SEISMIC RETROFIT USING CONTINUOUS FIBER SHEETS

Transcription

1 SEISMIC RETROFIT USING CONTINUOUS FIER SHEETS Yasuhiro MATSUZAKI 1, Katsuhiko NAKANO, Shigeru FUJII 3 And Hiroshi FUKUYAMA 4 SUMMARY The researh on seismi retrofit of RC members using ontinuous fiber sheets is rapidly advaning in Japan. These researh ativities have been aelerated sine 1995 Hyogo-ken Nanbu Earthquake beause this retrofitting tehnique has an advantage due to lighteight, flexibility, non-elding, and easy handling in onstrution orks. The most preferable retrofitting tehnique for the existing building being in use is jaketing RC olumns ith arbon or aramid fiber sheets beause of the advantage above. Reently many results of the researh on olumns ith ing all, short olumn ith spandrel all, earthquake resisting all, girder ith slab ( T-shaped girder ) and nonstrutural reinfored onrete all have been reported in Japan. This paper introdues, first of all, these experimental researh results and the seismi retrofitting effets by the ontinuous fiber sheets are omprehensively evaluated. Shear strength and the improvement of dutility is disussed to evaluate the effet of reinforement of ontinuous fiber sheets. And this paper reports on the omparative studies about several influening parameter on olumn apaities and the appliability of urrent RC design equations proposed by the Arhitetural Institute of Japan using the database. ased on the analysis, folloing findings ere obtained. (a) Shear strength of retrofitted olumns an be predited by the above design equations based on the same strut and tie models as for usual RC olumns. (b) Fators governing the lateral deformation apaity of retrofitted olumns ere disussed and an evaluation method to predit the dutility ratio as a funtion of the shear and flexural strength ratio as examined. As the summary, the problems hih should be solved, researh diretion and prospet are pointed out for the future researh on seismi retrofit by ontinuous fiber sheets. Researh needs for establishment of a design method hen various members are reinfored by ontinuous fiber sheets in the future are also disussed. INTRODUCTION The results of studies on the appliation of suh ne materials as the arbon and aramid fiber as strutural materials began to be made publi gradually in around the mid-1980s. In those days, efforts ere being made to establish a tehnology for forming suh ne materials into ontinuous fiber reinforing bars using resin bond, et. hih ould be substituted for the reinforing bars. y around 1993, it beame lear that arbon and aramid fibers ould be used as strutural materials. In the early 1990s, the durability of onrete strutures ame to attrat groing attention as a problem hih as strongly alling for the establishment of tehniques to repair or retrofit existing onrete strutures. This triggered researh and development on the appliation of ontinuous fiber sheet reinforement. On January 17, 1995, in Japan, the Hyogo-ken Nanbu Earthquake ourred, ausing damage to many of the strutures. As a result, the soial demand for seismi diagnosis and retrofit of existing reinfored onrete buildings beame strong. It aelerated the studies aimed to establish tehnology for Faulty of Engineering, Siene University of Tokyo, Tokyo, Japan ymatsu@rs.kagu.sut.a.jp Faulty of Engineering, Siene University of Tokyo, Tokyo, Japan knakano@rs.kagu.sut.a.jp Dept of Global Environment Engineering, Kyoto University, Kyoto, Japan fujii@arhi.kyoto-u.a.jp Senior Researher, Dr.eng., uilding Researh Institute, Tsukuba, Japan fukuyama@kenken.go.jp

2 strengthening existing buildings ith ontinuous fiber sheet reinforement. This studies on the appliation have been expanded to over not only olumns but also olumns ith ing alls, beams, earthquake resisting alls, et. For olumns, Japan uilding Disaster Prevention Assoiation (JDPA) and the uilding Center of Japan (CJ), respetively, have approved guide lines for design and onstrution of seismi retrofit by ontinuous arbon and aramid fiber sheets. In this paper, methods for evaluating the seismi retrofit of regular olumns ith ontinuous fiber sheets are desribed. Then, this paper desribes the latest results of those experimental studies and larifies the problems that must be solved before a truly reliable method for evaluating the seismi retrofitting effets ith ontinuous fiber sheets an be established. SEISMIC RETROFITTING EFFECTS OF CONTINUOUS FIER REINFORCEMENT AND EVALUATION OF EFFECTS Improvement of shear apaity The results of an experiment onduted to grasp the seismi retrofitting effet of ontinuous fiber sheets to improve the shear apaity of a beam are desribed belo [Araki et al.,1997a]. In this experiment, all the speimens had the same shear reinforement ratio [(p )r=0.19%], ith the amount of resin-bonded arbon and aramid fiber sheet reinforement varied. It an be seen that all the strengthened speimens markedly improved in shear apaity. Like the RC beams reinfored ith shear reinforing bars, the strengthened speimens shon an inrease in maximum shear apaity in proportion to an inrease in amount of shear reinforement, ( p σ ), alulated by the folloing equation-1. The above results indiate that the arbon and aramid fiber sheet are as effetive as the onventional shear reinforing bar and that their shear strengthening effet an be evaluated by the existing shear reinforement method. Hoever, the observed fiber strain at maximum shear is usually less than its potential elongation apaity. To evaluate the shear strengthening effet it is onvenient to define the effetive fiber strain. ( p σ ) = ( p ) r ( σ ) r + ( p ) ( σ ) (1) ( p )r : shear reinforement ratio of reinforing bars, ( p ) : shear reinforement ratio of fiber sheets, ( σ )r : yield strength of reinforing bars, ( σ ) : nominal tensile strength of fiber sheets ( nominal tensile strength : atalogue value onsidering the safety alloane of 3 σ [ σ : standard deviation ] ) Improvement of dutility The results of an experiment indiating that an regular olumn hih is shear-strengthened by ontinuous fiber sheets hanges in failure mode from shear failure to flexural failure and that the regular olumn markedly improves in dutility as the amount of shear reinforement is inreased are desribed belo [Kataoka et al.,1997]. Fig.1 Ultimate failure onditions of Column speimens Fig. Comparison of Q- δ relationship The ultimate failure onditions of speimens are shon in Figure 1. In this experiment, all the speimens strengthened by ontinuous fiber sheets had the same shear reinforing bar ratio [(p )r=0.13%], ith the amount of resin-bonded fiber sheet reinforement varied. Appliation of load as anti-symmetri moment ondition ith onstant axial fore (axial fore ratio: 0.). Figure shos envelopes of the relationship beteen shear fore (Q) and displaement ( δ ). The above results indiate that by a regular olumn strengthened by arbon or aramid fiber sheets, it is possible to improve the olumn s shear apaity and ause its failure mode to shift from shear failure to flexural failure, and that the olumn s dutility improves as the amount of shear reinforement is inreased.

3 Evaluation of seismi retrofitting effets of regular olumns Of the guidelines for design and onstrution of seismi retrofit method by ontinuous fiber sheets, some have obtained evaluation of JDPA and the others have obtained evaluation of CJ. asially, these guidelines are appliable to existing regular olumns of reinfored onrete strutures. To develop the design guidelines, the ommittee for Strutural Use of NFRM (Ne Fiber Reinforing Materials) in Arhitetural Institute of Japan (AIJ) has been olleting the test data. The ompiled database ontains a total of 36 olumn speimens ith retangular setions. Table 1 shos the lassifiation of the data regarding the types of the strengthening method and the observed failure modes for regular olumns ith deformed bars. Table shos the variations of the experimental parameters. Table 1 Classifiations of regular olumn data Speimens flexural Shear ond Shear failure after total types failure failure failure flexural yielding RC Carbon Wrap Aramid Wrap Steel Jaket total * deformed bar :13, round bar : 3 Table Variations of experimental parameters onrete sheet reinf. shear span to 0.0 strength ratio depth ratio Carbon Aramid σ [MPa] P f [%] M/(VD) min.-max Frequent range tensile reinf. axial stress Column Ratio ratio depth P t [%] P/(A σ ) D[mm] min.-max p f E f (GPa) Frequent range Fig.3 Fiber Strain at Maximum Shear Shear apaity 1) Shear apaity formula To alulate shear apaity, shear strength equation in the AIJ(Arhitetural Institute of Japan) s Design Guideline ased on Ultimate Strength Conept [AIJ, 1990] is used. This equation assumes the shear resisting mehanism as the sum of the truss mehanism and arh mehanism. It also assumes that the fiber sheet reinforement rapped around a olumn plays the part of a tension member in the truss mehanism as does the shear reinforing bars in the onrete. The formula is the folloing equation-. Q = b j p σ ot φ + tan θ 1 β b D ν σ / su Where t tan θ = β = ( ) ( ) ( L / D) + 1 L / D {( 1 + ot φ) ( p σ )} /( ν σ ) Observed average fiber strain b, D: idth and overall depth of the setion, L: lear span of the member, j t : distane beteen the top and bottom bars, σ : onrete strength, σ : shear reinforement strength, p : shear reinforement ratio, ν = 0.7 σ / 00 (N/mm ): effetive onrete ompressive strength ratio, φ : angle of the ompressive strut in the truss mehanism, {.0, j /( D tanθ), ν σ / ( p σ ) 1. } ot φ : min imum of 0 t ) Calulation of Σ(p σ ) in shear apaity formula The basi equation used for the alulation is the folloing equation-3. ( p σ ) = ( p ) r ( σ ) r + α α ( p ) ( σ ) 1 (3) α 1 : effetiveness oeffiient of shear reinforement by fiber sheets α : ratio of fiber sheet Young s modulus to reinforing bar Young s modulus Figure 3 shos the average fiber strains at maximum loads reported in several papers. The effetiveness of fibers dereases ith an inrease of reinforement amount. The analysis shos that the strength equation for RC is () 3

4 fundamentally available for retrofitted olumns by fiber rapping using the modulus of elastiity of fibers. Hoever, the influene of harateristi natures of eah fiber type, suh as the small elongation apaity, the poor resistane against diret shear and the smaller strength at the bent portions et. should be taken into aount. For example, α1 is determined ratio of effetive fiber strain to strain at ( σ ) or /3 of ( σ ) in ase of using arbon fiber sheets. 4) ond apaity formula Sear strength equation- in the AIJ s Design Guideline [AIJ, 1990] is applied. The maximum shear fore transferred by the truss ation is limited by either of the shear reinforement fore, the ompressive apaity of diagonal onrete strut or the main reinforement bond fore (Q sub ). It suggests that it is important to onsider the main reinforement bond apaity for shear strength evaluation. The effet of fiber sheet rapping on bond strength should be made lear. Reently onduted researh reported that the fiber sheet rapping as more effetive than hoops. Qsub = τ Where bu τ Σψ j + tanθ bu = τ t o + τ st ( 1 β) τ bu : bond strength, τ o reinforement, b D ν σ / : bond strength effeted by onrete, τ st Σ ψ : total nominal perimeter of the bars (4) : bond strength effeted by shear 5) ond strength ( τ st ) In the alulation of main reinforement bond strength, the shear reinforement ratio, p, is assumed as the sum of the shear reinforement ratio of steel bars and shear reinforement ratio of ontinuous fiber sheets. It is alulated by the folloing equation-5.for example, β 1 is determined 1.0 or 3.0 in several previous test speimens. τ st = K ( p ) ( p ) = ( p ) r + β β ( p ) σ Where 1 β : effetiveness oeffiient of bond reinforement by ontinuous fiber sheets 1 β : ratio of fiber sheet Young s modulus to reinforing bar Young s modulus Evaluation of dutility Ultimate dutility is alulated from shear safety margin (shear apaity of strutural member against shear fore obtained from flexural apaity) and the µ fator is defined by equation-6. µ = 10 ( Qsu / Qmu 1) [ 1 µ 5] (6) Q su : shear apaity, Q mu : shear forre obtained from flexural apaity The alulation results by using.3.1 Shear apaity are shon in Figure 4 and Figure 5. The speimens strengthened by ontinuous fiber sheets ere made assuming their effetive strength as the stress at the time hen the strain is 0.01 ( α 1 ( σ ) =350MPa, α =1.0 ). Figure 4 shos the orrespondene beteen evaluated and measured values of speimens. The database used in this examination as olleted in NFRM Committee. The alulation results nearly orrespond to the failure mode. Therefore, the evaluation formula is onsidered valid as one for evaluating the strength of regular olumns. Figure 5 shos the evaluation of dutility of speimens. Shear safety margin is roughly given by the linear equation shon in the Figure (5) pexp./ pmu Carbon Aram id SteelJaket RC p sumin / p mu Ru (rad.) Ru=ƒ Ê (1/150) ƒê=10(q su /Q mu -1) Carbon Aram id SteelJaket RC p su / p mu Fig.4 Correspondene beteen Fig.5 Evaluation of dutility alulated and measured values *Q exp.: measured maximum strength, Q mu : alulated flexural apaity *Q sumin : minimum of (Q su :alulate shear apaity, Q sub :alulated bond apaity) 4

5 PERFORMANCE OF STRENGTHENED RC MEMERS Strengthening effet of RC olumns under high axial fore In an experiment on the dutility of an olumn hih is strengthened by ontinuous fiber sheets, the axial fore ratio is normally set in the range 0.1 to 0.3 σ ( σ is ompressive strength of onrete.). ased on results, it has been reported that the dutility of suh regular olumn an be evaluated in nearly the same ay as the dutility of regular olumn hih are provided ith shear reinforing bars. Desribed belo are the results of an experimental study the strengthening effet of RC olumns under axial fore exeeding 0.3 σ, that is, in the range 0.4 to 0.6 σ [Hayashida et al., 1998]. The authors onlude that even hen the axial fore ratio in the range 0.35 to 0.6 σ, it is possible to seure an ultimate rotation angle of 1/30 by providing the member ith a suitable amount of fiber sheet reinforement. The results [Masuo et al., 1998] of another experiment indiate that the strengthening effet of arbon fiber sheet is inferior to that of steel plate due to the rupture of sheet and the bukling of main reinforement. Conerning the dutility of an RC olumn provided ith ontinuous fiber sheet reinforement under high axial fore, it an hardly be evaluated on a reliable basis due partly to the sarity of experimental data. It is hoped that studies on this subjet, inluding ability of ontinuous fiber sheet to prevent the main reinforement from bukling, ill be made in the future. Strengthening effet of RC olumns ith mortar finishing In providing existing buildings ith seismi retrofit, it is neessary to minimize the noise, vibration, dust, et. produed during exeution of the retrofitting ork. In addition, sine the ork is subjet to various limitations (e.g., time limit), an effiient ork method is alled for. In the ase of apartment houses, et., the retrofitting ork may have to be exeuted ithout evauating them. Literature [Fukuyama et al., 1998] presents that the arbon fiber sheet produes a shear strengthening effet regardless of the presene or absene of mortar. It is expeted that experimental results as to relationship beteen the strengthening effet and the type of finishing material, ondition of finish, and thikness of finishing material ill be aumulated so that a reliable method for evaluating the effet of reinforement an be established as early as possible. Strengthening effet of RC olumn ith spandrel A short olumn ith a spandrel or suspended all having a short inside span is subjet to stress onentration beause of its high rigidity. It is also suseptible to brittle shear failure beause of its poor dutility. For these reasons, in implementing seismi diagnosis and retrofit design of existing buildings, suh a short olumn alls for speial onsideration. Literature [Shiohara et al., 1998] presents the results of an experiment ith a short olumn hih as shear-strengthened by arbon fiber sheet. The speimen having a eak spandrel shos a stable flexural behavior, hen the olumn as strengthened so as to ause the spandrel to ollapse and the inside span to inrease gradually. Figure 6 (a) shos example of this failure. On the other hand, there are fe experimental studies on the use of ontinuous fiber reinforement to improve the strutural performane of a short olumn ith a spandrel in a right-angled diretion against a spandrel all. Figure 6 (b) shos examples of failure [Kobayashi et al., 1999]. The speimen having an only short olumn strengthened by arbon fiber sheets shos shear failure of the olumn ith a spandrel. The speimen having a strengthened olumn and spandrel by arbon fiber sheet shos good dutility till the rotation angle reahes 1/0 (Figure 7 (a)). If the appliation of ontinuous fiber reinforement an improve the strutural performane of those short olumns, it is expeted to beome a useful method in seismi retrofit design. Regular olumn(1) Reinfored olumn(1) Reinfored short olumn(1) Reinfored olumn() Reinfored short olumn() (a) Horizontal diretuion against the all (b) A right-angled diretion against the all Fig.6 examples of failure ith spandrel 5

6 (a) a short olumn and spandrel strengthened (b) an only short olumn strengthened Fig.7 Q- δ relationship ith spandrel (loading to a right-angle diretion against spandrel) Strengthening RC olumns ith side all Many of the olumns of existing buildings have side alls. When added ith side alls, regular olumns inrease in rigidity and beome subjet to stress onentration. In addition, as the shear span ratio dereases, they beome suseptible to brittle shear failure. Experimental studies on the use of ontinuous fiber reinforement to improve the shear apaity of olumns ith side alls have been arried on [Iso et al., 1997]. RC olumns ith side alls an be shear-strengthened ith ontinuous fiber sheet in various ays. For example, the sheet may be rapped around or applied only to the surfae, only the RC olumn or the hole unit of RC olumn ith side all is reinfored, et. therefore, it is hoped that an optimum reinforement method for a partiular purpose ill be larified and that a reliable method for evaluating eah of those methods ill be established. Shear-reinforement of T-beams It is onsiderably diffiult to shear-reinfored RC beams, hih are normally T-beams, that is, ombinations of a beam and a floor slab. When it omes to reinforing suh T-beams ith ontinuous fiber sheet, it is diffiult to rap them up in the sheet. Literature [Araki et al.,1997b] reports the results of an experiment onduted using T- beam speimens to measure the strengthening effet hen it as applied only to the surfaes of the beam underneath the slab. The relationship beteen the effet of reinforement and the position of bonded sheet end as measured. Figure 8 shos the method of appliation and fixing of th sheet. All the speimens reah a maximum shear apaity hen the sheet peels off at the time a rak opens along the root of the slab (Figure 9). These results indiate that the differene in the effet of sheet reinforement due to the differene in fixing method is unlear. In order to shear-reinfored T-beams positively, it is neessary to employ the losed type of sheet reinforement. Fig.8 Method of retrofit Fig.9 Q- δ ith T-beams Earthquake resisting all shear-reinfored ith ontinuous fiber sheet As a means of seismi retrofit of existing buildings, providing additional earthquake resisting alls or inreasing all thikness is often adopted. Hoever, these methods require extensive ork in the field, inluding form ork and onrete ork. y ontrast, the reinforement method in hih ontinuous fiber sheet is simply bonded to the surfae of alls is expeted to signifiantly improve the effiieny of reinforement ork. Literature [Yoshida et al.,1998] reports the results of experiments arried out to understand the strutural performane of earthquake resisting alls strengthened by ontinuous fiber sheet. Figure-10 shos the pattern in hih arbon fiber sheet as applied to a resisting all. Examples of the experimental results are shon in Figure-11. The speimen strengthened ith fiber sheet first inreases in maximum strength and shos good dutility. In addition, sine the appliation of fiber sheet restrains the ourrene and propagation of raks in the all, it is onsidered a promising reinforement method hih assures the performane of earthquake resisting alls. It is hoped that the aimed level of reinforement of earthquake resisting alls ith ontinuous fiber sheet 6

7 ill be defined and that a reliable method for evaluating the reinforing effet of ontinuous fiber sheet ill be established. bolt Column head and leg transverse layer 1 longitudinal layer and 1transverse layer 6mm thik steel plate Fig.10 Pattern of reinforement (CF3) Fig.11 Q-R relationship ith resisting all Non-strutural RC all retrofitted by fiber sheets In the past earthquakes, severe damages ere observed in RC frames (olumns and beams) ith ast-in-plae non-strutural RC alls (mullion, spandrel and side-all). Hoever, there are no evaluation methods for designing the stiffness and the apaity of this RC frame type. Further no effetive tehniques are proposed to ontrol the damage level of the ast-in-plae non-strutural RC all. Literature [Sugiyama et al.,1999] reports the results of experiments arried out to suggest a seismi retrofit tehnique using arbon fiber sheets for ontrolling the failure mode of the RC frame ith ast-in-plae non-strutural RC alls. The examples of raking patterns are shon in Figure 1 The authors onlude that the seismi retrofit tehnique using arbon fiber sheets improves the damage level of non-strutural RC alls in RC frame and the defletion of door opening and the residual rak idths an be ontrolled by retrofitting in arbon fiber sheets. (a) onventional RC speimen (b) strengthened speimen by arbon fiber sheets Fig.1 Crak patterns CONCLUSIONS When RC members are reinfored ith ontinuous fiber sheet, ho muh ill their flexural apaity, shear apaity, and dutility improve?. It is amazing to see that so many researhers have taken interest in these questions, arried out extensive experiments positively, and aumulated tangible researh results in suh a short period of time. It seems that the enormous impat the Hyogo-ken Nanbu Earthquake had on Japanese soiety triggered ative researh in this partiular field. Atually, hoever, in most spheres of researh in this field, experimental data is still insuffiient to allo for orret evaluation of strutural performane. In order for the appliation of ontinuous fiber sheet to beome a promising seismi retrofitting tehnology, it is important to learly understand the existing problems and determine the future diretion of strutural evaluation. In addition, ne tehniques to exeute reinforement ork effiiently ill have to be developed in the future. REFERENCES AIJ (Arhitetural Institute of Japan) (1990), Design Guidelines for Earthquake Resistant Reinfored Conrete uildings ased on Ultimate Strength Conept, AIJ, Tokyo Araki, N. and Matsuzaki, Y., et al. (1997a), Experimental Study on Shear Capaity of Reinfored Conrete eams using Sheet type Fiber, Proeedings of JCI, Vol. 19, No., pp Araki, N. and Matsuzaki, Y., et al. (1997b), Experimental Study on Shear Capaity of Retrofitted T-shaped RC eams ith Continuous Fiber Sheets, Summaries of tehnial papers of annual meeting of AIJ, Strutures 4, pp Fukuyama, H., et al. (1998), Strengthening Effet RC olumns ith Mortar Finishing, SRC olumns and RC olumns ith Walls by Continuous Fiber Sheets, Proeeding of JCI Symposium on CFRC, pp

8 Hayashida, N., et al. (1998), Experimental Study on RC Columns under High Axial Load ith Carbon Fiber Sheets, Proeeding of JCI Symposium on CFRC, pp Iso, M., et al. (1997), Experimental Study on Seismi ehavior of Reinfored Conrete Columns ith Wing Walls Retrofitted by Carbon Fiber Sheets, Proeedings of JCI, Vol. 19, No., pp Kataoka, T., et al. (1997), Experimental Study on Dutility of Reinfored Conrete Column using Sheet Type Fiber, Proeedings of JCI, Vol. 19, No., pp Kobayashi, M. and Matsuzaki, Y., et al. (1999), Experimental Study on Strutural Performane of Reinfored Conrete Column ith Spandrel Walls retrofitted by Carbon Fiber sheet, Proeedings of JCI, Vol. 1 Masuo, K., et al. (1998), Experimental Study on Strengthening Effet of Reinfored Conrete Columns under High Axial Load Using CFRP Sheets and Steel Plates, Proeeding of JCI Symposium on CFRC, pp Shiohara, H., et al. (1998), Retrofit of Reinfored Conrete Short Columns ith Spandrel Wall Using Carbon Fiber Sheets, Proeeding of JCI Symposium on CFRC, pp Sugiyama, T. and Matsuzaki Y., et al. (1999), Experimental Study on the Performane of the RC Frame Infilled Cast-In-Plae non-strutural RC Walls Retrofitted by Using Carbon Fiber Sheets, Proeedings of JCI, Vol. 1, No.3, pp Yoshida, N., et al. (1998), A Study on Retrofitting of Shear Wall ith Continuous Fiber Sheet, Proeeding of JCI Symposium on CFRC, pp