Research ArticleCyclic Behavior of Steel Beam to CFT Column Connections withGusset Plates

PengWang ,1 ZhanWang ,1,2 Jianrong Pan ,1,2 Yanjun Zheng ,1 and Deming Liu 1

1School of Civil Engineering and Transportation, South China University of Technology, Guangzhou 510641, China2State Key Laboratory of Subtropical Building Science, South China University of Technology, Guangzhou 510641, China

Correspondence should be addressed to Jianrong Pan; ctjrpan@scut.edu.cn

Received 5 June 2019; Accepted 21 October 2019; Published 16 November 2019

Academic Editor: Jiang Jin

Copyright © 2019 Peng Wang et al. 'is is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Beam-brace-CFT (concrete-filled tubular) column connections provide excellent performance in resisting seismic loads in high-risk areas. However, the load transmission mechanism of this type of connection still remains unclear, and there is a lack of studyon it.'erefore, in this paper, the mechanical behavior of beam-brace-CFTcolumn (BBC) connections penetrated by gusset plateswas evaluated through experiments and finite element analysis to resolve this issue.'e failure modes, strength, stiffness, ductility,and energy dissipation of this type of connection were studied. Experiment results indicated that the gusset plates in BBC (beam-brace-CFT) connections could effectively move the plastic hinge on beam away from the column face, reduce the strainconcentration between the beam end and column face, and notably improve the hysteretic behavior; the plastic rotation was ableto achieve at least 4% story drift angle before 20% strength degradation. Numerical studies were carried out and validated byexperiment results, and then the influence of the weld length and strengthening methods were investigated; some improvement ofdesign suggestions was proposed.

1. Introduction

For high-rise buildings, braced frame is a popular choice toresist the earthquake for its high lateral stiffness [1, 2].Moreover, in braced frame systems, concrete-filled tubular(CFT) columns are also commonly used because of theirfavorable strength and stiffness; the concrete fill in CFTcolumns not only contributes stiffness and compressivestrength but also restrains the steel tube from local buckling[3–5]. On the contrary, the steel tubes can be set as theshuttering of concrete so the labor cost could be decreasedduring the construction process. However, when con-necting CFTcolumns to beams and braces together, severalissues should not be ignored [6–9]. For example, theconnections should be capable to transfer all applied loadsas expectation; the loads must be reasonably distributed inthe steel tube and the concrete infill to ensure the compositeaction, and the connections must possess enough de-formation ability to maintain the ductility of the wholestructure.

Current design codes such as Eurocode 4 [10], AISCSpecification [11], and CIDECT [12] provide different ap-proaches to calculate the load distribution among themembers associated with connections. One popular CFTcolumn connection is using an internal diaphragm [13] andthrough diaphragm [14–16] which requires extra completepenetration welds around the perimeter of the tube at thebeam-column intersection and the beam flange. 'e waythat the diaphragm welded in the connection is too difficult,costly, and time consuming to be realized in practical en-gineering. Recently, a more convenient and economic BBCconnection was studied by Macrae et al. [17], Hassan et al.[18], and Hu et al. [19], in which the braces and beams areattached to a vertical gusset plate that penetrates through thecircular CFT column, and the force transfers through fric-tion and bearing mechanism at the bottom of the plate. 'econstruction of penetrating gusset plates increases the re-sistance and ductility of connections, ensures the compositeaction, and restrains the interface slip within the CFT col-umn between steel tubes and concrete infill. However, these

HindawiAdvances in Civil EngineeringVolume 2019, Article ID 3798671, 16 pageshttps://doi.org/10.1155/2019/3798671

researches did not focus on some significant seismic designissues on braced frame connections, such as the constructiondetails of beams, gusset plates, and CFT columns.

Previous seismic record and numerical analysis data showthat the secondary lateral force resisting capacity plays animportant role in collapse resistance in low-ductility structuresunder seismic activity. During the earthquake in Northridge,lots of steel frame structures still stayed up, although theirlateral resisting systems were damaged by seismic action,which owed to the inherent reserve capacity within structures.'is reserve capacity comes from the ductility of the joint,which is especially critical in the seismic design of moderateearthquake areas, and it should not be classified into the initiallateral resistance system [20–22]. Although this capacitywould take function in the severe earthquake zone, it worksbetter in low and moderate earthquake areas.

As mentioned above, beam-to-column connectionswhich have certain flexural resistance in CBFs providesignificant reserve capacity after the failure of the braces. Butprior researches on the flexural behavior of concentricallybraced frame (CBF) connections are limited to the studies ofKishiki et al. [23] and Stoakes and Fahnestock [21].Moreover, despite the convenient and economical pene-trated gusset plate connection employed in the BBC-CBFsystem, previous studies have only focused on the compositeaction between steel tube and concrete and dampers itself,ignoring the influence of other structural components suchas the gusset plate in the connection.

'e aim of this research is to investigate the cyclicperformance of beam-to-CFT connection with gusset platesby experimental and numerical simulations to study itsfailure modes, hysteresis characteristics, ductility, and en-ergy dissipation. 'ese connections are designed to reduceconcentration of stresses at the steel connection near theweld around the CFT column, to help the force transferbetween the beam and the column to be more smoothly, andto improve the overall ductility of these connections. In thisresearch, representative tests have been carried out to studythe seismic performance of connections.

2. Test Program

2.1. Test Specimens and Experimental Test Setup. Five testspecimens were designed in the research by using the sametype of hot-rolled beams (HM194×150× 6× 9 and steelgrade Q235) connected to medium-high round steel tubes(d� 273, t� 8, and steel Q235). 'e bending moment in theconnection area is formed by applying vertical loads to thebeam tip by using an MTS hydraulic loading jack. 'esection size of the beam and column corresponds to the full-size member in the frame structure, and the criterion of thestrong column with the weak beam was considered in thedesign. 'e ends of the beam and column in the test devicecorrespond to the inflection point of the deformation curvesof actual structural members, which can be simulated bysetting the pin boundary condition at both ends of the testmembers. In order to prevent the unexpected out-of-planedeformation of the specimens, transverse braces were in-stalled near the top and bottom beam flange.

'e details of the five specimens are illustrated in Figures 1and 2, and the configuration information is summarized inTable 1.'e specimen SJ-1 was used to assess the performanceof connection without the gusset plate and set as the controlgroup, in which the beam web directly penetrated through theCFT column and the root fillet weld at beam was located onboth sides of the top and bottom flanges. Specimen SJ-2 wasdesigned on the basis of the specimen SJ-1, but two extragusset plates were welded to the beam flange and column wallthrough double-side fillet welds, aiming at investigating theinfluence of gusset plates. For specimen SJ-3, a gusset plate waspenetrated through the CFTcolumn and inserted to the beamslot by fillet welds, and the root of slotted beam flange wasconnected to the column wall like the specimen SJ-1, the beamweb, and the gusset plate was connected by butt welds. Forspecimen SJ-4, the fillet weld between the beam flange andcolumn face was removed. 'e gap between the beam flangeand column wall was 20mm in SJ-5 according to SJ-4. 'econnection zone of the five specimens was all attached withstrain gauges, and the position of the strain gauges was ba-sically the same as that of the specimen SJ-3, and severallocations of the specimens were cleaned and dried to preparefor the installation of strain gauges, as illustrated in Figure 3.

Generally, the section size was controlled by the ultimatecapacity of the MTS loading system. During the cyclicloading until 0.01 rad, the specimens were designed to stay inrelatively elastic stage. Q235B steel was used in steel beams,columns, and gusset plates. Test samples for testing thematerial properties of steel were obtained from the samegroups of plates and tubes as specimens’. 'e coupon testresults are summarized in Table 2.

2.2. Instrumentation. As shown in Figure 4, the MTSloading device was mounted between the reaction steelframe and the beam tip and consisted of a 500 kN hydraulicactuator with a 200mm equal stroke for both upward anddownward directions. 'e displacement (δ1) was measuredby both a built-in displacement transducer of the hydraulicactuator and an external LVDT (linear variable differentialtransformer) named No.1, and the beam rotation (θb) wasrepresented by δ1/lb. Two displacement transducers (Nos. 2and 3) were installed to measure the column rotationaccording to the layout in Figure 5.

2.3. Test Procedure. 'e beam-column connection speci-mens were tested under the cyclic loading sequence rec-ommended by ANSI/AISC-341-16 [11]. 'e loadingprocedure consisted of ten stages. 'e displacement and thenumber of cycles at each stage are shown in Figure 6. 'edisplacement of the first three stages is 0.375%, 0.5%, and0.75% of story drift, for 6 cycles in each stage. 'e rest ofseven loading stages started with 1% of story drift, two cyclesfor each stage. 'e successive drift was increased by 0.5%from stage 4 to stage 6 and by 1% from stage 6 to stage 10. Aquasi-static loading was applied in a displacement-con-trolled mode in a relatively low rate to record the de-formation of the whole process during the specimenssubjected to load cycles.

2 Advances in Civil Engineering

(a) (b)

(c) (d)

(e)

Figure 1: Graphicmodels of the �ve specimens. (a) Specimen SP-1. (b) Specimen SP-2. (c) Specimen SP-3. (d) Specimen SP-4. (e) Specimen SP-5.

12

194

285 1090 10 90 10

150

14856

R136.5

(a)

12 200 253

194

253

1090 10 90 10285

R136.5

6 1485

150

(b)

Figure 2: Continued.

Advances in Civil Engineering 3

Table 1: Geometric detail of specimens.

Specimen Gusset plate Penetration �e weld between the beam ­ange and CFT Weld length between beam ­ange and gusset plateSJ-1 × Beam web Weld —SJ-2 √ Beam web Weld 200mmSJ-3 √ Gusset plate Weld 200mmSJ-4 √ Gusset plate — 200mmSJ-5 √ Gusset plate — 180mm

R136.5

12 200

485

700

253

194

253

1336.5

890200

10

10 90 10

6

150

(c)

12 200

253

194

253

200 89010 90 10

150

1200610

485

700

(d)

12 200

18020

485

700

253

194

253

1180

890180

10

10 90 10

6 150

(e)

700

1639

0

110

700

253

253

395

395200

10

10

9090

90

520

90

90

SP-2SP-3~SP-5

(f )

Figure 2: Connection details of the �ve specimens. (a) Specimen SP-1. (b) Specimen SP-2. (c) Specimen SP-3. (d) Specimen SP-4. (e)Specimen SP-5. (f ) Con�guration of the gusset plate.

2 1011

611

610

10

2

1

10

305830

581

33

1011

611

610

10

3030

10

90 90

30

30

2–2

109090

10

10

10

10

10

3–3

10

1–1

Figure 3: Layout of strain gauges distribution.

4 Advances in Civil Engineering

3. Test Observation

3.1. Failure Modes

3.1.1. Specimen SJ-1. �e ­exural yield of the beam �rstappears at the beam ­ange in the �rst cycle of the 1% storydrift angle; when the rotation reached 1.5% story drift angle,

there was a further yielding occurring in the beam ­ange. Itcan also be observed that a slight ­aking of yellow paint inthe beam ­ange near the beam end. During the last cycles of1.5% story drift angle, some visible microcracks were ini-tiated inside the �llet weld between the beam ­ange andcolumn wall. As load increased, the paint of outer surface ofthe beam ­ange was completely peeled o� during the lastcycles of 2% story drift angle of the test, and the low-cyclefatigue crack became more extensive. �e test on SJ-1 wasterminated at the �rst cycle of 4% story drift angle due tocomplete crack in �llet welds and extension to the columnweb. At the end of the test, there was no obvious localbuckling at the beam ­ange or web.�e test specimen duringthe 4% story drift angle is depicted in Figure 7(a).

3.1.2. Specimen SJ-2. Before the �rst cycle of stage 4,specimen SJ-2 remained elastic, and the yellow paint at theouter surface of the beam ­ange near the exterior edge of thegusset plate ­aked slightly. During the 3% and 4% story driftcycles, it can be observed that initial buckling was spread inthe beam ­ange, as seen in Figure 7(b). When the cycles of4% to 5% story drift angle were reached, the local buckling ofthe upper and lower beam ­anges appeared and the beamweb also started buckling. �ereafter, a minor crack oc-curred in the �llet weld between the gusset plate and beam­ange. When the load cycles reached the end of 0.06 rad,there was a large local buckling appearing at the beam

Table 2: Material properties of steel.

Material �ickness (mm) fy (MPa) fu (MPa) E (GPa) Elongation (%)Beam web 5.35 313.8 463.8 195.7 30.5Beam ­ange 9.28 281.8 443.2 195.9 37.4Gusset plate 9.31 275.1 443.3 191.1 39.4Steel tube 6.91 297.4 463.5 190.5 33.1

1

2

2

3

4

5

6

7

7

Hydraulic load cellReaction frame50-ton actuatorLateral brace

1234

Rigid floorSpecimenRoller

567

Figure 4: Experiment setup.

P2

P1

DT2

DT1

DT3

H

Lb

Figure 5: Layout of the displacement indicator.

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0 5 10 15 20 25–80

–60

–40

–20

0

20

40

60

80

0.02 radStage 6

Stage 5

Stage 4

Stage 3

Stage 2

Stage 1

0.02~0.06 rad

0.015 rad2 cycles

4 cycles0.01 rad

0.0075 rad6 cycles

0.00375 rad

6 cycles

6 cycles0.005 rad

Number of cycles

Disp

lace

men

t (m

m) 0.03 rad

Stage 7

0.04 radStage 8

0.05 radStage 9

0.06 radStage 10

Figure 6: Loading procedure.

+3.047e + 02+2.793e + 02+2.540e + 02+2.286e + 02+2.032e + 02+1.778e + 02+1.524e + 02+1.271e + 02+1.017e + 02+7.632e + 01+5.094e + 01+2.556e + 01+1.830e – 01

S. Mises(avg: 75%)

S. Mises(avg: 75%)

+3.047e + 02+2.793e + 02+2.540e + 02+2.286e + 02+2.032e + 02+1.778e + 02+1.524e + 02+1.271e + 02+1.017e + 02+7.632e + 01+5.094e + 01+2.556e + 01+1.830e – 01

(a)

S. Mises(avg: 75%)

S. Mises(avg: 75%)

+3.257e + 02+2.986e + 02+2.715e + 02+2.444e + 02+2.172e + 02+1.901e + 02+1.630e + 02+1.359e + 02+1.088e + 02+0.164e + 01+5.452e + 01+2.739e + 01+2.724e + 01

+4.370e + 02+4.006e + 02+3.642e + 02+3.278e + 02+2.915e + 02+2.551e + 02+2.187e + 02+1.823e + 02+1.459e + 02+1.095e + 02+7.313e + 01+3.675e + 01+3.579e – 01

(b)

S. Mises(avg: 75%)

S. Mises(avg: 75%)

+3.855e + 02+3.534e + 02+3.213e + 02+2.892e + 02+2.571e + 02+2.250e + 02+1.929e + 02+1.608e + 02+1.287e + 02+9.661e + 01+6.450e + 01+3.240e + 01+2.990e – 01

+3.855e + 02+3.534e + 02+3.213e + 02+2.892e + 02+2.571e + 02+2.250e + 02+1.929e + 02+1.608e + 02+1.287e + 02+9.661e + 01+6.450e + 01+3.240e + 01+2.990e – 01

(c)

Figure 7: Continued.

6 Advances in Civil Engineering

compression ­ange. Finally, the test was terminated duringthe cycles of 6% story drift angle due to the sudden fractureof the beam ­ange or the failure of welds due to excessivedeformations, along with the crack in the �llet weldspreading to the beam end. Figure 7(b) depicts specimen SJ-2 at 6% story drift angle.

3.1.3. Specimens SJ-3, SJ-4, and SJ-5. �e behavior of the lastthree specimens was similar, �rst yielding appeared in thebeam ­ange, followed by paints ­aking. A slight bucklingwas found in the beam ­ange during the cycles of 3% storydrift. With the cycles to 4% story drift, severe buckling withlarge displacement could be seen at the beam ­ange, and amicrocrack was initially at the beam ­ange near the gussetplate, as shown in Figures 7(c)–7(e). Finally, the specimensfailed by a trans�xion fracture in the beam ­ange and beamweb at the story drifts of 5%.

3.2. StrainReading. As shown in Figure 8, the letter Cmeansstrain reading in the CFT column along the column height,and the letter P represents the strain reading in the gussetplate and beam web along the vertical direction of speci-mens. Beyond that, the di�erent vertical loads at the beamend are expressed in the numbers contained in C-30 andP-60. �e outer surface of the tension beam ­ange is set asthe coordinate axis origin. Obviously, the value of the straingauge at the tension zone of the column would be positive.

For better comparison, the strain reading would be multi-plied by negative one.

Figure 8(a) illustrated that an earlier yield occurred inthe steel tube around the weld of the beam web through theconnection. No yielding was found in SJ-2 when the ap-plied load was 65 kN. When the beam end loads were100 kN, both the steel tube and the web enter the yield in SJ-2, and the stress on the steel tube is greater than that on thegusset plate. However, both SJ-3 and SJ-4 did not yield,which indicates that the penetrable gusset plate can relievethe stress concentration in the connection between thecolumn and the beam, but the beam web is yielding in SJ-5due to the shorter weld length between the beam ­ange andgusset plate.

4. Discussion of Test Results

4.1. Moment-Rotation Curves. In order to better analyze thedata collected in the experiments from di�erent specimens,the de�nition of the yield moment (My), the maximummoment (Mu), and the ultimate moment (Mf ) are illustratedin Figure 9, andMf is the bending moment corresponding tothe reduction of the bending moment resistance of theconnection to 0.85Mu. �e hysteretic curves of the speci-mens represented by the moment at column face versusstory drift angle were obtained by dividing the total load celldisplacement by the distance from the center of the load cellto the column centreline, as illustrated in Figure 10. Until thestory drift angle reached 4%, the test specimens with the

S. Mises(avg: 75%)

S. Mises(avg: 75%)

+3.396e + 02+3.113e + 02+2.830e + 02+2.547e + 02+2.265e + 02+1.982e + 02+1.699e + 02+1.416e + 02+1.133e + 02+8.501e + 01+5.673e + 01+2.844e + 01+1.483e – 01

+3.557e + 02+3.261e + 02+2.965e + 02+2.372e + 02+2.076e + 02+1.780e + 02+1.484e + 02+1.187e + 02+8.912e + 02+5.950e + 01+5.673e + 01+2.988e + 01+2.595e – 01

(d)

S.Mises(avg: 75%)

S.Mises(avg: 75%)

+3.398e + 02+3.115e + 02+2.832e + 02+2.549e + 02+2.266e + 02+1.983e + 02+1.700e + 02+1.417e + 02+1.134e + 02+8.512e + 01+5.673e + 01+2.852e + 01+2.267e – 01

+3.868e + 02+3.546e + 02+3.224e + 02+2.902e + 02+2.580e + 02+2.258e + 02+1.936e + 02+1.613e + 02+1.219e + 02+9.692e + 01+6.470e + 01+3.249e + 01+2.739e – 01

(e)

Figure 7: Loading procedure. (a) Failure mode of specimen SJ-1: experiment and FEM analysis. (b) Failure mode of specimen SJ-2:experiment and FEM analysis. (c) Failure mode of specimen SJ-3: experiment and FEM analysis. (d) Failure mode of specimen SJ-4:experiment and FEM analysis. (e) Failure mode of specimen SJ-5: experiment and FEM analysis.

Advances in Civil Engineering 7

–3000 –2500 –2000 –1500 –1000 –500 0 500 1000 1500–180

–120

–60

0

60

120

180

240

300H

eigh

t (m

m)

Strain (με)

C-20 C-30 C-65

P-20P-30P-65

(a)

Hei

ght (

mm

)

–1600 –1200 –800 –400 0 400 800 1200–180

–120

–60

0

60

120

180

240

300

Strain (με)

C-30C-65C-100

P-30 P-65P-100

(b)

Hei

ght (

mm

)

Strain (με)–500 –400 –300 –200 –100 0 100 200 300 400 500

–180

–120

–60

0

60

120

180

240

300

C-30 C-65 C-100

P-30P-65

P-100

(c)

Hei

ght (

mm

)

Strain (με)–600 –400 –200 0 200 400 600

–180

–120

–60

0

60

120

180

240

300

C-30C-65C-100

P-30P-65P-100

(d)

Hei

ght (

mm

)

Strain (με)–800 –400 0 400 800 1200 1600 2000

–180

–120

–60

0

60

120

180

240

300

C-30 C-65 C-100

P-30P-65P-100

(e)

Figure 8: Key parameters in the moment-rotation relationship of specimens. (a) SJ-1. (b) SJ-2. (c) SJ-3. (d) SJ-4. (e) SJ-5.

8 Advances in Civil Engineering

O

Mu

θ

M

Mf

My

θy θu θf

0.85Mu

Figure 9: Key parameters in the moment-rotation relationship of specimens.

–80 –60 –40 –20 0 20 40 60 80

–75

–50

–25

0

25

50

75

–0.06 –0.04 –0.02 0.00 0.02 0.04 0.06

–90

–60

–30

0

30

60

90

Load

(kN

)

Mom

ent (

kN·m

)

Displacement (mm)

Rotation (rad)

0.8Mbp

–0.8Mbp

SJ-1TESTSJ-1FE

(a)

Mom

ent (

kN·m

)

–80 –60 –40 –20 0 20 40 60 80Displacement (mm)

–0.06 –0.04 –0.02 0.00 0.02 0.04 0.06Rotation (rad)

–120–90–60–30

0306090

120

–120–90–60–300306090120

Load

(kN

)0.8Mbp

–0.8Mbp

SJ-2TESTSJ-2FE

(b)

–80 –60 –40 –20 0 20 40 60 80Displacement (mm)

–0.06 –0.04 –0.02 0.00 0.02 0.04 0.06Rotation (rad)

–120–90–60–30

0306090

120

–120–90–60–300306090120

–0.8Mbp

Load

(kN

)

0.8Mbp

SJ-3TESTSJ-3FE

Mom

ent (

kN·m

)

(c)

–80 –60 –40 –20 0 20 40 60 80Displacement (mm)

–0.06 –0.04 –0.02 0.00 0.02 0.04 0.06Rotation (rad)

–120–90–60–30

0306090

120

Load

(kN

)

–120–90–60–300306090120

Mom

ent (

kN·m

)

–0.8Mbp

0.8Mbp

SJ-4TESTSJ-4FE

(d)

Figure 10: Continued.

Advances in Civil Engineering 9

gusset plate exhibited stable and reliable hysteresis behaviorwith only small strength degradation.

�e speci�cation ANSI/AISC-341-16 [11] suggests thatthe composite special moment frame proposes a re-quirement that story drift angle should not be less than0.04 rad.

In addition, the calculated ­exural moment resistanceof connection determined at column face should equal atleast 0.8 nominal plastic moment resistance (Mbp) of thebeam at story drift angle of 0.04 rad in the compositespecial moment frame. �e story drift angles of specimensat di�erent stages are summarized in Table 3. SpecimensSJ-2 and SJ-3 achieved a total story drift for at least0.05 rad with maximum 15% strength degradation, whileSJ-4 and SJ-5 achieved at least 0.04 total story drift.�erefore, all the gusset plate connections meet the re-quirements of the ANSI/AISC-341-16 [11] which acceptsmaximum 20% strength degradation until 0.04 rad storydrifts for qualifying a connection for special momentresisting frames.

As illustrated in Figure 11, the bending moment at thelocation of the plastic hinge revealed that the connectionwith gusset plates had bearing moment that was 1.18 to 1.25times larger than Mbp in the gusset plate connection, re-spectively. Considering the strain hardening e�ect, thebending moment at the plastic hinge of specimens should belarger than the theoretical value Mbp. �e speci�cationANSI/AISC-341-16 [11] suggests an overstrength factor of1.1Ry� 1.21 to calculate the actual bending moment con-sidering the strain hardening phenomenon. As previouslydiscussed, this method can closely estimate the ultimatebending moment at the plastic hinge. And, the experimentalresults of bending moment at the plastic hinge is close to themethod suggested by ANSI/AISC-341-16 [11]; therefore, itcan be applied in the practical design of this type ofconnections.

4.2. Connection Rigidity. As suggested by CEN-1993-1-8[24], the connection rigidity and strength can be classi�edbased on their characteristic of their moment-rotationcurves; on the perspective of connection rigidity, a joint canbe evaluated as nominally pinned, rigid, or semirigid by itsinitial rotational sti�ness (Sj.ini) according to the boundariessuggested by EC3 part 1.8: a joint is nominally pinned ifSj.ini≥ 00.5EI/L, where E, I, and L are elasticity modulus,second moment of area, and the length of the steel beam,respectively. A rigid joint satis�es Sj.ini≥ kb EI/L, kb� 25EI/Lfor nonbraced frame and kb� 8EI/L for braced frame. Interms of strength, the minimum required connection re-sistance for partial-strength design is 0.25Mbp and the full-strength connection resistance is beyond Mbp. Two pa-rameters are introduced to, respectively, normalize themoment and rotation of the connection to facilitate analysisand comparison of data collected in the experiments. �ede�nition of the two parameters is as follows:

m �M

Mbp,

θ �θMbp

EIbLb.

(1)

For the aforementioned connection classi�ed method,the analysis results of test joint connection with di�erenttypes are shown in Figures 10 and 12. It indicates that the testjoints with the gusset plate could be regarded as a rigid andfull-strength connection in the braced frame; it also could beconsidered as semirigid with a full-strength connection inthe nonbraced frame.

4.3. Ductility. �e ductility of specimens is de�ned as theability of elastic-plastic deformation of specimens withoutobvious reduction of bearing capacity. In order to de�ne

–0.8Mbp

0.8Mbp

SJ-5TESTSJ-5FE

–0.06 –0.04 –0.02 0.00 0.02 0.04 0.06Rotation (rad)

–120–90–60–300306090120

Mom

ent (

kN·m

)

–120–90–60–30

0306090

120

Load

(kN

)

–80 –60 –40 –20 0 20 40 60 80Displacement (mm)

(e)

SJ-4SJ-5

SJ-1SJ-2SJ-3

–1.5

–1.0

–0.5

0.0

0.5

1.0

1.5

m

–120–90–60–30

0306090

120

Load

(kN

)

–80 –60 –40 –20 0 20 40 60 80Displacement (mm)

–0.06 –0.04 –0.02 0.00 0.02 0.04 0.06θ

(f )

Figure 10: Hysterical and skeleton curves. (a) SJ-1. (b) SJ-2. (c) SJ-3. (d) SJ-4. (e) SJ-5. (f ) Skeleton curves.

10 Advances in Civil Engineering

the ductility of the joint in this study, the ductility ratio (μ)is calculated by equation (2), where θy is the story driftangle when joint yielding θu is the story drift angle cor-responding to the bending moment resistance of theconnection reduced to 0.85Mmax. Although this

evaluation method may underestimate the ductility of theconnection to a certain extent, it is still reliable and ef-fective to compare the ductility among specimens. �ecomparison of the ductility ratios is summarized inTable 4.

Table 3: Moment and rotation at di�erent stages.

Specimen number θy (mm) My (kN) θmax (mm) Mmax (kN) θu (mm) Mu (kN)SJ-1 0.011 46.79 0.021 84.63 0.032 71.94SJ-2 0.010 70.46 0.043 119.71 0.056 101.76SJ-3 0.009 71.05 0.042 122.39 0.054 104.03SJ-4 0.010 74.47 0.043 127.41 0.046 108.30SJ-5 0.009 70.64 0.039 123.52 0.042 104.99

Beam

mom

ent Pl

astic

hin

ge

Mbp

Mdem

LpCo

lum

n fa

ce

Seismic demand

Distance from middle span

Figure 11: Bending moment diagram of the connection.

0.00 0.01 0.02 0.03 0.04 0.05θ

m

SJ-4SJ-5

SJ-1SJ-2SJ-3

0.0

0.1

0.2

0.3

0.4m = 24.77θ

m = 25θ

m = 22.93θ

m = 21.74θ

m = 16.63θ

m = 6.88θ

m = 0.5θ

(a)

0.00 0.01 0.02 0.03 0.04 0.05θ

m = 8θ

SJ-4SJ-5

SJ-1SJ-2SJ-3

m

0.0

0.1

0.2

0.3

0.4m = 24.77θ m = 22.93θ

m = 21.74θ

m = 16.63θ

m = 6.88θ

m = 0.5θ

(b)

Figure 12: Connection classi�cation by initial sti�ness. (a) Nonbracing frame. (b) Bracing frame.

Advances in Civil Engineering 11

μ �θuθy

. (2)

Ultrasonic testing was used to test the quality of the weldin the connection area before the test, and the quality of theweld is not fully guaranteed, for defects are inevitable in thewelds, which leads to brittleness of joints. Nonductilecracking or tearing affects the ductility of joint, whichfurther affects the seismic performance of specimens. An-other important point worth noting is that weld failure wasfound in SJ-1 with a lowest ductility ratio (μ� 2.91) due tothe fillet welds applied between the beam flange and columnwall instead of using complete joint penetration groovewelds. 'e ductility ratio of specimens with the gusset plate(SJ-2 and SJ-3) has greatest ductility ratios, as a result ofadditional force transferring path provided by the gussetplate. All the shear force and moment in specimens SJ-4 andSJ-5 were transferred by the gusset plate alone; consequently,stress concentration would be more serious that led topremature fracture around welds and weak cross section.

According to the Chinese Building Seismic Design CodeGB50011-2010 [25], which specifies the elastic story driftangle limit value [θe] and the elastic-plastic interlayer storydrift angle limit value [θp] in the multilayer and high-steelstructures are [θe]� 1/250 and [θp]� 1/50. As can be seenfrom the results in Table 4, almost all of the test specimensmeet the requirements for deformation checks under seismicloading.

4.4. Energy Dissipation. 'e energy absorption (dissipation)capability is one of the key points to evaluate the seismicperformance of a structure, and the total energy dissipationcapacity is defined as the envelope area of the hystereticcurve, which indicates the energy dissipation capacity.Equivalent viscous damping (EVD) is one of the three keyparameters evaluating the energy dissipation capacity ofconnection components. 'e method to calculate EVD isdefined in Figure 13(a). In equation(3), SABC and SCDA referto the upper half and lower half areas of the hysteresis curve,respectively. SOBE and SODF represent corresponding tri-angular areas:

he �12π

SABC + SCDA

SOBE + SODF. (3)

Figure 13(c) describes the changes of EVDs of the fourspecimens with increasing story drift angle; at the drift levelof 0.04 rad, the values of EVD for all tested specimens withthe gusset plate were above 0.4, which indicates good energydissipation capacity of all specimens.

Another two properties are the energy dissipated in eachstep (Ei) and accumulated energy dissipation (Et).Figures 13(b)–13(d) show Ei and Et of the five specimens; theaccumulated energy dissipations of the specimens with thegusset plate at failure varied between 50 kJ and 70 kJ. For thethree specimens with penetrated gusset plate show similarresponses at ultimate state; Et of specimen with the gussetplate at 0.03 rad was almost twice that of specimen SJ-1;therefore, the presence of the gusset plate could largelyenhance energy dissipation behavior. However, in com-parison with SJ-3 and SJ-4, the increase caused by weldbetween the column and beam flange was not obvious. It isworth noting that the overall performance of specimen SJ-2possessed the largest accumulated energy dissipations.

5. Finite Element Analysis

5.1. General. A finite element method (FEM) analysis modelwas established in ABAQUS to simulate specimens withoutgusset plate (SJ-1) and specimens with gusset plate (SJ-2, SJ-3, SJ-4, and SJ-5). 'e FEM results were verified by theexperimental dates. Further parametric analyses were con-ducted and accordingly some design suggestions werepresented.

5.2. Material Modeling of Steel and Concrete. 'e yieldstrength (fy) and the ultimate strength (fu) of steel wereadopted for every actual tensile coupon test result, assummarized earlier in Table 1. Poisson’s ratio was assumedto be 0.3. 'e constitutive relationship of steel adopts thethree-fold line model. In general, the concrete material canuse the damage plastic model, but the damage plasticitymodel is not applicable to the CFT column. In order toconsider the concrete restraint effect in CFT, the peak strainand transformation will be increased according to Han et al.[26]. 'e input stress-strain branch of the strain curve wasmodified, as shown in Figure 14.

5.3. Assessments of the Finite Element Models. In Figure 15,steel tubes, beam flanges, beam webs, gussets, and concretewere all meshed using 8-node linear brick incompatiblemode components (C3D8I), which reduced integration anduses hourglass control, and structured mesh controls wereused for all components.'e general mesh size for the entiremodel is 13mm, and there were at least three layers in thethickness direction. According to the previous study [9, 27],by using this mesh size for numerical analysis, better cal-culation results can be obtained with lower computationalcost. Surface-surface contact interaction was applied tosimulate the interaction between steel and reinforced con-crete by specifying the normal hard contact and tangentialfriction (with a friction coefficient of 0.6, in Coulombfriction model, maximum surface bond stress is 0.6MPa).'e interaction caused the concrete to separate from thesteel tube after partial buckling of the steel tube, and themerge characteristics are used to simulate the interactionbetween the steel tube, the beam, and the gusset. 'eboundary conditions are completely similar to the actual

Table 4: Ductility factor of specimens.

Specimen number θe/[θe] θp/[θp] μ

SJ-1 2.75 1.05 2.91SJ-2 2.50 2.30 5.60SJ-3 2.25 2.25 6.00SJ-4 2.50 1.80 4.60SJ-5 2.25 1.65 4.67

12 Advances in Civil Engineering

B

D

AOC

F E

M

θ

(a)

1 2 3 4 5 6 7 8 90

5

10

15

20

25

0.04 rad

Ei (k

J)

Load stage

SJ-2

SJ-4

SJ-1

SJ-3

SJ-5

(b)

0 1 2 3 4 5 6 7 8 9 100.2

0.3

0.4

0.5

0.6

Load stage

h e

0.04 rad

SJ-3

SJ-1SJ-2

SJ-4SJ-5

(c)

0 1 2 3 4 5 6 7 8 9 100

10

20

30

40

50

60

70E t

otal

(kJ)

Load stage

0.04 rad

SJ-1SJ-2SJ-3

SJ-5SJ-4

(d)

Figure 13: Comparison of energy dissipation and the hysteretic loop in the test specimens. (a) De�nition of a hysteretic loop. (b) Energydissipated during each step. (c) Equivalent viscous damping. (d) Accumulated energy dissipation.

0.0000 0.0005 0.0010 0.0015 0.0020 0.00250.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

σ (M

Pa)

ε

(a)

0.000 0.005 0.010 0.015 0.020 0.0250

5

10

15

20

25

30

35

ε

σ (M

Pa)

(b)

Figure 14: Stress-strain curves of the �lled concrete in the simulation.�e behavior for �lled concrete in tension (a) and in compression (b).

Advances in Civil Engineering 13

boundary conditions of the specimen. In order to simulatethe hinge constraint of the column end and the beam end,coupling constraints are generated at the end of the columnand the beam to eliminate unrealistic stress and strainconcentration. By limiting the out-of-plane motion of theupper and lower flanges of the beam to prevent overallinstability, lateral constraint was provided at the beam flangeat certain area during testing.

5.4. Comparison of Tests and Numerical Analysis. Table 5presents the FE analysis results in comparison with the testresults, and the results of the FE analysis agree with theexperimental behavior of the examined joints. Indeed, theplastic deformation mode identified during testing corre-sponds to the one observed in the FE model, and the re-sponse curves show good agreement in some ways of initialrotation stiffness (Sji) and plastic moment (Mp) [27]. 'estrain and stress distributions for SJ-1, SJ-2, SJ-3, and SJ-4are shown in Figure 10. Plastic hinges formed in the beamare observed in simulation results which coincide with thefailure modes found in test results.

5.5.Weld Length. In order to investigate the influence of thewelding length of the beam flange to the gusset plate, on thebasis of SJ-5, the welding length was modified in the sim-ulation, and the welding lengths were 190mm, 180mm,170mm, 160mm, 140mm, 120mm, and 100mm. 'enumerical models were numbered sequentially as SJ5-W1∼SJ5-W7, and the specimen SJ5-W2 and the specimenSJ-5 were the same models.

As summarized in Table 6, it could be found thatextending the welding length of the flange and the joint platein a certain range would help to improve the ultimatebending moment and the initial rotational stiffness of joints.

When the welding length of the beam flange and the jointplate reached the height of the section of the steel beam, theincrease of the ultimate bending moment and initial stiffnessis not obvious. 'erefore, the welding length between thegusset plate and beam flange should not be less than theheight of the connected beam.

5.6. Strengthen Pattern. In order to avoid or delay thefracture of the steel beam, on the basis of specimen SJ-5, theconnection area was strengthened by welding cover plateand web plate; as illustrated in Figure 16, the strengthenedmodel was named SJ5E. 'e specific size of the cover platewas 240mm× 50mm× 8mm, and the web plate was140mm× 120mm× 8mm, and the ultimate failure modesand Mises stress distribution of the connections and beamend are presented in Figures 16 and 17. By comparing thestress distribution of experimental specimens and

Beam

Loading point

Coupling constraint

URx = URz = 0Ux = Uy = Uz = 0

URx = URz = 0Ux = Uy = Uz = 0

Gusset plate

XY

Z

Figure 15: Finite element model and mesh.

Table 5: Validation between experiment and finite elementanalysis.

Specimens

Maximumdisplacement

(mm)Ultimate load (kN)

Initialstiffness Sj.ini/(kN·m·rad− 1)

FEM Exp FEM Exp FEM Exp

SJ-1 36.9 38.6/− 39.9 80.2 72.8/− 85.9 11963 12258

SJ-2 73.8 68.0/− 65.4 117.0 108.0/

− 113.6 28181 29616

SJ-3 61.5 66.9/− 64.6 116.7 111.3/

− 117.5 52062 44114

SJ-4 61.5 66.8/− 65.0 116.0 115.9/

− 121.0 44021 40845

SJ-5 61.5 61.2/− 48.8 115.3 112.0/

− 118.3 43081 38752

14 Advances in Civil Engineering

strengthened models, the plastic hinge forming in the steelbeam was moved to the edge of the cover plate and the webplate, away from the welding area of the steel beam and thejoint plate, and the possibility of steel beam fracture at theweld may be reduced at the stress path of the beam end [28].�e initial rotational sti�ness was signi�cantly enhanced instrengthen models; meanwhile, the ultimate moment re-sistance was slightly improved.

6. Summary and Conclusion

�is study involved an experiment to investigate the cyclicperformance of the CFTcolumn connection with penetratedgusset plate. �e experiment and numerical analysis resultspresented herein support the following conclusions:

(1) �e connection with a penetrated gusset plate ef-fectively reduced stress and strain concentration atthe weld between the column and beam and largelymoves the plastic hinges away from the column face.In addition, it could signi�cantly improve the duc-tility and energy dissipation capacity of the con-nection under cyclic loading, achieving a plasticrotation at least 4% total story drift and a maximumstrength reduction of 15%.

(2) �e connection without the gusset plate failed due tofracture at the �llet weld, and the penetrated beamweb connection with additional gusset plate failedfor an extreme plastic hinge in the beam.�e fracturewas found in the three penetrated gusset plateconnections due to the slot and heat-a�ected zone inthe beam with large plastic deformation in the beam.

(3) �e experimental results of bending moment at theplastic hinge is close to the method suggested byANSI/AISC-360-16 which adopts an overstrengthfactor of 1.1Ry� 1.21 to calculate the actual bendingmoment considered in the strain hardeningphenomenon.

(4) �e welding length between the beam ­ange andpenetrated gusset plate should not be less than theheight of the connected beam. �e specimen SJ-5reinforced by additional cover plates and web plateswill increase the sti�ness and ultimate moment andproperly enhance the ductility by moving the plastichinge away from the slot and heat-a�ected zone ofthe beam.

Data Availability

�e data used to support the �ndings of this study areavailable from the �rst author upon request.

Conflicts of Interest

�e authors declare no con­icts of interest.

Table 6: Percentage change of characteristic quantities of initial sti�ness and plastic moment resistance with di�erent weld lengths betweenthe gusset plate and beam ­ange.

Specimens Sj.ini/(kN·m·rad− 1) Percentage (%) Mu (kN·m) Percentage (%) Mp (kN·m) θp (rad)SJ5-W1 43984 30.4 123.5 16.7 106.3 2.42×10− 3SJ5-W2 43081 27.7 122.8 16.1 106.4 2.46×10− 3SJ5-W3 42165 25.0 121.7 15.0 106.4 2.53×10− 3SJ5-W4 41171 22.0 120.3 13.7 106.1 2.58×10− 3SJ5-W5 38969 15.5 116.8 10.4 102.5 2.63×10− 3SJ5-W6 36499 8.2 112.1 6.0 94.7 2.60×10− 3SJ5-W7 33741 — 105.8 — 86.0 2.55×10− 3

Note. Sj.ini is the initial rotational sti�ness; Mu is the moment resistance at 0.05 rad; Mp refers to plastic moment resistance; θp rotation corresponds to Mp;percentage means the increasing comparing to SJ-5-W7.

–3.868e + 02–3.546e + 02–3.224e + 02–2.902e + 02–2.580e + 02–2.258e + 02–1.936e + 02–1.619e + 02–1.291e + 02–9.692e + 01–6.470e + 01–3.240e + 01–2.739e + 01

S. Mises(avg: 75%)

Figure 16: Stress nephograms of specimens SJ5 and SJ5E.

0 50 100 150 200 250 300 350 400

200

250

300

350

400

450

S-M

ises (

MPa

)

Lp (mm)

0.03 SJ5E

0.05 SJ50.02 SJ5E

0.04 SJ5

0.02 SJ50.03 SJ5

0.04 SJ5E0.05 SJ5E

Figure 17: Stress path at beam end at di�erent rotations.

Advances in Civil Engineering 15

Acknowledgments

'is study was supported by the National Natural ScienceFoundation of China (Grant nos. 51638009; 51778241;51978279), the Fundamental Research Funds for the CentralUniversities (Grant nos. 2019MS121; 2019PY20; 2019ZD47),Chinese Postdoctoral Foundation of China (Grant no.2019M652898), and the Young Innovative Talents Programin Universities and Colleges of Guangdong Province (Grantno. 2018KQNCX006).

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