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Engineering Structures

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Cyclic loading tests of interior beam–column connections for concrete-filledthin-walled tube columns

Jong-Jin Lima, Tae-Sung Eoma,⁎, Jin-Won Kimb, Tae-Hyu Hab

a Dept. of Architectural Engineering, Dankook Univ., 152 Jukjeon-ro, Gyeonggi-do 448-701, Republic of Koreab POSCO Global R&D Center, 100, Songdogwahak-ro, Yeonsu-gu, Incheon 406-840, Republic of Korea

A R T I C L E I N F O

Keywords:Concrete-filled tube columnThin-walled tubeBeam-to-column connectionMoment connectionCyclic testComposite column

A B S T R A C T

In this study, the results of the cyclic loading tests of four interior connection specimens made of concrete-filledthin-walled tube column and U-section beam are presented. Through-beams and through-bars passing throughthe column were used as the connection details. The tests showed that both slip significantly occurred duringcyclic loading and thus the strength, ductility, and energy dissipation capacity of the specimens were sig-nificantly affected by the connection details. The web plates of the beam U-section, which were welded to thethin column tube wall, did not fully contribute to the connection strength, due to out-of-plane deformation andpremature weld fracture of the tube wall plates. Based on the test results, a modified plastic stress distributionmethod to estimate the connection strength was suggested. In addition, design and detailing considerations ofthe proposed through-beam and through-bar connection details are given.

1. Introduction

Various types of concrete-filled tube columns using thin steel plateshave been developed as an alternative for economical and efficientcomposite construction [10,19,21,22]. A typical example of suchcomposite columns is the octagonal concrete-filled tube column (OCFTcolumn) (see Fig. 1(a)). In the OCFT columns, the thin-walled tube isfabricated with 6.0–12.0 mm-thick C-shaped and planar plates bywelding. The rib plates, which are projected into the core at four cor-ners and encased by concrete, increase not only structural integritybetween steel and concrete, but also provide the stiffness requiredduring field erection process. Furthermore, the rib plates can act as alateral support against local buckling of the tube. In fact, the use ofconcrete-filled thin-walled composite members shown in Fig. 1 hasgrown recently because of the following advantages: (1) since the steelplates are thin it is convenient to fabricate various cross-sectionalshapes by bending; and (2) due to high structural efficiency of filledcomposite sections, the amount of steel used for the composite memberscan be reduced.

In the concrete-filled thin-walled tube columns such as the OCFTcolumn, moment transfer in the beam-to-column connection is of con-cern. When steel or composite beams are connected to the thin tubewall of the column, out-of-plane deformation occurs easily in the tubewall due to the pulling force induced by the beam moment [16]. Such

out-of-plane deformation hinders the transfer of the beam moment tothe column and consequently decreases the strength and stiffness of theconnection. In addition, fracture at the weld joint between the columntube wall and beam plates can occur easily. Thus, for the beam-to-OCFTcolumn moment connection, the conventional welded moment con-nection details are not enough to secure a full moment transfer from thebeam to the column. Instead, special connection details providingcontinuity of the force transfer inside the column are required.

A number of studies on the through-beam, through-plate, andthrough-bar details for the composite beam-to-column connectionswere performed in the past. Azizinamini and Prakash [6] suggested amoment connection details using through-beam to connect steel beamsto concrete-filled tube columns. Alostaz and Schneider [4] investigatedthe cyclic behaviors of various types of steel beam-to-concrete-filledtube column connections. The tests showed that the connection per-formance could be significantly improved when using through-beamdetails. Azizinamini and Schneider [7] investigated the behavior ofsteel beam-to-circular tube column connections under cyclic loading.By continuing the beam flanges through the column, the strength wassecured, but the energy dissipation was significantly reduced due to theexcessive slip. Beutel et al. [8] proposed through-bar connection de-tails, in which reinforcing bars welded to the top and bottom flanges ofthe beam were embedded into the concrete of the column. The testsshowed that such through-bar details are effective in improving the

https://doi.org/10.1016/j.engstruct.2019.04.007Received 16 April 2018; Received in revised form 1 April 2019; Accepted 2 April 2019

⁎ Corresponding author.E-mail addresses: doublej17@naver.com (J.-J. Lim), tseom@dankook.ac.kr (T.-S. Eom), jwkim10@posco.com (J.-W. Kim), hath@posco.com (T.-H. Ha).

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T

seismic performance of the connection. Hoang et al. [12] proposed anew extended end-plate connection for concrete-filled tube columns, inwhich long bolts passing through the column directly connected thebeam end-plates placed at both sides of the column. The tests showedthat the end-plate connection continued inside the column by long boltsare adequate to secure the stiffness and strength required during cyclicloading. Ou et al. [17] investigated the joint shear behavior of athrough-flange connection for steel beams to circular concrete-filledsteel tubular columns. Panel zone shear yielding occurred in the steelbeams and failure of the specimens was caused by fracture of thecolumn tube near the through-flange plates and crushing of concrete.Sheet et al. [20] investigated the cyclic behavior of extended end-platemoment connections for circular concrete-filled tube columns. Bybolting the end-plates to the column with through rods and letting thesteel beam pass through the column, the strength, ductility, and energydissipation capacity of the specimens were enhanced.

Based on the review of the previous studies, in this study, through-beam and through-bar details were considered for the interior con-nection of the OCFT columns (see Fig. 2). In the through-beam con-nection detail (see Fig. 2(a)), an H-section beam penetrates the com-posite column. Since the flange and web of the beam are continuous,the beam moment, as well as the shear, can be directly transferred tothe column. The transverse beam, which also penetrates the column,reduces the slip deformation of the beam occurring inside the column.In the through-bar detail proposed for the concrete-filled U-sectionbeams (see Fig. 2(b)), through-bars connected directly to the beam U-section flanges by welding are used, instead of continuing the beam U-section. Due to ribs and knots, bond resistance of the deformed through-bars within the column can be improved. The vertical shear of the beamis transferred to the column by the beam webs welded to the column

tube wall. For both the through-beam and through-bar details, addi-tional bars passing through the column are placed in the concrete slabso that the negative moment strength of the composite beam can beincreased. For the through-beam details, force transfer at the beam-to-column connection is obvious, but the details are complicated. On theother hand, for the through-bar details, the details are relatively simplebut the load introduction at the joint from the beam to the column isless obvious.

In this study, the seismic performance of the through-beam andthrough-bar details proposed for the interior beam-column connectionof the OCFT columns was investigated. For this, cyclic loading tests offour interior connection specimens were performed. Based on the testresults, effects of the proposed connection details on the cyclic behavior(i.e., strength, ductility, etc.) and failure mode of the specimens wereexamined. In addition, design considerations of the proposed through-beam and through-bar details were given.

2. Test program

2.1. Material strengths

Material strengths of the structural steel and reinforcing bars usedfor the test are shown in Table 1. The specified tensile strength of theweld metal was FEXX=610MPa. The compressive strengths of theconcrete, obtained from compression tests on the testing day of eachspecimen, were fc′=39.4MPa for IJ1 and IJ2, 38.6MPa for IJ3, and38.1MPa for IJ4.

2.2. Specimen details and test parameters

Four interior beam-to-column connection specimens, IJ1–IJ4, wereconsidered for the cyclic loading test. Specimen details and test para-meters are shown in Table 2 and Fig. 3. In all specimens, the height ofthe column between the top and bottom supports was 3030mm (=H).The cross-section of the OCFT column was 650mm×650mm in sizeand the tube wall thickness of the column was 6.0mm. The sectiongeometry of the column was close to square, rather than regular oc-tagon (refer to Fig. 1(a)). The overall length of the beam was 7400mm,and the shear span, defined as the distance from the column face toeither support of the beam, was 3055mm (=Ls). On top of the beamsection, a concrete slab, with a width 3000mm and a thickness 180mm(=hcf) was placed. Between the beam section and concrete slab, headedstuds (diameter 22.2 mm) were placed at a spacing 140mm. In theconcrete slab, twenty D13 bars (diameter db=12.7mm) were placed in

Nomenclature

Notation

A1 loaded area for consideration of bearing strengthA2 area of the lower base of the largest frustum under loaded

areaAb contact area of stopper angle to column tube wallAbf area of through-beam bottom flangebbf\ overall width of beam U-sectionbc,eff effective width of octagonal column tubedb diameter of bar or studfc′ compressive strength of concrete cylinderFEXX tensile strength of weld metalfy yield strength of reinforcing barsFy yield strength of steel plate, tube, or beam sectionH height of column between top and bottom pin supportshcf thickness of concrete slabhw overall height of beam U-section

Ls distance from column face to the nearest support of beamlw length of weld jointMnb nominal moment strength of beam including concrete slabMnc nominal shear-moment capacity for joint shear checkPn theoretical strength of connection (expressed as column

lateral load)Pu maximum test strength of connection (expressed as

column lateral load)s size of weld jointtbf, ttf, and tw thicknesses of top flange, bottom flange, and web,

respectively, in beam U-sectionVbs shear strength of steel through-beam for joint shear checkVcc and Vcs shear strengths of concrete and steel tube, respectively,

for joint shear check in OCFT columnVnb vertical reaction force at beam supportVuc column shear force for joint shear checkδ lateral drift ratioϕ resistance factor (or strength reduction factor)

Fig. 1. Concrete-filled thin-walled tube members.

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the longitudinal direction at a spacing of 150mm. In the transversedirection, D10 bars (db=9.5mm) were placed at spacings of 150mmnear the column and 300mm near the supports. As the transversebeam, a steel H-section (height 400mm, flange width 200mm, webthickness 9mm, and flange thickness 11mm) was used, and headedstuds (db=22.2mm) were placed at a spacing of 140mm for integritywith the concrete slab.

The specimen details described above were identical for all testspecimens. However, the cross-sectional shape and splice location ofthe beams and the connection details, such as through-beam andthrough-bar, were different specimen by specimen as follows (seeTable 2 and Fig. 3).

In IJ1 (see Fig. 3(a)), a steel H-section (height 482mm, flange width300mm, web thickness 11mm, and flange thickness 15mm) was usedfor the longitudinal beam. The H-section beam penetrated the column(i.e., through-beam detail) and then was spliced at a distance 600mmfrom the column face. At the column face, the flange and web of thebeam section were welded with the column tube wall. For the beamsplice, the inclined end-plate connection was used and the momentstrength at the splice was designed to be greater than the beam momentstrength (i.e., strong beam splice concept) [15]. In the inclined end-plate beam splice, the end-plates were projected into the concrete slabby 118mm.

In IJ2 (through-beam detail, see Fig. 3(b)), except that the H-sectionbeam near the supports was replaced by the concrete-filled U-sectionbeam, all specimen details were the same as those of IJ1. The overallheight and width of the beam U-section were 478mm (=hw) and300mm (=bbf), respectively, and the thicknesses of the web, top flange,and bottom flange were 6mm (=tw), 6 mm (=ttf), and 12mm (=tbf),respectively. For the splice between the beam U-section and H-section,

the inclined end-plate connection was used, and the beam splice wasdesigned based on the strong beam splice concept.

In IJ3 (through-beam detail, see Fig. 3(c)), the specimen detailswere the same as those of IJ2, except for the followings. First, to in-crease the negative moment strength, four D22 bars (db=22.2mm)passing through the column were added in the concrete slab. Second,the beam splice was placed at a greater distance 1200mm from thecolumn face. Third, at each column face, a steel angle (width 100mm,height 100mm, and thickness 13mm) of a length 250mm was placedbelow the beam bottom flange. Such angles were planned as a stopperto reduce possible slip deformation of the beam bottom flange insidethe column. The stopper angles were welded to the beam bottom flange,but were in contact with the column tube wall without welding. Thus,the stopper angles were activated by direct contact to the column faceonly under negative bending.

In IJ4 (through-bar detail, see Fig. 3(d)), the beam within 600mmfrom the column face was replaced with the concrete-filled U-sectionbeam. The overall height and width of the beam U-section were478mm (=hw) and 300mm (=bbf), respectively, and the flange andweb thickness of the beam U-section was 6mm. Since the bottom flangethickness was relatively smaller, the moment capacity of the beam wasdecreased, compared with IJ3. The beam U-section did not extendthrough the column. Instead, for continuity within the column, four andtwo D22 through-bars were used at locations of the bottom and topflanges, respectively. The through-bars were connected to the flanges ofthe U-section at distances 250mm–400mm from the column face byFlare-Bevel welding. In addition to the through-bars within the beamcore, four D22 bars passing through the column were also added in theconcrete slab, as was in IJ3. The top and bottom flanges of the beam U-section were not welded to the column tube wall; only the web plates of

Fig. 2. Proposed through-beam and through-bar details for OCFT column connections.

Table 1Material strengths.

Materials Yield strength Fy or fy (MPa) Tensile strength Fu or fu (MPa) Elongation (%)

OCFT column tube Plate 6T 509 537 19U-section for beams Plate 6T 509 537 19

Plate 12T 414 568 22H-section for beams Rolled section 424 553 22End-plate for beam splice Plate 27T 375 475 30Slab reinforcement D10 bars 535 624 19

D13 bars 476 681 21Through-bars D22 bars 460 655 24

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the beam U-section were welded with the column tube wall to transferthe vertical shear of the beam.

2.3. Loading method

The test setup used for cyclic loading test is shown in Fig. 4. TheOCFT column was pin-supported at the bottom. An actuator of a ca-pacity 2000 kN was installed laterally at the top of the column. Thecomposite beam including the concrete slab was roller-supported atboth ends and thus vertical displacement alone was restrained at thesupports. No support was provided for the transverse beam.

Lateral loading by the actuator was controlled by displacement.According to the loading sequence specified in AISC 341 [2], load cy-cles were repeated six times at lateral drift ratios δ=0.375%, 0.5%,and 0.75%, four times at δ=1.0%, and two times δ=1.5%, 2.0%,3.0%, 4.0%, 5.0%, 6.0%, and 7.0%.

3. Test results

3.1. Lateral load-drift ratio relationships

Fig. 5 shows the lateral load-drift ratio (P- δ) relationships of theconnection specimens. The lateral drift ratio δ was calculated by di-viding the lateral displacement at the loading point by the height of thecolumn (H=3030mm). As shown in Fig. 4, vertical and horizontaldisplacements such as δ1 and δ2 that occurred at the supports of thebeam and column were measured during the tests. Based on thosemeasurements, the lateral load-drift ratio relationships were calibratedso that slip deformations due to rigid body motion were eliminated.Thus the lateral displacement ratios in Fig. 5 were slightly less than thecontrolled displacement ratios.

In IJ1 with through-beam detail (see Fig. 5(a)), the overall yieldingof the connection occurred at about δ=1.87% and the maximumstrengths (Pu=+761 and −777 kN) were reached at δ= ±2.88%.The connection strength was decreased significantly in the second loadcycle at δ=3.88%, due to a premature fracture at the weld joint be-tween the beam flange and column tube wall. Significant pinching wasobserved throughout the cyclic behavior, which indicates that the beampassing through the column experienced significant slip deformations.

In IJ2 (see Fig. 5(b)), the overall behavior was the same as that ofIJ1, because the through-beam details in the beam-column connectionswere the same. However, the maximum strengths (Pu=+812 and−811 kN) were slightly increased.

In IJ3 with through-beam detail (see Fig. 5(c)), in which a stopperangle was placed below the bottom flange of the beam and four D22bars passing through the column were added in the concrete slab, themaximum strengths (Pu=+1054 and −1062 kN) and ductility weresignificantly increased, compared with those of IJ1 and IJ2. Theyielding of the connection occurred at about δ=1.96% as the beambottom flanges began to yield in tension at the column face, and themaximum strengths were reached at δ=+3.88% and −4.63%. As thestopper angles restrained the slip behavior of the beam inside thecolumn effectively, the connection strength was maintained untilδ=5.89%–6.90%.

In IJ4 with through-bar detail (see Fig. 5(d)), the yielding of the

connection occurred at about δ=1.83%, and the maximum strengths(Pu=+741 and −703 kN) were then reached at δ= ±3.87%. Themaximum strengths of IJ4 were about 30% less than those of IJ3 be-cause the moment strengths of the beam resisted by 4D22 through-barsat the column face were decreased. The post-yield ductile behaviorlasted until δ=5.84%–6.83%. However, pinching was significantthroughout the cyclic behavior, which indicates that bond slip occurredsignificantly along the through-bars inside the column.

3.2. Failure modes

Fig. 6 shows damage sequences and failure modes of each specimenthat occurred in each lateral drift ratio.

For IJ1 and IJ2, although the bottom flanges of the beam extendinginside the column were significantly pulled out under positive bending,out-of-plane deformation of the column tube wall plate did not sig-nificantly occur at δ=1.87%, as shown in Fig. 6(a1). However, thecolumn tube wall began to exhibit significant out-of-plane deformationat δ=2.88%, as shown in Fig. 6(a2), and thus fracture was initiated atthe weld joint between the beam bottom flange and column tube wall.Consequently, the weld fracture propagated into the column tube walland the slip deformation of the beam flange was ultimately greater than30mm at δ=4.88%, as shown in Fig. 6(a4).

For IJ3, as shown in Fig. 6(b1)–(b4), the stopper angle in contactwith the column tube wall effectively prevented the beam bottomflange in compression under negative bending from being pushed intothe column. Consequently, the slip deformation of the bottom flange inthe opposite side (i.e., pull-out deformation) was significantly reducedeven at large drift ratios of δ=2.89 and 3.88%, as shown in Fig. 6(b2).The behavior at the through-beams with the stopper angles is illustratedin Fig. 7. However, the slip deformation occurred significantly in thebottom flange at δ=6.90%, as the crushing of concrete inside thecolumn occurred and the stopper angle was then pushed in (seeFig. 6(b4)). Ultimately, fracture failure occurred at the weld joint andcolumn tube wall, as shown in Fig. 6(b3).

For IJ4 with through-bar detail, as shown in Fig. 6(c1)–(c2), al-though the flanges of the beam U-section were not welded to thecolumn tube wall, out-of-plane deformation occurred in the columntube wall at δ=1.83% and 2.85%, due to the pull-out force of the webwelded to the column tube wall. At δ=3.87%, fracture was initiated atthe weld joint of the web plate of the beam U-section, as shown inFig. 6(c1). Ultimately at δ=6.83%, the bond slip of the through-barsoccurred significantly inside the column.

Flexural cracks began to occur in the concrete slab under negativebending at δ=0.69%, and then the spalling of the concrete in contactwith the column tube wall occurred under positive bending atδ=2.88% and 3.88%, and the through slab bars were exposed withoutthe cover concrete, as shown in Fig. 8(a). Particularly, the concretespalling was concentrated on a limited slab width bc,eff, (i.e. the effec-tive width of the octagonal tube, see Fig. 1(a)). This is more evidentfrom radial cracks that occurred at the bottom surface of the slab.Furthermore, as shown in Fig. 8(b), the through-slab bars continuinginto the column (SR1) underwent large tensile and compressive plasticstrains during repeated load cycles, while the slab bars bypassing thecolumn (SR2) underwent only elastic tensile strains. These results

Table 2Test parameters.

Specimen IJ1 IJ2 IJ3 IJ4

Connection type Trough-beam Trough-beam Trough-beam Trough-barBeam sections (column face/support) H-/H-sections H-/U-sections H-/U-sections U-/U-sectionsBeam section at column face H 482×300×11×15 H 482×300×11×15 H 482×300×11×15 U 478×300×6×6Through-bars within beam (top/bottom) None None None 2D22/4D22Through-bars in slab None None 4D22 4D22Stopper angle at beam bottom None None L 100×100×13 None

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indicate that the effective width of the concrete flange was approxi-mately the same as the effective width of the column (bc,eff).

3.3. Strain measurements

Fig. 9 shows the steel strains measured from the beam sections andslab bars near the column face. On the left, the maximum strain profilesmeasured at δ= ±1.83%∼ ±4.88% are presented. The solid and

dotted lines are the maximum strain profiles for positive and negativebending, respectively. On the right, the strain histories at the beamsections and slab bars varying with δ are presented. Notice that theranges of lateral displacements at which the strains were recorded aredifferent specimen by specimen and gauge by gauge, because of gaugemalfunctions during the tests.

In IJ1, the strains of the top and bottom flanges, whether they werein tension or compression, were limited to less than 0.004mm/mm in

Fig. 3. Specimen details (mm).

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magnitude even at δ= ±3.88%, as shown in Fig. 9(a). This shows thatthe beam flanges passing through the column underwent significantslip. Since the slab bars were not continuous within the column, theirstrains were much less than the yield strain.

On the other hand, in IJ3 (see Fig. 9(b)), the strains of the bottomflange were significantly increased due to the use of stopper angles.This indicates that the stopper angles significantly decreased the slip ofthe bottom flange inside the column, and increased the flange strains asmuch as the decreased slip. Furthermore, the slab bars continuingthrough the column underwent large plastic strains in tension andcompression.

In IJ4 (see Fig. 9(c)), the strains measured from in the top andbottom flanges in contact to the column tube wall without weldingwere close to zero. This shows that the through-bars carried the tensileand compressive forces of the beam flanges inside column. For the webof the beam U-section not continuous inside the column, the tensilestrains were less than their yield strain (=0.0025mm/mm), while thecompressive strain exceeded the yield strain at the bottom. The strains

of the slab bars passing through the column underwent tensile andcompressive strains greater than their yield strain (=0.0023mm/mm).Strains in the through-bars were not recorded due to malfunction of thestrain gauges.

4. Strength evaluation of proposed connection details

4.1. Load-carrying capacity

The load-carrying capacity of the connections with through-beamand through-bar details were evaluated based on the plastic stressdistribution method. The stress distribution over the composite beamsection including the concrete slab is shown in Fig. 10. Fig. 10(a) and(b) are for through-beam and through-bar details, respectively. In eachfigure, the stress distributions for positive and negative bending arepresented on the left and right, respectively. When compared with theconventional plastic stress distribution method specified in AISC 360[1], the following modifications were made based the test observations.

(1) In the concrete slab subject to positive bending, the effective con-crete flange width was taken as bc,eff (i.e., the width of the flatportion of the column tube wall). This is because concrete crushingfailure early occurred at the interface with the column tube wall.

(2) The slab reinforcement passing through the column was assumed todevelop the yield strength (fy) in compression and tension for po-sitive and negative bending, respectively.

(3) Since the thickness of the column tube wall was small, the webplates of the beam U-section welded to the tube wall did not de-velop their full yield stress or strain, as shown in Fig. 9 [14]. Inaddition, considering the web plates were subjected to both shearforce and bending moment simultaneously, the contribution of theweb plates to the flexural strength was limited. Thus, approxi-mately, the design plastic stress of the web plate of the beam U-

Fig. 4. Test setup for cyclic lateral loading.

Fig. 5. Lateral load-drift ratio relationships by tests.

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section was reduced to 0.5Fy.(4) In the through-bar detail, it was assumed that the top and bottom

flanges developed their yield strength Fy only in compression. Thetensile stress of the flange was ignored because the flanges were notwelded to the column tube wall.

The calculated connection strengths Pn are presented in Table 3. Inthe table, the positive and negative composite beam moment strengths,Mnb

+ and Mnb−, respectively, were computed based on the proposed

plastic stress distributions and actual material strengths. The connec-tion strength Pn was then calculated from the moment equilibrium withrespect to the bottom pin support of the column, as follows (seeFig. 11).

⎜ ⎟=+

= ⎛⎝

⎞⎠

⎛⎝

+ ⎞⎠

+ − + −( )P

V V

HLH

M ML

( )

2nnb nb

Lnb nb

s

2

(1)

where L= length of the beam between the supports at both ends(=6760mm); H=height of the column from the bottom pin support tothe loading point (=3030mm); Ls=shear span length of the beamfrom either support to the nearest column face (=3055mm); and Vnb

+

and Vnb−=vertical reactions at the beam supports corresponding to

the positive and negative moment strengths Mnb+ and Mnb

−, respec-tively.

For IJ3 and IJ4 exhibiting the post-yield ductile behaviors, thecalculated strengths Pn agreed well with the test strengths Pu, as shownin Table 3. The Pu/Pn ratios were 1.03 and 1.04, which indicates thatthe proposed stress distributions in Fig. 10 were appropriate for thestrength estimation of the OCFT column connections with through-

Fig. 6. Failure modes observed from tests.

Fig. 7. Slip behavior of through-beam with and without stopper angles.

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Fig. 8. Concrete failure and bar strains in slab.

Fig. 9. Strains measured from beam steel sections and slab reinforcements.

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beam and through-bar details. However, for IJ1 and IJ2 in which thethrough-beam connection detail was used without stopper angle, thetest strengths Pu were only 84% and 89% of the calculated strengths(Pn). Such insufficient strength development might be attributed to theslip behavior of the through-beam that occurred inside the column.

4.2. Joint shear capacity

In the composite beam-column connections, the shear strength atthe beam-column joint is basically the sum of contributions of thecolumn and beam framing into the joint [1,5,9,17,18]. For the concrete-filled thin-walled tube columns such as the OCFT column, the shearstrengths resisted by structural steel and concrete, Vcs and Vcc, respec-tively, can be calculated as follows [1,13].

=V F h t2(0.6 )cs y ce (2)

= ′V f A1.66cc c c (3)

where Fy=yield strength of the column tube wall; hce=effective depthof the column tube wall except corner chamfering (see Fig. 12);t=tube wall thickness; fc′=compressive strength of the concrete; andAc=concrete area in the column section.

If steel beams penetrate into the composite column, such beams alsocontribute the joint shear strength. The shear strength resisted by the

Fig. 10. Proposed plastic stress distributions for composite beam moments at connection.

Table 3Connection strengths by beam flexural yielding.

Specimen IJ1 IJ2 IJ3 IJ4

Mnb+ and Mnb

−

(kN·m)1490 and1010

1490 and1010

1583 and1218

936 and963

Vnb+ and Vnb

− (kN) 488 and 331 488 and 331 519 and 466 306 and315

Pn (kN) 912 912 1022 693Pu/Pna 0.84 0.89 1.03 1.04

a The test strengths Pu is the mean of the negative and positive test strengths.

Fig. 11. Lateral load and support reactions.

Fig. 12. Moments and shear forces in columns and beams at beam-columnjoint.

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penetrating steel beam can be calculated as follows [1].

≈V F h t0.6bs y c w (4)

where Fy=yield strength of the beam web; hb=overall depth of thebeam steel section except the concrete slab thickness; and tw=webthickness of the web. For U-section beams, tw is taken as the twice theweb thickness.

According to the ASCE Task Committee [5], the joint shear strengthin the interior beam-column joint can be checked based on the fol-lowing moment equilibrium equations (see Fig. 12).

− ⎛⎝

⎞⎠

⩽M V h ϕMΣ Σ2nb uc

bnc (5)

where Mnb=nominal moment strength of the composite beam basedon the stress distributions in Fig. 10; Vuc=column shears at the top andbottom; ϕ=resistance factor for shear; and Mnc=nominal shear-mo-ment capacity for joint shear check; The shear-moment capacity Mnc isapproximately calculated as follows [11].

≈ + +M V V V h( )nc cs cc bs c (6)

The calculated joint shear demands and capacities of the specimensare presented in Table 4. In all specimens, the joint shear capacity-to-demand ratios (i.e., Mnc/[ΣMnb-ΣVuchb/2]) were greater than 1.5. Thiswas consistent with the test results that there was no significant dete-rioration observed at the beam-column joints during the tests. InIJ1–IJ3 with through-beam connection details, the contributions of thecolumn concrete (Vcchb) were 53% of the shear-moment capacity Mnc.In IJ4 with through-bar connection detail, the contribution of thecolumn concrete was greater than 70% of the shear-moment capacityMnc. These indicate that for the composite thin-walled tube columnconnections such as the OCFT column, the concrete contribution wascrucial to secure safety for joint shear.

It is noted that, for specimens IJ1–IJ3 using the through-beam de-tails, beam forces were introduced directly to the concrete and tubewall of the column. Thus, for the through-beam details, the forcetransfer at the joint was clear. For specimen IJ4 using the through-bardetail, beam forces were introduced first to the tube wall of the column,and then the tube wall forces were transferred to the concrete of thecolumn through bond between steel and concrete. Particularly, the ribplates of the OCFT column embedded within the concrete played animportant role for the force transfer. Although the force transferthrough bond between steel and concrete is generally deficient, theforce transfer in IJ4 was enough because the embedded rib plates of theOCFT column provided an enhanced bond resistance. No damage re-lated to the shear transfer between the concrete and tube plate was notobserved.

5. Detailing of stopper angle for through-beam connection

For the OCFT column connection having the through-beam detail,the stopper angle is critical to reduce slip deformation and securestrength and ductility, as shown in Fig. 7 and Table 1. Particularly, ifslip deformation occurs in the through-beam and the thin tube wall ofthe OCFT column consequently tear out (refer to Fig. 6(b3) and (b4)),the column axial load-carrying capacity can be significantly degraded.Thus, design and detailing of the stopper angle that is welded to thethrough-beam bottom flange should be carefully carried out as follows.

• The weld size and length between the stopper angle and beambottom flange are determined such that the weld strength is greaterthan the yield strength of the beam bottom flange as follows [1].

⩾ϕ F s l F A(0.6 )(0.7 )EXX w y bf (7)

where FEXX=tensile strength of filler metal; s and lw=size and lengthof the weld joint; Fy and Abf=yield strength and area of the beambottom flange; and ϕ=resistance factor (=0.75).

• The contact area of the stopper angle to the column tube wall, Ab, isdetermined such that the bearing strength of the concrete inside thetube is not less than the yield strength of the beam bottom flange asfollows [3].

⎧⎨⎩

′ ′ ⎫⎬⎭

⩾ϕ f A AA

ϕ f A F Amin (0.85 ) , ·2(0.85 )c b c b y bf2

1 (8)

where A1= loaded area for consideration of the bearing strength(=Ab); A2= area of the lower base of the largest frustum under theloaded area, contained wholly within the column; and ϕ=resistancefactor (=0.65).

• To spread bearing stresses evenly over the contact area of thestopper angle, stiffener should be provided for the angle legs. Inaddition, the leg of the stopper angle should not be welded to thecolumn tube wall.

6. Conclusions and design considerations

In this study, the through-beam and through-bar details at the in-terior beam-column connection for concrete-filled thin-walled tubecolumns were proposed and their seismic performances were in-vestigated through the cyclic loading tests. The major findings are asfollows.

(1) For the conventional through-beam connections, the strength wasnot fully developed due to slip deformation of the through-beamoccurring inside the column. However, by using stopper anglesreducing the slip of the bottom flange of the beam, the connectionstrength and ductility were significantly improved. Slip deforma-tions occurring inside the column were significant both in thethrough-beam and through-bar connections, and thus the energydissipation capacity of the connections were significantly reducedby pinching.

(2) For both the through-beam and through-bar connections, the webof the steel beam section welded to the column tube wall did notcontribute to the moment strength with their full yield strength.This is because the slip of the through-beams and through-bars, andthe out-of-plane deformation of the column tube wall hinderedstress development in the beam web during repeated load cycles.

(3) Under positive bending, the spalling and crushing of the concrete inthe slab were concentrated only within a limited width in directcontact with the column tube wall. Under negative bending, onlythe through-slab bars continuing within the column underwentlarge plastic strain reversals, while the other slab bars bypassing thecolumn remained in elastic strains.

Based on these investigations, design and detailing considerations ofthe proposed through-beam and through-bar connections for thin-walled tube columns are given as follows.

(1) For the through-beam connection, stopper angles should be placedat the bottom flange of the beam, without welding to the columntube wall. The size of the stopper angle and the strength of the weld

Table 4Shear check in beam-column joint.

Specimen IJ1 IJ2 IJ3 IJ4

− ( )M VΣ Σnb uchb2

(kN·m) 2060 2060 2308 1565

V hcs b (kN·m) 802 802 802 802V hcc b (kN·m) 1922 1922 1903 1890V hbs c (kN·m) 883 883 883 –

= + +M V h V h V h[ ]nc cs b cc b bs c (kN·m) 3607 3607 3588 2692−M M V h/[Σ Σ( /2)]nc nb uc b 1.75 1.75 1.55 1.72

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joint to the beam bottom flange should be sufficient to resist theyield strength of the beam bottom flange. For the through-barconnection, the top and bottom flanges of the beam U-section arenot welded to the column tube wall; however, the webs of the beamU-section need to be welded to the column tube wall to securevertical shear transfer from the beam to the column.

(2) The moment strength of the composite beam including the concreteslab can be calculated based on the conventional plastic stressdistribution method, with the following modifications. First, thestress of the beam web is reduced to 0.5Fy for both negative andpositive bending. Second, the effective width of the concrete slabunder positive bending is taken as the width of the flat portion inthe tube wall. Third, under negative bending, only through-slabbars continuing within the column contributes to the beam momentstrength.

(3) In the beam-column joint, the joint shear strength is estimated bysumming all resistances of the concrete and steel in elements of thecomposite column, such as the tube wall and infilled concrete. If thethrough-beam detail is used, the contribution of the beam webcontinuing within the column can be also taken into account.

Acknowledgement

This work was supported by the research grants of the NationalResearch Foundation of Korea (NRF-2018R1A6A1A07025819) and theMinistry of Land, Infrastructure, and Transport (19AUDP-B106327-05).

Appendix A. Supplementary material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.engstruct.2019.04.007.

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