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SISMIC RETROFIT OF STRUCTURES USING CARBON FIBERS

VIII SNIS

Katsumi Kobayashi (1), Toshiyuki Kanakubo(2) and Yasuo Jinno (3)

RESUMEN Un sistema sísmico de la modificación que usa la hoja de la fibra del carbón (SR-CF) se introduce. Ha sido desarrollado desde 1997 por la colaboración de sectores privados y de universidades. Esto se caracteriza con CF-anchors que pueda alcanzar envolverse cerrado para las columnas con la pared y las vigas con la losa así como columnas peladas. Hasta ahora, más de 200 edificios adoptaron este método y más de 5000 m2 de superficie concreta fueron envueltos con la hoja de la fibra del carbón (CF) y CF-anchors. Se ha evaluado para ser un confiable y el método prometedor y él de la construcción fueron dados una concesión técnica del desarrollo de AIJ en 2003.

ABSTRACT A Seismic Retrofit system using Carbon Fiber sheet (SR-CF) is introduced. It has been developed since 1997 by a collaborating team of private sectors and universities chaired by Dr. Shiro Morita, Professor Emeritus, Kyoto University. This is characterized with CF-anchors that can achieve the closed wrapping for columns with wall and beams with slab as well as bare columns. Up to now, more than 200 buildings adopted this method and more than 5000 m2 of concrete surface was wrapped with Carbon Fiber sheet and CF-anchors. It has been evaluated to be a reliable and promising construction method and it was given an AIJ Technical Development Award in 2003.

INTRODUCTION The continuous fiber materials including carbon fibers appeared in the construction field as a structural material in late 1980s, and the first international symposium, FRPRCS, was held in Vancouver, Canada coupled with ACI spring convention in 1993. At the early stage of research and development, the continuous fiber materials got attention as a new structural material to develop a new structural system in Japan and FRP-bars were in main use. On the other hand, especially in North America, non-corrosive characteristic of FRP-bars was the greatest advantage and it was studied and expectd as a substitution of steel bars in the area where they suffered the deterioration of structures due to deicing salt, However in Japan, the development of seismic retrofit method using continuous fibers started at the same period. Figure 1 and Fig.2 are the seismic retrofit examples at early stage. (1) Fac. Engineering, University of Fukui, Japan, katsumi@anc.anc-d.fukui-u.ac.jp (2)

Fac. Engineering, Tsukuba University, Japan, kanakubo@kz.tsukuba.ac.jp

(3)

Technical Research Institute, Shimizu Corporation, jinno@sit.shimz.co.jp

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Fig.1 Shear strengthening with Aramid Fiber Tape

Fig.2 Shear strengthening with Carbon Fiber Strands Winding

After Kobe earthquake 1995, the needs of seismic strengthening increased and the developments of seismic retrofit methods using continuous fibers were energetically conducted by private sectors, governmental institutions and academic institutions on the basis of the accumulated research results since before Kobe Earthquake. JSCE compiled the technology developed by private sectors and published a Recommendation of Design and Construction for the civil structures (JSCE, 2000). Japan Building Disaster Prevention Association (JBDPA) added the seismic retrofit method using continuous fibers to their menu of seismic retrofit schemes, and published Seismic Retrofit Design and Construction Guideline (JBDPA, 1999). AIJ research committee compiled research results and constructed the database of experimental data, and published a Design and Construction Guideline (AIJ, 2002) on the basis of the analysis of the database (Katsumata et al. 1999; Matsuzaki et al. 1999; Fujii et al. 1999). Design and Construction Guidelines of JBDPA and AIJ cover all the individual construction methods using continuous fibers, and gives an evaluation method for the shear capacity and ductility gain. In this paper, SR-CF system as an individual construction method will be introduced considering a convenience of applications to real buildings. The structural elements to be strengthened in a building has a large variety in their shapes and carbon fiber (CF) sheet wrapping technology for columns with wall, beams with slab, shear wall, etc will be needed as well as for the bare columns and the beams without slab. SR-CF construction method using CF sheet has been developed since 1997 by a collaborating team of private sectors and universities chaired by Dr. Shiro Morita, Professor Emeritus, Kyoto University, including the authors (Kobayashi et al. 2001; Matsuzaki et al. 2001; Masuo et al. 2001; Jinno et al. 2001; Jinno et al. 2002a, b). This is characterized with CF-anchors that can achieve the closed wrapping for columns with wall, beams with slab, shear walls, etc as well as bare columns. Up to now, more than 200 buildings adopted this method and more than 5000 m2 of concrete surface was wrapped with CF sheet and CF-anchors. It has been evaluated to be a reliable and extensive construction method and it was given an AIJ Technical Development Award in 2003.

CARBON FIBERS USED IN SEISMIC RETROFIT The diameter of carbon fiber filament is 7 to 10μm. 3000(3K), 6000(6K), 12000(12K) or 24000(24K) of filaments are bundled and it forms a strand. CF strands are woven with transverse glass or nylon threads to keep the shape of a sheet. A feature of CF strand and CF sheet is show in Fig. 3. The specified

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VIII Simposio Nacional de Ingeniería Sísmica, Tlaxcala, México, Septiembre de 2004

mechanical properties of CF sheet is shown in Table 1. The tensile strength and the Young’s modulus are tested using a test specimen as shown in Fig. 4. Table 1 Specification of characteristics of Carbon Fiber Sheet 200 Type CF sheet 300 Type CF Sheet 2 Density (g/m ) 200 300 Thickness for design (mm) 0.111 0.167 Tensile strength (N/mm2) 3400 3400 5 Young’s modulus (N/mm2) 2.3×105 2.30×10

CF strand CF sheet

Fig.3 Feature of CF strands and CF sheet CF sheet with resin

Gripping tab

Strain gage

60mm

Direction of filaments

60mm

250mm Fig. 4. Test specimen for tension test A bundle of tens strands is used for CF-anchors. The strand usually contains 1.0% of sizing agent to keep assembling extremely fine filaments. However this sizing agent disturbs good impregnation of epoxy resin into filaments. CF-anchors show better performance when the amount of sizing agent is more reduced. So, the strand with 0.2% of sizing agent is used for CF-anchor. CF-sheet and CF-anchors show their performance when they are coupled with epoxy resin and forms the fiber reinforced plastic, and bond with concrete. Before installation of CF sheet, the primer is given to the concrete surface to improve the adhesion between CF sheet and concrete. The primer must have the adhesive strength more than 1.5N/mm2. The epoxy resin used to install CF sheet must securely impregnate with CF sheet and guarantees that CF sheet can show its mechanical properties shown in Table 1. The tensile strength, bending strength and shear strength of epoxy resin are specified to be more than 29N/mm2, more than 39N/mm2 and more than 10N/mm2, respectively.

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SHEAR CAPACITY AND DUCTILITY GAIN BY CF SHEET WRAPPING

Concept of Japanese seismic performance evaluation method and seismic retrofit

Shear coefficient

Japanese seismic capacity evaluation method was developed in 1977 and revised in 1990 and 2001 (JBDPA, 2001). It is based on Newmark’s equal energy criterion for inelastic response. To evaluate both elastic and elastoplastic structures to have same seismic capacity index, µ ⋅ δ y is converted to F-index, that is F = (2 µ − 1) , and the seismic capacity is defined as the product of the shear coefficient (C) and the F-index as shown in Fig. 5. For example, a story is idealized as a series of vertical members such as in Fig. 6-(a). The member A is a brittle column with shear failure mode. The member B is a flexural column with less ductility. Shear force displacement relation is schematically shown in Fig. 6-(b). The relation between the story shear and horizontal displacement is described by superimposing the member A and B as shown in Fig. 6-(c). If the vertical load on the member A can’t be redistributed to the other members when the member A fails in shear, the story will collapse (Fig. 6-(d)). If the vertical load is redistributed, the structure can be displaceable up to the deformation limit of the member B, and the seismic capacity is evaluated for the shaded area of Fig. 6-(e).

Ce

F = (2 µ − 1)

C

δy

µ ⋅δ y Displacement Fig. 5 Elastic and elasto-plastic response

A

B

A

Story Shear

Capacity of members

Fig. 6-(a) Idealized Story of a Building

B

A B

Story Drift Fig. 6-(c) Relation between the story shear and Story drift

Horizontal Displacement Fig. 6-(b) Shear force-displacement relations of members

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VIII Simposio Nacional de Ingeniería Sísmica, Tlaxcala, México, Septiembre de 2004

Response criterion Story Shear

Story Shear

Response criterion A B

Story Drift Fig. 6-(d) Collapse of strory after after shear failure of brittle member A

A B

Story Drift Fig. 6-(e) Collapse of strory at the deformation limit of the member B

The seismic capacity is compared with the response criterion, Iso. C ⋅ F ≤ or ≥

1 I so φ

(1)

The seismic capacity is evaluated for each story and compared with the expected response of the story that is more magnified at higher story. 1/φ is the magnification factor of the response. Equation (1) is transformed as follow, E o = φ ⋅ C ⋅ F ≤ or ≥ I so

(2)

If the mass and rigidity of each story uniformly distribute along the height, φ is approximately expressed by Eq.(3),

φ=

n +1 n+i

(3)

Here, n is the total number of stories and i is the number of the target story. Eo is the basic seismic capacity index. The total seismic capacity index (Is) is calculated by multiplying with the reduction factors due to the irregular structural design (SD- index) and the deterioration of materials (T-index). Just in case that there is basement stories, the response will be reduced and SD-index becomes an advantage factor. (4)

Response criterion

In Fig. 6-(e), the shaded area doesn’t reach the response criterion. The member A is required to have shear capacity gain and to yield, and exhibit large deformability. The member B is required to have more ductility. For this reason, the evaluation of the shear capacity and the ductility is required for the seismically retrofitted members with any retrofitting scheme as well as CF sheet wrapping method as shown in Fig. 6-(f).

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Story Shear

I s = Eo ⋅ S D ⋅ T

Capacity & Ductility gain Ductility gain

Story Drift Fig. 6-(f) Needs of capacity and ductility gain


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When the total seismic capacity index (Is) is smaller than the criterion (Iso), the shear capacity and the ductility gain must be achieved by retrofitting scheme by the following equation. ∆ (C ⋅ F ) =

I so − I s φ ⋅ SD ⋅T

(5)

Evaluation of shear capacity gain by SR-CF system The evaluation method of shear capacity for bare columns strengthened by SR-CF system is based on the “Design Guidelines for Earthquake Resistant RC buildings Based on Ultimate Strength Concept” (AIJ, 1990). This evaluation method basically consists of the shear forces carried by truss mechanism and arch mechanism. It is assumed that the CF sheet and CF Anchor installed in the hoop direction of the column work together with the existing steel hoops and form a truss mechanism. The first term of Eq.(6) expresses the truss mechanism. The amount and strength of shear reinforcements is represented as the summation of those of steel hoops and CF sheet. At the ultimate state, steel hoops reach the yield strength. However, the CF sheet does not always show its full strength, and the stress on the CF sheet does not distribute uniformly. In order to superimpose the contribution of steel hoops and the CF sheet to the shear capacity, the coefficient, νcf, to express the effective strength of CF sheet was newly introduced. The coefficient is given by functions of the simply summed up amount of shear reinforcement and the concrete compressive strength as show in Eq.(11). The relations of Eq.(11) is illustrated in Fig.7. The value of coefficient, νcf, decreases, when the ratio of the simply summed up amount of shear reinforcement to the concrete strength increases. Q su = b ⋅ j t ⋅ ¦ ( p w ⋅ σ w ) cot + tan ⋅ (1 − ) ⋅ b ⋅ D ⋅ν c ⋅ σ B / 2

(6)

tan θ = ( L / D) 2 + 1 − L / D

(7)

β = (1 + cot 2 φ ) ⋅ Σ( pw ⋅ σ w ) /(ν c ⋅ σ B )

(8)

cot φ = min 2.0, jt /( D ⋅ tan θ ), ν c ⋅ σ B / Σ( p w ⋅ σ w ) − 1.0

(9)

¦(p

w

⋅ σ w ) = p ws ⋅ σ wy + p wcf (ν cf ⋅ σ wcf )

(10) p ws ⋅ σ wy + p wcf ⋅ σ wcf

ν cf = 1.0

σB

ν cf = −0.42

ν cf = 0.98

p ws ⋅ σ wy + p wcf ⋅ σ wcf

p ws ⋅ σ wy

σB

+ .28

0.66 ≤

σB + p wcf ⋅ σ wcf

1.53 ≤

6

p ws ⋅ σ wy + p wcf ⋅ σ wcf

σB p ws ⋅ σ wy + p wcf ⋅ σ wcf

σB

< 0.66

< 1.53

(11)


VIII Simposio Nacional de Ingeniería Sísmica, Tlaxcala, México, Septiembre de 2004

where, Qsu : shear capacity b : width of member D : depth of member L : clear span length of member jt : distance between tensile main bars and compressive main bars pws : shear reinforcement ratio of steel hoop σwy : yield strength of steel hoop pwcf : shear reinforcement ratio of CF sheet σwcf : design tensile strength of CF sheet σB : concrete compressive strength (N/mm2) νc : effectiveness coefficient of concrete compressive strength (= 0.7 – σB / 200) νcf : effectiveness coefficient of CF sheet tensile strength

Fig. 7 Effectiveness coefficient of CF sheet strength

Fig. 8 Comparison between column test results and evaluated shear capacity

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The strain on the CF sheet were measured in some laboratory tests in which the columns strengthened by CF sheet were given cyclic shear force and bending moment. Measured strain showed 10000 to 12000 µ at the ultimate state. Though the actual tensile strength of CF sheet is over 4000N/mm2 by coupon tests, the design strength is evaluated as 2300 N/mm2 according to the test results. Figure 8 represents the comparison between the evaluated shear capacity (Qsu) by Eq.(6) and the observed maximum shear force (Qexp) by column tests. Both values are normalized by the calculated shear force at bending yield (Qmu). The evaluation by Eq.(6) is reasonably verified by the test results. Evaluation of Ductility Gain by SR-CF system

It is possible to evaluate the ductility factor (µ = δu / δy) using AIJ shear capacity method (AIJ, 1990). In this method, the shear capacity after bending yield is reduced according to the increase of displacement by controlling the values of cotφ (angle of compressive strut in truss mechanism) and νc (effectiveness coefficient of concrete compressive strength). It is supposed that the ultimate displacement (δu ) can be evaluated at the point where the reducing shear capacity becomes smaller than bending capacity, and this method is adaptable only for shear failure mode. However, in the laboratory tests, many columns strengthened by CF sheet showed ductile behavior even though they failed in splitting bond failure mode. The cover concrete falls down at the early stage for the ordinary RC columns with splitting bond failure mode. However, the columns strengthened by CF sheet supposedly do not show that phenomenon due to the confining effect by CF sheet. In many of design guidelines, for example published by JBDPA (JBDPA, 1999), an evaluation method using shear and splitting bond capacity index is supported. The shear and the splitting bond capacity index are defined as the ratio of shear capacity and the splitting bond capacity to the bending capacity, respectively. It is known that the ductility factor becomes larger as the capacity index becomes larger in experience. The following equation is introduced to evaluate ductility factor by analyziing the test results. Figure 9 shows relations between the capacity index and the observed ductility factor in column tests. This shows adaptable results not only for the members with longitudinal deformed bars but also for those with round bars.

Fig. 9 Relationship between the capacity index and the observed ductility factor

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VIII Simposio Nacional de Ingeniería Sísmica, Tlaxcala, México, Septiembre de 2004

§Q · µ = 10 ⋅ ¨¨ su − 0.9 ¸¸ © Qmu ¹

(12)

Here, µ is the ductility factor. However, µ is evaluated to be 1.0 when the ratio of the splitting bond capacity to the bending capacity (Qsub /Qmu) is in the following range. Qsub /Qmu < 0.8 in case that the deformed bars are used for the longitudinal bars Qsub /Qmu < 0.6 in case that the round bars are used for the longitudinal bars CF-ANCHORS

Variety of Structural Elements in Building

The number of bare columns and beams in a building is relatively small and there is much variety such as columns with wing wall, columns with spandrel/suspended wall, columns with shear wall, beams with slab etc. When columns have walls, the wrapped CF sheet is split to both side of the wall. Several ideas to achieve a closed wrapping through the walls were proposed. It is, however, difficult to achieve a closed wrapping without removing the walls partially. This would reduce the strength and rigidity of the columns with walls.

Penetrating hole

Opening angle ≦90° Overlapping length Overlapping length ≧10mm ≧10mm

Embedding length ≧150mm

Spacing of CF-anchor ≦200mm

Splicing length ≧200mm

Wrapping for a bare column Wrapping with CF-anchor for shear wall

Wrapping with CF-anchor for column with wing wall

Fig. 10 Variety of Structural Elements and applications of CF-anchor

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CF-Anchor is an idea to connect the wrapped CF sheet split by walls, remaining the wall in the original condition as shown in Fig.10. Penetrated holes are drilled on the wall closely to the column surface. A bundle of CF strand is put through the hole, and then CF strand is opened in a shape of fan at both outside of the hole. The distributed strand in a shape of fan is spliced with CF sheet on the column. There is an advantage that CF-anchors hardly affect the strength and the rigidity of the walls. A column strengthened by this system would not fail in shear mode, even after the failure of the walls. The walls would work as energy absorption elements. In early stages, the rigidity and the strength as columns with walls could be expected, and after the failure of the walls, a performance of bare columns with high ductility could be expected. CF sheet wrapping with CF-anchor is also applicable to beams with slab. A half of CF-anchor shown in Fig.10 may become an embedded type CF-anchor. This is used for anchoring CF sheet on the wall surface to the circumference frame (Jinno et al. 2002a).

Specification of CF-anchor

A standard detail of CF-anchor system is shown in Fig.11. As for the amount of fibers contained in CFanchor, it should be more than that in main CF sheet at least, and is specified more than 1.25 times of that in the main CF sheet for columns and beams. Exceptionally it is same as that in the CF sheet on the wall because the design tensile strength of CF sheet on the wall is evaluated small. CF-anchors with larger splicing length (L) have larger anchor strength. A larger CF-anchor splicing length means also a small opening angle of the fan, and it should be limited to less than 90 degrees. By overlapping of the fan shaped portions of adjoining CF-anchors, the outside strands on the fan works more effectively. So, the fan shaped portions of adjoining CF-anchors should be arranged so that they surely overlap more than 1cm. The spacing of CF-anchor is limited less than 20cm. The perpendicular CF sheet is required under the fan shaped portion of CF-anchor so that the outside strands on the fan works effectively. Its amount would be more than one third of main CF sheet.

Main CF sheet

20cm or less 90 degrees or less 1cm or more

Perpendicular CF sheet L 20cm

Wing wall

Fig.11 Standard detail of CF-anchor system

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VIII Simposio Nacional de Ingeniería Sísmica, Tlaxcala, México, Septiembre de 2004

CONSTRUCTION PROCEDURE FOR CF SHEET

Removal of existing finishing material

The existing finishing of members is removed to expose structural concrete surface. If some damage is found in the structural concrete, it must be repaired because the column with damage cannot be improved so much as expected in the retrofit structural design. When piping materials or some finishing material cannot be removed, the use of CF-anchor is possibly discussed. Figure 12 is an example of a column retrofitted by CF sheet and CF-anchors without removing the window frame. The removal of finishing makes much dust and noise and it is very inconvenient for building users and owners. If the finishing mortal has enough strength and bonds with structural concrete in good condition, it is not always necessary to remove it and CF sheet is possibly installed on the finishing mortal. Here a precise investigation is absolutely needed.

Fig.12 CF sheet wrapping without removing the window frame

Arrangement of and treatment of concrete surface

The smooth concrete surface is needed not to make wrapping defects such as wrinkles, voids, air cells, etc. Moreover, the sharp edges on the surface are removed and the corners are rounded. Grinding of concrete is usually employed for this work as shown in Fig.13. However, grinding makes much dust and noise and the application of a new mortal is preferred as shown in Fig.14. The resin putty is used to fill the air cells and to treatment small area of concrete surface. After smoothing, the primer is spread to improve the bond strength between CF sheet and concrete. Note that the resin putty is applied after primer spreading.

Fig.13 Smoothing by grinder

Fig.14 Smoothing by mortal

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1

h

2

hl

3 4

H

5 6 7 8 9

10 11

Step 1 Step 5

Step 4

12 13 14

Step 2 Step 6

Step 3

15 16 17 18

>100mm 2nd Layer

1st

Fig.15-(b) Pre-cut of CF sheets

Fig.15-(a) Wrapping program

Wrapping with CF sheet with CF-anchor or Wrapping with CF sheet and CFanchor

Marking positions of penetrating holes Drilling of penetration holes

without CF-anchor

Preparation of CF-anchor

Spreading of primer Adjustment of gap between concrete surface and holes

Adjustment of surface with resin putty Marking CF sheet layout line on the concrete surface with CF-anchor Pre-cutting of sheet

Installation of CF sheet in the longitudinal direction

CF sheet Wrapping and Resin Impregnation with CF-anchor

Installation of CF-anchor

Curing Finishing Materials

Fig. 16 Work flow diagram of CF sheet installation

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VIII Simposio Nacional de Ingeniería Sísmica, Tlaxcala, México, Septiembre de 2004

Installation of CF sheet

The work flow diagram of SR-CF system is shown is Fig. 16. The drawing of a wrapping plan is made at first as shown in Fig.15-(a). When the CF sheets are spliced with lap joints, the lapping length must be more than 100mm and the location of lap joints must distribute on every side of the members. According to the wrapping plan, CF sheets are pre-cut with scissors as shown in Fig.15-(b). The base of epoxy resin is mixed with the harder using a hand-powered mixer before CF sheet installation work. The right mixing ratio is essential because an inadequate mixing ratio reduces resin strength. The uniformity of mixing, that is no locally inadequate mixing ratio, is also important. A wrong mixing will cause many trouble with this technique. If the resin reaches the pot life, it must be discarded.

Fig.17 CF sheet Wrapping Along with the marking, the mixed epoxy resin is spread on a target area on the concrete. A precut CF sheet is put on the spread epoxy resin and pressed by hands, a rubber spatula and a plastic roller in order not to remain the defects such as air cells, wrinkles, resin mass, unbonded part, etc. as shown in Fig.17. To make a lap joint, the resin is spread on the lapping area and the wrapping sheet is glued and pressed. After 10 to 30 minutes, the resin impregnation may be completed and the viscosity of impregnated resin may increase, and then some resistance of installed CF sheet against position changing is expected. At the end of wrapping, the resin is spread again as a top coating on the installed CF sheet. After 0.5 to 1.0 day of initial curing under ordinary temperature, the finishing materials can be applied. The onsite quality checking, that is appearance inspection of wrapping sheet, is given. If a defect is found, it is adequately repaired depending on the extent of the defect. A tension test of FRP sampled at the construction site is done in the laboratory as a regulation.

CONSTRUCTION METHOD OF CF-ANCHOR

CF anchors are made of CF strands. The number of strands and the diameter of penetration holes depend on the total density of CF sheet installed on the concrete surface and the spacing of CF-anchors. If the amount of carbon fibers is 1.25 times of the main splicing CF sheet, a standard specification is shown in Table 2.

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Table 2 Standard specification of the number of strands and the diameter of penetration hole Total density Spacing of CF-anchors (mm) of CF sheet CF 100 150 200 (g/m2) strand Diameter of holes (mm) Diameter of holes (mm) Diameter of holes (mm) (The number of strands) (The number of strands) (The number of strands) 200 φ8 (32) φ10 (48) φ12 (64) 300 φ10 (48) φ13 (72) φ14 (96) 12K 400(200+200) φ12 (64) φ14 (96) φ17 (128) 600(300+300) φ14 (96) φ18 (144) φ20 (190) 900(300×3) φ18 (144) φ22 (214) φ25 (286)* 200 -φ10 (24) φ12 (32) 300 φ10 (24) φ12 (36) φ14 (48) 24K 400(200+200) φ12 (32) φ14 (48) φ17 (64) 600(300+300) φ14 (48) φ18 (72) φ20 (96) 900(300×3) φ18 (72) φ22 (108) φ25 (144)* * Splicing length (L) of CF-anchor is required more than 250mm

CF strand is winded around two arms as shown in Fig 18. Both ends of the bundled CF strands are tied with wire. The construction procedure for columns with wall is illustrated in Fig.19. The penetrating holes are drilled on the wall as closer as possible to the column (Fig.19-(a)). CF sheet is installed along the column length on which the main CF sheet on the column and CF-anchor are spliced (Fig.19-(b)). Next, the main CF sheet is glued in the transverse direction of the column (Fig.19-(c)). The center part of the bundled CF strands, that comes into the penetrating hole, is impregnated with epoxy resin in advance (Fig.19-(d)). After spreading epoxy resin on the main CF sheet, the bundle of CF strands is put through the penetrating hole using a wire that is attached at the end (Fig.19-(e)). In the last step, the end of CF strands is cut off and the wire is removed. Then, the CF strands are spread in the shape of fan and the epoxy resin is put again. The spliced part is pressed by a plastic roller to splice with the main CF sheet tightly (Fig.19-(f)). For the embedded type CF-anchor, the wire at the end of the strand is put through a small pipe, and the strands are inserted into the penetrating hole using the pipe. The pipe works as a route where the air comes out when the bundle of strands with resin is inserted.

CF strand

Wire Fig.18 A way to make a CF-anchor

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VIII Simposio Nacional de Ingeniería Sísmica, Tlaxcala, México, Septiembre de 2004

Wall

Wall Drill

Main CF sheet

Column

Column

(a)

(b)

(c) CF-anchor

CF-anchor Epoxy resin

Wire Plastic roller

Wire

(d)

(e)

(f)

Fig. 19 Construction procedure of CF-anchor for columns with wall

APPLICATION OF SR-CF SYSTEM TO THE SHEAR WALL

Method for strengthening shear wall

The SR-CF system strengthens walls with the CF sheet diagonally glued on the surface of the wall. The edges of CF sheet are fixed to the peripheral column, beam, and floor slab by CF-anchors. This method gives the performance as tensile braces to the CF sheet, and consequently increases the shear resistance of the wall. The process of strengthening a wall using CF-anchors is shown in Fig.20.

15

(1) Smoothen the surface of the wall . Drill holes on the peripheral columns, beam, and floor for inserting CF-anchors. (2) Apply primers on the surface of the wall.


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(1) Smoothen the surface of the wall panel by removing soft section. Drill holes for installing CF-anchors on the peripheral frame (side columns, beam, and the floor slab). (2) Apply the primer on the surface of wall panel. (3) Laminate carbon fiber sheets on the wall surface. The fibers of the sheets should be alined to the diagonal direction. Apply sheets so that the fibers of each sheet are in the cross-diagonal direction. CF sheets in one diagonal direction work for one directional horizontal force. Laminate the required numbers of layers. (4) Prepare CF-anchors by bundling the same amount of carbon fiber strands as those contained in the CF sheets. (For example, a CF sheet of 200mm wide and 300 g/m2 in density contains carbon fibers almost equivalent to 38 of "24K" strands.) If two layers of such CF sheets is installed, CFanchor made of 19 x (200 / 100) x 2 = 76 strands is needed for every 200 mm wide CF sheet. (5) Immerse the upper half of the CF-anchors into epoxy resin, and insert the ends into the holes drilled on the column, beam and slab. (6) Apply epoxy resin on the CF sheets and spread the remaining half of the CF-anchors like fans on the CF sheets, and then splice with CF sheets by immersing epoxy resin. The center line of each fan should coinside with the direction of fibers of CF sheet. Since CF sheets are installed in the crossdiagonal directions, the CF-anchors on the beam are devided into two groups at the center of the beam, and oriented to the direction of the opposite corner. The CF-anchors on the column are similarly divided into two groups.

(3) Adhere CF sheets on the surface of the wall.

(4) Bundle CF strands to make a CF-anchor. (5) Insert the CF-anchor that is immersed with epoxy resin into a hole. (6) Spread the CF strands and adhere to the CF sheet.

Fig.20 Process of strengthening walls

Evaluation of shear capacity gain by SR-CF system

To estimate the shear force gained by CF sheets, the wall is assumed to be deformed into a parallelogram as shown in Fig.21. The shear deformation is dominant and the bending deformation is ignored in the model. The CF sheets, diagonally laminated on the walls, are regarded as tensile braces installed on the existing walls. Assuming that the strain on the carbon fiber sheets distributes uniformly over the entire wall surface, the shear force gained by CF sheets is expressed by Equation (13). The tensile force of the CF sheets is transmitted to the upper and lower beams and the columns on both sides. The horizontal component of the force that is transmitted to the upper beam is the shear force gained by CF sheet. According to the experimental results, the design tensile strength of the CF sheets was evaluated to be 680 N/mm2. Qcf = L ⋅ t cf ⋅ σ cf ⋅ sin θ ⋅ cos θ

(13)

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VIII Simposio Nacional de Ingeniería Sísmica, Tlaxcala, México, Septiembre de 2004

σ cf = E cf ⋅

δ ⋅ sin θ ⋅ cos θ h

(14)

where, Qcf : shear force gained by CF sheets, L H tcf σcf Ecf δ θ

: clear length of the wall, : clear height of the wall, : thickness of CF sheets : design tensile strength of CF sheets (=680 N/mm2) : Young’s modulus of CF sheets, : horizontal displacement at the top of the wall : angle of the fibers of CF sheet on the wall

L

Fixed to the beam dL⋅sinθ⋅t cf ⋅E cf ⋅εcf

δ dL⋅sinθ

Strain of CF sheet εcf Elastic modulus Ecf

dL h θ

Fixed to the column dL⋅sinθ⋅t cf ⋅E cf ⋅εcf

θ

Fixed to the column dL⋅sinθ⋅t cf ⋅E cf ⋅εcf

Assumed deformation of a wall and strain of CF sheet

Fixed to the beam dL⋅sinθ⋅t cf ⋅E cf ⋅εcf

Equilibrium of forces on CF sheet

Fig.21 Model for calculating the shear force gained by the CF sheets

Evaluation of ductility gain by SR-CF system

The evaluation method for the ductility of the wall is based on " Standards for Evaluation of Seismic Capacity and Comments for Existing Reinforced Concrete Buildings (JBDPA 2001)". The failure modes of walls are classified into flexural failure, shear failure and uplift rotation mode in SR-CF system. Walls which may fail in flexure and uplift rotation mode are evaluated to have larger ductilities than those which may fail in shear mode. The SR-CF system intends to increase just the shear capacity of walls, and not to increase the ductility directly. However, it is possible to changes the failure mode of the wall from the shear failure mode to the uplift rotation mode by increasing the shear capacity of the wall, resulting in increase of the ductility. Even though the shear failure mode changes to the flexural failure mode, it should not be allowed to expect the larger ductility gain in the strengthening design because the flexural capacity may also increase by the installation of CF sheet.

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A CASE STUDY

The applicability of SR-CF system was studied for a 9-story university building in Tokyo. It has two basement stories and 2-story penthouse, and was directly supported by the ground. The construction age was 1968. It was just after the Tokachi-oki earthquake and before the revision of seismic design code in 1971. The 3rd to 9th floors have same plan. The result of the seismic capacity evaluation described in the above (JBDPA, 2001) is shown in Table 3. Except for the 9th-story, the seismic capacity did not reach the criterion where the total seismic capacity index is 0.72. Two columns of 8th story, six columns of 7th story, ten columns of 3-6th stories were retrofitted by SR-CF system. The 3rd floor plan is shown in Fig.22 together with the retrofitted columns. The first and the second stories were excluded from the discussion because they had very irregular structural configulation. The original columns with shear failure mode changed to the ductile columns by the retrofit using CF sheet wrapping. All the retrofitted columns were secured to have the ductility factor over 5.0 and F-index increased to 3.2. As a result of application of CF sheet wrapping with CF-anchors, all the stories in X- and Y-direction over 3rd story satisfied the criterion.

Retrofitted column by CF sheet

5,400 F

5,400 E

5,400 D

5,400 C

Fig.22 The 3rd floor plan and retrofitted columns

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5,400 B

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VIII Simposio Nacional de Ingeniería Sísmica, Tlaxcala, México, Septiembre de 2004

Table 3 Results of siemic capcity evaluation before and ffter retrofit by SR-CF system Before retrofit After retrofit using Retrofitted Eo Is Eo Is Story X-axis Columns Y-axis X-axis Y-axis X-axis Y-axis X-axis Y-axis 9 1.03 1.09 0.85 0..91 0 1.03 1.09 0.85 0.91 8 0.72 0.78 0.67 0.72 2 0.93 0.78 0.86 0.72 7 0.43 0.63 0.40 0.65 6 0.82 0.79 0.75 0.81 6 0.37 0.58 0.38 0.60 10 0.85 0.78 0.87 0.90 5 0.37 0.56 0.38 0.58 10 0.78 0.83 0.80 0.85 4 0.34 0.43 0.35 0.44 10 0.71 0.81 0.73 0.83 3 0.32 0.42 0.33 0.43 10 0.78 1.01 0.81 1.04 2 0.56 0.44 0.52 0.41 Not discussed 1 0.54 0.45 0.44 0.46

CONCLUSION

The seismic retrofit work is not welcome because it requires inconvenience to the building owners and the users, for example, high cost, noise, dust, risk of fire accident, and specially moving from their office during retrofit works. Expecting to reduce such inconvenience, CF sheet wrapping technology has been developed. The advantages of CF sheet wrapping are lightweight, flexibility, non-welding and easy handling. The introduced SR-CF system using CF sheet is characterized with CF-anchor, and it can be applicable all the structural members. The evaluation of shear capacity and ductility gain of bare columns and the wall was representatively introduced in this paper. Of course, that for the columns with wing walls, columns with spandrel wall, columns with suspended wall, and beams with slab is included in “Design guidelines for SR-CF system (SR-CF System Research Association, 2001).” The basic concept of the evaluation is based on the ordinary evaluation equations for reinforced concrete members, and the reasonable summation of shear reinforcing effect of the existing steel shear reinforcement and the externally bonded CF sheet. The experimental verification for the evaluations can refer to the literatures, (Matsuzaki et al. 2001; Masuo et al. 2001; Jinno et al. 2001, Jinno et al. 2002a, b ). From a case study, it was found that a vulnerable building could be retrofitted to satisfy the criterion using just CF sheet wrapping technology in combination with CF-anchors. SR-CF system has gotten a good reputation that is reliable and promising. The formation of CF-anchor will need some craftsman skill. Now pre-manufactured CF-anchors with well handling are studied and available.

REFERENCES

Architectural Institute of Japan (1990), “Design Guidelines for Earthquake Resistant RC buildings Based on Ultimate Strength Concept”, November (in Japanese) Architectural Institute of Japan (2001), “Design and Construction Guideline of Continuous Fiber Reinforced Concrete,” March (in Japanese)

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Fujii, S. Matsuzaki Y., Nakano K. and Fukuyama H. (1999), “Japanese State of the art on seismic retrofit by fiber wrapping for building structures, Evaluations,” Fourth International Symposium on Fiber Reinforced Polymer Reinforcement for Reinforced Concrete Structures, ACI, SP-188, pp.895-906 Japan Building Disaster Prevention Association (1999), “Seismic Retrofit Design and Construction Guideline of Existing Reinforced Concrete and Steel Encased Concrete buildings,” September Japan Building Disaster Prevention Association (2001), “Standards for Evaluation of Seismic Capacity and Comments for Existing Reinforced Concrete buildings (in Japanese)” Japan Society of Civil Engineers (2000), “Recommendation for Design and Construction of Concrete Structures Using Continuous Fiber Reinforcing Materials (English translation is available),” Concrete Library 101, July Jinno Y., Tsukagoshi H. and Yabe Y. (2001), ”RC Beams with Slabs Strengthened by Carbon Fiber Sheets and Bundles of CF Strands,” Fifth International Symposium on Fiber Reinforced Polymer Reinforcement for Reinforced Concrete Structures, Vol.2, Cambridge UK, pp.981-988, July Jinno Y. and Tsukagoshi H. (2002), ”Sesmic Strengthening of Reinforced Concrete Walls by SR-CF System --Methods and Effect of Shear Strengthening by Carbon Fiber Sheets and CF-anchors--,” First fib Congress 2002, Session6, pp.109-118 Jinno Y. and Tsukagoshi H. (2002), ”Seismic Strengthening of Reinforced Concrete Beams with Slabs by Carbon Fiber Sheet and CF-anchor,” SEWC2002, Technical Session T8-3-a-1, pp.1-8 Katsumata K., Kobayashi K., Morita S. and Matsuzaki Y. (1999), “Japanese State of the art on seismic retrofit by fiber wrapping for building structures, Technologies and research and development activities,” Fourth International Symposium on Fiber Reinforced Polymer Reinforcement for Reinforced Concrete Structures, ACI, SP-188, pp.865-878 Kobayashi K., Fujii S., Yabe Y., TsukagoshiI H. and Sugiyama T. (2001), “Advanced wrapping system with CF-anchor --Stress transfer mechanism of CF-anchor --,” Fifth International Symposium on Fiber Reinforced Polymer Reinforcement for Reinforced Concrete Structures,Vol.2, Cambridge UK, pp.379388, July Masuo K., Morita S., Jinno Y. and Watanabe H. (2001), “Advanced Wrapping system with CF-anchor -Seismic strengthening of RC columns with wing walls --,” Fifth International Symposium on Fiber Reinforced Polymer Reinforcement for Reinforced Concrete Structures,Vol.1, Cambridge UK, pp.299308, July Matsuzaki Y., Nakano K., Fujii S. and Fukuyama H. (1999), “Japanese State of the art on seismic retrofit by fiber wrapping for building structures, Research,” Fourth International Symposium on Fiber Reinforced Polymer Reinforcement for Reinforced Concrete Structures, ACI, SP-188, pp.879-893 Matsuzaki, Y. Nakano K., Fukuyama H. and Watanabe S. (2001), ”Advanced Wrapping System with CF-Anchor -- Shear Strengthening of RC Columns with spandrel wall --,” Fifth International Symposium on Fiber Reinforced Polymer Reinforcement for Reinforced Concrete Structures, Vol.2, Cambridge UK, pp.813-822, July

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SR-CF System Research Association (2001), “Design guidelines for SR-CF System (in Japanese),” February

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Profile for Rodney Bellido de Luna

Refuerzo Sismico con FRP  

Reforzamiento estructural con fibra de carbono, anclajes con FRP, refuerzos sismicos de estructuras

Refuerzo Sismico con FRP  

Reforzamiento estructural con fibra de carbono, anclajes con FRP, refuerzos sismicos de estructuras

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