Database investigation on bond performance of interior beam-column joints with high-strength reinforcement

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1 Dataase investigation on ond performane of interior eam-olumn joints with high-strength reinforement H.C. Chen Graduate Shool of Engineering Siene and Tehnology, National Yunlin University of Siene and Tehnology, Yunlin, Taiwan. H.J. Lee & T.C. Tsai National Yunlin University of Siene and Tehnology, Yunlin, Taiwan NZSEE Conferene ABSTRACT: The use of high-strength reinforement in onrete strutures has many advantages suh as laor and ost savings. Whenever higher strength reinforement is used, the ond and development length eomes ritial prolems for design of onrete strutures, espeially for joints of moment-resisting frames. This paper reviewed existing design riteria of development length for the straight eam ars within eam-olumn joints and reommended to extend the ond requirements of NZS 3101 for the use of Grade 690 reinforing ars. Aording to an extensive dataase investigation, the validity of the design equations of NZS 3101 is assessed y hysteresis performane of eam-olumn joint tests olleted from laoratories in Unites States, Japan, New Zealand, and Taiwan. Pratial design reommendations are drawn for Grade 690 reinforing ars eing use as longitudinal reinforement passing through joints of moment-resisting frame. 1 INTRODUCTION Speial moment-resisting frames are widely used for the design of reinfored onrete uilding strutures in moderate to high seismi zones. If properly detailed, the plasti hinge an e arranged to develop at the eam regions adjaent to the joint when the frame sujeted to large lateral loads, as shown in Figure 1(a). During the formation of these eam plasti hinges, extremely high ond stresses an e developed along the straight eam ars passing through the joint, eause these ars may e fored to yield in tension at one olumn fae and e lose to yield in ompression at the opposite olumn fae. One ertain degree of ond deterioration ourred within the joint, these eam ars may slip within the joint under large load reversals. Signifiant ond slip is not desirale eause it redues the stiffness and energy dissipation apaity of eam-olumn onnetions. Some ond deterioration is inevitale and should e aepted. However, if the ond deterioration is severe, the ar tension will penetrate through the joint and develop in the eam ompression zone on the opposite side. This means that oth top and ottom eam ars are in tension at the olumn fae and then large ompression fores will transfer to the onrete of the eam ompression zone. As shown in Figure 1(), onrete rushing at eam ends may our onsequently and followed y signifiant redution on eam flexural strength and dutility. Hakuto et al. (1999) ever demonstrated the detrimental effet of ond deterioration y analytial studies and onluded that ond deterioration should e onsidered in the design of eam-olumn joints. d h (a) Beam hinging mehanism () Bond slip and onrete rushing at joint faes Figure 1. Bond failure at interior eam-olumn joints for earthquake resistane.

2 Although ond performane of eam ars passing through interior joints have een extensively studied sine 1980s, the development length requirements for eam-olumn joints still differ remarkaly among the urrent ACI 318 Code (ACI ) in United States, the AIJ Guideline (AIJ 1999) in Japan, and the NZS 3101 Standard (NZS ) in New Zealand. For seismi design of interior eam-olumn joints in speial moment-resisting frames with normal weight onrete, the ACI 318 Code requires a minimum olumn dimension of 20 times the largest diameter of eam ars parallel to that olumn dimension. This riterion was ased on an evaluation of availale tests in 1980s. Zhu and Jirsa (1983) reviewed yli loading response of 18 interior eam-olumn joints with normal strength onrete and reinforement. While ACI 318 Code set a simple riterion of 20-ar diameters, oth the AIJ Guideline and the NZS 3101 Standard estalish the minimum ratios of olumn dimension to eam ar diameter as a funtion of material strengths and the olumn axial stress. The philosophy ehinds these requirements are ased on elaorate studies on the energy dissipation apaities of eam-olumn joints in Japan and New Zealand. High-strength onrete has een used in many uilding strutures in Japan (Aoyama 2001), partiularly, for olumns with limited arhitetural dimensions and high axial load at the lower levels. Whenever high-strength reinforement is used for eam longitudinal reinforement passing through a eam olumn joint, either a large olumn depth or a small permissile diameter of eam ars would make design and proportion diffiult. To provide a promising solution, this paper ompares existing ond requirements in international onrete design odes and then validates proper design equations using a large dataase of eam olumn joint tests. Laoratory testing performanes suh as strength, stiffness, and energy dissipation apaity of eah eam olumn joint speimen are evaluated aording to ACI standards (ACI ) for speial moment frames. Finally, a viale set of design equations for the development length in eam olumn joints is reommended to ahieve aeptale ond performane for speial moment frames. 2 EXISTING DESIGN CRITERIA 2.1 Geneti formula For the use of Grade 500E reinforement in New Zealand, Brooke and Ingham (2013) reviewed existing design riteria for the reinforement anhorage length at interior eam-olumn joints. During the formation of the adjaent eam hinging, the stresses on the eam ar may ahieve α o f y in tension at one fae of the joint and κα o f y in ompression at the opposite fae of the joint, as shown in Figure 2. Average ond fores T 1 = A s,top a o f y Q C 2 with flange in ompression C s2 = A s,top k 2 a o f y Typial A s,top A s,ot and then k 1 k 2 C s1 = A s,ot k 1 a o f y C 1 Q h Column depth d joint T C 1 T 2 = A s,ot a o f y Beam ar diameter 2 C L h Q jd s2 Q L L 1 Figure 2. Horizontal shear and ond fores ating on the joint onrete. By assuming an average ond stress on the eam ar along the olumn depth, the ond requirements for preventing exessive ond slip of eam ars in joints are given as follows. πd h α p u πd 2 h (1+κ) α o f y d 4 α p u 4 α of y (1 + κ) or (1a) where h = olumn depth; d = maximum ar diameter of the eam ars passing through the joint; f y =is (1) 2

3 speified yield strength of the reinforement; u = average ond stress on the eam ar in the joint; α p = fator aounting for the enefit effet of olumn axial ompression on ond strength; α o = overstrength fator of the eam ars; and κ= ratio of ar ompressive stress to the ar tensile stress. 2.2 Existing design equations Bond requirements on the asis of Eq. (1) an e found in AIJ Guideline (AIJ 1999) and NZS 3101 (NZS 2006). Tale 1 ompares aove design riteria with the design reommendations proposed y Brooke and Ingham (2013) and this paper. Among the existing design riteria, differenes an e found for the (1 + κ) term of ar stress eing developed in the joint, average ond strength u along the ar, and the fator α p of the olumn axial stress on ond strength. Originally, NZS 3101 uses an average ond strength of 1.5 f MPa, whih is 60% of the peak loal ond strength of 2.5 f MPa oserved y Eligehausen et al. (1983), and two additional modifiation fators, α f and α t, to onsider the idiretional loading and the top ar effets, respetively. Paulay and Priestley (1992) desried detail development of aove α fators. Reently, Brooke and Ingham (2013) assemled a dataase of 93 interior eam-olumn joint tests to assess the suitaility of existing design riteria for the ond development length in joints and onluded that the existing riteria annot reflet the ond failure oserved in experiments. They proposed to modify the asi ond strength from 1.5 f to 1.25 f MPa and the orresponding equations of κ and α p for updating NZS 3101, as shown in Tale 1. The α p fator proposed y Brooke and Ingham (2013) is relatively onservative with a upper limitation of 1.20 for high axial load onditions. Following prior investigation, this paper reommends to omit the α f and α t terms and set the ond strength u =1.5 f, whih are demonstrated with satisfatory ond performane in laoratory testing. Tale 1. Comparison of existing design equations for the development length in interior joints. Design riteria AIJ 1999 NZS 3101 Brooke & Ingham 2014 Reommended Bar stress fator 1 + κ 1 + A s,ot A s Bond strength u (MPa) Axial stress fator α p (f ) P A g f A s A s,top 1.8 α f α t 1.5 f P A g f α o A s,top A s α o α f α t 1.25 f P A g f α o A s,top A s α o 1.5 f P A g f 1.20 Note: With limitation of A s,top A s,ot, where A s,ot = area of ottom eam ars; A s,top = area of top eam ars; A s = area of the ar group, A s,top or A s,ot, ontaining the ar for whih development length is eing alulated; α o = overstrength fator for eam ars; α f = 1.0 for a eam ar passing through a joint sujeted to unidiretional loading, and α f = 0.85 for i-diretional loading; Bar loation fator α t = 0.85 for a top eam ar where more than 300 mm of fresh onrete is ast elow the ar, α t = 1.0 for all other ases. P= axial ompression fore on olumn; A g = gross area of olumn; f = onrete ompressive strength. For many years, Grade 420 (f y =420 MPa) steel reinforement has een the standard for reinfored onrete onstrution in Taiwan as well as in the United States. Whenever high-strength steel reinforing ars are used in the onrete strutures, ond requirements eome ritial issues for design and proportion. ACI-ASCE Committee 352 (2002) reommends a reasonale multiplier of f y 420 MPa for the minimum olumn depth of 20d for higher grade reinforement. h d 20 f y 420 (2) 3

4 For omparison of the existing design equations, a referene ruiform eam-olumn joint is assumed to have eam hinging adjaent to the joint faes, a least axial ompression of 0.2A g f, a pratial eam reinforement ratio A s,ot A s,top of 0.75, equal ar diameter for top and ottom reinforement, and ar f y of 420 or 690 MPa. Under suh onditions, the minimum olumn depths with respet to the ottom ar diameter are ompared in Figure 3. Clearly, the ond requirements of AIJ Guideline (1999) are very onservative, and those of NZS 3101 are relatively less onservative. The reommendations of Brooke and Ingham (2013) still results in a olumn depth similar to those of NZS 3101 (2006). The minimum olumn depth reommended y ACI 352R (2002) is also displayed in Figure 3, whih may e onservative for normal strength onrete ut e too onservative for high strength onrete. Figure 3. Comparison of minimum olumn depth for (a) Grade 420 ars with α o = 1. 25; () Grade 690 ars with α o = DATABASE INESTIGATION AND PERFORMANCE EALUATION 3.1 Dataase of eam-olumn joint tests Lee and Hwang (2013) presented a dataase for reverse yli tests of reinfored onrete eam-olumn joints in speial moment frames y extensively reviewing the related papers pulished in Japan, United States, New Zealand, and Taiwan. Aout 200 interior joints were assemled in this dataase. All speimens were reinfored onrete onentri eam-olumn suassemlages isolated from infletion points of eams and olumns, and tested under quasi-stati yli lateral loading (typial repeated yles for eah drift ratio ranged from one to three) to simulate the earthquake-introdued fores ating on the joints. Test results of eam-olumn joints were lassified in three asi failure modes inluding: Beam flexure failure ( B failure), Joint shear failure without yielding of eam ars ( J failure), Joint shear failure with yielding of eam ars ( BJ failure). The modes of B-, BJ-, and J-failures are well-aepted in Japan for the development of design guidelines for eam-olumn joints (Kitayama et al. 1991). Besides aove three asi failure modes, some joint speimens were reported as BJa failure, whih is refer to ond or anhorage failure along the eam ars in the joint. jh, m f / f / / 470 P / A g f 0.06 h / d 500/ 25 ya n jh, m / 0.78 f / f ya 31/ 473 P / A g f 0.10 h / d 400/ 19 n Figure 4. Cyli loading response of (a) BJ-failure and () BJa-failure joint speimens tested in YunTeh. 4

5 Figure 4 shows two interior eam-olumn joints tested y first authors and his olleagues at YunTeh in Tawian. Both speimens reahed eam yielding at aout 1.5% to 2% drift ratio, ut Speimen B1 eventually failed in joint shear at 6% drift ratio while Speimen EW0 exhiited a very pinhed hystereti urve sine the ond failure ourred in the 3% drift yles. Notaly, oth speimens had a olumn depth of 20 ar diameters and similar ar yield strength, ut hysteresis performane of the Speimen B1 is etter than that of Speimen EW0, although the later had a lower target shear stress in the joint. Oviously, the key parameter make Speimen B1 perform etter is attriuted to its higher onrete strength (greater ond resistane in the joint). It seems the hysteresis performane of eam-olumn joints is also related to the ond stress of the eam ar in the joint. Although the onsequene of ond failure is not as severe as that of shear failure in joints, it is still preferred to prevent the ond failure within the design earthquake level, whih is aout 3.5% drift apaity for the strutural testing. This paper intends to determine a viale set of design equations to over the use of Grade 690 reinforement in joints of speial moment frames. Therefore, a suset of availale test data from the dataase of Lee and Hwang (2013) was investigated in this paper aording to ACI , the aeptane riteria for moment frames (ACI Committee 374, 2005). Hystereti performanes inluding strength, stiffness, and energy dissipation apaity for eah test speimen were evaluated for assessing existing design odes and reommendations for the development length in eam-olumn joints. 3.2 Aeptane riteria for testing performane of eam-olumn joints ACI Committee 374 (2005) reported a testing protool and aeptane riteria for strutural omponents of speial moment frames. For aeptane, test results of the third omplete yle to a limiting drift ratio not less than 3.5% should satisfy: 1. Strength at peak displaement shall not e less than 75% of the maximum peak strength in the same loading diretion; 2. Seant stiffness etween drift ratios of -1/10 and +1/10 of the limiting drift ratio shall not e less than 5% of the initial stiffness otained from the first yle; and 3. Energy dissipation in the third yle of limiting drift ratio shall not e less than 12.5% of the idealized elastoplasti energy of that drift ratio. For Speimen EW0 shown in Figure 4(), the third 4% drift yle had a peak strength equal to 64% of the maximum peak strength, a seant stiffness etween ±0.4% drift ratios equal to 1% of the initial stiffness, and an relative energy dissipation ratio of 19%. Oviously, hysteresis performane of Speimen EW0 is not aeptale eause it does not satisfy the three aeptane riteria given y ACI The poor hysteresis performane of Speimen EW0 an e attriuted to the ond failure along eam ars in the joint ourred in 3% drift yles. The seletion of numer of yles at eah drift ratio depends on the judgment of the researhers and the partiular degradation harateristis of the system eing tested. More reently, ACI 374.2R-13 (ACI Committee ) reported that a minimum of two yles at eah deformation level is suffiient to onsider the damage assoiated with the numer of yles at a given drift level. For the assessment of the ond performane for eah test speimen, therefore, the seond (or third, if availale) yle at a drift ratio etween 3.5% and 4% were reprodued. Therefore, this paper seleted 59 interior joints from the dataase of Lee and Hwang (2013) aording to following onditions: BJ or BJa failure speimens; straight eam ars passing through the joint with ar f y exeeding 400 MPa; yli loading response have a minimum of two yles at a drift ratio exeeding 3.5%. After sreening, a total amount of 65 interior joint speimens (46 BJ and 19 BJa data) are evaluated in this paper. Figure 5 displays the range of measured onrete strength and ar yield strength for the seleted BJ and BJa failure speimens. Oviously, ond failure is likely to our for the omination of higher strength reinforement and normal-strength onrete. Within the test dataase, there is no BJafailure speimens availale for onrete strength exeeding 100 MPa. 5

6 Figure 5. Range of joint onrete strength and ar yield strength in the seleted suset of test data. 3.3 Assessment of ond requirements Aording to the three aeptane riteria given y ACI , the seond (or third, if availale) yle at a drift ratio aout 4% for eah test speimen is evaluated and lassified as aeptale or unaeptale performane. The aeptale test data satisfy the aforementioned three aeptane riteria while the unaeptale may only meet one or two of the three aeptane riteria. Figure 6(a) shows the relations of hysteresis performane to the ratio of experimental-to-nominal joint shear strength and the ratio of provided-to-required olumn depth. The vertial axis of Figure 6 is the maximum experimental joint shear fore ( jh,m ), whih an e ak-alulated from the eam moments in equilirium with the peak maximum lateral loads, divided y the nominal joint shear strength ( n = 1.25 f A j ) speified in ACI 318 (2014) for interior joints without transverse eams. Therefore, test data fall in Quadrat 3 and 4 had experimental joint shear stresses elow the permissile value of 1.25 f MPa and expeted to e apale of preluding the premature shear failure. jh, m n, ACI jh, m n, ACI h d a a f s a 6 p o y f h d asao f y max a p 6 f, 20 (a) Eq.(2) with u =1.5 f MPa () Set u =1.5 f MPa and a minimum olumn depth of 20d Figure 6. Relations of hysteresis performane to the ratio of experimental-to-nominal joint shear strength and the ratio of provided-to-required olumn depth. The horizontal axis of Figure 6(a) is provided olumn depth in experiments divided y the required 6

7 olumn depth reommended in this paper. In other words, test data fall in Quadrat 1 and 4 had olumn depth exeeding the minimum olumn depth, alulated using a asi ond strength of 1.5 f MPa omined with the fators (1 + κ) and α p proposed y Brooke and Ingham (2013), and expeted to e apale of preluding premature ond failure. Ideally, test data with unaeptale performane shall not appear in Quadrat 4 eause oth the joint shear and ond stress are kept within permissile limits. However, as shown in Figure 6(a), there are six unaeptale data in Quadrat 4, indiating that the use of asi ond strength of 1.5 f MPa may e unonservative for ertain onditions. One of the solutions to improve onservation is to redue the asi ond strength from 1.5 f to 1.25 f MPa, as proposed y Brooke and Ingham (2013), and then inrease 20% of the required olumn depth, as shown y the vertial dash line in Figure 6(a). Clearly, only one unaeptale test data exeeds 1.20 times the proposed ond development length (vertial dash line). However, many aeptale test data also fall etween 1.0 and 1.20 times the proposed ond development length, indiating the redution of asi ond strength from 1.5 f to 1.25 f MPa may also e too onservative for some onditions. Another simple promising way is to improve the safety y setting a minimum olumn depth of 20 ar diameters instead of reduing the asi ond strength. As shown in Figure 6(), the limitation of minimum 20d riterion moves those test data with low f y f ratios toward the left quadrants and sustantially improves the distriution of the unaeptale data. Aording to data oservation in Figure 5, it is onluded that either the minimum olumn depth of 20d or the redution of asi ond strength from 1.5 f to 1.25 f MPa would give similar safety to prelude unaeptale hysteresis performane. Finally, the minimum olumn depth proposed herein are ased on test data otained from relatively onservative ond onditions of the eam ars, whih extend through an isolated ruiform eamolumn joints without transverse eams and slas. There is lak of experimental evidene of ond failure oserved from indeterminate frame with floor slas, whih may ounter the slip of eam ars passing through the frame joints. Definitely, a larger h d ratio ould redue the potential eam ar slip within the interior joints during a major earthquake, ut it would also lead to larger olumns and/or smaller diameter ars in groups, whih makes design and onstrution diffiult. Therefore, the minimum h d ratios speified for speial moment frames y odes and standards are ased on the judgment of how well the hysteresis ehaviour expeted at a design interstory drift is. After a design earthquake attak, a moment frame with ond failure in joints may e too flexile under a moderate earthquake. Beause it is unlikely to e repair ond failure, a eam-olumn joint has etter e well-proportioned to avoid ond failure ourred during a design-ased earthquake. Although ond performane of eam ars passing through interior joints have een extensively studied sine 1980s, the development length requirements for eam-olumn joints still differ remarkaly among the international design odes and standards. This paper only provides reommendations for joints of speial moment frame designed and detailed aording to ACI 318 ode. 4 DESIGN RECOMMENDATIONS AND CONCLUSIONS Based on dataase investigation, this paper reommends a viale set of design equations for the development length of straight eam ars passing through the joints of speial moment frame. The proposed design equations are validated y the testing performane of eam-olumn joints at a drift ratio aout 4%, where the hysteresis ehaviour is evaluated y the aeptane riteria speified in ACI Code and standards. For ahieving an aeptale ond performane, a minimum olumn depth-to-eam ar diameter ratio an e related to availale ond strength in the joint, reinforement ratio in eams, and olumn axial loads. Using ommon ond design equations, it is reommended to use a asi ond strength of 1.5 f MPa with two modifiation fators proposed y Brooke and Ingham (2013), aounting for effets of reinforement ratio and olumn axial loads. Besides, the minimum olumn depth-to-eam ar diameter ratio should not e taken less than 20. 7

8 5 REFERENCES ACI (Amerian Conrete Institute) Building ode requirements for strutural onrete and ommentary. ACI , Farmington Hills, MI. ACI-ASCE Committee Reommendations for design of eam-olumn onnetions in monolithi reinfored onrete strutures. ACI 352R-02, Amerian Conrete Institute, Farmington Hills, MI. ACI Committee Aeptane riteria for moment frames ased on strutural testing and ommentary." ACI , Amerian Conrete Institute, Farmington Hills, MI. ACI Committee Guide for testing reinfored onrete strutural elements under slowly applied simulated seismi loads. ACI 374.2R-13, Amerian Conrete Institute, Farmington Hills, MI. AIJ (Arhitetural Institute of Japan) Design guidelines for earthquake resistant reinfored onrete uildings ased on inelasti displaement onept, Tokyo (in Japanese). Aoyama, H Design of modern highrise reinfored onrete strutures, World Sientifi, Singapore. Brooke, N. & Ingham, J Seismi design riteria for reinforement anhorages at interior RC eam-olumn joints. J. Strut. Eng., /(ASCE)ST X , Eligehausen, R., Popov, E.P., & Bertero, Loal ond stress-slip relationships of deformed ars under generalized exitations. UCB/EERC-83/23, University of California, Berkeley CA. Hakuto, S., Park, R., & Tanaka, H Effet of deterioration of ond of eam ars passing through interior eam-olumn joints on flexural strength and dutility. ACI Strut. J., 96(5), Kitayama, K., Otani, S., & Aoyama, H Development of design riteria for r interior eamolumn joints. SP-123: Design of eam-olumn joints for seismi resistane, Amerian Conrete Institute, Farmington Hills, MI, Lee, H.J., & Hwang, S.J High-strength onrete and reinforing steel in eam-olumn onnetions. Pro., Strutures Congress 2013, ASCE, Pittsurgh, PA, doi: / NZS (Standards New Zealand) Conrete strutures standard part 1-the design of onrete strutures. NZS 3101:2006, Wellington, New Zealand. Paulay, T. & Priestley, M.J.N Seismi design of reinfored onrete and masonry uildings, Wiley, New York. Zhu, S., & Jirsa, J. O. (1983). A study of ond deterioration in reinfored onrete eam-olumn joints. PMFSEL Report No. 83-1, Phil M. Ferguson Strutural Engineering Laoratory, University of Texas at Austin, Austin, TX. ACKNOWLEGEMENT This study is supported y the grants from the Ministry of Siene and Tehnology in Taiwan and ompleted with help of graduate students, Ms. Ying-Ru Lin, Ying-Chiey Chi, and Wei-Ching Huang in the National Yunlin University of Siene and Tehnology. The authors are also grateful to their help and support. A detail list of the test data and dataase investigation will e pulished the PhD thesis of Dr. His-Ching Chen in July,