Composite Structures 120 (2015) 262–274

Contents lists available at ScienceDirect

Composite Structures

journal homepage: www.elsevier .com/locate /compstruct

Design procedure of GFRP-infill panels to improve the strengthof steel-frame structures

http://dx.doi.org/10.1016/j.compstruct.2014.09.0670263-8223/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author at: Dept. of Civil Engineering, Gyeongsang NationalUniversity, 501 Jinju-daero, Jinju 660-701, South Korea. Tel.: +82 55 759 0538; fax:+82 55 772 1799.

E-mail address: jskim0330@gmail.com (J. Kim).

Minho Kwon a, Jinsup Kim b,⇑, Wooyoung Jung c, Hyunsu Seo b

a Dept. of Civil Engineering, ERI, Gyeongsang National University, Jinju 660-701, South Koreab Dept. of Civil Engineering, Gyeongsang National University, Jinju 660-701, South Koreac Dept. of Civil Engineering, Gangneung-Wonju National University, Gangneung 210-702, South Korea

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

Article history:Available online 12 October 2014

Keywords:GFRP infill panels (GIPs)Design procedureStrengthInfill frameSteel frame

A design procedure of GFRP-infill panels (GIPs) was proposed for use in steel-frame structures. The designgoal for these panels was to increase the strength of the steel-frame structures. The GIP was composed ofa GFRP plate and a stiffener, in which the cross-sectional shape was assumed to be a box. Each GIP wasintended to strengthen the existing steel frame and to provide emergency support for damaged steelframes under seismic loads. To achieve these goals, separate types of GIPs were made, that also providedeasy transportation and construction. In this study, GIPs were designed and fabricated using the proposeddesign procedure. The maximum strength obtained in the experiments corresponded to the expecteddesign strength. It was demonstrated that the proposed design procedure for GIPs could be used toimprove the performance of infill-frame structures.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction the behavior of the new panels. After that study, construction

Fiber reinforced polymer (FRP) composite materials have theadvantages of being lightweight and having a high stiffness-to-weight ratio, good chemical resistance, high fatigue strength andpotentially high resistance to environmental degradation. Manyresearchers have shown that structural elements strengthened byFRP composites enhanced the performance of existing and ofdamaged frame structures [1–14]. Therefore, in this study,FRP-composite panels were developed to enhance the structuralperformance of infill-frame structures. One of the developmentobjectives was to create FRP-composite panels for use in a powerplant constructed with a steel frame. FRP-infill panels are intendedfor use in places where there would be a high risk of fire fromwelding, where there would be chemical effects, where the struc-tures should not increase significantly in weight, and where thebuildings were intended for semi-permanent use (e.g., nuclearpower plant, chemical-plant, and ocean-side-steel structures, aswell as common steel-structure and plant-type-factory buildings).

Jung and Aref [15] was the first to develop infill FRP panels. Thepanels they developed were applied to steel-frame structures withbolted joints, and numerical studies were performed to evaluate

demand steadily increased for strengthening techniques usingFRP-infill panels. However, current studies on FRP-composite-infillpanels have not provided enough information to meet these risingdemands. Also, an effective procedure to satisfy specific designgoals had not yet been proposed for FRP panels.

In this study, a proposal is presented to use glass-fiber rein-forced polymer (GFRP) infill panels to enhance the strength ofexisting walls, or to replace damaged walls when they must bestrengthened or repaired in emergencies. In order to use GFRP-infill panels (GIPs) in construction, a simple method for predictingtheir behavior and an effective design-procedure, are required.However, there is not presently any design procedure proposedspecifically for GIPs to be used on steel-frame structures. For thesereasons, a novel procedure applicable to the design of GFRP-composite panels, is herein proposed. The proposed designprocedure was developed to achieve specific goals for improvedperformance of framed structures. Finally, the performance ofsteel-frame structures strengthened with the proposed GIP systemwas evaluated experimentally.

2. Design procedure for GFRP-infill panels

2.1. In-plane behavior of GFRP-infill panels (GIPs)

The behavior of infill-frame structures is complex and difficultto understand. Both the infill-material properties and the

Fig. 1. In-Plane behavior of the infill-frame structure.

Fig. 2. Cross-sectional shape of the single GIP.

M. Kwon et al. / Composite Structures 120 (2015) 262–274 263

interactions between the frame and infill are important factors inthe overall stiffness and strength of an infill-frame. The design pro-cedure proposed in this study is a general outline of the equiva-lent-strut mechanism developed from previous research ontraditional infill-frame structures.

Several approximation methods have been proposed todetermine the lateral stiffness of infill-frame structures. Amongthese, simplified methods based on an equivalent-strut model havebeen proposed [16–19]. In the case of an infill-frame with perfectbonding conditions, the lateral stiffness of the infill subjected toshear deformation is modeled by a set of pin-ended diagonal struts

Table 1Design procedure for GIPs.

Step Description

1 Determine material2 Calculate angle3 Determine target force4 Determined contact ratio by numerical method5 Calculate effective plate width

6 Calculate effective plate length

7 Assume FRP thickness8 Calculate area9 Calculate axial force

10 Check target force11 Divide thickness of plate12 Assume core thickness13 Calculate equivalent stiffness

14 Calculate plate buckling load

15 Check target force

running in both directions. The diagonally compressive loads onthe infill are used to obtain the lateral stiffness.

The stress of a panel distributed diagonally can be analyzedusing the contact length between the infill and the frame. The ver-tical and horizontal (ah is vertical contact length, aL is horizontalcontact length) contact lengths are calculated based on the interac-tion distributions. Hendry [18] proposed an equation of effectivestrut width, w, based on the contact lengths.

w ¼ 0:5ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffia2

h þ a2L

qð1Þ

Stafford-Smith and Carter [16] stated that the contact length of abeam, aL, can be assumed to be roughly half the span length becausevariations in beam stiffness have little effect on structural behavior.An exact mathematical solution for frame/infill contact lengths wasderived by Saneinejad and Hobbs [19]. They considered the inter-face shear forces, as well as both the elastic and plastic behaviorof the material, in the derivation. Based on the proposed diagonalstrut stability, and the proposed geometry of the strut angle, theeffective length and strut angle of the diagonal band can becalculated using the contact lengths and geometry of the infill.The effective length, leff, is arranged as in Eqs. (2) and (3), basedon the research of Saneinejad and Hobbs [19].

leff ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffið1� ahÞ2h02 þ l02

qð2Þ

tan h0 ¼ h0 � ahh0

l0ð3Þ

2.2. Design assumptions

The design goal for the GIP was to increase the performance of asteel-frame structure under seismic load. Each panel consisted of aGFRP-plate and stiffeners. The cross-section of such a GIP can bedesigned to achieve the level of strengthening needed for the steelframe.

The in-plane behavior of the infill-frame was assumed to be acompression strut, as shown in Fig. 1. As shown in Fig. 1, aH is ver-tical contact length ratio, aL is horizontal contact length ratio andHp and Lp are the height and width of the GFRP panel. weff and leff

are effective width and length of the compression strut. There wereseveral basic assumptions made in the design of the new GIP. Itwas assumed that the applied in-plane loads are transferred onlythrough the GFRP-plate in the GIP. The GFRP-stiffener wasassumed only to resist buckling. The effective maximum strain

Formula Remarks

Ef, ef, Hp, Lp

h = tan�1(Hp/Lp)PH

aH, aL

weff ¼ 12

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiðaHHpÞ2 þ ðaLLpÞ2

qleff ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffið1� aHÞ2H2

p þ ð1� aLÞ2L2p

qtAs = weff � tNf = EfAsef

Nf P PH= cos h If N.G., go to Step 7h = 0.5tc

ðEIÞeq ¼ weffEf h3

6 þ 2Ef h h2þ c

2

� �2� �

PE ¼p2ðEIÞeq

ðleff Þ2

PE P PH= cos h If N.G., go to Step 12

Fig. 3. Flow chart.

Fig. 4. Test specimen details (Unit = mm).

264 M. Kwon et al. / Composite Structures 120 (2015) 262–274

(ef) of the GFRP material was assumed to be 0.002 (considering asafety factor); even though the maximum strain was defined as0.004 in ACI-318 [20]. It was assumed that the cross-sectional areawould not change during deformation. The cross-section of thecore materials was ignored in the calculation of the panel cross-section. The core materials in the cross-section were ignored inthe cross-sectional area. The GFRP material was treated as ahomogeneous and isotropic material.

2.3. In-plane load

The panel can receive an axial load up to the value of thelaminate’s in-plane strength. In terms of strain, the material will

not fail in compression as long as the predicted strain ef is less thanthe laminate’s compressive-failure strain. In terms of stress,because the strain ef is the same in the plate and the stiffener(under the assumption of a perfect bond) the load becomes

Nf ¼ Ef Asef ð4Þ

where Nf is the load per unit width (N/m) on the plate, Ef is the elas-tic moduli of the plate, and As is the plate section area.

2.4. Contact length

The contact length between the GIP and the steel frame is animportant parameter for panel design. In previous research aboutGFRP panels [15], only vertical contact length was considered inthe calculation of effective width and effective length. However,horizontal contact length also occurred through the lateral loadingforce. Those contact lengths were dependent on the strength andshapes of the structure. So, in this study, the contact-length values,the contact length and contact length ratio, were determined fromthe numerical results obtained by the ABAQUS [21] general-purpose finite-element code.

2.5. Effective width and length

The effective width (weff) of the compression strut formed in theGIP is derived by

weff ¼12

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiaHHp� �2 þ aLLp

� �2q

ð5Þ

after which the effective length (leff) of the compression strutbecomes

leff ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1� aHð Þ2H2

p þ 1� aLð Þ2L2p

qð6Þ

h ¼ tan�1ðHp=LpÞ ð7Þ

and where aH is vertical contact length ratio, aL is horizontal contactlength ratio and Hp and Lp are the height and width of the GFRPpanel.

Table 2Dimensions and sectional properties.

Standard sectional dimension (mm) Sectional area (cm2) Unit weight (kg/m) Moment of inertia (cm4) Radius of gyration (cm) Section modulus (cm3)

H � B tweb tflange r A W Ix Iy Rx Ry Zx Zy

194 � 150 6 9 13 39.01 30.6 2,690 507 8.3 3.61 277 67.6125 � 125 6.5 9 10 30.31 23.8 847 293 5.29 3.11 136 47

Table 3Material properties of SS400.

Yieldstrength(MPa)

Ultimatestrength(MPa)

Elongation(%)

Elasticmodulus(MPa)

Density(ton/mm3)

SS400 344 464 26.5 20,000 7.8E�9

Fig. 5. GFRP fabric tension test coupon.

0 3000 6000 9000 12000 15000 180000

100

200

300

400

500

Strain (10-6)

Stre

ss (M

Pa)

GF91-1 GF91-2 GF91-3 GF91-4 GF91-5

Fig. 6. Tension test results of GFRP fabric test coupon.

Table 4Material properties of GFRP fabric test coupons.

Strength at Max. load (MPa) Strain at Max. load (�10�6)

GF91-1 433 14,971GF91-2 436 15,047GF91-3 439 14,954GF91-4 433 14,886GF91-5 490 16,316

Average 446 15,235

M. Kwon et al. / Composite Structures 120 (2015) 262–274 265

2.6. Equivalent flexural rigidity

The main loads required to design the cross-section were in-plane and buckling loads. The in-plane load determined the thick-ness, while the buckling load determined the moment of inertia, aswell as stiffener spacing and thickness. A box-shaped cross-sectionwas proposed for the GIPs, which provided advantages such as asymmetric cross-section and ease in increasing its moment of iner-tia. Fig. 2 descripted the cross-section shape of a single GIP, whereB is the width of a single GIP, h is the thickness of a GFRP plate, c isthe core depth, and Wc is the thickness of a GFRP-stiffener.

Modulus of elasticity (GPa) Thickness (mm) Width (mm)

30.9 1.95 10.1231.1 1.94 10.0631.4 1.92 10.1531.9 1.90 10.3032.2 1.85 10.02

31.5 1.91 10.13

<Panel model> <Infilled-frame specimen>

Fig. 7. FEM model of a simple panel specimen.

0 5 10 15 20 25 300

100

200

300

400

500

Loa

d (k

N)

Displacement (mm)

Load

Fig. 8. Load and displacement relationship obtained from simple-panel analysis.

Fig. 9. Stress distribution of the simple panel.

Table 5Contact lengths of design parameters.

Length (mm) Ratio (%)

aH 225.0 15aL 616.8 30

Fig. 11. Assembled GIPs.

266 M. Kwon et al. / Composite Structures 120 (2015) 262–274

If the section is symmetric, the neutral surface will be located inthe middle of the surface. The equivalent flexural rigidity, (EI)eq, ofthe cross-section would then be as indicated in Eq. (8).

ðEIÞeq ¼ weffEf h3

6þ 2Ef h

h2þ c

2

� �2" #

ð8Þ

Fig. 10. Experimental GIPs (Unit = mm).

GFRP Panel

Angle 30mmx30mm

Drilling

Beam

Column

Bolt Set

Fig. 12. Installation design of GIPs.

1. Preparing GFRP sheet

2. Pre-fabrication

3. Fabrication

4. Pressing and finishing

Fig. 13. Manufacturing process for the GIP.

Fig. 14. Orientations of FRP plate laminate layers.

Fig. 15. GFRP plate tension test coupons.

0 2000 4000 6000 8000 10000 120000

50

100

150

200

250

300

350

400

Stre

ss (

MP

a)

Strain (10-6)

FRP 1 FRP 2 FRP 3 FRP 4 FRP 5

Fig. 16. Stress–strain relationship of GFRP plate tension test coupons.

M. Kwon et al. / Composite Structures 120 (2015) 262–274 267

where Ef is the Young’s moduli of the plate, weff is the effectivewidth of the plate, and h and c are the thickness of GFRP plateand core depth of the cross-section, respectively.

2.7. Buckling load

Under an in-plane load Nf, the entire panel buckles as a columnif its sides are unsupported. The buckling load (PE) of the panelbecomes

PE ¼p2ðEIÞeq

ðleff Þ2ð9Þ

where leff is the effective length and (EI)eq is the equivalent flexuralrigidity.

2.8. Design procedure for the GIP

Table 1 shows the detailed steps in the design procedure for theGIP, and Fig. 3 presents a flow chart of the design procedure.

3. Experimental tests

3.1. Steel frame specimen

A steel-frame specimen was designed, and its joint type wasassumed to be a welded joint, such as have been used in most

268 M. Kwon et al. / Composite Structures 120 (2015) 262–274

steel-frame structures for a long time. The steel frame specimenwas constructed using H-beams, and the beam and column sec-tions were selected by moments-of-inertia. Fig. 4 presents thedesign of the steel-frame specimen. The cross-sectional informa-tion is summarized in Table 2. H-beams made of SS400 materialswere used in the construction of the steel frame specimen, andthe properties of the SS400 materials are summarized in Table 3.

3.2. Material properties of GFRP

Glass fiber fabric sheet was used in this study. The elastic mod-ulus and maximum strain are important properties of the GFRPmaterial in structural design. The maximum strength, elastic mod-ulus and maximum strain of the glass fabric were obtained fromthe test results of GFRP fabric test coupon, which were tested byASTM-D638 [22]. A GFRP fabric test coupon is made of 8 compositeplies, which are made from glass fibers fabric and epoxy bindingagent. Each composite ply is 0.25 mm thick, making a 2 mm com-posite strip. Within each composite ply, ninety percent of the glassfibers are placed along longitudinal direction while ten percent aredistributed in transverse direction. Five GFRP fabric test couponsare prepared as shown in Fig. 5 and tested under tensile loading

Table 6Material properties of GFRP plate test coupon.

Stress at Max. load (MPa) Strain at Max. load (�10�6)

FRP 1 340 10,414FRP 2 373 11,503FRP 3 366 11,290FRP 4 320 9,812FRP 5 344 10,640

Average 349 10,732

Fig. 17. Tes

to evaluate their mechanical properties using a MTS universal test-ing machine.

All tests are performed by displacement control and the rate ofloading is 5 mm/min as ASTM-D3039 [23]. The sampling rate is1.0 Hz. Each specimen is 10.13 mm in width and 1.91 mm in thick-ness. Fig. 6 are presented all testing results. As shown in Fig. 6, thestress–strain responses of GFRP fabric test coupons are essentiallylinear. All test results are summarized in Table 4. The average val-ues of the tensile strength and elastic modulus are 446 MPa and31.5 GPa, respectively.

3.3. Design of the GIP

The cross-section of the GIP was designed to increase the lateralstrength of the steel-frame structure by the amount of 100 kN. Theplate and core thicknesses needed to meet the design objectiveswere determined, and the cross-sectional dimension of the GIPwas computed using the proposed design procedure. Detailed cal-culations according to the design procedure are summarized in theAppendix A.

Fig. 7 shows the FE model of the GFRP-composite panel used inthe numerical study. The GFRP-composite panel in the FE model

Modulus of elasticity (GPa) Width (mm) Thickness (mm)

33.7 25.083 3.18733.1 25.070 3.19333.4 25.073 2.98333.6 25.093 3.20033.3 25.090 3.190

33.4 25.081 3.151

t setup.

M. Kwon et al. / Composite Structures 120 (2015) 262–274 269

was assumed to define the contact length between the steel frameand the GFRP-panel. The GFRP-composite panel was modeled byone part of a homogenous, solid model and was considered toreflect the material properties of the GFRP composite. The contactlength was determined at the maximum strength indicated by theresults of the numerical study. Pushover analysis was performed,and the surface between the frame and GFRP-panel was modeledusing a contact model. The friction between the two materials

Fig. 18. Instrumentation locations.

< OSF >

< RSF >

Fig. 19. Test specimens.

was ignored. Fig. 8 shows the load–displacement relationshipobtained from the numerical study. The maximum load of379.3 kN occurred after 21.9 mm of lateral displacement. Fig. 9shows the stress distribution of the infill-frame specimen at themaximum load, at which point the contact lengths were measured.The contact length was determined at a GFRP strain of 0.002. Thecontact lengths obtained from the numerical study are summa-rized in Table 5. The vertical contact length was 225 mm and thehorizontal contact length was 616.8 mm. These values were usedto design the GIP. Also, the contact lengths ratio were determinedfrom the numerical study (aH = 0.15, aL = 0.03). The effective widthand effective length of the compressive strut were calculated bycontact length variables. The effective width was 328.28 mm andeffective length was 1922.74 mm.

The optimal thickness of both the GFRP-plate and section corewas determined using the repeated-calculation method to satisfythe purpose load. The optimal thickness of the GFRP, accordingto the proposed design procedure, was 6 mm. Due to the shapeof the cross-section, the thickness of each plate became 3 mm,and the distance between each plate was 52 mm. Since the GIPswere intended to enhance the strength of existing walls, and toreplace damaged walls, their design also considered the need forurgent construction and emergency repairs. For these reasons,sperate types of GIPs were designed to provide easy transportationand easy construction. Fig. 10 presents the final cross-sectiondesign of the GIP. In this study, two GIPs were used to experimen-tal test. Those two GIPs were bonded by epoxy bond. Fig. 11 showsthe assembled GIPs to use in experimental test. Bolts and steelangles were used to install the GIPs that were to assume in-planebehavior. Fig. 12 depicts the installation design of the GIPs.

< OSF >

< RSF >

Fig. 20. Final deformed shapes.

Fig. 21. Local behavior of GIPs.

270 M. Kwon et al. / Composite Structures 120 (2015) 262–274

3.4. Manufacturing of the GIP

The GIPs were manufactured, using appropriate manufacturingprocesses, in a factory. Fig. 13 shows the manufacturing process.Twelve plies of GFRP fabric sheet were used to fabricate the3-mm-thick FRP plate. Fig. 14 shows the orientation of each lami-nate layer. A MTS universal testing machine was used to test thetensile test coupons. Fig. 15 shows the test coupons. The stress–strain relationship obtained from the tensile test is plotted inFig. 16, and the test results are listed in Table 6. The yield strengthand elastic modulus of the GFRP plate were 349 MPa and 33 GPa,respectively. The yield strength and maximum strain of GFRP platewas decreased since it was the orientation of each laminate layer.However, the elastic modulus of the GFRP plate and GFRP fabriccoupon were almost the same.

3.5. Test setup

Fig. 17 presents the experimental test setup. Lateral loads wereimposed by a top-loading block using a 1000-kN hydraulic actua-tor. The support conditions of the steel-frame specimen wereassumed to be hinges. Four hinge-supports were placed on thetop and bottom of each steel-frame specimen, and four auxiliaryguide rollers were installed to prevent out-of-plane deformation.The specimens and instruments were attached to a reaction walland floor. Fig. 18 shows the locations of the instrumentation.

Two linear variable differential transformers (LVDTs) were placedon the upper and lower beams to measure lateral displacement,and relative-lateral displacement and inter-story drift were calcu-lated using those LVDTs. One LVDT was placed at the center of theGIP, and out-of-plane displacement was measured using this LVDT.For each steel-frame-structure specimen, five strain gauges wereplaced on the columns, and five on the beams. Forty strain gaugeswere placed on the surface of the GIP.

A quasi-static cyclic load was performed to evaluate the influ-ence of the GIP on the steel frame. The displacement load wasapplied through a loading arm over the specimen, and wasincreased by approximately 0.2% of the drift ratio in each loadingstep. The steps were repeated twice to improve experimentalaccuracy.

Two specimens were experimentally investigated in this study.There was a steel frame without a GIP (OSF), and the other steel-frame specimen that were strengthened by two GIPs (RSF).Fig. 19 shows photos of the two steel-frame specimens.

4. Experimental results

4.1. Deformed shape

The OSF specimen was the reference used to evaluate thestrengthening effect. Fig. 20 shows the deformed shape of the spec-imens at the final stage of test. The top and bottom beams showed

-100 -80 -60 -40 -20 0 20 40 60 80 100-200

-150

-100

-50

0

50

100

150

200L

oad

(kN

)

Relative Lateral Displacement (mm)

OSF

< OSF >

-100 -80 -60 -40 -20 0 20 40 60 80 100-200

-150

-100

-50

0

50

100

150

200

Loa

d (k

N)

Relative Lateral Displacement (mm)

RSF

< RSF >

Fig. 22. The load and relative lateral displacement relationships.

-100 -80 -60 -40 -20 0 20 40 60 80 100-200

-150

-100

-50

0

50

100

150

200

Loa

d (k

N)

Relative Lateral Displacement (mm)

OSF RSF

Fig. 23. Result comparison for envelope curve.

M. Kwon et al. / Composite Structures 120 (2015) 262–274 271

flexural deformation in OSF. The RSF specimen was strengthenedwith GIPs. In the RSF, plastic deformation of these parts occurredat both ends of the beam because the flexural and shear stress inthe beam member was increased by the GIPs, and it was locallybuckled at the right bottom corner. Fig. 21 presented the localbehavior of the GIPs.

Table 7Experimental test results.

Max. strength in north direction Max. strength in south direc

Step Load (kN) Step Load (kN

OSF 20 74.58 �20 76.05RSF 9 191.00 �11 168.36

4.2. Strength

Fig. 22 shows the relationship between the load and the rela-tive-lateral-displacement. Envelope curves for each specimen arearranged in Fig. 23. The maximum strength of the GFRP-strengthened specimens (RSF) increased, in comparison with theOSF specimen. The maximum strengths of the specimens are sum-marized in Table 7. The maximum load of the RSF specimen wasapproximately 2.39 times that of the OSF specimen (a gain of�104.4 kN). The GIPs were locally buckled at the corner.

4.3. Behavior of the GIP

The in-plane strains of the GIP were measured. Those strainswere named FX1–FX20 for the bottom, and FY1–FY20 for the rightside of the GIP. Fig. 24 shows the peak strain at the bottom andright side of the GIP in RSF. Furthermore, a large strain was distrib-uted to the corner of the GIP. The compression and tension behav-ior of the GIP was also obtained since it exhibited local buckling.

5. Discussion

5.1. Increased strength

The increased strength was compared with the design load, asshown in Table 8. The GIPs had been designed with the goal ofincreasing the lateral load by more than 100 kN. After the designprocess, the lateral load was expected to increase byapproximately 100.25 kN. With this value as the reference, theRSF specimen exhibited 104.11% of the planned increase in lateralload.

5.2. Effective contact length

The effective width of the compressed struts in the GIPs wasdetermined from the effective contact length measured at maxi-mum strength. The effective contact lengths between the steelframe and GIPs were determined from strain results that weregreater than 0.002. Table 9 summarizes the effective contactlengths between the steel frame structure and GIPs. The effectivecontact-length ratio between the column and the GIP was approx-imately 17.5%, and that between the beam and GIP was approxi-mately 5.1% because of vertical stiffener, so, contact-length ratioof beam was smaller than that of column. The design variablesand variables observed in the experiment are summarized inTable 10. The steel-frame structures and GIPs were assumed tobe in total contact in the design. However, they were not in perfectcontact due to construction and manufacturing errors, and theboundary conditions of the GIPs also did not exactly match thosespecified in the design.

5.3. Out-of-plane behavior of the GIPs

The GIPs were governed by the in-plane and buckling loads.Out-of-plane deformation was governed by the buckling load,and was measured by the DT-3 LVDT installed at the center ofthe GIPs. Fig. 25 shows the out-of-plane displacements and

tion Average Max. load (kN) Ratio Increased load (kN)

)

75.31 1 �179.68 2.39 104.37

0 2 4 6 8 10 12 14 16 18 20-3000

-2000

-1000

0

1000

2000

3000

RSF

Stra

in (

10-6

)

Gauge Position Number (FX)

-1 -2 -3 -4 -5 -6 -7 -8 -9 -10

0 2 4 6 8 10 12 14 16 18 20-3000

-2000

-1000

0

1000

2000

3000

RSF

Stra

in (

10-6

)

Gauge Position Number (FX)

-11 -12 -13 -14 -15 -16 -17 -18 -19 -20

(South direction)

0 2 4 6 8 10 12 14 16 18 20-3000

-2000

-1000

0

1000

2000

3000

RSF

Stra

in (

10-6

)

Gauge Position Number (FX)

+1 +2 +3 +4 +5 +6 +7 +8 +9 +10

0 2 4 6 8 10 12 14 16 18 20-3000

-2000

-1000

0

1000

2000

3000

RSF

Stra

in (

10-6

)Gauge Position Number (FX)

+11 +12 +13 +14 +15 +16 +17 +18 +19 +20

(North direction)

(a) Bottom

0

2

4

6

8

10

12

14

16

18

20

-8000 -4000 0 4000 8000

RSF

Strain (10-6)

Gau

ge P

osit

ion

Num

ber

(FY

)

-1 -2 -3 -4 -5 -6 -7 -8 -9 -10

0

2

4

6

8

10

12

14

16

18

20

-8000 -4000 0 4000 8000

RSF

Strain (10-6)

Gau

ge P

osit

ion

Num

ber

(FY

)

-11 -12 -13 -14 -15 -16 -17 -18 -19 -20

(South direction)

0

2

4

6

8

10

12

14

16

18

20

-8000 -4000 0 4000 8000

RSF

Strain (10-6)

Gau

ge P

osit

ion

Num

ber

(FY

)

+1 +2 +3 +4 +5 +6 +7 +8 +9 +10

0

2

4

6

8

10

12

14

16

18

20

-8000 -4000 0 4000 8000

RSF

Strain (10-6)

Gau

ge P

osit

ion

Num

ber

(FY

)

+11 +12 +13 +14 +15 +16 +17 +18 +19 +20

(North direction)

(b) Right side

Fig. 24. Strain distribution of GIPs.

272 M. Kwon et al. / Composite Structures 120 (2015) 262–274

relative-lateral-displacement relationship of the strengthenedspecimens (RSF). Table 11 summarizes the out-of-plane displace-ments obtained at maximum load and shows that buckling of the

GIPs had occurred. This out-of-displacement was caused by theconstruction error, but also caused by the effect from local bucklingof GIPs.

Table 8Comparison of increased loads.

Specimen Increased lateral load Design load Ratio(kN) (kN) (%)

RSF 104.37 100.25 104.11

Table 9Effective contact lengths.

Specimen Position Effective contact length (mm)

North direction South direction Average

RSF Column 337.5 187.5 262.5Beam 104.0 104.0 104.0

Ratio (%) Column 22.5 12.5 17.5Beam 5.1 5.1 5.1

Table 10Comparison of effective contact length.

Value Design Experiment

Length (mm) Ratio (%) Length(mm) Ratio (%)

aH 225.0 15 262.5 17.5aL 616.8 30 104.0 5.1

-100 -80 -60 -40 -20 0 20 40 60 80 100-20

-16

-12

-8

-4

0

4

8

12

16

20

Out

-Of-

Pla

ne D

ispl

acem

ent

(mm

)

Relative Lateral Displacement (mm)

RSF

Fig. 25. Out-of-plane behavior of the GIP.

Table 11Out-of-plane displacement.

Specimen Loading direction Max. out-of-plane displacement (mm)

RSF North (+) 2.61South (�) �15.94

M. Kwon et al. / Composite Structures 120 (2015) 262–274 273

6. Conclusions

The main purpose of this study was to develop a simplifieddesign procedure for GIPs. The design goal for these panels wasto increase the strength of steel-frame structures. GIPs weredesigned to resist both in-plane load and buckling load from theassumed behavior and stress distribution. Each GIP was composedof a GFRP-plate and a stiffener, of which the cross-sectional shapewas assumed to be a box. The experimental GIPs used in this studywere designed and fabricated using the proposed design

procedure. The maximum strength obtained in the experimentscorresponded to the expected design strength. Also, designassumption was confirmed from experimental results. Finally,the proposed design procedure for GIPs can be used to improvethe performance of infill-frame structures.

Acknowledgements

This research was supported by a grant (11 Cutting-edge UrbanC10) from the Cutting-edge Urban Development Program fundedby the Ministry of Land, Transport and Maritime Affairs of theSouth Korean government.

Appendix A

A.1. Determine the material and target structure

Hp ¼ 1500 mmLp ¼ 2056 mmEf ¼ 31;500 MPaef ¼ 0:002

A.2. Determine the target force

PH ¼ 100 kN

A.3. Define the contact length (from the FEM)

aH ¼ 0:15aL ¼ 0:30

A.4. Calculate the effect width and effect length

weff ¼12

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiaHHp� �2 þ aLLp

� �2q

¼ 12

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi0:15� 1500ð Þ2 þ 0:30� 2056ð Þ2

q¼ 328:28 mm

leff ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1� aHð Þ2H2

p þ 1� aLð Þ2L2p

q

¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1� 0:15ð Þ2 1500ð Þ2 þ 1� 0:30ð Þ2 2056ð Þ2

q¼ 1922:74 mm

h ¼ tan�1ðHp=LpÞ ¼ tan�1ð1500=2056Þ ¼ 36�

A.5. Assume the FRP thickness

t ¼ 6 mmh ¼ 0:5t ¼ 3 mm

As ¼ weff � t ¼ 328:28� 6 ¼ 1969:67 mm2

Nf ¼ Ef Asef ¼ 31500� 1969:67� 0:002 ¼ 124:089 kNPH= cos h ¼ 100= cosð36�Þ ¼ 123:607 kN

Check

Nf ¼ 124:089 kN P 123:607 kN ¼ PH= cos h OK

FN ¼Nf

PH= cos h¼ 124:089

123:607¼ 1:002

PH�Nf ¼ Nf � cos h ¼ 124:089� cos 36� ¼ 100:246 kN

274 M. Kwon et al. / Composite Structures 120 (2015) 262–274

A.6. Assume the core thickness

c ¼ 52 mm

ðEIÞeq ¼ Ef �weffh3

6þ 2h

h2þ c

2

� �2" #

¼ 31500� 328:2833

6þ 2� 3

32þ 52

2

� �2" #

¼ 46967797:771 kN �mm2

PE ¼p2ðEIÞeq

l2eff

¼ p2 � 46967797:771 kN �mm2

ð1922:74 mmÞ2¼ 125:389 kN

Check

PE ¼ 125:389 kN P 123:607 kN ¼ PH= cos h OK

FS ¼PE

PH= cos h¼ 125:389

123:607¼ 1:014

PH�E ¼ Pe � cos h ¼ 125:389� cos 36� ¼ 101:442 kN

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