013Z02) The 1047297nancial support is highly appreciated The authors

also thank Dr Yongjin Li for his assistance on the experiment work in

this paper and Mr Tingmin Mu for providing the photos in Fig 1 of the paper

References

[1] Han LH An YF Performance of concrete-encased CFST stub columns under axialcompression J Constr Steel Res 20149362ndash76

[2] An YF Han LH Behaviour of concrete-encased CFST columns under combinedcompression and bending J Constr Steel Res 2014101314ndash30

[3] Rasmussen LJ Baker G Large-scale experimental investigation of deformable RC boxsections J Struct Eng ASCE 1999125(3)227ndash35

[4] An YF Han LH Zhao XL Experimental behaviour of box concrete-encased CFSTeccentrically loaded column Mag Concr Res 201365(20)1219ndash35

[5] An YF Han LH Zhao XL Analytical behaviour of eccentrically-loaded concrete-encased CFST box columns Mag Concr Res 201466(15)789ndash808

[6] An YF Han LH Roeder C Flexural performance of concrete-encased concrete-1047297lledsteel tubes Mag Concr Res 201466(5)249ndash67

[7] Wang G Study on 1047298exural behaviors of steel tube con1047297ned concrete members andcomposite steel tube con1047297ned concrete members [Master Dissertations] BeijingChina Tsinghua University 2004[in Chinese]

[8] Galal K Yang Q Experimental and analytical behavior of haunched thin-walled RCgirders and box girders Thin-Walled Struct 200947(2)202ndash18

[9] Yuan A DaiH SunD Cai J Behaviors of segmental concrete boxbeams with internaltendons and external tendons under bending Eng Struct 201348623ndash34

[10] GB50010-2010Code for design of concrete structures BeijingChina ChinaBuildingIndustry Press 2010[in Chinese]

[11] Lu H Han LH Zhao XL Analytical behavior of circular concrete- 1047297lled thin-walledsteel tubes subjected to bending Thin-Walled Struct 200947(3)346 ndash58

[12] Hibbitt Karlson Sorenson Inc ABAQUS Version 65 theory manual users manualveri1047297cation manual and example problems manual 2005

[13] Hoshikuma J Kawashima K Nagaya K Taylor AW Stressndashstrain model for con1047297nedreinforced concrete in bridge piers J Struct Eng ASCE 1997123(5)624ndash33

[14] Attard MM Setunge S Stress ndashstrain relationship of con1047297ned and uncon1047297nedconcrete ACI Mater J 199693(5)432ndash42

[15] Han LH Yao GH Tao Z Performance of concrete-1047297lled thin-walled steel tubes under

pure torsion Thin-Walled Struct 200745(1)24ndash

36

Fig 22 Strength calculation of core concrete

Fig 23 Simpli1047297cation on xecore


8172019 han2015_2


[16] Nawy EG Reinforced concrete a fundamental approach Fifth ed Upper SaddleRiver New Jersey Pearson Education Inc 2003

[17] ACI 318-11 Building code requirements for structural concrete and commentaryDetroit (USA) American Concrete Institute 2011

[18] Eurocode 2 Design of concrete structures-part 1-1 general rules and rules forbuildings BS EN 1992-1-12004 European Committee for Standardization 2004

[19] Chana PS Investigation of the mechanism of shear failure of reinforced concretebeams Mag Concr Res 198739(141)196ndash204

[20] Han LH Yao GH Zhao XL Tests and calculations for hollow structural steel (HSS)stub columns 1047297lled with self-consolidating concrete (SCC) J Constr Steel Res 200561(9)1241ndash69


8172019 han2015_2


tested The test parameters for the composite member specimens

included

(a) Diameter of steel tube (D) from 749 mm to 1022 mm

(b) Sectional height (H ) from 840 mm to 1260 mm

Table 1 summarizes the specimen dimensions and properties The

specimen labels listed in Table 1 are de1047297ned as follows (1) The initial

character ldquoBrdquo indicates the box section (2) The second character is an

ldquoHrdquo (if any) for specimens without inner CFST (3) The 1047297rst Arabic nu-

meral stood for the different group with the same section type and

(4) The second Arabic numeral (if any) meant the different specimen

in the same group All specimens had the same 1047298ange and web thick-

ness of 87 mm and 66 mm respectively All specimens had longitudinal

reinforcement of 64 mm diameter and stirrups with 64 mm diameter

and 65 mm spacing Detailed information for size geometry and rebar

placement in cross section is shown in Fig 2 All the specimens were6 m in length The 1047297rst and second groups of specimens with the label

ldquoB1rdquo and ldquoB2rdquo had the same dimension of cross section (B times H ) but

different steel tubes The second and fourth groups of specimens with

the label ldquoB2rdquo and ldquoB4rdquo had the same inner CFST and width of cross

section but the sectional height (H ) was different

Standard tensile coupon tests were conducted to measure the mate-

rial properties of the steel tubes and rebar The measured average yield

strength ( f y) the ultimate strength ( f u) the modulus of elasticity (E s)

and Poissons ratio are listed in Table 2 Two different self-consolidating

concrete (SCC) mixes were used with the higher strength concrete

used as concrete 1047297lled inside the CFST components The maximum size

of coarse aggregate was 25 mm Six 150 mm cubes were cast for each

batch of concrete and cured in conditions similar to that of the related

specimens and tested to measure the compressive strength of the

concrete The physical properties of the concrete are given in Table 3

To construct the specimen the cold-formed steel tubes of inner CFST

initially were cut and machined to the required length 1047297rst and the

outside surfaces of the tubes were brushed to remove any rust and

loose debris Then the four tubes were welded to two 10 mm thick

steel end plates to achieve the design geometry Four holes were

drilled into one of the end plates for casting concrete The steel tubes

were sloped at about 30deg to the horizontal during concrete placement

and self-consolidating concrete (SCC) was poured into the inner steel

tubes As soon as the inner CFST was prepared the rebarswere installed

and outer concrete waspoured Theouter RC componentwas fabricated

by two steps ie (1) the bottom 1047298ange and parts of the webs under

the middle 1047297rst and (2) then the top 1047298ange and remainder of the

webs

A general view of the test setup is given in Fig 3 The load wasapplied by a 5000 kN capacity hydraulic jack through a rigid steel

beam with four-point loading In-plane displacements were measured

at locations along the specimen by three displacement transducers

as shown in the 1047297gure Strain gauges were used for each specimen to

measure strains in the steel tube and longitudinal bar at the mid-span

of each specimen as shown in Fig 2 A load increment of 120th of the

estimated ultimate load was used for each load step and each step

was held constant for about 2 min The strain load and the de1047298ection

measurements were automatically recorded

Nomenclature

A Cross-sectional area of the whole section

Acore Cross-sectional area of core concrete in CFST

Ahol Cross-sectional area of the hollow part

Al Cross-sectional area of longitudinal bar

As Cross-sectional area of steel tube of CFST

Asc Cross-sectional area of CFST (= Acore + As)

Aout Cross-sectional area of outer concreteav Length of shear span

B Sectional width

D Outer diameter of steel tube

Di Diameter of core concrete

E c Elastic modulus of concrete

E s Elastic modulus of steel

f cu Concrete cube strength

f c Concrete cylinder compressive strength

f yl Yield strength of longitudinal bar

f ys Yield strength of steel tube

f u Ultimate strength of steel

H Sectional height

L Length of the specimen

M Moment

M cr Moment when cracks occur

M rc Moment of the outer box RC component

M cfst Moment of the inner CFST component

M ser Moment at the serviceability limit state

M u The ultimate moment

M y Yield moment

N cfst Load of the inner CFST component

N rc Load of the outer box RC component

SI Strength index given by M ue minus cecfst M ue minus rc

α l Longitudinal bar ratio (= Al Aout + Al)

α s Steel ratio of CFST (= As Acore)

w Width of concrete crack

um Mid-span de1047298ection

ε Strain

ξ Con1047297nement factor frac14 As f ys Acore f ck

of the inner CFST component

Fig 1 Practical applications of concrete-encased CFST box members


8172019 han2015_2



221 Failure mode





































in the load













Table 1


No Specimen

label

Section

dimension

B times H (mm)

Inner tube

D times t (mm)

M cr

(kN m)


M ue

(kN m)

M serM ue


M uc M uc M ue

1 B1-1 956 times 1260 749 times 26 370 580 870 1370 0423 1343 1238 0904

2 B1-2 956 times 1260 749 times 26 365 560 860 1347 0416 1321 1238 0919

3 B2-1 956 times 1260 1022 times 29 372 798 1233 1637 0487 1605 1380 0843

4 B2-2 956 times 1260 1022 times 29 366 798 1190 1609 0496 1577 1380 0858


7 B4-2 956 times 840 1022 times 29 180 362 650 894 0405 2104 795 0889


Mean 0879




8172019 han2015_2





construction













































































Table 2






Table 3



(kgm3)

Coarse aggregate

(kgm3)

Fine aggregate

(kgm3)

Fly ash

(kgm3)

Breeze

(kgm3)

Water

(kgm3)

Water reducer

(kgm3)

f cu 28day

(Nmm2)

f cu

(Nmm2)

C50 290 1090 740 65 85 170 63 523 617

C70 400 960 720 172 ndash 144 69 584 742



8172019 han2015_2


a)

b)

c)

d)

e)

f)

g)

h)



8172019 han2015_2


























M ue‐rc

eth1THORN










as H increased





















8172019 han2015_2


















(a) (b)

(c) (d)


a) b)




8172019 han2015_2

































ratio (av H ) is 3

































is in tension


a) b)




8172019 han2015_2







































001























a) b)




8172019 han2015_2


















a) b)


a) b) c)


a) b)



8172019 han2015_2









development








420 Nmm

2

and DB = 01ndash



crease in M u




















a) b)

c)d)



8172019 han2015_2









Aminus

Ahol





























elements and



















a modi1047297














M core frac14X











y frac141 thorn q x

01ξ

minus1

ξge112eth THORN x













8172019 han2015_2




8172019 han2015_2



and (11)



xecore

eth11THORN






























test

5 Conclusions


research

























a)

b)


a) b)



8172019 han2015_2









members

Acknowledgements







References

















36




8172019 han2015_2








8172019 han2015_2



221 Failure mode





































in the load













Table 1


No Specimen

label

Section

dimension

B times H (mm)

Inner tube

D times t (mm)

M cr

(kN m)


M ue

(kN m)

M serM ue


M uc M uc M ue

1 B1-1 956 times 1260 749 times 26 370 580 870 1370 0423 1343 1238 0904

2 B1-2 956 times 1260 749 times 26 365 560 860 1347 0416 1321 1238 0919

3 B2-1 956 times 1260 1022 times 29 372 798 1233 1637 0487 1605 1380 0843

4 B2-2 956 times 1260 1022 times 29 366 798 1190 1609 0496 1577 1380 0858


7 B4-2 956 times 840 1022 times 29 180 362 650 894 0405 2104 795 0889


Mean 0879




8172019 han2015_2





construction













































































Table 2






Table 3



(kgm3)

Coarse aggregate

(kgm3)

Fine aggregate

(kgm3)

Fly ash

(kgm3)

Breeze

(kgm3)

Water

(kgm3)

Water reducer

(kgm3)

f cu 28day

(Nmm2)

f cu

(Nmm2)

C50 290 1090 740 65 85 170 63 523 617

C70 400 960 720 172 ndash 144 69 584 742



8172019 han2015_2


a)

b)

c)

d)

e)

f)

g)

h)



8172019 han2015_2


























M ue‐rc

eth1THORN










as H increased





















8172019 han2015_2


















(a) (b)

(c) (d)


a) b)




8172019 han2015_2

































ratio (av H ) is 3

































is in tension


a) b)




8172019 han2015_2







































001























a) b)




8172019 han2015_2


















a) b)


a) b) c)


a) b)



8172019 han2015_2









development








420 Nmm

2

and DB = 01ndash



crease in M u




















a) b)

c)d)



8172019 han2015_2









Aminus

Ahol





























elements and



















a modi1047297














M core frac14X











y frac141 thorn q x

01ξ

minus1

ξge112eth THORN x













8172019 han2015_2




8172019 han2015_2



and (11)



xecore

eth11THORN






























test

5 Conclusions


research

























a)

b)


a) b)



8172019 han2015_2









members

Acknowledgements







References

















36




8172019 han2015_2








8172019 han2015_2





construction













































































Table 2






Table 3



(kgm3)

Coarse aggregate

(kgm3)

Fine aggregate

(kgm3)

Fly ash

(kgm3)

Breeze

(kgm3)

Water

(kgm3)

Water reducer

(kgm3)

f cu 28day

(Nmm2)

f cu

(Nmm2)

C50 290 1090 740 65 85 170 63 523 617

C70 400 960 720 172 ndash 144 69 584 742



8172019 han2015_2


a)

b)

c)

d)

e)

f)

g)

h)



8172019 han2015_2


























M ue‐rc

eth1THORN










as H increased





















8172019 han2015_2


















(a) (b)

(c) (d)


a) b)




8172019 han2015_2

































ratio (av H ) is 3

































is in tension


a) b)




8172019 han2015_2







































001























a) b)




8172019 han2015_2


















a) b)


a) b) c)


a) b)



8172019 han2015_2









development








420 Nmm

2

and DB = 01ndash



crease in M u




















a) b)

c)d)



8172019 han2015_2









Aminus

Ahol





























elements and



















a modi1047297














M core frac14X











y frac141 thorn q x

01ξ

minus1

ξge112eth THORN x













8172019 han2015_2




8172019 han2015_2



and (11)



xecore

eth11THORN






























test

5 Conclusions


research

























a)

b)


a) b)



8172019 han2015_2









members

Acknowledgements







References

















36




8172019 han2015_2








8172019 han2015_2


a)

b)

c)

d)

e)

f)

g)

h)



8172019 han2015_2


























M ue‐rc

eth1THORN










as H increased





















8172019 han2015_2


















(a) (b)

(c) (d)


a) b)




8172019 han2015_2

































ratio (av H ) is 3

































is in tension


a) b)




8172019 han2015_2







































001























a) b)




8172019 han2015_2


















a) b)


a) b) c)


a) b)



8172019 han2015_2









development








420 Nmm

2

and DB = 01ndash



crease in M u




















a) b)

c)d)



8172019 han2015_2









Aminus

Ahol





























elements and



















a modi1047297














M core frac14X











y frac141 thorn q x

01ξ

minus1

ξge112eth THORN x













8172019 han2015_2




8172019 han2015_2



and (11)



xecore

eth11THORN






























test

5 Conclusions


research

























a)

b)


a) b)



8172019 han2015_2









members

Acknowledgements







References

















36




8172019 han2015_2








8172019 han2015_2


























M ue‐rc

eth1THORN










as H increased





















8172019 han2015_2


















(a) (b)

(c) (d)


a) b)




8172019 han2015_2

































ratio (av H ) is 3

































is in tension


a) b)




8172019 han2015_2







































001























a) b)




8172019 han2015_2


















a) b)


a) b) c)


a) b)



8172019 han2015_2









development








420 Nmm

2

and DB = 01ndash



crease in M u




















a) b)

c)d)



8172019 han2015_2









Aminus

Ahol





























elements and



















a modi1047297














M core frac14X











y frac141 thorn q x

01ξ

minus1

ξge112eth THORN x













8172019 han2015_2




8172019 han2015_2



and (11)



xecore

eth11THORN






























test

5 Conclusions


research

























a)

b)


a) b)



8172019 han2015_2









members

Acknowledgements







References

















36




8172019 han2015_2








8172019 han2015_2


















(a) (b)

(c) (d)


a) b)




8172019 han2015_2

































ratio (av H ) is 3

































is in tension


a) b)




8172019 han2015_2







































001























a) b)




8172019 han2015_2


















a) b)


a) b) c)


a) b)



8172019 han2015_2









development








420 Nmm

2

and DB = 01ndash



crease in M u




















a) b)

c)d)



8172019 han2015_2









Aminus

Ahol





























elements and



















a modi1047297














M core frac14X











y frac141 thorn q x

01ξ

minus1

ξge112eth THORN x













8172019 han2015_2




8172019 han2015_2



and (11)



xecore

eth11THORN






























test

5 Conclusions


research

























a)

b)


a) b)



8172019 han2015_2









members

Acknowledgements







References

















36




8172019 han2015_2








8172019 han2015_2

































ratio (av H ) is 3

































is in tension


a) b)




8172019 han2015_2







































001























a) b)




8172019 han2015_2


















a) b)


a) b) c)


a) b)



8172019 han2015_2









development








420 Nmm

2

and DB = 01ndash



crease in M u




















a) b)

c)d)



8172019 han2015_2









Aminus

Ahol





























elements and



















a modi1047297














M core frac14X











y frac141 thorn q x

01ξ

minus1

ξge112eth THORN x













8172019 han2015_2




8172019 han2015_2



and (11)



xecore

eth11THORN






























test

5 Conclusions


research

























a)

b)


a) b)



8172019 han2015_2









members

Acknowledgements







References

















36




8172019 han2015_2








8172019 han2015_2







































001























a) b)




8172019 han2015_2


















a) b)


a) b) c)


a) b)



8172019 han2015_2









development








420 Nmm

2

and DB = 01ndash



crease in M u




















a) b)

c)d)



8172019 han2015_2









Aminus

Ahol





























elements and



















a modi1047297














M core frac14X











y frac141 thorn q x

01ξ

minus1

ξge112eth THORN x













8172019 han2015_2




8172019 han2015_2



and (11)



xecore

eth11THORN






























test

5 Conclusions


research

























a)

b)


a) b)



8172019 han2015_2









members

Acknowledgements







References

















36




8172019 han2015_2








8172019 han2015_2


















a) b)


a) b) c)


a) b)



8172019 han2015_2









development








420 Nmm

2

and DB = 01ndash



crease in M u




















a) b)

c)d)



8172019 han2015_2









Aminus

Ahol





























elements and



















a modi1047297














M core frac14X











y frac141 thorn q x

01ξ

minus1

ξge112eth THORN x













8172019 han2015_2




8172019 han2015_2



and (11)



xecore

eth11THORN






























test

5 Conclusions


research

























a)

b)


a) b)



8172019 han2015_2









members

Acknowledgements







References

















36




8172019 han2015_2








8172019 han2015_2









development








420 Nmm

2

and DB = 01ndash



crease in M u




















a) b)

c)d)



8172019 han2015_2









Aminus

Ahol





























elements and



















a modi1047297














M core frac14X











y frac141 thorn q x

01ξ

minus1

ξge112eth THORN x













8172019 han2015_2




8172019 han2015_2



and (11)



xecore

eth11THORN






























test

5 Conclusions


research

























a)

b)


a) b)



8172019 han2015_2









members

Acknowledgements







References

















36




8172019 han2015_2








8172019 han2015_2









Aminus

Ahol





























elements and



















a modi1047297














M core frac14X











y frac141 thorn q x

01ξ

minus1

ξge112eth THORN x













8172019 han2015_2




8172019 han2015_2



and (11)



xecore

eth11THORN






























test

5 Conclusions


research

























a)

b)


a) b)



8172019 han2015_2









members

Acknowledgements







References

















36




8172019 han2015_2








8172019 han2015_2




8172019 han2015_2



and (11)



xecore

eth11THORN






























test

5 Conclusions


research

























a)

b)


a) b)



8172019 han2015_2









members

Acknowledgements







References

















36




8172019 han2015_2








8172019 han2015_2



and (11)



xecore

eth11THORN






























test

5 Conclusions


research

























a)

b)


a) b)



8172019 han2015_2









members

Acknowledgements







References

















36




8172019 han2015_2








8172019 han2015_2









members

Acknowledgements







References

















36




8172019 han2015_2








8172019 han2015_2








han2015_2

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