Behaviour of Hollow Concrete-filled Steel Tubular

7/30/2019 Behaviour of Hollow Concrete-filled Steel Tubular

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ISSN 13923730 print / ISSN 18223605 online

JOURNAL OF CIVIL ENGINEERING AND MANAGEMENT

http:/www.jcem.vgtu.lt

2007, Vol XIII, No 2, 131141

BEHAVIOUR OF HOLLOW CONCRETE-FILLED STEEL TUBULARCOMPOSITE ELEMENTS

Artiomas Kuranovas1, Audronis Kazimieras Kvedaras2Dept of Steel and Timber Structures, Vilnius Gediminas Technical University,

Saul tekio al. 11, LT-10223 Vilnius, Lithuania

E-mail:[email protected];

[email protected]

Received 8 Aug 2006; accepted 6 Oct 2006

Abstract. Behaviour of composite steel-concrete elements in various loading stages is quite well analysed by theoretical

investigations and experiments. Concrete-Filled Steel Tube (CFST) is one of many composite elements used at present incivil engineering. Different approaches and design philosophies were adopted in different design codes for it. But forhollow CFST elements, which are more effective than ordinary CFST, any code does not provide informationabout how to design these elements. Further investigations of hollow composite CFST elements are needed.In loadingstage, when a particular level of stresses exists, an interaction between steel tube and concrete core appears and therefore acomplex stress state of element takes place, which increases the load-bearing capacity of the whole composite element.This interaction between components of CFST elements is reached because of different material properties, such as Pois-sons ratio, elasticity modulus etc. In this article reasons of the above-mentioned complex stress state appearance and be-haviour of hollow CFST element components in different load stages of compressed stub structural member are analysed.The test results are presented in diagrams, tables. Previous researches of other investigators are summarised. Differencesand similarities in behaviour of solid concrete and composite elements and hollow members with different number of con-crete core layers are discussed.

Keywords: composite structures, centrifuging, hollow concrete-filled steel tubes, interaction of components, behaviour,concrete triaxial stress state, Poissons ratio, micro-cracking.

1. Introduction

Steel members have the advantages of high tensilestrength and ductility, while concrete members may beadvantageous in compressive strength and stiffness.Many researchers agree that CFST members utilise theadvantages of both steel and concrete [1]. They arecomprised of a steel hollow section of circular or rec-tangular shape filled or centrifuged with plain or rein-forced concrete (Fig 1).

Fig 1. Various types of composite columns: concrete en-cased steel (CES) (a), CFST (b), combination of CES andCFST (c), hollow CFST sections (d) and double skin sec-

tions (e)

CFST columns have many advantages over rein-forced concrete columns. The major benefits of concretefilled columns are: a) steel column acts as a permanent andintegral formwork; b) steel column provides external rein-forcement, c) steel column supports several levels of con-struction prior to concrete being pumped and d) higherstrength and stiffness compare with RC columns of thesame materials properties.

The main effect of concrete is that it delays the localbuckling of the tube wall and the concrete itself, in therestrained state, and is able to sustain higher stresses and

strains when unrestrained.A hollow CFST member has some advantages in re-spect of a solid CFST: a) a lower concrete consumption andtotal permanent load to building, b) inside columns canpipes, cables and other services of the building be installed,c) higher relative compressive strength, d) easier andcheaper assembling.

Hollow CFTS members can be also produced with aconcentrically layered concrete core (double layered,triple layered etc) that increases the strength capacity ofthe whole member [2].

These composite columns can be also used for theresisting outside pressure, such as ocean waves, ice; in

seismic regions because of excellent earthquake-resistantproperties such as high strength, high ductility, and largeenergy absorption capacity.


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Different approaches and design philosophies havebeen adopted in different design codes [3]. In Japan, thestandard for designing of composite columns isbased on a simple method of superposition that uses theallowable stresses of the materials or the workingstress method. ACI-318 adopts the traditional rein-

forced concrete approach. AS 3600-1994 also uses theconcept of reinforced concrete design. The AISC-LRFDis based on the concept of structural steel. The Eurocode4, being a dedicated code for composite construction,combined the design approach of both structural steel-work and reinforced concrete columns. All these codesprovide a design procedure just for CFST, CES compos-ite columns and any code does not mention hollow CFSTelements. Generally, these codes are adopted for hollowsection as for solid member.

Concentrically layered hollow CFST elements(Fig 2b) are more effective than ordinary hollow ele-ments, because of the interaction between surfaces of

concrete layers which appears after spinning [4]. Thisinteraction appears independently on the type of loadingapplied to such hollow CFTS element and on the in-creased load-bearing capacity of components [5].

As mentioned above, there exists between CFSTcomponents some interaction and any code does not takeit into account, except for confinement effect and averageinterface shear stress associated with the residual loaddue to friction. Any adhesive bond is taken into account.

a) b)

Fig 2. Cross-section of CFTS element with (a) single- and(b) double layered concrete core

So at present it is a lack of information for designers.Further investigations, tests, FEM and structural analysesare necessary.

2. Structural behaviour of hollow CFTS columns

Many researchers agree that most composite struc-tural elements are subjected to a multi-axial stress state[3]. The response of concrete in CFST elements varieswidely for different stress states, and it is therefore im-portant to know how the concrete behaves in differentmulti-axial stress states.

The structural behaviour of hollow CFST membersis complex because of the interaction between the steeltubular shell and the hollow concrete core. The pioneertest research effort on the structural behaviour of CFSTmembers was made by J. S. Sewel [6]. He observed thatthe ultimate axial resistance of CFST columns is greaterthan the sum of resistance of separately tested steel andconcrete components. Further investigations of CFSTelements were performed by a great number of research-ers of whom K.Klppel and W.Goder [7], H. R. Salaniand J. R. Sims [8], R. W. Furlong [9] may be men-tioned. Their investigations discovered that the in-crease in load-bearing capacity of CFST elements ismainly caused by the confining effect of steel tube on theconcrete core. The different Poissons ratios of steel tubeand concrete core are considerably affected by the struc-tural behaviour of CFST elements.

Almost all researchers agreed that during the initialloading stage (as for solid as well as for hollow CFSTmembers) the concrete Poissons ratio is lower than thatof steel (Fig 3a), and the steel tube has no confining ef-fect on the concrete core. As longitudinal strains increase,the lateral expansion of concrete gradually becomesgreater than that of steel tube (Fig 3b). Hoop and radialstresses are equal. At this stage, it is considered that thesteel tube becomes bi-axially and solid concrete core tri-axially stressed [10, 11], but for the hollow CFST 3D

Fig 3. Stress condition in steel tube and concrete core atdifferent stages of loading: ca > (a), ca


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stress state is on the inside concrete core and steel tubeinterface. At the same time, the steel is compressed in thelongitudinal, radial and is tensioned in the hoop direc-tions. For double-layered CFST (especially for layersfrom different concrete grades) the greater transversaldeformations of internal layer cause lateral pressure on

the external core layer. Thus stresses of both concretelayers redistribute, and on external concrete core layermaybe analysed as behaving in triaxial and internal, corelayer - in biaxial states. On the other hand, researchers[12] suggest for axially and laterally compressed solidbody to consider biaxial stress state as a simplified modelof triaxial state.

In the first stage of axial compression the steel tubeas a component of CFST element sustains a greater partof the load until it starts yielding. At this point a loadtransfer from steel tube to the concrete core starts, and afurther increase of load is exhibited by only concrete coreuntil it reaches its micro-cracking compressive strength

[5, 13, 14]. After this stage of loading, a redistribution ofload from concrete core to the steel tube takes place. Atthis point the steel exhibits a hardening behaviour as in anuniaxial stress-strain hardening relationship.

The theoretical and practical investigations accord-ing to [14] show that the behaviour of concrete in CFSTelements becomes more ductile with lateral pressure in-creasing due to delaying micro-cracking. It is also inter-preted that during compression the force transfer throughconcrete core is accomplished by bridging forces betweenthe aggregates, and to a large extent by shear stresses,which are transferred over more or less inclined micro-cracks. The lateral pressure force balances the bridging

forces that reduce the tensile force and thereby delay therise and growth of bond cracks, therefore formation ofcracks becomes more difficult. According to [14], whencracks are present, the confining stress will slow downshear sliding of cracks, due to an increase of the frictionalshear stresses and stresses because of aggregate interlock.Furthermore, the reduced micro-cracking shear slidingleads to a less crack opening and thus reduces lateralexpansion. A delayed damage due to confining pressureof maximum value is reached at zero volumetric strain ofconcrete.

Another point of view to micro-cracking of concretecomposite members is presented in [15]. In composite

steel and concrete structures, where one of main load-bearing components is concrete, at various loading stagesappear its compaction, resolution and failure processes,which lead to micro-cracking. During loading of concrete

usually two parametrical points 0crf and

crf are fixed.

Investigations allow defining consistent pattern relation-ships and various important concrete deformation proc-

esses between these points. 0crf value of concrete

strength notices the start of micro-cracking process and

crf fixing the moment when of relative volume incre-

ment reaches zero. Independence of concrete structure

parametric point

crf is usually fixed when stresses inconcrete core reach 0,50,8 cf value. Then concrete vol-

ume starts to increase, and at ac a concrete corebegins to stress a steel tube. For a certain period the mi-cro-crack propagation process starts slowing down. How-ever, further load increase leads to concrete core pressurerising on the steel shell. This phenomenon is consideredas one of the main reasons of strengthening the concrete

confined in the steel tube. At parametric point

crf theincrement of relative volume reaches zero, and trans-versal strains of concrete core start to increase signifi-cantly. It is experimentally determined [16, 17] thatstructure of loaded concrete depends on the specimenform. Concrete tubes have a greater micro-cracking level

crf than prism or cylindrical specimens, because the

values of transversal and longitudinal strains of concretetubes are 1,21,3 times greater on internal than on exter-nal surfaces. For CFST elements under compression a

parametrical point crf maybe determined by condition

0= [16]:

2211

221

2 )2()2(

cia

emciema

dd

dd

= , (1)

where 2/)( 111 iem = mean increment of relativelongitudinal strains at hollow CFST element on the external

e1 and internal i1 surfaces, determined at correspon-

ding loading step ; e2 , i2 transversal strains atexternal and internal surfaces of element respectively.

For the behaviour determining CFST members un-der axial compression the problem of axial symmetry isused. The behaviour of axial loaded centrifuged CFST

members is more complicated than of a solid, becausenone of stresses are uniformly distributed through thecross-section [5] (Fig 4a), but the stress distribution pat-tern in the steel tube is the same both in solid and hollowcomposite cross-sections [11].

For 3D stress state the strain-stress relationship maybe represented by generalised Hooks law, and the ex-pression of longitudinal strains is derived from it:

Etrzz /)]([ = . (2)

In plastic stage, the distribution of radial stressacross hollow concrete core according to Lame problemmaybe analysed as linear:

)/()(biicicer

rrrr =

. (3)

The distribution of hoop stress t at plastic stage is dueto rectangular diagram and its value defined by value ofradial stresses due to Laplace equation:

),1/( = iirt (4)where cicei rr /= relative concrete core thickness.

Behaviour of multi-layered hollow CFST elementsis more complicated than of single-layered ones becauseof additional interaction between concrete core layers(Figs 4 b, c). Eqs (2), (3) and (4) used for double-andtriple-layered elements are valid only for the internallayer. As mentioned above, the 3D stress state is specific

to external and media layers of hollow concrete core andstress determined as for solid member [5].


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a) b) c)

Fig 4. Stress distribution in concrete core of centrifuged (a), one- (b) double-, and (c) triple-layered CFST elements

According to [18], diagrams of principal stresses ofmulti-layered CFST elements at contact surfaces (steel-concrete, concrete-concrete) are stepped because ofchanges in stiffness and appearance of additional internalforces in the area of contact between different concretecore layers (Figs 4 b, c). The shape of diagram remainsthe same as for single-layered element. Values of stepsare determined by formula:

iii = , (5)

where i experimentally determined coefficient of

stiffness and i tangential stress in i-contact surface.These steps are mathematically compensated [19]:

Ejz ANE /= , (6)

where == Aj

jjE EAEdxdyA stiffness of the whole

element and jj EA , cross-sections and elasticity

modulus ofj medium.Difference in Poissons ratios of composite element

symmetrically composed from different materials duringcompression increases the load-bearing capacity of wholeelement independently of signs of components Poissonsratios by increment of stiffness 33K value:

>= A z dxdyK 0033 , (7)where 0 axial stress of auxiliary problem plain de-formation solution. So the increased axial stress in com-

posite element is determined by (8):)/( 33KANE Ejz = . (8)

According to [19] axial compression in compositeelement produced from the same material with differentPoissons ratios generate lower axial stresses, than in caseof bar produced from different materials, but with samePoissons ratios. Therefore the first component of com-posite bar can be loaded with a greater force until reach-ing the same axial stresses as in the second component.This is valid for elastic materials. As mentioned above,during compression a pressure to the tube appears. So forCFST elements a plastic stage starts at an internal layerand ends at the external one [19].

Some researchers pay attention to a more complexbehaviour of CFST element and explain that during axialloading not only normal, radial and tangent stresses ap-pear, but also shear stresses at surfaces between compo-nents of CFST [18, 20]. The mechanisms by which shearstresses maybe transferred over the interface from steeltube to the concrete core and in opposite way are quitewell described [9, 10, 14], and assessed as adhesion, in-terface interlocking and friction. These 3 mechanisms areoften referred to as the natural bond [2124].

3. Experimental investigations

3.1. Properties of materials and specimens

Steel circular hollow sections (CHS) were used inmanufacturing the specimens. Yield and ultimate strengthof CHS were determined by standard steel plate couponsand rings tests. The coupons and rings were cut off fromeach steel tube. According to test results the, S355 steelgrade was found for CHS. The concrete mixture for sin-gle- and double-layered specimens was designed for


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compressive cube strength at 28 days of approx 30 MPa.The used mixture proportions are presented in Table 1.For fine aggregate the quartz sand of 0,2 mm main grainsize and Portland cement of CEMII/A-L 42,5N grade asbinder material were used. The initial cement/water ratioand slump of concrete cone were taken from recommen-

dations [4], but because of very fine aggregates the re-quired slump for centrifugation with these proportions ofconcrete components was not achieved. So for achievingthe needed slump additional water was used (Table 1,number in brackets). After centrifuging the residual waterquantity was measured, and it was obtained that duringmulti-layered centrifuging more residual water is pressedout from the concrete mixture. For determining the con-crete, mechanical properties cubes and prisms weremanufactured from the same concrete mix by vibrating.Single- and double-layered centrifuged CFST were curedin laboratory with environmental humidity of 21 % andtemperature 16,1 C. Concrete cores in centrifuged

specimens were isolated from environmental action bypolyethylene film at the ends of steel tube. Internal envi-ronmental humidity was 82 %. CFST members after 28days of curing were cut to smaller specimens of ~437 mmin length. For determining the mechanical properties ofsingle- and double-layered centrifuged concrete cores thesteel shells from some specimens were taken off. Hollowconcrete and CFST elements were tested under an axialcompression.

3.2. Force-strain relationships of different elements

For the improvement that in hollow CFST elementsduring loading the stresses are redistributed in a complexway as mentioned above, there were manufactured andaxially compressed specimens of annular cross-section:single- (1CFST) and double-layered (2CFST) CFSTmembers (Fig 5c), single- (1CT) and double-layered(2CT) concrete members (Fig 5b) and empty steel tubesST (Fig 5a). All their longitudinal L and transversal T strains measured by vertical and horizontal strain-gages,and load-bearing capacity were fixed by testing machinescale. These test results are presented in Table 2 and by

,, FF diagrams (Figs 59).The analysis of results shows that during multilayered

centrifuging an interaction between components of CFST

element appears and increases strength at least by 10 %.These results have shown that composite effect of single-layered specimens was ~6 %, for double-layered ~12 %.Diagrams F of ST members are presented in Fig 5afrom which can be noted that yield stresses appear near thevalue determined by testing of rings 340, =cyf MPa and

coupons 361, =tyf MPa. Buckling of ST members ap-

pears immediately after reaching the yield stresses.Fig 5b represents diagrams F of single- and dou-

ble-layered CT. Curves 1, 1 in Figs 5b, c represent valuesof longitudinal and transversal strains measured on asingle-layered internal, 3, 3 on external concrete core

a)

b)

c)Fig 5. Diagrams iF for ST (a); CT (b) and CFST (c)elements


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Table 1. Quantities of materials used for manufacturing CT and CFST specimens

Quantities of materials

No Length ofspecimen,

mm

Number ofspecimens

Initial length ofspun member,

mm

Cement,kg

Fineaggregate,

kg

Water,kg

Cement:aggregateand W/C

ratio

Slump ofconcretemix, cm

Quantityof water

pressuredout frommix, kg

219x5(1) 437 12 5540 74,1 150,5 40,2(8,5) 10,0 8,342,4 86,1 23,0(4,9) 6,2

219x5(2) 437 12 554037,1 75,5 20,1(4,2)

10,55,3

Total quantity of materials for spun specimens 153,6 312,1 83,3C219(1) 4 10,0

4 C219(2)

100x100x1004

1,9 3,8 1,0310,5

P219(1) 3 10,0

3 P219(2)

400x100x1003

_

7,6 15,2 4,12

1:2,030,543

10,5

Total quantities of materials for concrete prisms andcubes 11,4 22,8 4,8

Calculated total quantities of materials for cubes, prisms

and spun specimens340,8 692,1 145,6

Used quantities of materials for cubes, prisms and spunspecimens with a compaction coefficient of 20 % 409,0 830,5 174,7

surfaces. The curves 2, 2 and 4, 4 represent strains meas-ured on double-layered CT members in internal and exter-nal surfaces respectively. The same notation of curves isused for CFST elements too. The obtained results of F relationships for single- and double-layered elements shownearly the same shape and inclination of curves, but failurestrains for double-layered CT are nearly twice and for dou-ble-layered CFST members by 20 % greater than for sin-gle-layered members respectively.

Longitudinal and transversal strains of single- and

double-layered CT elements of annular cross-section areof the same shape and inclination respectively, but values

at the same compression force are approx by ~25 % lessfor double-layered elements. For CFST elements onlylongitudinal strains of concrete are on internal surfaceand transversal strains on steel tube have the same shapesand inclinations for single- and double-layered elements.Other strains on corresponding surfaces of single- anddouble-layered members are of different shape and incli-nation. That confirms an existence of additional interac-tion between components of double-layered CFSTelements, and redistribution of stresses distributes

through the thickness of concrete core section.

Table 2. Geometrical parameters and test results of single- and two-layered CFST, CT and ST elements

Steel tube Concrete coreSpecimen

at ,

mmaD ,

mmaA , 10

-4

mm2ceD ,

mm1ct , mm 2ct , mm cA , 10

-4

mm2

exp,uN ,

kNexp,uf ,

MPa

1CFST11CFST21CFST3

5,05,04,9

220219220

33,833,633,1

210,0209,0210,2

28,527,027,1

162,5154,4155,9

167916581658

103,3107,4106,4

Ave(3)= 1665,0 105,7

2CFST12CFST22CFST3

5,15,15,1

219220220

34,334,434,4

208,8209,8209,8

16,015,216,2

15,115,915,0

173,6174,6175,1

202120662080

116,4118,3118,8

Ave(3)= 2055,7 117,91CT11CT21CT3

210,1211,8210,4

27,426,227,1

157,2152,9155,7

559670653

35,643,841,9

Ave(3)= 627,3 40,42CT12CT22CT3

208,8209,8209,6

15,515,015,0

14,015,013,7

166,2169,5163,1

753690769

45,340,747,2

Ave(3)= 737,3 44,4ST1ST2

ST3

4,94,9

5,0

219219

220

33,033,0

33,8

942927

942

285,6281,3

279,0Ave(3)= 937,0 281,9


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It is possible to maintain that behaviour of externalconcrete layer in 3D stress state like in solid member andinternal layer is behaving as in 2D state. Steel tube inboth cases may be accepted as in biaxial stress state. Lon-gitudinal strains at internal surface of concrete core are1,28 for single- and 1,72 times greater for double-layered

CFST specimens than longitudinal strains of steel tube.But transversal strains on external surface of steel tubefor single- and double-layered elements are 15 % and53 % respectively lower than on internal concrete core.As longitudinal strains increase, the lateral expansion ofconcrete core becomes gradually greater than that in 2Dstress state. This shows that redistribution of stresses inmulti-layered element increases the strength and ductilityof CFST elements.

3.3. Poissons ratios of CFST and CT elements

For behaviour analysis of hollow CFST and CTelements Poissons ratio and axial force diagrams F are presented in Fig 6 a, b. Average value of Poissonsratio for steel tube was 0,28.

Poissons ratio of steel shell and concrete core ischanging during load applications. This ratio defined onthe base of strains measurements on external and internalsurfaces is distributing due to similar shapes in single-and double-layered CFST members (Fig 6b). The curves

F of CT elements on internal and external surfaces ofsingle- and double-layered members are not of the sameshape (Fig 6a). An average value of Poissons ratio oninternal and external surfaces of single- and double-layered CT elements (Fig 6a) is approx equal to 0,175.

Initial Poissons ratio values on the internal surface ofsingle-layered elements are less than that on external one,hereupon for double-layered members is opposite. Bothsurfaces achieve the same values of at 1/3 of ultimateload of single- and double-layered elements. Surfaces ofdouble-layered CT elements reach the same Poissonsratio at 0,82 level of the ultimate load. Under an ultimateload concrete core Poissons ratio value is greatest and oninternal surfaces equal to 0,255 for single- and double-layered CT elements. Maximum values on external surfa-ces are 0,175 and 0,163 for single- and double-layeredelements respectively. Average values of concrete coreon internal and external surfaces are 0,15 and 0,18 for

single-layered, 0,18 and 0,16 for double-layered ele-ments. From diagram F (Fig 6b) for CFST elementsmaybe noted that maximum values of Poissons ration areobtained on internal and external surfaces at levels of 1/3and 1/5 respectively for single- and double-layered mem-bers. Minimum values of ratios are obtained at the ulti-mate load 0,1 and 0,2 on internal for single-layered,0,34 on external for double-layered elements. Averagevalues of Poissons ratio on internal and external surfacesare of 0,25 and 0,38 for single-, 0,29 and 0,42 for double-layered CFST elements, respectively. Curves 1 and 2 onFigs 6a, b represent the relationships F on internalconcrete core of single- and double-layered CT and CFST

elements respectively, and curves 3 and 4 on externalsurfaces. Analysis of curves on Fig 6b shows that Pois-

sons ratio average values of double-layered CFST ele-ments are greater than those of single-layered members.

For a more detailed analysis of single-and double-layered CT and CFST elements behaviour dia-grams were plotted on Figs 7a, b, c and d. Fig 7a repre-sents diagrams of single-layered CT specimens. It can be

noted that Poissons ratio on external surface (curves 1,1') of CT members (Figs 7a, b) initially starts frommaximum and diminishes to its minimum values at ulti-mate strain, and on internal surface it is an opposite situa-tion (curves 2, 2'): Poissons ratio initially starts atminimum value and reach it maximum extreme value atultimate strain. External and internal surfaces achievedthe same value of Poissons ratio at ~1/2 level of ultimatestrains. Similar behaviour is seen in transversal direction.The initial situation of double-layered CT elements isdifferent in respect of single-layered members (Fig 7b).Poissons ratio on external surface (curves 1, 1) reachesmaximum value at approx 2/3 of ultimate strain and on

internal surface (curves 2, 2) reaches maximum value atinitial loading stage. At 1/2 and 1/3 of ultimate strain thePoissons ratio for internal and external surfaces reachesthe same value of 0,1600,165 at level of 0,850,90 ofultimate strains. For CFST elements (Figs 7c, d) situationdiffers considerably from that of CT elements. Theshapes of these elements diagrams on external andinternal surfaces are quite similar. Maximum values ofPoissons ratios on external surfaces are reached at initialloading stage at approx level of 0,15 ultimate strains, andminimum values at initial strains. Poissons ratio on in-ternal surface achieved the maximum value at level 0,35,and minimum at ultimate strains. This behaviour is typi-

cal for single- and double-layered elements.The difference in behaviour between single- and

double-layered CFST elements shows an appearance ofinteraction. Poissons ratio of concrete internal surface insingle-layered elementreaches a lower value than that inthe double-layered element.

3.4. Elasticity modules of CFST and CT elements

Figs 8a, b represent diagrams E-F for single- anddouble-layered CT and CFST elements, respectively.

Elasticity modulus ECT of internal (curve 1) andexternal (curve 3) surfaces of single-layered CT elements

does not greatly vary during loading, and its approximateaverage value is 35,0 GPa for both surfaces. For double-layered CT elements at initial loading stage, elasticitymodulus on internal surface reaches its ultimate value(63,7 GPa), and diminishes to a mean value with loadincreasing at 1/5 of ultimate load approach.

It may be noted that a mean value ofECT on externalsurface (curve 4 on Fig 8a) of double-layered CT ele-ments is twice greater than that of single-layered ones.That may be explained by a higher density of externalconcrete layer [4] achieved because of centrifuging proc-ess. Therefore the curve 2 (Fig 8a) coincides with thecurves of single-layered elements and shows that no

changes in stress state or in structure have appeared com-paring with the ones of single-layered elements. For a


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a) b)Fig 6. Diagrams

F(a) for single- and (b) two-layered CT CFST elements

a) b)

c) d)

Fig 7. Diagrams of (a) single, (b) double-layered CT elements, (c) single- and (d) double-layered CFST elements


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a)

b)Fig 8. Diagrams ECTFof CT (a) and ECFSTFof CFST(b) elements

single-layered CFST elements situation is the same as forthe CT elements. Curves of elasticity modulus on internal(curves 1 and 3 on Fig 8a) surfaces repeat each other andreach the mean value of 48,5 GPa. Differently from CTelements, elasticity modulus curves ECFST on externalsurface (curve 3 on Fig 8b) of single-layered CFST ele-ments repeats only the shape of curves 1 and 2(Fig 8b) with ~15 GPa a greater value. This shows that a

greater density of external concrete layer [4] is not themain reason of greater ECFST because an interaction be-tween layers causes stress redistribution in elements;therefore it becomes more ductile and may resist thegreater strains. It may be noted that ECFSTvalue of exter-nal steel shell in double-layered CFST elements (curve 4on Fig 8b) has a greater value than the one of single-layered members rising up with load reaching its peakvalue of ~90 GPa close to failure.

For determining the ECT, ECFST parameters Hookeslaw and total cross-section area (area of cross-section forinternal and for external surfaces was taken as total areaof the whole cross-section) were used.

Elasticity modulus of ST elements was determinedand corresponds to 212 GPa value.

3.5. Micro-cracking and volumetric strains of CT andCFST element

According to [14, 1617] micro-cracking of concrete

starts significantly increasing at parametric point crf ,

when concrete volumetric strains v or its relative volu-

metric increment , calculated by (1), reach zero value.The experimental results (Figs 9a, b) show that ofsingle-layered CT and CFST elements reach zero value atlevel of approx uCTCFST f/97,0 )( (curve 1). Hereupon

double-layered elements do not cross zero axes at all. Itshows that in single-layered CFST elements micro-cracksconsolidate 97 % of ultimate strength, and failure ofstructural elements takes place soon.

a)

b)Fig 9. Diagrams ui f/ for CT (a) and CFST (b)

elements


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Hereupon macro-cracks in double-layered elementsappear only on internal layer, and in external layer afterinspection no macro-cracks were found at all (curves 2 inFigs 9a, b). Collapse of these elements takes place laterthan of single-layered ones because cracks on their inter-nal layer do not develop into an external concrete layer;

just propagation of cracks proceed along the perimeter ofcontact surface between the two concrete layers.Diagrams of volumetric increments of CT single-

and double-layered elements are approximately of thesame value and shape until lower micro-cracking paramet-ric point of uCT f/52,0 value. After this point of

double-layered elements starts growing up faster and at

uCT f/72,0 its value starts to decrease. Volume of sin-

gle-layered CT elements starts to decrease at~ uCT f/57,0 and reaches a zero increment at about

uCT f/97,0 , that shows an appearance of an upper limit

of micro-cracking crf . As it was mentioned above, of

double-layered elements does not cross zero axes at all.Differently from diagrams of CT elements the dia-

grams of CFST show that maximum volumetric incre-ment of double-layered members is increasing less thanthat of single-layered CT elements. Relative volumetricincrement of single-layered CFST element reacheshigher value than that of the double-layered elements;therefore situation is opposite with respect of CT ele-

ments. Lower parametric point 0crf for CFST members is

reached at uCFST f/44,0 level.

Comparing our own test results with the results of[16, 17], it can be noted that behaviour of single-layeredCT and CFST elements is nearly the same, but in [16, 17]CT elements reach the second parametric point at level

uCT f/82,0 and CFST elements at uCFST f/75,0 .

Such differences in these results may be explained that anextra-fine grain concrete mix was used for authors ownspecimens, and for specimens of [16, 17] an ordinaryconcrete mix was applied.

4. Conclusions

Theoretical and experimental investigations showthat behaviour of hollow CFST elements is more compli-cated than that of solid ones, because of complex stressstates none of stresses in hollow concrete core are evenlydistributed through the thickness of its cross-sections. Forsingle-layered CFST elements the triaxial stress state isachieved only at the contact surface between the concretecore and steel shell. An internal hollow concrete core ofdouble-layered CFST elements is in the same stress stateas of single-layered members, but an external layer isanalysed as being in 3D stress state.

Researchers [18, 19] using materials of differentproperties tried to explain and evaluate this phenomenonby mathematical equations (47). According to [18],during loading between neighbouring layers of concrete

core in normal stress diagrams at place of interactionsurface the step appears, and authors [18, 19] suggesthow to evaluate this step.

Test results (Table 2, Figs 59) improve the proposi-tions of [5, 18, 19] that multi-layered elements hadgreater load-bearing capacity with respect to single-layered hollow CFST elements. This increase in strengthis explained by appearance of additional interaction be-tween neighbouring concrete layers under loading condi-

tions. For double-layered CFST members greaterdeformations of an internal layer cause confinement of anexternal layer; therefore their stresses redistribute and anexternal concrete layer may be analysed as in 3D stressstate.

Behaviour of single- and multi-layered CFST ele-ments differs essentially. Multi-layered elements resistnot only greater loads but, being more ductile, they havebetter such characteristics as modulus of elasticity, den-sity etc. Failure of such elements is more ductile, there-fore structures from such elements may be used moresafely.

Experimental investigations presented in this article

may help for further investigations and design of hollowsingle- and multi-layered CFST elements. Prediction oftheir behaviour at various loading situations is possible.

References

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TUIAVIDURI BETONERDI PLIENINI VAMZDINI STRYP ELGSENA

A. Kuranovas, A. K. Kvedaras

S a n t r a u k a

Daniausiai naudojam vairiai apkraut pilnaviduri betonerdi plienini vamzdini element elgsena teorikaipakankamai isamiai ianalizuota ir pagrsta bandymais. Tuiaviduriai betonerdiai plieniniai strypiniai elementai(TBPS) yra viena i toki kompozitini element ri. Pasaulyje naudojami vairs norminiai dokumentai ir projekta-vimo rekomendacijos yra pritaikytos pilnaviduriams BPS. Tuiaviduri element, kurie neretai yra efektyvesni nei pil-

naviduriai, tyrim

ir j

elgsenos teorin s analiz s n ra gausu. Apkrovimo metu, kai pasiekiamas tam tikrastempi

b

vis,tarp kompozitinio elemento komponent atsiranda sveika. Tod l susikuria sud tingesnis tempi bvis bei padid ja ele-mento laikomoji galia. i sveika atsiranda d l naudojam mediag skirting savybi, toki kaip Puasono koeficientas,tamprumo modulis ir t. t. Straipsnyje pateiktos sud tingo tempi bvio atsiradimo prieastys bei j elgsenos, veikiantvairioms gniudomosioms apkrovoms, ypatumai. Aprayti ir ianalizuoti bandymo metu gauti rezultatai, kurie pateiktigrafik ir lenteli pavidalo. Betonini ir kompozitini vienasluoksni ir dvisluoksni TBPS element elgsenos panaumaiir skirtumai aptarti ir palyginti su pilnaviduriais BPS.

Reikminiai odiai: kompozitin s konstrukcijos, betonerdiai plieniniai vamzdiai, tuiaviduriai, centrifugavimas,sveika tarp komponent, elgsena, betono triais bvis, Puasono koeficientas.

Artiomas KURANOVAS. PhD student at Dept of Steel and Timber Structures. Vilnius Gediminas Technical University,Saul tekio al. 11, LT-10223 Vilnius, Lithuania. Ph 2745228, fax 2398800. E-mail: [email protected] graduate of Civil Engineering at Vilnius Gediminas Technical University (2002). MSc of Civil Engineering (2004) at

Vilnius Gediminas Technical University. Research interests: structural mechanics, composite elements and behaviour oftheir components, engineering software for structural elements design.

Audronis Kazimieras KVEDARAS. Prof Dr Habil at the Dept of Steel and Timber Structures, Director of the Innova-tory Scientific Institute of Special Structures Kompozitas of Vilnius Gediminas Technical University, Saul tekio al. 11,LT-10223 Vilnius, Lithuania. E-mail: [email protected] interests: steel, composite steel-concrete and timber structures. Member of IABSE and ASCCS, NATO invitedexpert (1996, 2000).

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Behaviour of Hollow Concrete-filled Steel Tubular