EXPERIMENTAL INVESTIGATION ON RECTANGULAR SRC COLUMNS WITH MULTI-SPIRAL CONFINEMENTS

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1 4th International Conferene on Earthquake Engineering Taipei, Taiwan Otober 12-13, 26 Paper No. 49 EXPERIMENTL INVESTIGTION ON RECTNGULR SRC COLUMNS WITH MULTI-SPIRL CONFINEMENTS C.C. Weng 1, Y.L. Yin 2, J.C. Wang 3, C.Y. Liang 4 and C.M. Huang 4 BSTRCT Presented herein is a study on ompression tests of a series of full-sale retangular SRC (steel reinfored onrete) olumns onfined with a new type of multi-spiral ages. The multi-spiral is a devie of five interonneted spiral ages, named 5-spirals or Yin s spirals in this study, with a large irular spiral at the enter and four small ones at the orners. The innovation of applying the 5-spirals to retangular SRC olumns is to take its superiority in onrete onfinement as well as its effiieny in automati prodution for the preast onstrution industry. The major parameters of this study inluded the ost effetiveness of the multi-spirals, and the strength and dutility of the spirally onfined SRC olumns. s ompared to the reinfored onrete olumn tied with traditional retilinear hoops, the test results owed that, with signifiant ost savings of the onfinement steel, the SRC olumns onfined with 5-spirals demonstrated exellent performanes in both strength and dutility. Keywords: Retangular SRC Column; Multi-Spiral Confinement; 5-Spirals; Preast Constrution; Compression Test; Compressive Strength; Dutility Improvement; Cost Effetiveness. INTRODUCTION With the fast advanes in onstrution tehnology, it has beome inreasingly popular in Taiwan to onstrut buildings with omposite strutural members. The SRC (steel reinfored onrete) olumn is one of the major omposite strutural members, whih renders a building with the advantages of the strength and stiffness of reinfored onrete as well as the dutility of strutural steel. dditional merits of the SRC olumn extend to that the onrete also protets the imbedded steel setion from loal bukling and fire damages (Weng and Wang, 25). Figure 1 ows two types of SRC olumns ommonly seen in Taiwan s onstrution sites, in whih the embedded strutural steel an be a ross-h or a box setion. It is notied that the onfinement reinforements in retangular SRC olumns typially onsists of retilinear perimeter hoops (IJ, 21). Eah of the hoops is formed with a single steel bar and losed at both ends by two hooks with 135-degree bends. The onfinement reinforements in the olumn are designed to hold the longitudinal bars in position and to provide the olumn with ear strength and passive onfinement Professor, Department of Civil Engineering, National Chiao Tung University, 11 Ta-Hsueh Road, Hsinhu, 3, Taiwan frankweng@mail.ntu.edu.tw Professor, Department of Civil Engineering, National Taiwan University, No. 1, Roosevelt Road, Setion 4, Taipei, 16, Taiwan; CEO and Chief R&D Offier, Ruentex Group, 14F, No. 38, Se. 2, Bade Road, Taipei, 14, Taiwan. samuel@mail.ruentex.om.tw Manager, R&D Division, Runhorn Preteh Engineering Co., Ltd., 1F, No. 38, Se. 2, Bade Road, Taipei, 14, Taiwan. Graduate Researh ssistant, Department of Civil Engineering, National Chiao Tung University, 11 Ta-Hsueh Road, Hsinhu, 3, Taiwan. 1

2 stress for the ore onrete. However, experienes from the field pratie indiated that the hoops with 135-degree bends are not easy to setup in the SRC olumns, and the entire proess is heavily relied on skilled labors, whih is time-onsuming and expensive. Fig. 1 Retangular SRC olumns with retilinear perimeter hoop onfinements It is known that the onfining effiieny of the onfinement age in a reinfored onrete olumn is influened by both the geometry and the spaing of the onfinement steel (Darwin, 1977; Mander et al., 1988; Pantazopoulou, 1998). s ompared to the retilinear hoops, the irular spirals in the olumns have been own to be more effetive in onrete onfinement (Shah et al., 1983; Sheikh and Tokluu, 1993). In addition, automati prodution of the spiral ages are ommon in today s preast onstrution industry. Prodution of the ontinuous spirals and assembly of the ages an be arried out easily in the fatory. It is beause of the ost effetiveness and the lower demand of skilled labors whih make the spiral ages a potential ompetitor. However, appliations of the irular spirals to the retangular ross-setion olumns are not ommon in today s engineering pratie. Fig. 2 Retangular SRC olumn ross-setions with 5-spiral onfinements s own in Fig. 2, the innovation of the multi-spiral onfinements was originally proposed by Y.L. Yin for prefabriated retangular reinfored onrete olumns (Yin et al., 24). The multi-spiral is a devie of five interonneted spiral ages, named 5-spirals or Yin s spirals in this study, with a large entral spiral at the enter and four small ones at the orners. The idea of using the 5-spirals in retangular SRC olumns is to take its superiority in onrete onfinement and its effiieny in automati prodution in prefabriated strutural members. In pratie, longitudinal bars in the prefabriated olumns are loated at four orners of the ross-setion to reate more working spae at the beam-olumn joint. The use of four small spiral ages at the orners suessfully eliminates the barrier for extending the appliation of irular spirals to the retangular ross-setion olumns. The objetive of this researh is to investigate experimentally the effiieny of applying the idea of multi-spiral onfinements to the retangular SRC olumns. The major parameters of this study 2

3 inluded the ost effetiveness of the multi-spirals, and the strength and dutility of the SRC olumns. It is also hoped that this experimental investigation will provide further insight on the mehanial behavior and failure mehanism of the retangular SRC olumns with multi-spiral onfinements. DESIGN RULES FOR CONFINEMENT REINFORCEMENTS The following setions briefly review the rules used in this study for the design of onfinement reinforements in the RC and SRC olumn speimens: 1. CI-318 Code The CI-318 Code (25) is one of the most widely aepted design guides for reinfored onrete strutures. For a spirally onfined reinfored onrete olumn, it is stated in setion 21.4 of the CI Code that the volumetri ratio,ρ s, defined as the ratio of the volume of spiral reinforement to the volume of ore onrete, all not be less than the following requirements: g f s.45 1 (1) f yh and f s.12 (2) f yh in whih f is the ompression strength of onrete; f yh is the yield stress of spiral steel; g and are, respetively, the gross and the onfined ore onrete areas. For a reinfored onrete olumn tied with retilinear hoops, it is stated that the ross-setional area of the onfinement reinforement,, all not be less than and f g.3 1 f yh h f.9 f yh where s is the spaing; h and h are, respetively, the width and the area of onfined ore onrete. 2. Taiwan SRC Code In 24, the Ministry of Interior of Taiwan publied her first offiial edition of the design ode for SRC buildings (MOI, 24). To take into aount the benefiial effet provided by the embedded steel setion in aring the axial load of SRC olumn, the Taiwan SRC Code adopted a similar design approah for omposite olumns used in the ISC Seismi Provisions (25). For a spirally onfined SRC olumn, it is suggested that the volumetri ratio,ρ s, of spiral reinforement all not be less than and g f s f ys s f yh Pn u f s f ys s.12 1 f yh P n u in whih f ys and s, respetively, are the yield stress and the ross-setional area of the steel setion; (P n ) u is the nominal axial apaity of the SRC olumn. (3) (4) (5) (6) 3

4 (P n ) u = f ys s +.85 f + f yr r (7) In addition, for a SRC olumn tied with retilinear hoops, it is suggested that the ross-setional area of the onfinement reinforement,, all not be less than and 3. Weng s Formula f g s f ys f yh h P n u f s f ys.9 1 f yh P n u s own in Fig. 3, the ondition of onrete onfinement in a SRC olumn is quite different from that of an ordinary reinfored onrete olumn (Weng et al., 1998). It is observed from the figure that, due to the onfining effet to the ore onrete provided by the flanges of the steel setion in the SRC olumn, the onrete is subjeted to different degrees of onfinement. The onrete in SRC olumn an be ategorized into three areas: (a) The highly onfined onrete; (b) the ordinarily onfined onrete; and () the unonfined area (Weng et al., 24). (8) (9) Highly Confined rea Ordinarily Confined rea Unonfined rea Longitudinal Bars Steel Setion Hoop Confinement Fig. 3 Conditions of onrete onfinement in a SRC olumn (Weng et al., 24) In reognition of the onfining ontribution provided by the flanges of the steel setion, Weng et al. proposed a new set of formulas for the design of onfinement reinforements in SRC olumns to aount for this benefiial effet (Weng et al., 24). It is proposed that 1. For a spirally onfined SRC olumn, the volumetri ratio,ρ s, of spiral reinforement all not be less than the followings: and f P P s f yh n u g s h f P s P h. 12 s 1 f yh n u P P 2. For a SRC olumn tied with retilinear hoops, the ross-setional area of the onfinement reinforement,, all not be less than (1) (11) 4

5 and f g Ps P h f yh h P n u f Ps P h.9 1 f yh P n u (12) (13) where (P n ) u is the nominal axial apaity of SRC olumn determined from equation (7). P s and P h are, respetively, the axial apaities provided by the steel setion and the highly onfined onrete. P P f s ys s h.85 f h where h is the area of highly onfined onrete, as the dark aded zone own in Fig. 3. It is noted that the braket at the end of equations (1) to (13) is a redution fator whih aounts for the ontribution of the flanges of the steel setion in onfining the ore onrete in the SRC olumn. EXPERIMENTL PROGRM s own in Table 1, a total of eleven full-sale ort olumns were tested in this study, inluding two RC olumns and nine SRC olumns. ll olumn speimens are 6 mm square and 12 mm height. Two types of onfinement in the SRC olumns were investigated, in whih speimens SRC1 and SRC2 were tied with the traditional retilinear hoops; speimens SRC3 to SRC9 were designed with the 5-spirals. Speimens SRC1 and SRC2 were served as the benhmark for omparison purpose, and they were given the same amount of steel setion and longitudinal bars to provide the same magnitude of expeted ompressive strength as designed for the SRC olumn speimens. Besides, the RC olumn speimens, RC1 and RC2, were also designed for omparison purpose, given the required amount of longitudinal bars to provide the same expeted ompressive strength as for the SRC olumns. The speimen RC1 was tied with the retilinear hoops; and the speimen RC2 was onfined with the 5- spirals. In Table 1, the last olumn indiates the design guides used to determine the amount and spaing of the onfinement reinforement for eah speimen. The redution fator represents the perentage of ost effetiveness of onfinement reinforement for a speimen designed aording to Taiwan SRC Code or Weng s formula, relative to the amount of onfinement needed if designed with the CI-318 requirements. In this study, the spaing of the onfinement reinforement varies from 75 to 11 mm. The smallest redution fator is 65% for speimens SRC2 and SRC5; both were designed aording to Weng s formula. In addition, the volumetri ratio and the weight per unit length of onfinement reinforement for eah speimen are also own in the table. The volumetri ratios vary from.81% to 1.67%; and the weight per unit length of onfinement reinforement ranges between 235 to 45 N/m. The steel apes in the SRC olumn speimens inluded the welded built-up ross-h and box setions. Steel plates of 6, 9 and 1 mm thik were used, and the yield stresses vary from 411 to 445 MPa. The ompressive strength of the normal weight onrete is 41.1 MPa. The #8(D25) and #9(D29) deformed bars with yield stresses of 442 and 43 MPa, respetively, were served as longitudinal reinforements. The #3(D1) and #4(D13) deformed bars with yield stresses of 485 and 463 MPa, respetively, were used as the onfinement reinforements. s required by Taiwan SRC Code, one #4 supplementary longitudinal bar was plaed in the middle of eah side of the SRC olumn, but was ut 5 mm ort at eah end. It is note that, for the effiieny of onfinement and automati prodution, the #4 bar was seleted to form the large spiral ages and the #3 bar was used for the small ones. Figure 4 ows two photos of the setup of the full-sale ompression test of the ort olumns. 58,8 kn (6, metri ton) hydrauli jak was used to apply the axial ompressive fore at a (14) (15) 5

6 onstant strain rate of.3 mm/se (25 με/se). The expeted maximum load to be applied to the speimens was about 21, kn. To ahieve a more uniform load distribution on the speimen, a steel end ap was mounted on eah end of the olumn. During the test, a LVDT (linear variable differential transformer) extensometer was attahed along the side of the speimen to monitor the axial ortening. To measure the variations of strains within the speimens, strain gages were glued on the seleted surfaes of the steel setion and the reinforing bars before asting the onrete. Table 1 Design details of olumns tested in this study Column Cross-Setion Speimen Designation Hoop/Spiral Small Cirle Large Cirle Hoop/Spiral Spaing ( mm ) Volumetri Ratio ρ s Weight of Redution Hoop/Spiral Fator ( N / m ) Design Guide RC1-H-CI-9 # % 45 1% CI-318 Code RC2-Y-CI-75 #3 # % 36 1% CI-318 Code SRC1-HC-TWN-75 # % % Taiwan SRC Code SRC2-HC-WENG-9 # % % Weng s Formula SRC3-YC-CI-75 #3 # % 36 1% CI-318 Code SRC4-YC-TWN-95 #3 # % % Taiwan SRC Code SRC5-YC-WENG-115 #3 # % % Weng s Formula SRC6-YC-S1-75 #3 # % 36 1% Spaing = SRC1 SRC7-YC-S2-9 #3 # % % Spaing = SRC2 Note : SRC8-YB-TWN-95 #3 # % % Taiwan SRC Code SRC9-YB-WENG-11 #3 # % % Weng s Formula (1) Column dimensions:ll olumn speimens are 6 mm square and 12 mm height. (2) Steel setions in SRC olumns: (a) Cross-H: 2H ,ρ sr = 2.91% ; (b) Box setion: ,ρ sr = 2.94% (3) Longitudinal bars in SRC olumns: (a) Hoop olumns (SRC1 & SRC2): 12 #9 (D29),ρ r = 2.15% ; (b) Spiral olumns: 16 #8 (D25),ρ r = 2.25% (4) Longitudinal bars in RC olumns (RC1 & RC2): 16 # 8 (D25) and 12 # 9 (D29);ρ r = 4.4% RESULTS ND DISCUSSIONS Figure 5 makes a omparison between the load-displaement urves of the two reinfored olumns, RC1 and RC2, in whih both speimens were designed with same amount of longitudinal bars, and the onfinement reinforements were arranged to meet the CI-318 requirements. It is observed that the ultimate ompressive strengths of the two olumns are quite lose; however, the dutility of the spiralonfined olumn, RC2, is obviously better than that of the hoop-tied olumn, RC1. These observations are in onsisteny with the test results reported by Wang in 24. It is also noted from Table 1 that the weight of onfinement reinforement used for speimen RC2 is 36 N/m. It is muh more osteffetive than that of the speimen RC1, whih takes 45 N/m. 6

7 (a) The 588 kn ompression test mahine (b) Compression test of a SRC olumn Fig. 4 The ompression test mahine and the setup of the experiment In Fig. 6, three more load-displaement urves obtained from the SRC olumns, SRC3, 4, and 5, were added to Fig. 5. For the purpose of omparison, all test speimens were initially designed with the same expeted ompressive strength. It is observed from Fig. 6 that the three SRC olumns all owed slightly higher ultimate apaities than those of the RC olumns. More signifiantly, the dutility of the three SRC olumns was found to be muh better than that of the RC olumns. It is also important to observe from Table 1 that the onfinements of olumns SRC4 and SRC5 were designed aording to Taiwan SRC Code and Weng s formula, respetively. The amount of spiral steel used for these two SRC olumns were only 283 and 235 N/m, respetively, whih are muh less that those of olumns RC1 and RC2. From the redution fators own in Table 1, for olumn SRC5 whih was designed aording to Weng s formula, only 65% of spiral reinforement is needed as ompared to the olumn designed with CI Code. These observations demonstrated the advantages in dutilityimprovement as well as in ost effetiveness of applying the 5-spirals to retangular SRC olumns. Compressive Fore (kn) RC1 RC2 Compressive Fore (kn) SRC3 SRC4 SRC5 RC1 RC Displaement (mm) Fig. 5 Comparison of load-displaement urves between two RC olumns: RC1(hoops) vs. RC2(5-spirals) Displaement (mm) Fig. 6 Comparison of load-displaement urves between RC and SRC olumns: RC1(hoops) vs. RC2, SRC3,4,5(5-spirals) In order to ompare the performanes between the SRC olumns tied with the retilinear hoops and those onfined with the 5-spirals, Figs. 7(a) and (b) ow the differenes of the load-displaement behavior of olumns SRC1 versus SRC 6 and olumns SRC2 versus SRC7, respetively. It is observed that, given the same spaing of onfinement reinforement, the strength and the dutility of the spiral-onfined SRC olumns are all better than those of the hoop-tied SRC olumns. 7

8 3 SRC1 3 SRC2 Compressive Fore (kn) 2 1 SRC6 Compressive Fore (kn) 2 1 SRC Displaement (mm) (a) Spaing of onfinement: both 75mm Displaement (mm) (b) Spaing of onfinement: both 9mm Fig. 7 Load-displaement urves of SRC olumns with different types of onfinement: SRC1 and SRC2 (with hoops) vs. SRC6 and SRC 7 (with 5-spirals) 8 8 RC1 SRC2 Stress (MPa) 6 4.7P.163 u Pu Stress (MPa) P.2317 u Pu Strain (a) Spaing of hoops in RC1: 9mm Stress (MPa) Pu Strain (b) Spaing of hoops in SRC2: 9mm SRC4.7 P.546 u Strain () Spaing of spirals in SRC4: 95mm Fig. 8 Comparison of dutility performanes between RC and SRC olumns: RC1 (hoops); SRC2 (hoops); SRC4 (5-spirals) 8

9 Figures 8(a), (b) and () ow a omparative study on the dutility performanes among three different types of olumns: the traditionally hoop-tied RC and SRC olumns, speimens RC1 and SRC 2, respetively, and the spiral-onfined SRC olumn, speimen SRC4. dutility index, μ, is defined herein as the ratio of the axial strain measured at 7% of the post-peak load,ε.7pu, to the strain reorded at the peak load, ε Pu. It is observed from the figures that the dutility indexes are 1.82, 3.6 and 4.77 for speimens RC1, SRC2 and SRC4, respetively. The above observations indiated that the spiral-onfined SRC olumn, SRC4, demonstrated the best quality in absorbing inelasti strain energy. The apability of sustaining large deformation without quik loss of strength after the peak load of the SRC olumn with 5-spirals is one of the important harateristis for ahieving suessful seismi resistane. To summarize, the ultimate strength and dutility index of the olumns tested in this study are own in Table 2. Table 2 Ultimate strength and dutility index of the olumns tested in this study Speimen RC1 RC2 SRC1 SRC2 SRC3 SRC4 SRC5 SRC6 SRC7 SRC8 SRC9 (P u ) test (KN ).7 P u Pu In addition, the photos presented in Figs. 9(a) and (b) may provide an explanation to the phenomenon of quik axial strength deterioration after reahing the peak load, as observed from the hoop-tied olumn, RC1. It is seen from Fig. 9(a) that the 135-degree hooks of the retilinear hoops were opened up and resulted in bukling of the longitudinal bars. On the ontrary, as own in Fig. 9(b), the 5- spirals and the ore onrete in the spiral-onfined olumn, speimen SRC4, were found to be able to maintain in a relatively sound ondition after the peak load. (a) Separation of the 135 hooks (b) The 5-spirals remained sound Fig. 9 Comparison of failure modes between olumns with different types of onfinement: (a) Column RC1 with retilinear hoops; (b) Column SRC4 with 5-spirals 9

10 SUMMRY ND CONCLUSIONS series of full-sale retangular SRC olumns with multi-spiral onfinements were tested under monotoni ompression. For omparison purpose, two reinfored onrete olumns of the same size were also tested. The following onlusions an be drawn based on the test results of this study: 1. The new type of multi-spiral onfinements, named 5-spirals or Yin s spirals whih was innovated by Y. L. Yin in 24, has been experimentally proven to be able to suessfully extend its appliation to retangular SRC olumns. 2. s ompared to the RC and SRC olumns tied with retilinear hoops, the SRC olumns onfined with 5-spirals demonstrated exellent performanes in both strength and dutility. 3. The retangular SRC olumns with 5-spirals owed signifiant apability of sustaining large deformation without quik deterioration of axial strength after reahing the peak load, whih is one of the important harateristis for ahieving suessful seismi resistane. 4. s ompared to the CI-318 requirements for onfinement reinforements, the test results indiated that the Weng s formula an provide signifiant ost benefit in the savings of the usage of onfinement steel. 5. The experimental results owed that, with satisfatory performanes in strength and dutility, the spiral onfinements designed aording to the Weng s formula takes only 65% of the amount of onfinement reinforements needed if designed with the CI Code. 6. In general, the test results have demonstrated the advantages in dutilityimprovement as well as in ost effetiveness of applying the newly innovated 5-spirals to retangular SRC olumns. CKNOWLEDGMENTS The finanial support of Runhorn Preteh Engineering Co., Ltd. is gratefully aknowledged. Thanks are also extended to China Engineering Consultant for providing the test faility for this investigation. REFERRENCES CI (25). Building Code Requirements for Strutural Conrete (CI 318) and Commentary (CI 318R). merian Conrete Institute, Farmington Hills, Mihigan. IJ (21). Standards for Strutural Calulation of Steel Reinfored Conrete Strutures. rhitetural Institute of Japan, Tokyo. ISC (25). Seismi Provisions for Strutural Steel Buildings, merian Institute of Steel Constrution, Chiago, Illinois. Darwin, D., and Peknold, D.. (1977). Nonlinear Biaxial Stress-Strain Law for Conrete, J. of Eng. Meh. Div., SCE, 13(2), Mander, J.B., Priestly, M.J.N., and Park, R. (1988). Theoretial Stres-Strain Model for Confined Conrete, J. of Strut. Div., SCE, 114(8), MOI (24). Building Code for Design of Steel Reinfored Conrete (SRC) Strutures. Ministry of Interior, Taipei, Taiwan. (in Chinese) Pantazopoulou, S.J., (1998). Detailing for Reinforement Stability in RC Members, J. of Strut. Eng., SCE, 124(6), Shah, S.P., Fafitis,., and rnold, R., (1983). Cyli Loading of Spirally Reinfored Conrete, J. of Strut. Div,. SCE, 19(7), Sheikh, S.., and Tokluu, M.T., (1993). Reinfored Conrete Columns Confined bycirular Spirals and Hoops, CI Strut. J., 9(5), Wang, P.H., (24). The Study of New Confinement Details for Retangular Conrete Columns, Master s Thesis, Dept of Civil Eng., National Taiwan Univ., Taipei, Taiwan. (in Chinese) Weng, C.C., and Wang, H.S., (25). Seismi Behavior of Steel Beam to Steel Reinfored Conrete Column Connetions, The 4th Int. Conf. on dv. in Steel Strutures, Shanghai, China, June. Weng, C.C., Wang, H.S., Li, R., and Liang, C.Y., (25). Experimental Study on Seismi Behavior of Hoop- Confined SRC Columns, epted, J. of Chinese So. of Strut. Eng., Ot. (in Chinese) Weng, C.C., Yen, S.I., and Lin, C.C., (1998). Influene of Steel Setion on Hoop Confinement in Conrete- Enased SRC Columns, J. of Chinese So. of Hydraulis and Civil Eng., 1(2), (in Chinese) Yin, Y.L., Chang, K.C., and Wang, J.C., (24). Experimental Studies ofretangular Columns with Innovative Spiral Confinements, The 17th KKCNN Symp. on Civil Eng., yuthaya, Thailand, De. 1