EARTHQUAKE RESISTANT DESIGN CRITERIA FOR REINFORCED CONCRETE INTERIOR BEAM-COLUMN JOINTS

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1 This paper was plished in the Proeedings, Paii Conerene on Earthqake Engineering, Wairakei, New Zealand, Agst 5-8, 1987, Vol. 1, pp EARTHQUAKE RESISTANT DESIGN CRITERIA FOR REINFORCED CONCRETE INTERIOR BEAM-COLUMN JOINTS Kazhiro Kitayama, Shnske Otani and Hiroyki Aoyama University o Tokyo, JAPAN SUMMARY A set o earthqake resistant design riteria are proposed or reinored onrete interior eam-to-olmn onnetions taking into aont the expeted deormation limits o a ilding. Althogh the shear ailre in the eam-olmn onnetion is assoiated with the hange in the shear transer mehanism ased y the ond deterioration o eam ars within the onnetion, it is not easile to prevent the deterioration. Thereore, some ond deterioration shold e permitted, the riteria o whih were determined on the asis o nonlinear earthqake response analyses o ildings with good and poor hysteresis energy dissipation. At the same time, the inpt shear to a eam-olmn joint is restrited in proportion to onrete ompressive strength. The role o the lateral reinorement is onsidered to onine the onnetion rather than to resist shear. INTRODUCTION A reinored onrete (R/C) ilding in Japan has een designed or a large earthqake load, whih normally reslts in wide olmns. Hene, the shear stress in a eam-olmn onnetion and ond stress along the eam longitdinal reinorement are kept low even in ase o a large earthqake. The damage o eam-olmn s-assemlages was sarely oserved in the past earthqakes. Thereore, the design o R/C eam-olmn joints has not een reqired in the Arhitetral Institte o Japan Standard or Strtral Callation o Reinored Conrete Strtres[1]. The rationalization o the design allation, the se o higher strength materials might make it possile to onstrt new types o ildings with smaller olmn setions, espeially y the adoption o an ltimate strength design proedre relying on the dtility. Then eam-olmn joints may e proven to e a weak point, and seismi design provisions may eome neessary or R/C eam-olmn joints in Japan. This paper dissses the earthqake resistant design o an interior eam-olmn joint in a weak-eam strong-olmn plane rame strtre. PERFORMANCE REQUIREMENTS OF BEAM-COLUMN JOINTS A eam-olmn joint shold not ail dring a strong earthqake ease a) the gravity load mst e sstained in the joint, ) a large dtility and energy dissipation an not e expeted in the joint, and ) a joint is diilt to repair ater an earthqake. However, some damage shold e tolerated i the damage does not ase ill eets on the overall ehavior o the strtre. The design o a joint against the gravity load and lexre is atomatially satisied i the olmn reinorement is ontined throgh the joint. However, the shear in a joint an e signiiantly dierent rom that in the olmn nor the eam, and the joint shold e designed against a rittle shear ailre althogh some shear raking may e aepted. At the same time, the anhorage o eam ars shold e properly maintained to develop the weak-eam strong-olmn earthqake resistane mehanism. The shear ailre o a joint panel and signiiant slippage o eam ars within a joint shold e prevented p to a sale limit o strtral deormation, whih this paper aritrarily deines as a eam dtility o or or a

story drit angle o 1/50, whihever is smaller. SHEAR MECHANISMS IN BEAM-COLUMN JOINT Ations on an interior joint rom onneting eams and olmns are shown in Fig. 1. Shear transer mehanisms in a joint have een desried y Palay et al.

2 story drit angle o 1/50, whihever is smaller. SHEAR MECHANISMS IN BEAM-COLUMN JOINT Ations on an interior joint rom onneting eams and olmns are shown in Fig. 1. Shear transer mehanisms in a joint have een desried y Palay et al.[2] as shown in Figs. 2.a, and 2.. These are alled "main strt mehanism" and "s-strt mehanism" in this paper. The main strt is ormed along the main diagonal o the joint panel as the resltant o the horizontal and vertial ompression stresses ating at the eam and olmn ritial setions. Note that the main strt exists withot regard to the ond sitation o eam ars within the joint. The s-strt mehanism is ormed y diagonal ompression stresses distrited niormly within the panel region. The diagonal strt stresses mst alane with the tensile stress in the vertial and horizontal reinorement and the ond stresses ating along the eam and olmn exterior ars. There may e a third transer mehanism (trss mehanism) as shown in Fig. 2.. This is a trss mehanism ormed y the lateral Fig. 1: Design Stresses in interior eam-olmn reinorement, diagonal onrete strts and the olmn onnetion exterior reinorement. I the ond is peret along the eam reinorement, the main strt mehanism arries a part o shear nearly Fig. 2: Shear transer mehanisms in joint eqal to the ompression ore, e.g., C 1 in Fig. 1, and the s-strt mehanism arries the part nearly eqal to the eam tension ore T 2. Thereore, the ontrition o the s-strt mehanism to shear resistane is omparale with that o the main strt mehanism. Note that the s-strt mehanism is possile only when a good ond stress transer is maintained along the eam and olmn reinorement. However, it is not easile to maintain peret ond along the eam reinorement ater eam lexral yielding, and the ond deterioration starts rom the tension side o the eam reinorement. One the ond along the eam reinorement deteriorates, the s-strt mehanism starts to lose shear transer aility, and gradally the eetiveness o the joint lateral reinorement will e lost. The main strt mehanism arries the entire shear in the joint. The shear transer mehanism in a eam-olmn joint hanges with the ond deterioration along the eam reinorement. Note that the tension ore, not transerred to the joint onrete y the ond, o the eam reinorement mst e resisted y the onrete at the ompression ae o the joint, inreasing the magnitde o ompression stresses in the main strt. Bease the strt onrete is weakened y the reversed yli loading and ease the ompressive strength is reded y the inreasing tensile strain perpendilar to the diretion o the main

3 strt, the shear apaity o the main strt dereases and eventally ails in shear ompression. The prinipal role o the lateral reinorement in this ase is to onine the raked joint ore onrete. The trss mehanism may e eetive at this stage. An index, alled "eam ar ond index", is introded in this paper to indiate the possiility o ond degradation along the eam reinorement. The average ond stress over the olmn width or simltaneos yielding o the eam reinorement in tension and ompression on the two sides o the joint is expressed as ollows, y d = (1) 2 h where y : yield strength o eam ars (MPa), d : diameter o eam ars and strength is assmed to vary with the sqare root o the onrete ompressive strength easiility o ond degradation may e expressed y a ond index BI, deined as h : olmn width. I the ond ' in MPa, the BI = (2) ' The index inreases or higher eam ar strength, larger diameter o eam ars, narrower olmn width, and weaker onrete strength. The ond deterioration is more likely to or or a higher index vale. JOINT MECHANISM AND TEST RESULTS Two series o hal-sale plane interior eam-olmn s-assemlages (alled J- and C-series) were tested at the University o Tokyo (Res. 4 and 8). Tale 1: Properties o speimens J1 and C1 Speimen J1 J3 C1 C2 (a) Beam (200x300 mm) Top Bars 8-D13 8-D13 12-D10 12-D10 p t (%) Bot. Bars 4-D13 4-D13 6-D10 6-D10 p t (%) y (MPa) () Colmn (300x300 mm) Total Bars 16-D13 16-D13 16-D13 16-D13 Axial stress (kg/m 2 ) y (MPa) () Connetion Hoops@(mm) 2-D6@75 4-D6@25 2-D6@70 4-D6@45 p w (%) Conrete strength (MPa) BI index v / ' Note: p t ; tensile reinorement ratio, p w ; we reinorement ratio, y : yield stress o reinorement, : onrete strength, BI: deined in Eq. (2), and v / ': inpt shear stress normalized y.

The test reslts an demonstrate some eatres o the shear transer mehanisms o a joint. The properties o speimens are smmarized in Tale 1. The overall dimensions were ommon in the two series.

4 The test reslts an demonstrate some eatres o the shear transer mehanisms o a joint. The properties o speimens are smmarized in Tale 1. The overall dimensions were ommon in the two series. Note that the tensile reinorement ratio o the speimens was qite large ompared to that ommonly sed in a rame strtre ease the speimens were designed to develop high shear stresses in the joint ater eam yielding. However, the amont o the eam top reinorement in the C-series speimens was ontrolled y the ar spaing reqirements. From the speiiations, the ond along the eam reinorement is expeted to e ritial in J-series speimens, while the ond sitation was improved in C-series speimens y sing lower strength and narrower eam ars. Crak patterns o speimens J1 and C1 are ompared in Fig. 3 at the end o the tests. As expeted, 1, speimen J1 ailed in shear in the joint at a story drit angle o 1/23, when the shell onrete spalled o. Speimen C1 old maintain the joint panel to the end o the test and developed an ideal eam hinging at the eam ends, althogh many shear raks were oserved in the joint panel. It shold e pointed ot, however, that the joint shear stress developed in Speimen J1 was approximately 1.25 times larger than that in Speimen C1. The nmer o diagonal shear raks was less in Speimen J1, and X-shaped raks gradally opened along the main diagonal o the joint panel with an inrease o deormation. Speimen C1 developed many ine diagonal raks niormly distrited over the joint panel. The story shear-story drit relations are ompared or speimen J1 and C1 in Fig. 4. Speimen J1 exhiited a pinhing hysteresis shape espeially ater a story drit o 1/46, while Speimen C1 developed a good spindle-shape hysteresis. Fig. 3: Crak patterns ater tests. Fig. 4: Story shear-drit relations The rak patterns in a joint spport the onept o the main strt and s-strt shear transer mehanisms assoiated with good and poor onding along the eam reinorement. Note that Speimen J1 developed shear raks y the s-strt mehanism, t the shear raks in the main diagonal eame dominant at a larger deormation. This was nderstood that the s-strt mehanism was lost with the ond deterioration along the eam reinorement, and the prinipal stress onentrated along the main strt to ase shear ailre. On the other hand, the main strt and s-strt mehanisms were maintained in speimen C1 and diagonal

5 ompression stresses distrited niormly in the panel onrete. The amont o lateral reinorement was inreased in Speimen J3 and C2 rom the orresponding Speimens J1 and C1, respetively, maintaining the remaining parameters o the speimens (Tale 1). Strains in joint lateral reinorement are ompared in Fig. 5. In Speimens J1 and J2 with relatively poor ond sitation, the strains were omparale p to a story drit angle o 1/46, where a pinhing was oserved, althogh the lateral reinorement ratio was inreased rom 0.27 % to 1.27 %. Whereas, larger strains were oserved in Speimen C1 than in Speimen C2 y hanging a lateral reinorement ratio rom 0.27 % to 0.90 %. Fig. 5: Strains in joint lateral reinorement The role o the lateral reinorement in the two series appears dierent depending on the degree o the ond deterioration along the eam reinorement. Note that the strain in Speimen J3 ontined to inrease ater the ond deterioration, the phenomenon o whih annot e explained y the loss o the s-strt mehanism. The trss mehanism in Fig. 2 mst have partially ontrited to the joint shear resistane, and also the lateral reinorement ated to onine the raked ore onrete. An inreasing amont o lateral reinorement was oserved to rede the shear deormation and rak width in the joint panel, t the dierene appeared at a story drit angle greater than 1/46 in C-series tests. EFFECT OF BOND DETERIORATION ON RESPONSE It was onsidered that the ond deterioration o eam ars within a joint is not desirale or the reasons as ollows; (1) Pinhing appears in the hysteresis shape reding the energy dissipating aility o eams, (2) The ompressive stresses inrease at the eam ritial setion, and inrease the diagonal ompression stress amplitde, asing shear distress in a joint ater eam yielding, and (3) The region o eam reinorement nder tension inreases within the onnetion, inreasing the eam deormation de to the additional rotation at the ritial setion ased y eam ar slip within a joint. It is important to note that the ond deterioration and shear resistane are losely related. The inlene o the energy dissipation apaility at the eam ends on earthqake responses is stdied to disss the permissiility o the eam ar slip within a joint. The earthqake response analyses were arried ot y a ompter program DANDY[7]. Eah memer was represented y a one-omponent model, in whih an inelasti rotational spring was plaed at memer ends. A eam-olmn joint was assmed to e rigid. The hysteresis models plaed at eam ends were seleted to simlate the pinhing ehavior ased y the ond deterioration along the eam reinorement (Takeda-Slip hysteresis model). Takeda model was sed to simlate a good ond sitation with a spindle-shape hysteresis (Fig. 6). The skeleton rves o oth models were ommon, t the vales o parameters or the hysteresis shape were hosen rom J- and C-series tests to stdy the eet o derease in hystereti area on the response; an eqivalent visos damping ratio h eq, ratio o the dissipated energy within hal a yle to 2 times the strain energy at peak o an eqivalent linearly elasti system, was 0.25 or Takeda model and 0.15 or Takeda-Slip

6 model at a dtility ator o 4.0. The h eq vale o 0.10 was also sed in Takeda-Slip model or omparison. The additional deormation ased y the pll-ot o eam ars rom a joint is not onsidered here. Fig. 6: Hysteresis models Fig. 7: Displaement response histories at roo level For-, seven- and sixteen-story s-strtres, removed rom a rame strtre y tting o the eams raming into an interior olmn at inletion points, were designed to orm weak-eam strong-olmn rame strtres sing Japanese Bilding Standard Law and assoiated reglations. The heights o the strtres were 14.0, 25.0, and 56.0 m, respetively. The ndamental periods or design were 0.28, 0.50, and 1.12 se., respetively. The total weights were 140 ton, 247 ton and 642 ton. From nonlinear stati analyses nder monotonially inreasing loads o an inverted trianglar distrition, the ase shear oeiients at the ollapse mehanism were allated to e 0.30 or or-story strtre, 0.28 or seven-story strtre and 0.22 or sixteen-story strtre. Inpt earthqake motions were the 1940 El Centro (NS) reord and the 1952 Tat (S69E) reord. The intensities o grond motions were seleted so that the maximm memer dtility ators were approximately 4.0 at eam ends or the strtres sing Takeda model (Tale 2). Tale 2: Intensity o ase motion (nit in m/se 2 ) Nmer o Stories El Centro NS Tat S69E 4-story story story Note: Vales within parentheses mean the maximm aeleration in gal.

7 Fig. 8: Attained dtility ators at eam ends

8 Time-histories o displaement responses at the roo level are shown in Fig. 7 and attained dtility ators at eam ends are shown in Fig. 8. The distrition o eam-end dtility demands o a strtre with Takeda model is similar to that with Takeda-Slip model (h eq =0.15). The hange in the h eq vale o Takeda-Slip model rom 0.15 to 0.10 did not aet the dtility demand at eam ends. Althogh the displaement response waveorms o the strtres with Takeda-Slip model deviated rom those o the strtre with Takeda model, the attained maximm response displaements were omparale or the three strtres. From the reslts o earthqake response analyses, the eet o hysteresis energy dissipating apaity on the response was ond relatively small or a range o eqivalent visos damping ratio rom 0.10 to 0.25 at dtility ator o 4.0. Thereore, some ond deterioration o eam ars within a joint may e tolerale. LIMITATION OF BEAM BAR BOND INDEX The eam ar ond index and the eqivalent visos damping ratio h eq are ompared or the plane eam-olmn s-assemlages tested at the University o Tokyo (S-, J- and C-series tests [Res. 8, 3, 4]) at a story drit angle o 1/92 rad and 1/46 rad in Fig. 9. The solid line was derived rom the least sqares method to it the data. The roken lines in Fig. 9() show the h eq vales sed in the nonlinear analyses, i.e., h eq = 0.25, 0.15 and In Fig. 9(a), the speimens with open symols developed eam yielding at the story drit angle o 1/92 rad, while the speimens with solid symols did not. The h eq vales do not show a orrelation with the eam ar ond index vales at the story drit angle o 1/92 rad. However, the h eq vales tend to derease with an inreasing eam ar ond index vale at the story drit angle o 1/46 rad (Fig. 9()). I an allowale deormation level o R/C rame strtres is taken to e a story drit angle o 1/50 rad, the eam ar ond index shold satisy Eq.(3) to ensre the eqivalent visos damping ratio o 0.10, as indiated in the earthqake response analyses. ' 1.6 (3) where nit o oth is otained. and ' are in MPa. Sstitting in Eq.(1) into Eq.(3), the ollowing expression h d y (4) 3.2 ' This reommendation with regard to the ratio o the olmn width to the eam ar diameter is ompared with oth the NZS 3101:1982 ode and the ACI-ASCE 352 reommendation [9] in Fig. 10. The h / d ratio is moderated in this reommendation in omparison with the NZS 3101:1982 ode provision. LIMITATION OF INPUT SHEAR INTO A JOINT Ater some ond deterioration along the eam reinorement in a joint, the shear stress within a panel region is arried dominantly y the main strt mehanism. The shear ompression ailre in the main strt mehanism may e retarded y restriting the level o shear stress. The joint lateral reinorement ratio is ompared with the vale v/ ' or plane eam-olmn s-assemlage test speimens, reported in Res. [3, 4, 8, 10-13], in Fig. 11, in whih v is the maximm joint shear stress oserved in the test. The eetive joint area to resist shear is deined as the olmn depth mltiplied y the average o the eam and olmn widths. The lateral reinorement ratio was deined as the total ross-setional area o lateral reinorement within a joint divided y the olmn width and the distane ( = 7/8 d) etween resltant tension and ompression ores at the eam ritial setion. From the igre, the joint shear stress v mst e limited as given in Eq. (5) to prevent shear ailre ater eam lexral yielding;

9 v < 0.25 (5) ' Fig. 9: Eqivalent visos damping ratio eam ar ond index relations Fig. 10: Reqired olmn width to eam ar diameter The shear ailre in a joint panel orred in spite o the amont o lateral reinorement i v / ' is greater than Bt shear ailre ater eam yielding orred at a story drit angle o greater than 1/25 rad. When the allowale drit is assmed to e 1/50 rad, the limitation o v / ' may e limited y I an inpt shear stress v exeeds 0.25 ', the joint ailed in shear irrespetive o the amont o lateral

10 reinorement within a joint. On the other hand, the lateral reinorement ratio o 0.27 % is siient when the inpt shear stress v is less than 0.25 '. From these test reslts, 0.30 % is reommended as the reqired minimm lateral reinorement ratio o a joint. The lateral reinorement within a joint is expeted to onine the panel onrete. Fig. 11: Joint lateral reinorement ratio - normalized inpt shear stress relation CONCLUDING REMARKS The hange o the joint shear transer mehanism ased y the ond deterioration along eam reinorement was pointed ot. The ratio o the olmn width to the eam ar diameter mst e limited as ntion o the strength o eam ars and onrete strength. The design shear stress shold e limited to prevent shear ompression ailre ater the ond deterioration along the eam reinorement. A minimm amont o lateral reinorement mst e plaed within a joint to onine the onrete o the main strt. REFERENCES [1] Arhitetral Institte o Japan, "AIJ Standard or Strtral Callation o Reinored Conrete Strtres (in Japanese)", revised in [2] Palay, T., R. Park and M. J. N. Priestley, "Reinored Conrete Beam-Colmn Joints Under Seismi Ations", ACI Jornal, Novemer 1978, pp [3] Otani, S., Y. Koayashi, M. Tamari and H. Aoyama, "Shear Resistane o Reinored Conrete Beam-Colmn Connetions in Weak-Beam Frames (in Japanese)", Reports, Annal Meeting, Arhitetral Institte o Japan, Otoer 1984, pp [4] Kitayama, K., K. Krs, S. Otani and H. Aoyama, "Behavior o Beam-Colmn Connetions with Improved Beam Reinorement Bond", Transations o The Japan Conrete Institte Vol. 7, 1985, pp [5] Amerian Conrete Institte, "Bilding Code Reqirements or Reinored Conrete (ACI )", Detroit, revised in [6] Standard Assoiation o New Zealand, "New Zealand Standard Code o Pratie or the Design o Conrete Strtres", NZS 3101, [7] Kaeyasawa, T., S. Otani and H. Aoyama, "Nonlinear Earthqake Response Analyses o R/C Wall-Frame Strtres", Transations o The Japan Conrete Institte Vol. 5, 1983, pp [8] Koayashi, Y. and S. Otani, "A Stdy on Beam Bar Bond in Reinored Conrete Beam-Colmn Joints Part 1 (in Japanese)", Reports, Annal Meeting, Arhitetral Institte o Japan, Septemer 1983, pp

11 [9] ACI-ASCE Committee 352, "Reommendations or Design o Beam-Colmn Joints in Monolithi Reinored Conrete Strtres", ACI Jornal, May-Jne 1985, pp [10] Noghi, H. and H. Terasaki, "Experimental Stdy on RC Beam-Colmn Joints Sjeted to Seismi Stresses (in Japanese)", Smmaries o Tehnial Papers o Annal Meeting Strtres II, Arhitetral Institte o Japan, Otoer 1985, pp [11] Tada, T. and T. Takeda, "Researh on Reinorement o Beam-Colmn Joint Panel or Seismi Resistant Reinored Conrete Frame part 1 (in Japanese)", Jornal o Strtral and Constrtion Engineering No. 352, Arhitetral Institte o Japan, Jne 1985, pp [12] Hamada, D., T. Kamimra and S. Hayashi, "Experimental Stdy on Beam-Colmn Joints in Reinored Conrete Strtres part 1 (in Japanese)", Reports, Annal Meeting, Arhitetral Institte o Japan, Septemer 1978, pp [13] Meinheit, D. F. and J. O. Jirsa, "The Shear Strength o Reinored Conrete Beam-Colmn Joints", CESRL Report No. 77-1, The University o Texas at Astin, Janary 1977.

SEISMIC PERFORMANCES OF REINFORCED CONCRETE FRAMES UNDER LOW INTENSITY EARTHQUAKE EFFECTS

th World Conerene on Earthquake Engineering Vanouver, B.C., Canada August -, 00 Paper No. 0 SEISMIC PERFORMANCES OF REINFORCED CONCRETE FRAMES UNDER LOW INTENSITY EARTHQUAKE EFFECTS Bing Li and Tso-Chien