Polymers 2015, 7, 1716-1731; doi:10.3390/polym7091480
OPEN ACCESS
polymers ISSN 2073-4360 www.mdpi.com/journal/polymers Article
An Experimental Study on Non-Compression X-Bracing Systems Using Carbon Fiber Composite Cable for Seismic Strengthening of RC Buildings Kang Seok Lee School of Architecture, Chonnam National University, Gwangju, 500-757, Korea; E-Mail:
[email protected]; Tel.: +82-62-530-1645; Fax: +82-62-530-1630 Academic Editors: Alper Ilki and Masoud Motavalli Received: 22 May 2015 / Accepted: 31 August 2015 / Published: 9 September 2015
Abstract: Cross-bracing (X-bracing) is one of the most popular methods of seismic retrofitting, and has been shown to significantly increase the structural stiffness and strength of buildings. Conventional steel X-bracing methods typically exhibit brittle failure at the connection between the brace and the building, or buckling failure of the braces. This study investigated the structural properties of a new type of non-compression X-bracing system using carbon fiber composite cable (CFCC). This non-compression X-bracing system uses CFCC bracing and bolt connections between structural members and the terminal fixer of the CFCC, instead of conventional steel bracing. The aim is to overcome the brittle and buckling failures that can occur at the connection and bracings with conventional steel X-bracing methods. We carried out cyclic loading tests, and the maximum load carrying capacity and deformation were investigated, as well as hysteresis in the lateral load–drift relations. The test results revealed that the CFCC X-bracing system installed in reinforced concrete frames enhanced the strength markedly, and buckling failure of the bracing was not observed. Keywords: carbon fiber composite cable; X-bracing; reinforced concrete; reinforcement; seismic strengthening; seismic capacity
1. Introduction Existing earthquake-resistance methods focus primarily on encasing reinforced concrete (RC) columns using steel or fiber composite materials. A number of reports over the past two decades have shown that the most efficient strengthening method to resist the lateral earthquake loads for RC buildings
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is to use steel bracing, and retrofitted steel bracing can strengthen earthquake-damaged buildings and increase the seismic capacity of existing RC buildings [1–7]. Viswanath et al. [8] analytically showed that X-bracing significantly increases the structural stiffness of buildings, and reduces the maximum inter-story drift. Triangulated- and X-bracing systems have been used in five-story RC buildings, without considering seismic design, and a significant strengthening effect was found [9]. Scale tests for seven-story RC buildings reinforced using steel bracing have been reported, and an enhancement of vertical and horizontal resistance was found just prior to buckling failure of the bracing in response to earthquake loadings [10]. An RC school building was strengthened using post-tensioned rod braces, which increased the moment capacity rather than increasing the ductility [11]. Direct connection has been shown to exhibit favorable earthquake resistance compared with indirect connection [1,12–15]. Seismic reinforcement using steel braces typically exhibits brittle failure at the connections or buckling failure of the braces. There have been a number of reports of methods to address the buckling failure of the braces. These have mostly focused on the buckling restrained braced (BRB) system. An analytical study to calculate the confidence parameters of buckling failure of a four-story steel office building was carried out, and indicated that carefully assessing the braced steel frames was particularly important, especially to prevent collapse [16]. In Japan, BRB frames have become increasingly popular because of the favorable earthquake resistance [17]. A new type of bidirectional-resistant ductile end diaphragm using BRB for applications in bridges was introduced [18]. A BRB device encased in steel tubing and mortar has been reported [19], and the authors found effective performance in response to cyclic loading. A buckling restrained braced frame (BRBF) with a pinned moment connection has been reported, favorable performance in mitigating residual drift, soft-story deformation, and preventing undesirable failure modes in the beam-column connection, and was analyzed using nonlinear dynamic analysis [20]. A nine-story building reinforced with a special concentrically braced frame (SCBF), a buckling restrained braced frame (BRBF) and a mega-braced frame (MBF) was simulated to evaluate the seismic performance of these different bracing configurations. The MBF series exhibited the most effective resistance to lateral drift, even though it was relatively low-weight [21]. A method to inhibit buckling failure of the braces has been reported, which uses non-compression braces installed with sliding equipment at the brace connection [22]. Recently, Ju et al. [23] reported a technique for non-compression X-bracing systems using carbon fiber anchors for seismic strengthening of RC structures. A carbon fiber is a bundle of yarn-type carbon. Using cyclic load tests, they verified that the carbon fiber anchors have advantages compared with conventional steel X-bracing systems, including an improved strength-to-weight ratio of carbon fiber, compression-free bracing, and confinement using carbon fiber sheets at the brittle concrete-steel joint area. They also reported that further work is needed to optimize details of the connection between the carbon fiber bracing and columns. In this paper, we describe a new technique for non-compression X-bracing using carbon fiber composite cable (CFCC) for seismic strengthening of RC structures. This CFCC X-bracing system consists of CFCC bracing and bolt connections between the structural members and the terminal fixer of the CFCC, rather than the conventional X-bracing system, which is commonly used with steel bracing. We report the seismic performance of an RC frame reinforced with CFCC X-bracing using cyclic loading
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tests, andtests, the maximum load carrying deformation were characterized, as well as as hysteresis loading and the maximum load capacity carrying and capacity and deformation were characterized, well as of the lateral load–drift relation. hysteresis of the lateral load–drift relation. 2. Seismic Strengthening Strengthening Using 2. Seismic Using CFCC CFCC X-Bracing X-Bracing 2.1. CFCC 2.1. CFCC Tokyo of carbon carbon fibers fibers and Tokyo Rope Rope [24] [24] developed developed CFCC, CFCC, aa structural structural reinforcing reinforcing cable, cable, formed formed of and thermosetting resins with a polyester skin, and using a new composite technology to create a stranded thermosetting resins with a polyester skin, and using a new composite technology to create a stranded cable. Thus, the the structural structural behavior behavior of of CFCC CFCC is is similar similar to to that that of of carbon carbon fiber fiber strands, strands, which which has cable. Thus, has no no plastic behavior. plastic behavior. Figure shows optical of the the CFCC, CFCC, including including the the terminal terminal fixer. fixer. CFCC CFCC exploits exploits the excellent Figure 11 shows optical images images of the excellent mechanical properties of ofcarbon carbonfiber, fiber,and andexhibits exhibitsa high a high tensile strength (approximately equal to mechanical properties tensile strength (approximately equal to that that of steel strands for prestressed concrete, as discussed high tensile modulus to of steel strands for prestressed concrete, as discussed later), later), a high atensile modulus (similar(similar to that of that steel strands for prestressed concrete), is lightweight (approximately massstrands), of steel steelof strands for prestressed concrete), is lightweight (approximately one-fifthone-fifth the massthe of steel strands), good corrosion acid and alkali resistance), is non-magnetic, flexible (the has good has corrosion resistanceresistance (high acid(high and alkali resistance), is non-magnetic, flexible (the stranded stranded configuration the cables to beeasily easilycoiled), coiled), and and aalow expansion rate (the configuration of the of cables allowsallows themthem to be lowlinear linear expansion rate coefficient of linear expansion is approximately 1/20 of that of steel). (the coefficient of linear expansion is approximately 1/20 of that of steel).
(a)
(b)
(c)
(d)
Figure 1. Optical images of standard CFCC (Tokyo Rope, [24]): (a) 19-strand, 25.5 mm Figure 1. Optical images of standard CFCC (Tokyo Rope, [24]): (a) 19-strand, 25.5 mm diameter and (b) seven-strand, 12.5 mm diameter; (c) A terminal fixer for a single cable made diameter and (b) seven-strand, 12.5 mm diameter; (c) A terminal fixer for a single cable made by expansive material filling method (ES-type) and (d) a terminal fixer for multiple cables by expansive material filling method (ES-type) and (d) a terminal fixer for multiple cables (EM-type). Reprinted with with permission permission from from reference reference [24]. [24]. Copyright Copyright 2013, 2013, Tokyo Tokyo Rope. (EM-type). Reprinted Rope. Figure 2 shows the load-elongation characteristics of standard CFCC, as well as those of steel strand for prestressed concrete and aramid fiber reinforced plastic (AFRP) [24]. The tensile modulus of CFCC is approximately twice that of AFRP, and approximately equal to that of steel strand for prestressed concrete. CFCC has has aa high high tensile tensile modulus, modulus, which which is is similar similar to to that that of of steel steel strand strand for for prestressed prestressed concrete. CFCC concrete. Table the concrete. Table 11 lists lists the the material material properties properties of of standard standard CFCC. CFCC. The The tensile tensile elastic elastic modulus modulus of of the
Polymers 2015, Polymers 44 Polymers 2015, 2015, 777 Polymers 2015, 7 1719 Polymers 2015, 7 4 2 2 As will be described later, the ultimate load at failure standard CFCC is in the range 130–160 kN/mm standard standard CFCC CFCC is is in in the the range range 130–160 130–160 kN/mm kN/mm2... As As will will be be described described later, later, the the ultimate ultimate load load at at failure failure 2 of seven-strand seven-strand CFCC withrange diameter ofkN/mm 15.2 mm, mm, usedbe indescribed this study, study, is 270 270 kN. of aa130–160 diameter of 15.2 in this is kN. standard CFCC isCFCC inisthe range kN/mm .2.As will be described later, ultimate at failure standard CFCC in with the 130–160 As used later, thethe ultimate loadload at failure of seven-strand CFCC a diameter 15.2mm, mm,used used in in this this study, of seven-strand CFCC withwith a diameter ofof 15.2 study,isis270 270kN. kN. 200 200 200 200
150 150 150
Load (kN)
Load (kN) Load Load(kN) (kN)
150
100 100 100 100
50 50 5050
0 00 0 0 00 0
Aramid Fiber Reinforced Plastic Aramid Fiber Reinforced Plastic Aramid Fiber Reinforced Plastic Aramid Fiber Reinforced Plastic Standard CFCC Standard CFCC Standard CFCC Standard CFCC Steel Strand forfor Prestressed Concrete Steel Strand for Prestressed Concrete Steel Strand Prestressed Concrete Steel Strand for Prestressed Concrete
11
11 1
2 22
2
Elongation (%)
3
33 3
4
44 4
Elongation (%) Elongation Elongation (%) (%)
Figure 2. Load–elongation carbonfiber fibercomposite composite cable (CFCC) Figure 2. Load–elongationrelations relations of of standard standard carbon cable (CFCC) Figure 2. Load–elongation relations of standard carbon fiber composite cable (CFCC) Figure 2. Load–elongation relations of standard fiber cable (CFCC) Figure 2. Rope, Load–elongation relations ofpermission standard carbon carbon fiber composite composite cable (CFCC) (Tokyo Rope, [24][24] 2013). Reprinted with fromreference reference [24]. Copyright 2013, (Tokyo 2013). Reprinted with permission from [24]. Copyright 2013, (Tokyo Rope, [24] 2013). Reprinted with permission from reference [24]. Copyright 2013, (Tokyo Rope, (Tokyo Rope, [24] 2013). 2013). Reprinted Reprinted with with permission permission from from reference reference [24]. [24]. Copyright Copyright 2013, 2013, Tokyo Rope. Tokyo Rope.[24] Tokyo Rope. Tokyo Tokyo Rope. Rope. CFCC reinforcement has been used mainly in the field of civil engineering, including tendons in
CFCC reinforcement has been used mainly in the field of civil engineering, including tendons in CFCC reinforcement has been used mainly in the field of civil engineering, including tendons in pre-stressed concrete bridges, cantilever cables and cables of theengineering, pre-stressed concrete bridges, CFCC reinforcement has used in the of including tendons CFCC reinforcement has been been used mainly mainly in external the field field of civil civil engineering, including tendons in in pre-stressed concrete bridges, cantilever cables and external cables of the pre-stressed concrete bridges, tendons of the ground anchors, and stay cables for strengthening the catwalks of bridges [24]. pre-stressed concrete bridges, cantilever cables and external cables of the pre-stressed concrete bridges, pre-stressed pre-stressed concrete concrete bridges, bridges, cantilever cantilever cables cables and and external external cables cables of of the the pre-stressed pre-stressed concrete concrete bridges, bridges, tendons of the ground anchors, andfor stay cableshave for strengthening the catwalks of study, bridges [24]. CFCC CFCC reinforcement techniques buildings not yet been investigated. In this we describe tendons of the ground anchors, and stay cables for strengthening the catwalks of bridges [24]. tendons tendons of of the the ground ground anchors, anchors, and and stay stay cables cables for for strengthening strengthening the the catwalks catwalks of of bridges bridges [24]. [24]. reinforcement techniques for buildings have not yetCFCC been investigated. In this study, webuildings. describe for the for the first time the technique of X-bracing using for seismic reinforcement of RC CFCC reinforcement techniques for buildings have not yet been investigated. In this study, we describe CFCC CFCC reinforcement reinforcement techniques techniques for for buildings buildings have have not not yet yet been been investigated. investigated. In In this this study, study, we we describe describe first time the technique of X-bracing using CFCC for seismic reinforcement of RC buildings. for the first time the technique of X-bracing using CFCC for seismic reinforcement of RC buildings. for the first time the technique of X-bracing using CFCC for seismic reinforcement of RC buildings. for the first time the technique of1.X-bracing using CFCC for seismic reinforcement of RC buildings. Table Specification of the standard CFCC cables. Table 1.φ Specification the standard cables. Configuration and Diameter Effective crossofof Tensile strengthCFCC Nominal mass Tensile elastic Table 1. Specification the standard CFCC cables. Table 1. Specification of the standard CFCC cables. Table 1. Specification of the standard CFCC cables. a b 2 2 diameter
(mm)
section (mm )
at break (kN)
density (g/m)
modulus (kN/mm )
Diameter Effective cross Tensile strength Nominal mass Tensile elastic Configuration and Configuration and Diameter Effective cross Tensile strength Nominal mass Tensile elastic U, 5.0 5.0 φφ 15.2 cross 38 strength 30 167 Configuration and Effective mass Tensile Configuration andφ Diameter Diameter φ Effective cross Tensile Tensile strength Nominal Nominal mass Tensile elastic elastic diameter aaaa ϕ (mm) section bbbb(mm2222) at break (kN) density (g/m) modulus (kN/mm2 ) 222 a 7.5 φ b (mm 2)) diameter (mm) section (mm at break (kN) density (g/m) modulus (kN/mm 1 × 7, 7.5 31.1 76 60 155 diameter (mm) section at break (kN) density (g/m) modulus (kN/mm diameter (mm) section (mm ) at break (kN) density (g/m) modulus (kN/mm2))) 1 × 7, 10.5 φ 10.5 57.8 141 111 155 167 U,5.0 5.0ϕ φ 5.0 15.2 38 30 167 U, 5.0 15.2 38 3030 U, 5.0 φφ 5.0 15.2 38 167 U, 5.0 5.0 15.2 38 30 167 17,× 7.5 7, 12.5 φ 12.5 76.0 18476 145 60 155 155 1 × φ 7.5 31.1 ×× 7, 7, 7.5 φφ 7.5 31.1 76 155 111ˆ 31.1 76 6060 155 7, 7.5 7.5 ϕ 7.5 31.1 76 60 155 1 × 7, 15.2 φ 15.2 115.6 270 221 155 1 × 7, 10.5 φ 10.5 57.8 141 111 155 ×× 7, 7, 10.5 φφ 10.5 57.8 141 111 155 111ˆ 57.8 141 111 155 7, 10.5 10.5 ϕ 10.5 57.8 141 111 155 1 × 7, 17.2 φ 17.2 151.1 350 289 155 × 7, 12.5 φ 12.5 76.0 184 145 155 × 7, 12.5 φ 12.5 76.0 184 145 155 111 ˆ 7, 12.5 ϕ 12.5 76.0 184 145 155 1 ×17,× 12.5 φ φ 76.0 184 19, 20.5 20.5 206.2 316 410 145 137 155 7, 15.2 15.2 115.6 270 221 111 ϕ 15.2 115.6 270 221 155155 ×× φφ 15.2 115.6 270 1ˆ ×17, 7,×15.2 15.2 φ φ 15.2 115.6 270 221 155 19, 25.5 25.5 304.7 467 606 221 137 155 1 × 7, 17.2 φ 17.2 151.1 350 289 111ˆ ϕ 17.2 151.1 350 289 155 ××17, φ 17.2 151.1 350 289 155 7,×17.2 17.2 φ 17.2 151.1 350 289 155 19, 28.5 φ 28.5 401.0 594 777 137 155 111 ˆ 19, 20.5 ϕ 206.2 316 410 137 × 19, 20.5 φ 20.5 206.2 316 410 137 φφ φ 20.5 206.2 316 1 × 37, 35.5 591.2 841 1,185410 127 137 1 ×× 19, 20.535.5 20.5 206.2 316 410 137 111 ˆ 19, 25.5 ϕ 304.7 467 606 137 × 19, 25.5 φ 25.5 304.7 467 606 137 1 × 37, 40.0 φ 40.0 798.7 1,200 1,529 145 × 19, 25.5 φ 25.5 304.7 467 606 137 1 × 19, 25.5 φ 25.5 304.7 467 606 137 a 111 ˆ 19, 28.5 ϕ 401.0 594 777 137 U is the abbreviation of uni-, φ shows the diameter, and 1 × 7, 1 × 19, and 1 × 37 are a cable formed with 7, × 19, 28.5 φ 28.5 401.0 594 777 137 × 19, 28.5 φ 28.5 401.0 594 777 137 1 × 19, 28.5 φ 28.5 401.0 594 777 137 b 19, and 37 strands, respectively. Effective cross section is defined as the cross sectional area of composite 111ˆ 37, 35.5 ϕ 35.5 591.2 841 1,185 127 × 37, 35.5 φ 35.5 591.2 841 1,185 127 × 37, 35.5 φ 35.5 591.2 841 1,185 127 1 × 37, 35.5 φ 35.5 591.2 841 1,185 127 materials, i.e., carbon40.0 fibers and thermosetting resins, excepting the polyester skin.1,529 111ˆ 37, 40.0 ϕ 40.0 798.7 1,200 145 × 37, 40.0 φ 798.7 1,200 1,529 145 40.0 798.7 1,200 1,529 145 1 ×× 37, 37, 40.0 40.0 φφ 40.0 798.7 1,200 1,529 145 a aaaa U is the abbreviation of uni-, φ shows the diameter, and 1 × 7, 1 × 19, and 1 × 37 are a cable formed with 7, UU isisis the ofof uni-, ϕφφshows 37are areaaacable cableformed formedwith with7, the abbreviation uni-, shows the diameter, and 7, ×× 19, U theabbreviation abbreviation of uni-, showsthe thediameter, diameter,and and111ˆ××7, 7,111ˆ 19, and and 11 ׈ × 37 37 are cable formed with 7, b b bb 7,19, 19, and strands, respectively. cross section is defined as the cross sectional area of composite b Effective 19, and 37 respectively. Effective cross section is defined as the cross sectional area of composite and 37 strands, respectively. section is defined as the cross sectional area of composite 19, and 37 strands, respectively. Effective cross section is defined as the cross sectional area of composite materials, i.e., carbon fibers and thermosetting resins, excepting thethe polyester skin. materials, i.e., carbon fibers and thermosetting resins, excepting the polyester skin. materials, i.e., carbon fibers and thermosetting resins, excepting polyester skin. materials, i.e., carbon fibers and thermosetting resins, excepting the polyester skin.
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2.2. CFCC X-Bracing 2.2. CFCC X-Bracing The CFCC X-bracing system for RC buildings consists of flat-plate and protrusion configurations, The CFCC X-bracing system for RC buildings consists of flat-plate and protrusion configurations, asasshown in Figure 3. Following coring 30-mm-diameter holes in the existing beam, steel devices shown in Figure 3. Following coring 30-mm-diameter holes in the existing beam, steel devices for for connecting flat-plateand andprotrusion-type protrusion-typereinforcements reinforcementswere weremechanically mechanicallyinstalled installed using using connecting of of thetheflat-plate prestressing bars through throughthe theholes holesat at both edges of beam. the beam. The was CFCC was cross-braced prestressingsteel steel bars both edges of the The CFCC cross-braced using a using a terminal fixer, as shown in Figure 3c, and bolted to steel connecting devices. The flat-plate terminal fixer, as shown in Figure 3c, and bolted to steel connecting devices. The flat-plate and and protrusion CFCC X-bracing systems a bolt-nut connection method, enables control protrusion typetype CFCC X-bracing systems use ause bolt-nut connection method, whichwhich enables control over over the deflection of the bracing. the deflection of the bracing.
(a)
(b) 1435 240 300 M 39 x 4
Terminal Fixer
60
835
CFCC (EM-type): 1x7 -15.2 (GC : 270 kN)
60
240 300 M 39 x 4
Terminal Fixer
M39 x 4(pitch)
(c)
Figure X-Bracing: (a)(a) Flat-plate andand (b) Figure3.3.Seismic Seismicreinforcement reinforcementmethods methodsusing usingCFCC CFCC X-Bracing: Flat-plate protrusion configurations. (c) Details of anofindividual CFCC reinforcement. (b) protrusion configurations. (c) Details an individual CFCC reinforcement.
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The structural behavior of the CFCC X-bracing is indicated in Figure 3a. When a lateral force acts, as shown by P1, the ascending brace experiences a tensile force T 1 and the descending brace has no compressive force; i.e., C1 = 0. For a lateral force P2, the descending brace has a tensile force T 2 and the ascending brace has no compressive force, so C2 = 0. There is no compressive stress on the CFCC X-bracing system in response to lateral loading. 3. Structural Tests 3.1. Material Characteristics of Concrete, Steel Rebar and CFCC The specified compressive strength of the concrete used in the tests was 24 MPa. The results of cylindrical compressive tests were 19.0 MPa for 14 days and 24.3 MPa for 28 days, as listed in Table 2. The specified yield strength of the steel reinforcing bar (rebar) used in the test specimens was 400 MPa. Two diameters of rebar were used: D13 for the main rebar of the column and D6 for the hoop. The tensile strength of steel rebar from using a universal testing machine (UTM) was 567.2 ˘ 8.2 MPa for D13 and 538.0 ˘ 3.0 MPa for D6. Table 2. Material properties of the concrete. Specified strength (MPa)
24 Average
Compressive strength (MPa) 14 days 28 days 19.3 23.1 18.3 19.9 19.4 29.9 19.0 24.3
The properties of the 1 ˆ 7, 15.2 ϕ used here are listed in Table 1. We used a terminal fixer for multiple cables (EM). The diameter of the CFCC was ϕ = 15.2 mm, the effective cross section was 115.6 mm2 , the tensile strength at break was 270 kN, the nominal mass density was 221 g/m, and the tensile elastic modulus was 155 kN/mm2 . 3.2. Design, Variables and Fabrication of Test Specimens Specimens were designed based on the frame of a typical Korean school building [25], and fabricated for cyclic load tests to investigate the reinforcement properties of the CFCC X-bracing system. The building was a three-story RC structure designed in the 1980s, without seismic reinforcement. The structure consisted of RC frames with spandrel brick walls in the longitudinal direction, and in-filled brick walls in the transverse direction. One span of the exterior frame in the longitudinal direction corresponds to columns, the beam and the spandrel brick wall, as shown in Figure 3. This was selected for the tests because the seismic capacity in the transverse direction with the in-filled brick walls is higher than that in the longitudinal direction [26]. Three types of specimen were designed and fabricated for the experimental tests: a control test specimen (not reinforced), as shown in Figure 4, a test specimen reinforced with CFCC X-bracing using the flat-plate, as shown in Figure 3a, and a test specimen strengthened with the CFCC X-bracing using the protrusion configuration, as shown in Figure 3b. Table 3 lists details of each of the specimens.
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Figure 4. Details of the control specimen. Figure 4. Details of the control specimen. Table 3. Summaryof ofthe thespecimens specimensused usedinin this study. RCFR: Control specimen without CFCC Table 3. Summary this study. RCFR: Control specimen without CFCC X-bracing; CFCC-1: CFCC X-bracing X-bracing (flat-plate); (flat-plate); CFCC-2: X-bracing; CFCC-1: CFCC CFCC-2: CFCC CFCC X-bracing X-bracing (protrusion). (protrusion). Column clear Specimens Column clear Specimens height, h hoo (mm) (mm) height,
RCFR RCFR CFCC-1 CFCC-1 CFCC-2 CFCC-2
1680 1680 1680 1680 1680 1680
Column Column depth, depth, D (mm) D (mm)
/D hhoo/D
Strengthening Strengthening material material
210 210 210 210 210 210
6.72 6.72 6.72 6.72 6.72 6.72
-CFCC 7, 15.2 15.2 φϕ CFCC 11 ˆ × 7, CFCC 1 ˆ 7, 15.2 φϕ CFCC 1 × 7, 15.2
Strengthening Strengthening Connection Connection types method types method -Flat-plate Flat-plate Protrusion Protrusion
- Bolt-nut system Bolt-nut system Bolt-nut system Bolt-nut system
All All specimens specimens had had identical identical dimensions dimensions and and arrangements arrangements of of rebar. rebar. The The cross-section cross-section of of the the columns columns EachEach specimen was was clear height to to thethe depth ho/D was 210 210 ׈300 300mm, mm,and andthe theratio ratioofofcolumn column clear height depth ho = /D6.72. = 6.72. specimen prepared with 10-D13 type SD40 with shear reinforcement D6 steel bars 180-mm was prepared with 10-D13 type main SD40rebar, mainreinforced rebar, reinforced with shear reinforcement D6atsteel bars intervals. wereBeams fabricated install thetoCFCC werewhich designed have same at 180-mmBeams intervals. weretofabricated installX-bracing, the CFCC which X-bracing, weretodesigned to behavior a stub with at theatop, in Figure 4. The stub, with a high stiffness, the have samewith behavior stubasatshown the top, as shown in Figure 4. The stub, with a was highinstalled stiffness,atwas top of each specimen to provide confinement the columns. spandrel brick wall wasbrick 480 wall mm installed at the top of each specimen to providefor confinement for The the columns. The spandrel high, andmm constructed using B-type brick; i.e.,brick; 190 mm long,mm 90long, mm 90 wide 57and mm The was 480 high, and constructed using B-type i.e., 190 mmand wide 57thick. mm thick. compressive strength of the was was 8 MPa. The compressive strength ofbrick the brick 8 MPa. 3.3. Testing Testing Procedure 3.3. Procedure The main main purpose purpose of of the the tests tests was was to to investigate investigate the seismic resistance resistance of of the the non-compression non-compression CFCC CFCC The the seismic X-bracing in in terms terms of of the the maximum maximum load-carrying load-carrying capacity, capacity, deformation, hysteresis of the lateral lateral X-bracing deformation, and and hysteresis of the load–drift relations. load–drift relations.
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Figure 5 shows the the testtest set-up forforcyclic Loadswere wereapplied applied using actuators a Figure 5 shows set-up cyclicloading loading test. Loads using twotwo actuators with with a capacity of 300 actuatorwith withaacapacity capacity of columns were subjected to a to a capacity of 300 kN,kN, andand oneone actuator of 500 500kN. kN.The Thetwo two columns were subjected constant vertical of 219 duringcyclic cycliclateral lateral loading loading using 300-kN actuators. The cyclic constant vertical loadload of 219 kNkN during usingthe thetwo two 300-kN actuators. The cyclic lateral loading was controlled by the lateral displacement of the 500-kN actuator. lateral loading was controlled by the lateral displacement of the 500-kN actuator.
Figure 5. 5. Experimental forthe thecyclic cyclicloading loading tests. Figure Experimentalconfiguration configuration for tests. The load cycles were repeatedthree threetimes times at at the the lateral of of 0.08%, 0.1%, 0.12%, The load cycles were repeated lateral drift driftangles angles(R)(R) 0.08%, 0.1%, 0.12%, 0.15%, 0.2%, 0.25%, 0.31%, 0.4%, 0.49%, 0.62%, 0.79%, 1%, 1.24%, 1.54%, 2%, and 3.33%. Then, 0.15%, 0.2%, 0.25%, 0.31%, 0.4%, 0.49%, 0.62%, 0.79%, 1%, 1.24%, 1.54%, 2%, and 3.33%. Then, at each loading step, the lateral drift angle (R) was increased to approximately 1.2 times of the previous at each loading step, the lateral drift angle (R) was increased to approximately 1.2 times of the previous one, repeating the load cycle three times. Table 4 lists the loading cycles applied to the each specimen. one, repeating the load cycle three times. Table 4 lists the loading cycles applied to the each specimen. Table 4. The loading cycles used in the experiments. Loading step
Table 4. The loading cycles used in the experiments. 1
2
Drift angle R (%) Loading cycles Lateral drift δ (mm) Drift angle R step (%) Loading LateralLoading drift δ cycles (mm) Drift angle R (%) Loading step Lateral drift δ (mm) Loading cycles
1–3 1 0.08 1–3 1.34 0.08 9 1.34 25–27 0.49 9 8.4 25–27
4–6 2 0.1 4–6 1.68 0.1 10 1.68 28–30 0.62 10 10.5 28–30
7–9 3 0.12 7–9 2.10 0.12 11 2.10 31–33 0.79 11 13.4 31–33
10–12 4 0.15 10–12 2.63 0.15 12 2.63 34–36 112 16.8 34–36
Drift angle R (%) Lateral drift δ (mm)
0.49 8.4
0.62 10.5
0.79 13.4
1 16.8
Loadingstep cycles Loading
3
4
5
13–155 0.2 13–15 3.36 130.2 3.36 37–39 1.2413 21 37–39
1.24 21
6
7
8
16–186 19–21 7 0.25 0.31 16–18 19–21 4.2 5.25 0.25 14 15 0.31 4.2 43–455.25 40–42 1.5414 2 15 26.3 40–42 33.643–45
1.54 26.3
2 33.6
22–24 8 0.4 6.7222–24 16 0.4 46–486.72 3.33 16 56 46–48
3.33 56
4. Failure Sequence and Lateral Load–Drift Curves For all specimens, the compressive behavior of the CFCC X-bracing was out-of-plane. This was originally expected to be compression-free, so that buckling failure of the braces would not occur. The behavior of the control and CFCC X-bracing strengthened specimens differed significantly. The flat
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plate-type and the protrusion-type specimens exhibited similar evidence of failure, with a similar crack appearance, and load–drift relations; however, the CFCC X-bracing specimens did not exhibit damage resulting from tensile stress. In the following discussion, we focus on the crack and failure patterns in terms of the lateral drift and load–drift relations during the final stages of the test. Each loading step was identical during the three loading cycles. Table 5 lists the test results of the mechanical characterization following different loading steps, and Table 6 lists the maximum shear strengths and drifts for the three specimens. Table 5. Results of the mechanical loading tests for the specimens. Specimens Load (kN)
RCFR
CFCC-1
CFCC-2
37 68 ´65 96 148 163 99 29 46 263 ´228 ´40 65 ´293 ´241
Drift (mm)
Drift angle R (%)
Observation
2.6 5.3 ´5.3 10.5 26.3 32.9 44.0 1.7 2.6 33 31.2 1.34 2.6 32.3 33
0.15 0.31 0.31 0.62 1.54 2.0 2.6 0.1 0.15 1.96 1.8 0.08 0.15 1.92 1.96
Flexural cracks Shear cracks Shear cracks Shear cracks > 3 mm in width Large shear cracks and peeling of concrete cover Ultimate state Shear collapse Flexural cracks Shear cracks Ultimate state Shear failure Flexural cracks Shear cracks Ultimate state Shear failure
Table 6. The maximum strengths and drifts of the specimens. Specimens RCFR CFCC-1 CFCC-2
Positive V max a (kN) δmax b (mm) 163 263 275
Negative V max a (kN) δmax b (mm)
32.9 33 31.2 a
´160 ´257 ´293
´33.6 ´26.2 ´32.3
Final failure mode Shear failure and collapse of the columns Shear failure of the columns Shear failure of the columns
V max : Maximum shear strength; b δmax : Drift at the maximum point.
4.1. Unbraced Control Specimen (RCFR) Figure 6 shows the RCFR specimen following the final cyclic load test, and Figure 7 shows load–drift curves for the RCFR specimen. The first observed crack occurred at a positive load of 37 kN, and a small flexural crack appeared in the bottom column faces after three cycles at the fourth loading step (R = 0.15%). Cracking was not observed in the center of the column. Flexural cracks extended into the middle of the column after step four. Following the seventh loading step (R = 0.31%), with a load of both positive 68 kN and negative ´65 kN, shear cracks were observed at the top faces of the columns, and a number of diagonal shear cracks appeared, some of which were more than 3 mm wide. When the applied load reached 148 kN, at the fourteenth positive loading step (R = 1.54%), more large shear cracks
themiddle middleofofthe thecolumn columnafter afterstep stepfour. four.Following Followingthe theseventh seventhloading loadingstep step(R (R==0.31%), 0.31%),with withaaload loadofof the bothpositive positive68 68kN kNand andnegative negative−65 −65kN, kN,shear shearcracks crackswere wereobserved observedatatthe thetop topfaces facesofofthe thecolumns, columns, both Polymers 2015,ofof 7diagonal 1725 andaanumber number diagonalshear shearcracks cracksappeared, appeared,some someofofwhich whichwere weremore morethan than33mm mmwide. wide.When When the and the appliedload loadreached reached148 148kN, kN,atatthe thefourteenth fourteenthpositive positiveloading loadingstep step(R (R==1.54%), 1.54%),more morelarge largeshear shearcracks cracks applied wereobserved, observed,with withincreased increasedwidths. widths.We Weobserved observedpeeling peelingfailure failuredue duetotoshear shearforces forcesfrom fromthe theconcrete concrete were cover.This Thisisislikely likelytotobebethe theresult resultofofinsufficient insufficientshear shearconfinement. confinement. cover. insufficient shear confinement.
Figure 6.6.The The RCFR specimen following the cyclic Figure6. TheRCFR RCFRspecimen specimenfollowing followingthe thecyclic cyclicloading loadingtest. test. Figure loading test. RCFR: Control Specimen RCFR: Control Specimen
200 200 150 150
(32.9,163) (32.9,163)
+Vmax +Vmax Ultimate Strength Point (Positive) Ultimate Strength Point (Positive)
Load (kN) Load (kN)
100 100 50 50 0
0
Shear Collapse Shear Collapse of Frame of Frame +δmax +δmax
-δmax -δmax
-50 -50
-100 -100
Ultimate Strength Point (Negative) Ultimate Strength Point (Negative) -Vmax -Vmax
-150 -150 -200 -200
(-33.6, -160) (-33.6, -160) -40 -40
-30 -30
-20 -20
-10 -10
0
0
10 10
20 20
30 30
40 40
Drift (mm) Drift (mm)
Figure 7. Load–drift relations for the RCFR sample. Figure7.7.Load–drift Load–driftrelations relationsfor forthe theRCFR RCFRsample. sample. Figure Shear thethe toptop of both columns following the application of a load of 99 of kN,99with Shearcollapse collapseoccurred occurredatatat both columns following theapplication application load kN, Shear collapse occurred the top ofofboth columns following the ofofaaload of 99 kN, awith lateral drift ofdrift 44.0 mm (R = 2.6%). The maximum load capacity of the frame of the RCFR specimen lateral 44.0 mm (R==2.6%). 2.6%). Themaximum maximum loadcapacity capacity the frame theRCFR RCFR with aalateral drift ofof44.0 mm (R The load ofofthe frame ofofthe was a positive of 163 load kN, with163 a lateral drift aof lateral 32.9 mm = 32.9 2.0%)mm (Table 6).2.0%) The maximum specimen wasload positive kN, with with drift(R (R == (Table 6). 6). specimen was aa positive load ofof 163 kN, a lateral drift ofof 32.9 mm (R 2.0%) (Table positive load capacity was similar to the maximum negative load capacity of 160 kN, with a of lateral Themaximum maximum positive load capacity was similarto tothe themaximum maximum negative load capacity of160 160drift kN, The positive load capacity was similar negative load capacity kN, ofwith 33.6 mm. lateraldrift driftofof33.6 33.6mm. mm. with aalateral 4.2. Flat-Plate Specimen (CFCC-1) Figure 8 shows a photograph of the CFCC-1 specimen following the cyclic loading test, and Figure 9 shows the load–drift curves. The CFCC-1 specimen had the flat-plate type X-bracing system (Figure 3a). Cracks first appeared with a positive load of 29 kN, and a small flexural crack appeared in the top
4.2.Flat-Plate Flat-PlateSpecimen Specimen(CFCC-1) (CFCC-1) 4.2. Figure88shows showsaaphotograph photographof ofthe theCFCC-1 CFCC-1specimen specimenfollowing followingthe thecyclic cyclicloading loadingtest, test,and andFigure Figure99 Figure Polymers 2015, 7 1726 showsthe theload–drift load–driftcurves. curves.The TheCFCC-1 CFCC-1specimen specimenhad hadthe theflat-plate flat-platetype typeX-bracing X-bracingsystem system(Figure (Figure 3a). shows 3a). Cracks first first appeared appearedwith with aa positive positive load load of of 29 29 kN, kN, and and aa small small flexural flexural crack crackappeared appeared in in the the top top and and Cracks bottom column facesfaces following two cycles cycles of the the second loading step (R (R 0.1%). The flexural flexural cracks and bottom column following two cycles ofsecond the second loading step (R = 0.1%). The flexural bottom column faces following two of loading step == 0.1%). The cracks increased in number number and width, width, and and shear cracks appeared following the fourth loading step step cracks increased in number and width, shear cracks appeared followingthe thefourth fourth loading loading increased in and and shear cracks appeared following step (R= 0.15%),with withaaapositive positiveload loadof of46 46kN; kN;these theseshear shearcracks cracksextended extendedinto intothe themiddle middleof ofboth bothcolumns. columns. (R 0.15%), with positive (R ==0.15%), load of 46 kN; these shear cracks extended into the middle of both columns. Themaximum maximumload loadcapacity capacity of the CFCC-1 specimen was positive load of263 263 kN,kN, withwith lateral drift The maximum load capacityof ofthe theCFCC-1 CFCC-1specimen specimen was a positive load of 263 a lateral The was aapositive load of kN, with aalateral drift of33 33of mm (R 1.96%) (Table6(Table 6and andFigure Figure 9). Shear9). failure occurred thetop topand and button ofand bothbutton columns drift 33(R mm (R = 1.96%) 6 and9). Figure Shear failureatatoccurred at button the topof of of mm ==1.96%) (Table Shear failure occurred the both columns under negative load of 228 228 kN, kN, with with lateral drift of 31.2 31.2drift mmof(R (R 1.8%), as shown in in Figure Figure in 9. both columns under a negative load of 228 kN, with a lateral 31.2 mm (Ras = shown 1.8%), as shown under aa negative load of aa lateral drift of mm == 1.8%), 9. should beshould notedthat that buckling failure ofthe theCFCC CFCC X-bracing wasnot notobserved. observed. TheCFCC CFCCThe X-bracing Figure 9. be It be noted thatfailure buckling failure of the CFCC X-bracing was not The observed. CFCC ItItshould noted buckling of X-bracing was X-bracing systemfor forsystem RCframes frames wasframes therefore antherefore effectivean reinforcement techniquethat that canmarkedly markedly increase the X-bracing for RC wasan effective reinforcement technique that can markedly system RC was therefore effective reinforcement technique can increase the shearstrength. strength. increase the shear strength. shear
Figure The CFCC-1 specimen Figure8. 8.The TheCFCC-1 CFCC-1specimen specimenfollowing followingthe thecyclic cyclicloading loadingtests. tests. Figure 8. following the cyclic loading tests. CFCC-1:Flat FlatPlate PlateType Type CFCC-1:
300 300 250 250 200 200
(33.0,263) 263) (33.0,
+Vmax +V max UltimateStrength StrengthPoint Point(Positive) (Positive) Ultimate
150 150
Load Load(kN) (kN)
100 100 50 50 00
-δ-δmax max ++δδmax max
-50 -50
-100 -100
ShearFailure FailureofofFrame Frame Shear
-150 -150 -200 -200 -250 -250
UltimateStrength StrengthPoint Point(Negative) (Negative) Ultimate -V -Vmax max
(-26.2, -257)
-300 (-26.2, -257) -300 -40 -30 -20 -40 -30 -20
-10 -10
00
10 10
20 20
30 30
40 40
Drift(mm) (mm) Drift
Figure 9. Load–drift relations for sample CFCC-1. Figure9. 9.Load–drift Load–driftrelations relationsfor forsample sampleCFCC-1. CFCC-1. Figure 4.3. Protrusion-Type Specimen (CFCC-2) Figure 10 shows a photograph of the CFCC-2 sample following the cyclic loading tests, and Figure 11 shows load–drift curves. The CFCC-2 test specimen was reinforced using the protrusion-type X-bracing system, as shown in Figure 3b. The first crack was observed with a negative load of ´40 kN and with
4.3. Protrusion-Type Protrusion-Type Specimen Specimen (CFCC-2) (CFCC-2) 4.3. Figure 10 10 shows shows aa photograph photograph of of the the CFCC-2 CFCC-2 sample sample following following the the cyclic cyclic loading loading tests, tests, and and Figure Figure 11 11 Figure Polymers 2015, 7 1727 shows load–drift load–drift curves. curves. The The CFCC-2 CFCC-2 test test specimen specimen was was reinforced reinforced using using the the protrusion-type protrusion-type shows X-bracing system, system, as as shown shown in in Figure Figure 3b. 3b. The The first first crack crack was was observed observed with with aa negative negative load load of of −40 −40 kN kN X-bracing aand lateral of 1.34 This a was slight flexural crack,crack, whichwhich appeared in the andand bottom of and with drift lateral drift mm. of 1.34 1.34 mm.was This was slight flexural crack, which appeared in top the top top and bottom with aa lateral drift of mm. This aa slight flexural appeared in the bottom the column faces after three cycles ofof the of the the column faces after three cycles of thefirst firstloading loadingstep step(R(R (R== =0.08%). 0.08%).Flexural Flexuralcracks cracks also also appeared of column faces after three cycles the first loading step 0.08%). Flexural cracks also appeared following the second and third loading steps, and shear cracks appeared following the fourth following the second and third loading steps, following the loading following the second and third loading steps, and shear cracks appeared following the fourth loading loading step step (R (R = = 0.15%) 0.15%) with with aaa positive positive load load of of 65 65 kN. kN. The The shear shear cracks cracks increased increased in in number number and and extended extended into into step (R = 0.15%) with positive load of 65 kN. The shear cracks increased in number and extended into the middle middle of of both both columns columns following following the the fourth fourth loading loading step. step. Compared Compared with with the the control control specimen, specimen, the Compared with the middle of both columns following the fourth loading step. the control specimen, however,there therewere wereaaasmall smallnumber numberof ofcracks, cracks,which whichwere wereless lesswide. wide. however, however, there were small number of cracks, which were less wide. The maximum maximum load load capacity capacity of of specimen specimen CFCC-2 CFCC-2 was was aaa negative negativeload loadof of´293 −293 kN kN with with aaa lateral lateral drift drift The The maximum load capacity of specimen CFCC-2 was negative load of −293 kN with lateral drift of 32.3 32.3 mm mm (R (R = = 1.92%) 1.92%) (Table (Table 666 and and Figure Figure 11), 11), which which is is similar similar to to specimen specimen CFCC-1. CFCC-1. Shear Shear failure failure of of 32.3 mm (R = 1.92%) (Table and Figure 11), which is similar to specimen CFCC-1. Shear failure occurred at at the the top top and and bottom bottom of of both both columns, columns, and and bucking bucking failure failurewas wasnot notobserved. observed. occurred columns, and bucking failure was not observed.
Figure 10. The CFCC-2 CFCC-2 specimen specimen following following the the cyclic cyclic load load tests. tests. Figure 10. The following the cyclic load tests. CFCC-2: Non-embeddable Non-embeddable Type Type CFCC-2:
350 350 300 300
(31.2, 275) 275) (31.2,
+Vmax +V max
250 250 200 200
Ultimate Strength Strength Point Point (Positive) (Positive) Ultimate
Load Load(kN) (kN)
150 150 100 100 50 50 00 -50 -50 -100 -100 -150 -150 -200 -200
--δδmax max +δδmax + max Shear Failure Failure Shear of Frame of Frame Ultimate Strength Strength Point Point (Negative) (Negative) Ultimate
-250 -250 -300 -300
(-32.3, -293) -350 (-32.3, -293) -350 -40 -40
-30 -30
-Vmax -V max -20 -20
-10 -10
00
10 10
20 20
30 30
40 40
Drift (mm) (mm) Drift
Figure 11. Load–drift relation for the CFCC-2 specimen. Figure 11. 11. Load–drift Load–drift relation relation for for the the CFCC-2 CFCC-2 specimen. specimen. Figure 5. Strength and Deformation Figure 12 shows envelop curves of the lateral load–drift relations for the RCFR, CFCC-1 and CFCC-2 specimens. The larger of the maximum positive and negative load values were used here (see Figures 7, 9 and 11). Table 7 lists maximum strength and deformation capacities. The strength
5. Strength and Deformation Figure 12 shows envelop curves of the lateral load–drift relations for the RCFR, CFCC-1 and Polymers 7 CFCC-22015, specimens. The larger of the maximum positive and negative load values were used1728 here (see Figures 7, 9 and 11). Table 7 lists maximum strength and deformation capacities. The strength ratio (SR)(SR) is defined as the of maximum loadload VmaxVofmaxspecimens with with the CFCC X-bracing to that the ratio is defined as ratio the ratio of maximum of specimens the CFCC X-bracing to of that control specimen, and the ratioratio (DR)(DR) indicates the ratio of drift at the maximum point δmax ofRCFR the RCFR control specimen, anddrift the drift indicates the ratio of drift at the maximum point of the specimens withwith the the CFCC X-bracing to that of the RCFR control specimen. δmax of the specimens CFCC X-bracing to that of the RCFR control specimen. 350
Maximum shear strength 300
293 kN 263 kN
Load (kN)
250 200
163 kN
150 100
RCFR CFCC-1 CFCC-2
50 0
0
10
20
30
40
Drift (mm)
Figure 12. The envelope of the load–drift relations of the three specimens. Figure 12. The envelope of the load–drift relations of the three specimens. Table 7. Summary of the strengths and deformation capacities. Table 7. Summary of the strengths and deformation capacities. Specimen Specimen
Maximum shear strength Maximum shear strength a V max Vmax(kN) (kN) a
RCFR RCFR CFCC-1 CFCC-1 CFCC-2 CFCC-2
163 163 263 (61.3%) 263 (61.3%) 293 (79.8%) 293 (79.8%)
Drift at the maximum point Strength ratio Drift at the maximum point Strength ratio b δmax (SR) δmax(mm) (mm) (SR) b 32.9 32.9 33.0 33.0 32.3 32.3
Drift ratio Drift ratio c (DR) (DR) c
(163/163) 1.001.00 (32.9/32.9) 1.001.00 (163/163) (32.9/32.9) (263/163) 1.001.00 (33/32.9) 1.611.61 (263/163) (33/32.9) 1.80 (293/163) 0.98 (32.3/32.9) 1.80 (293/163) 0.98 (32.3/32.9)
a a
Dataininparentheses parenthesesindicate indicatea astrengthening strengtheningeffect effectininterms termsofofshear shearstrength strengthcompared comparedwith withthe theRCFR RCFRcontrol control Data b specimen;b SR SRisisthe theratio ratioofofmaximum maximumload loadV max Vmaxofofspecimens specimensstrengthened strengthenedwith withCFCC CFCCtotothat thatofofthe theRCFR RCFR specimen; c c controlspecimen; specimen; DR DR is is the thethe maximum point δmaxδof strengthened with CFCC control the ratio ratioofofthe thedrift driftat at maximum point of specimens strengthened with maxspecimens to thattoofthat theofRCFR control specimen. CFCC the RCFR control specimen.
Themaximum maximumshear shearstrength strengthofofspecimens specimensCFCC-1 CFCC-1was was263 263kN, kN,and andthat thatofofCFCC-2 CFCC-2was was293 293kN; kN; The thisrepresents representsananincrease increaseofofa afactor factorofofapproximately approximately1.6–1.8 1.6–1.8(i.e., (i.e.,60%–80%) 60%–80%)larger largerthan thanthe theRCFR RCFR this controlspecimen specimen(where (wherethe themaximum maximumshear shearstrength strengthwas was163 163kN). kN).The Thedrift driftratio ratio(DR) (DR)ofofthe theCFCC CFCC control X-bracedspecimens specimenswas wasapproximately approximately1.0, 1.0, which which means means that that the X-braced the maximum maximum drift drift was wassimilar similartotothat thatof modes of of three three specimens specimenswere weresimilar; similar; ofthe thecontrol controlRCFR RCFR specimen. specimen. This Thisisis because because the the failure failure modes i.e., shear failure for the CFCC X-braced specimens and shear collapse of both columns for the RCFR control specimen. The results of strength and deformation capacities mentioned above show that the CFCC X-bracing system for RC frames is an effective retrofitting technique to provide increased strength, which is the most promising approach for low- to medium-rise RC buildings. If the ductility is insufficient, an adequate strength can reduce the inelastic displacement [27].
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6. Conclusion We have investigated the structural properties of CFCC X-bracing systems designed to improve the earthquake resistance of RC structures using cyclic loading tests. The CFCC X-bracing system proposed here has numerous advantages compared with conventional steel X-bracing systems, including an improved strength-to-weight ratio of the CFCC, compression-free bracing, and connection using bolt-nut joints, which can control the deflection of the X-bracing. These structural properties enable enhanced seismic resistance of RC frames. The results of this investigation can be summarized as follows. The control RCFR specimen reached maximum load capacity at positive and negative loads of approximately 160 kN, with a lateral drift of 33 mm (R = 2.0%). Shear collapse occurred at the top of both columns under a load of 99 kN, with a lateral drift of 44.0 mm (R = 2.6%). The frame of a typical Korean RC school building was selected for designing the RCFR specimen, which represents a shear failure mode. The building was a three-story RC structure designed in the 1980s without seismic resistance features. The maximum load capacity of specimen CFCC-1, which was reinforced with CFCC X-bracing with a flat plate-type configuration, was a positive load of 263 kN, with a lateral drift of 33 mm (R = 1.96%). The CFCC-2 specimen, with the protrusion-type X-bracing configuration exhibited a maximum strength with a negative load of ´293 kN with a lateral drift of 32.3 mm (R = 1.92%), which is similar to that of specimen CFCC-1. Both of the CFCC X-braced system exhibited shear failure modes at the top and button of both columns, buckling failure of the bracing was not observed. The strength capacity of both CFCC-1 and CFCC-2 specimens increased by a factor of approximately 1.7 compared with that of the RCFR control specimen. The deformation capacity of the CFCC X-braced specimens, however, was approximately equal to that of the RCFR control specimen. These strength and deformation capacities indicate that the CFCC X-bracing system is an effective retrofitting technique to provide increased strength. The reinforcement effect depends on the bracing material. It follows that the target strength capacity can be designed by choosing the appropriate CFCC X-bracing system. This approach to seismic strengthening can eliminate the buckling failure modes of conventional steel X-bracing systems. However, further work is required to optimize the details of the connection between the CFCC bracing and the columns or beams, as well as to design the seismic retrofitting procedures for existing RC buildings. This research was a preliminary approach regarding the use of new braces in carbon fibers and focused on an experimental investigation to study applicability of a new type of non-compression CFCC X-bracing system proposed in this study. As a recommendation for future work, however, additional theoretical and analytical studies are needed to justify the seismic behavior of the proposed CFCC X-bracing system. Acknowledgments This work was supported by National Research Foundation of Korea grant 2013R1A1A2009761, funded by the Ministry of Education, Korea.
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