AIJ Proposal of Ultimate Strength Design Requirements for RC Buildings with Emphasis on Beam-column Joints

Transcription

1 This paper was pulished in ACI SP-123 Design of Beam-column Joints for Seismic Resistance, James O. Jirsa, Editor, 1991, American Concrete Institute, pp AIJ Proposal of Ultimate Strength Design Requirements for RC Buildings with Emphasis on Beam-column Joints Shunsuke Otani Synopsis: The Architectural Institute of Japan (AIJ) pulished "Design Guidelines for Earthquake Resistant Reinforced Concrete Buildings ased on Ultimate Strength Concept (draft)" in Octoer, 1988, as a first attempt to develop an ultimate strength design procedure in Japan. The paper introduces general concept of the design procedure, and explain in detail the deign requirements and ackground information for reinforced concrete eam-column joints of the AIJ guidelines. Based on experimental evidence, the amount of lateral reinforcement in the joint is required significantly reduced from the ACI requirements. Keywords: reinforced concrete, eam-column joints, shear, ond, anchorage, design requirements, earthquake resistance, ultimate strength design, Japan. Shunsuke Otani, associate professor, Department of Architecture, University of Tokyo, graduated from University of Tokyo, and otained M. Sc. and Ph. D. degrees from University of Illinois at Urana-Champaign. He was awarded with the 1990 AIJ Prize for his work on Nonlinear Earthquake Response Analysis. INTRODUCTION The first Japanese uilding code, Uran Building Law, was promulgated in 1919, to regulate uildings in six major cities. Structural design, ased on the allowale stress design procedure, was outlined in Building Law Enforcement Regulations enacted in Earthquake resistant design with a seismic coefficient of 0.10 was introduced in 1924 in Building Law Enforcement Regulations after the 1923 Kanto earthquake. Building Standard Law, applicale to all uildings in Japan, was proclaimed in 1950, in which two levels of allowale stress levels were used for long-term gravity conditions and for short-term earthquake situations; i.e., the allowale stresses in earthquake loading were increased to full yield strength for reinforcement and to two-third compressive strength for concrete. The seismic coefficient was increased to 0.20 reflecting the increase in the allowale stresses. Building Standard Law Enforcement Order was revised in 1981, maintaining the allowale stress design procedure, to specify an ultimate lateral load resistance of each story. 1

2 The Architectural Institute of Japan (AIJ) pulished "AIJ Standard for Structural Calculation of Reinforced Concrete Structures (Ref. 1)" in After many efforts to develop an ultimate strength design procedure, the AIJ pulished "Design Guidelines for Earthquake Resistant Reinforced Concrete Buildings ased on Ultimate Strength Concept (draft) (Ref. 2)" in The AIJ guidelines were drafted y Seismic Design Su-committee (chairman: T. Okada) of Reinforced Concrete Committee (chairman: Y. Kanoh), and was formally approved y Structures Committee, AIJ, in July, The application is limited to a regular uilding less than 45 m high. The guidelines have not otained legal support from the Ministry of Construction. The guidelines are ased on the capacity design concept. AIJ GUIDELINES FOR EARTHQUAKE RESISTANT DESIGN It is desirale for a structure to e free from earthquake damage provided such a structure could e constructed at a reasonale cost. The AIJ guidelines accept that some damage should e tolerated in a case of an intense earthquake, ut propose to avoid negative performance of a uilding, such as, a) large plastic deformations, ) concentration of damage in limited locations, and c) rittle failures. Yield Mechanism A uilding structural system of either moment resisting frames or a structural wall-moment resisting frame dual system shall e designed so that a total yield mechanism is controlled y eam-yielding (Fig. 1). The locations where yield hinges are intended to develop shall e planned to ensure a) a required lateral resistance of the uilding, and ) sufficient structural deformation capacity. The locations where yield hinges are not intended shall e provided with sufficient resistance to avoid hinges at these locations. Nonlinear earthquake response analyses indicated that the overall deflection of a structure is comparale for different distriutions of damage within a uilding. Therefore, the smaller is the numer of yield hinges to develop in a structure, the larger is the concentration of plastic deformation at each yield hinge (Fig. 2). Consequently, the eam-yielding total mechanism requires the smallest plastic deformation at each yield hinge. Assuring Deformation A uilding shall e provided with lateral load resistance and stiffness sufficient to limit "a story drift angle to e less than 1/100 rad" even during an intense earthquake. These requirements were stipulated to avoid damage to non-structural elements, and to ensure human safety and evacuation. A sufficient stiffness is also necessary to form the planned yielding mechanism prior 2

3 to the limiting story drift angle. Considering uncertainties in earthquake characteristics and structural response, the uilding shall e detailed to deform to an "Assuring Deformation" without decay in resistance (Fig. 3); e.g., a story drift angle of 1/66 rad for a moment-resisting frame structure and 1/75 rad for a wall-frame structure. In other words, eams, columns, structural walls, and eams connected to a structural wall should e detailed to deform to a memer rotational angle of 1/50, 1/67, 1/75, and 1/40 rad, respectively, if yield hinges are expected. Reliale and Upper Bound Strengths Two levels of the ultimate strength are defined; i.e., (1) "reliale strength": lower ound strength of a section or memer calculated using sectional dimensions and specified material strengths, and (2) "upper ound strength": flexural strength at a hinge section evaluated y taking into consideration all possile factors which contriute to the strength, such as, statistical upper ound material strengths, additional construction steel, reliaility of strength evaluation methods, and spread of effective sla width for eams and orthogonal wall width for columns. The effect of strain hardening is not considered ecause the memer deformation is limited. The design action in the non-yielding regions of the structure is increased to recognize a) the upper ound strength of yielding memers, ) the contriution of higher modes (dynamic effect), and c) the effect of i-directional earthquake response. Mechanism Design and Assuring Design Design for earthquake loading is carried out in two steps: i.e., (1) "Mechanism Design": the reliale strength shall e provided at the planned yield hinges for the design ending moment determined y a linear structural analysis under earthquake loading using reduced memer stiffness, and modified y moment redistriution, and (2) "Assuring Design": the reliale strength shall e provided at the non-yielding region for the design action determined y a nonlinear plastic analysis at the formation of the planned yielding mechanism. The upper ound strength at the planned yield hinges magnified y factors which account for the dynamic effect and i-directional response should e used for an Assuring Design. PERFORMANCE CRITERIA OF BEAM-COLUMN JOINTS "AIJ Standard for Structural Calculation of Reinforced Concrete Structures (Ref. 1)" does not require the design of eam-column joints. The lack of significant earthquake damage oserved in reinforced concrete joints in Japan is the proale reason for this; however, the damage in a joint may have een hidden ehind architectural coverage, or premature column failure may have reduced the action in the joint. The design of the eam-column joint has een reviewed for the guidelines. 3

4 The AIJ guidelines require that the eam-column joint shall not e selected as a planned location of yield hinge in the Mechanism Design ecause a) the joint as a part of column must sustain the gravity load, ) large hysteretic energy dissipation and deformation capacity are difficult to achieve in the joint, and c) the joint is difficult to repair after an earthquake. A eam-column joint should e designed to avoid failure in shear of the joint panel and to avoid failure of the anchorage of the eam and column reinforcement. Significant shear stresses are put into the joint from the connecting eams and columns causing diagonal cracking of joint concrete and yielding of lateral reinforcement. In addition, the eam reinforcement yields at the face of the joint under the planned eam-yielding mechanism causing ond deterioration along the eam reinforcement within the joint. The comined effects result in a reduction in stiffness and energy dissipation capacity with increased deflection. The lateral reinforcement in the joint cannot eliminate the source of the stiffness degradation; therefore, the joint size and eam reinforcement (diameter and strength) must e altered to improve the joint performance. Therefore, the AIJ guidelines require that "Beam-column connection shall e designed to maintain its integrity to the assuring deformation (1/66 rad for a moment resisting frame, and 1/75 rad for a frame-wall dual structure), and to prevent a significant stiffness reduction and slip-type hysteretic ehavior caused y load reversals." DESIGN FOR SHEAR Design Principles The AIJ guidelines require that reliale shear strength, V j, used in the Assuring Design. V ju, shall e greater than design shear, Design Shear Design joint shear, V j, should e calculated y the following expression; V = T + C ' + C ' V j s c c = T + T' V c (1) where the symols are explained in Fig. 4. The upper ound strength must e used in evaluating tension forces, T and T ', in the reinforcement. Column shear, V, may e taken as an average of the upper and lower story column shears. The value of input joint shear may differ in the positive and negative directions of earthquake loading especially when the eam reinforcement 4 c

5 is anchored in the joint. Reliale Shear Strength The shear resisting mechanism may e thought to consist of truss action and a diagonal concrete strut action (Ref. 3, Fig. 5). The truss mechanism is formed y the ond stress transfer along the column and eam longitudinal reinforcement, tensile resistance of lateral reinforcement and compressive resistance of uniform diagonal concrete struts in the joint panel. The diagonal concrete strut mechanism is formed y the major diagonal concrete compression in the joint caused y compression and shear on concrete acting at the joint oundary. The lateral reinforcement increases joint shear resistance in the truss mechanism, ut will not in the diagonal strut mechanism. Average joint shear stress, which D j : column depth, v ju, is evaluated using the effective joint shear area j x j : effective width of eam-column connection, given y Eq. (2). D j, in = + + (2) j a1 a2 where, : eam width, a1, a2 : the smaller of one-quarter of column depth (e.g., shaded area on right side in Fig. 6) and one-half of distance etween eam and column faces (e.g., shaded area on left side in Fig. 6) on either side of the eam. Interior joint test results of 68 specimens, tested etween 1966 and 1988 in Japan and other countries, were compared with actual concrete strength in Fig. 7. Twenty-four specimens failed in joint shear efore eam flexural yielding (solid square symols; J-type). Forty-four specimens failed in shear after eam flexural yielding (open circle symols; BJ-type), ut at a story drift angle greater than 1/50 rad eyond the Assuring Deformation; hence the ehavior of BJ-type specimens were judged acceptale in the AIJ guidelines. The lower ound shear strength of J-type specimens is approximately 0.3 times the concrete compressive strength in a concrete strength range from 210 to 360 kgf/cm 2 (3,000 to 5,000 psi). Recent studies reveal that the lateral reinforcement is not as effective on joint shear resistance as previously considered. The effect of lateral reinforcement is presented in Fig. 8 for J-type test specimens (joint shear failure without eam flexural yielding). The ordinate represents the average shear stress at shear failure divided y the concrete compressive strength, and the ascissa represents the product of lateral reinforcement ratio and yield strength of lateral reinforcement. Those specimens tested as a series y the same researcher(s) are connected y straight lines. Some researchers reported an increase in resistance with the amount of lateral reinforcement; however, a majority of researchers did not. 5

6 The numer of exterior joint specimens failing in joint shear was small. Five specimens failed in joint shear efore eam flexural yielding (J-type) and seven specimens failed in joint shear after eam flexural yielding (BJ-type) as shown in Fig. 9. The lower ound shear strength of J-type specimens is approximately 0.18 times the concrete compressive strength. Tests of interior and exterior joints that failed in shear demonstrated that the joint shear strength depends strongly on the concrete strength, ut did not depend apprecialy on the amount of lateral reinforcement. Therefore, the AIJ guidelines assume the diagonal strut action to e a dominant shear resisting mechanism in the joint, and define the reliale shear strength, V, of a eam-column connection to e proportional to the compressive strength f ' of concrete; c ju V = k f ' D (3) ju c j j where, k : factor dependent on shape of a eam-column connection; i.e., 0.30 for an interior eam-column connection, and 0.18 for an exterior eam-column connection. Effective Joint Shear Area The dominant shear resisting mechanism is assumed to e the diagonal strut action in the AIJ guidelines; therefore, the entire column depth is assumed to e effective in an interior joint. However, the horizontally projected length of a 90-degree hook is used in an exterior joint ecause the compression strut is assumed to start at the corner of the end y earing against core concrete. The area effective to resist joint shear may not e as large as the column's entire cross area especially when the width of girders is narrow compared to that of the column. If the eams and columns are connected with eccentricity, torsional moment may e developed in the columns and eams, causing the reduction in stiffness y torsional cracking or the reduction in joint volume effective to resist shear. The shear resistance of a joint is known to decrease when the depth of connecting eams exceeds twice the column depth. Therefore, the AIJ guidelines intend to limit the effective joint shear area in an eccentric joint y Eq. (2). Minimum Joint Shear Reinforcement Although experimental results demonstrated small dependency of the joint shear resistance on the amount of joint lateral reinforcement, some lateral reinforcement is required (1) to avoid diagonal tension failure of the joint, (2) to improve the ductility of the joint y confining the cracked core concrete, and (3) to protect column corner ars from ond splitting failure. Therefore, the AIJ guidelines require that lateral reinforcement ratio, p, of a eam-column connection shall e not less than 0.002, and shall satisfy Eq. (4). jh 6

7 p jh V j (4) V ju in which V j : design joint shear, V ju : reliale joint shear strength given y Eq. (3). Effect of Transverse Beams The unloaded transverse eams are known to increase the shear resistance and stiffness of a joint. For example, Fig. 10 compares the shear resistance of three-dimensional and two-dimensional eam-column su-assemlages with respect to an index λ defined y Eq. (5); L DL λ = (5) 2 D D where, L : width of the transverse eam, D L : depth of the transverse eam, ut limited y depth of the longitudinal eam, D : depth of the longitudinal eam, and D : depth of the column. However, a joint in a real uilding is sujected to i-directional response during an earthquake, forming yielding hinges in the longitudinal and transverse eams at all faces of the joint. Therefore, the eneficial effect of the transverse eams may e reduced. To address this issue, the AIJ guidelines recommend that the transverse eam effect may e considered only when the transverse eams do not yield at the face of the joint, and that the shear resistance may e increased y the factor β, where β = λ (6) The effect of the transverse eam shall not e considered if the transverse eam exists only on one side of the joint. Vertical Reinforcement in Joint The AIJ guidelines do not require the placement of vertical reinforcement in a joint ecause at least one intermediate column longitudinal reinforcing ar is placed in a column section. An experimental study on exterior al reinforcement especially under a large axial load (Ref. 4). Bi-directional Earthquake Response It is desirale to design a joint to resist simultaneous i-directional earthquake loading conditions, in which the joint input shear ecomes larger than that under uni-directional earthquake loading. Minami et al. (Ref. 5) tested three-dimensional eam-column joint specimens under horizontal 7

8 load reversals in an inclined direction. The shear resistance, defined as a vectorial sum of the longitudinal and transverse shear resistances, increased when axial load was zero, ut it increased little under an average axial stress of 0.2 times the concrete strength (Fig. 11). In three-dimensional eam-column joint tests (Ref. 6), in which the joint failed in shear after developing flexural yielding at eam ends, the resistance and deformation capacity were similar for a specimen sujected to separate longitudinal and transverse load reversals and a specimen sujected to diagonal load reversals. Therefore, the AIJ guidelines suggest that the joint might e designed for the two principal directions independently. ANCHORAGE OF BEAM AND COLUMN REINFORCEMENT Anchorage Method Beam longitudinal reinforcement normally passes through an interior joint, and is anchored with a 90-degree standard hook in an exterior joint. Hysteretic energy dissipation deteriorates with the yielding penetration of eam reinforcement into the joint and with the ond deterioration along the reinforcement. The ond deterioration may e delayed in the interior joint y passing the eam reinforcement in the confined concrete core, and y limiting the ond stress level in design. The eam reinforcement should e anchored within the joint core in the exterior joint. The tension in eam reinforcement often causes cone-type failures at the eam-column interface, resulting in a reduction in anchorage length. However, this ehavior is not clearly understood, and the critical section for anchorage of the eam reinforcement may e taken at the face of the column. The guidelines require that the longitudinal reinforcement of eams shall pass through the column core or anchored in columns with a 90-degree standard hook. Development length of a ar may e measured from the critical section at the column face for eam reinforcement and at the eam face for column reinforcement. Anchorage with 90-degree Hook It is often difficult to distinguish a shear failure and an anchorage failure for tests conducted on exterior joints; however, the anchorage failure often exhiits earing crushing of concrete at the end, or splitting along the reinforcement. In some cases anchorage failure results in a sudden loss of resistance and poor hysteretic energy dissipation characteristics. 8

9 Recent studies on the anchorage with a 90 degree hook point out the importance of the horizontal projection length efore the end. The extension of the reinforcing ar eyond 12 ar diameters from the end is not effective for anchorage. Therefore, the straight portion of the ar efore the end should e as long as possile. There have een several proposals to evaluate the anchorage resistance of reinforcement with a 90-degree hook. Fujii et al. (Ref. 7) assumed that the ond stress transfer would e lost along the horizontal portion at an early stage, and that the anchorage resistance would e attained when the earing stress along the end reached the concrete earing strength (Fig. 12), and proposed the expression elow: P= wd f ear θ h sin h j (7) in which π w= 1.41 β r cos 4 θ 1 dh θ = tan j dh 1 r β = d f ear = + r+ d C α = 16.1 d As γ = s = αγ 0 1 f c ' where, d : ar diameter (cm), h : distance etween column inflection points (cm), r : radius of end (cm), 1: emedment length (cm), f c ' : compressive strength of concrete (kgf/cm 2 ), A s : area of lateral reinforcement (cm 2 ), s : spacing of lateral reinforcement (cm), j : distance etween tensile and compressive resultants of the eam (cm), and C 0 : cover thickness to the centroid of the column longitudinal reinforcement measured at the column side (cm). The average of the calculated to the oserved for 73 specimens was 0.95 with a coefficient of variance of Exterior joint specimens with eam ottom ars ent downward into the lower column normally exhiited smaller flexural resistance under positive ending than that under negative ending. 9

10 Such reinforcement detailing requires a large amount of lateral reinforcement at the top of the lower column. It is more rational to end the eam ottom reinforcement upward into the joint. The AIJ guidelines require that the eam longitudinal reinforcement shall e extended eyond the mid-depth of column with a 90-degree standard hook. Extension of a 90-degree standard hook shall e placed in the connection. The cover concrete for an exterior joint tends to spall off when the extension tail of the eam reinforcement lines up with the column longitudinal reinforcement, causing the loss of anchorage resistance. However, little test data are availale on the necessary cover thickness for the extension. Bars Passing through Joint Average ond stress τ of eam longitudinal reinforcement over the joint width is expressed y Eq. (8); a τ a σ s s a = (8) D φs where a s and φ s : area and perimeter of a eam ar, σ s : difference in eam ar stresses at the joint faces, and D : depth of the column. The area and perimeter are expressed y ar diameter d in Eq. (8), then σ s d τ a = (9) 4D If the ond strength is assumed to e proportional to the square root of concrete strength, and the stress difference σ is determined for simultaneous yielding in tension and compression at the ends; s D d σ y (10) µ f ' c where, σ y : yield strength of the eam ar. The guidelines do not specify the value of µ in Eq. (10). It is hard to prevent the ond deterioration for the concrete and reinforcement strengths, ar sizes, and column dimensions commonly used in Japan. The ond deterioration along the eam reinforcement causes the following prolems; (1) reduction in hysteretic energy dissipation capacity results in increased earthquake response, (2) eam deformation is increased prior to eam flexural yielding, (3) large eam end rotation 10

11 accelerates concrete crushing at the critical section, and (4) repair of the ond deterioration is difficult. However, if the eam critical region is properly confined, and if the eam reinforcement is anchored in the opposite side eam across the joint, the loss of ond in the joint may not lead to a sudden reduction in the eam resistance. A study of earthquake responses with different hysteresis models (Ref. 8) reveals out that the largest response is not sensitive to the hysteretic energy dissipation capacity. The study proposed a value of µ in Eq. (10) of 10 to maintain the hysteretic energy dissipation capacity at a story drift angle of 1/50 rad. It should e noted, however, that the maximum response amplitude may not e influenced y the decay in the hysteretic energy dissipation. The numer of large amplitude oscillation certainly increases with the decay. The value of µ may e further increased ecause a) the increase in structural resistance due to the development of the upper ound strength at yield hinges will decrease the structural response, ) the consequence of ond deterioration is less significant than that of shear failure, c) the ond deterioration in a limited numer of joints may not cause ill effect on the structural response as long as the majority of joints can maintain their stiffness and resistances. Therefore, some deterioration can e tolerated as long as the earthquake response can e controlled. Earthquake response analyses of frames (Ref.8) indicate that the slipping hysteretic characteristics do not increase response in the displacement range considered in the AIJ guidelines; i.e., an approximate story drift angle of 1/100 rad. The AIJ guidelines require that the ar size to memer depth ratio shall e determined not to cause significant stiffness reduction or slip-type hysteretic ehavior under load reversals where eam or longitudinal reinforcement is intended to yield at oth faces of the connection. ACKNOWLEDGMENT The AIJ Design Guidelines for Earthquake Resistant Reinforced Concrete Buildings ased on Ultimate Strength Concept was drafted y Seismic Design Su-committee of Reinforced Concrete Committee, AIJ. The su-committee consists of T. Okada as chairman, S. Otani, T. Kuo and S. Nomura as secretary, and M. Ohkuo, N. Kani, T. Kaeyasawa, S. Sugano, A. Shiata, O. Joh, S. Nakata, M. Hirosawa, Y. Matsuzaki, K. Minami, T. Yamauchi, K. Watanae, and F. Watanae. The author wishes to acknowledge the work y the committee memers. REFERENCES 11

12 1. Architectural Institute of Japan, AIJ Standard for Structural Calculation of Reinforced Concrete Structures (in Japanese), 1933, 1937, 1947, 1949, 1958, 1962, 1971, 1975, 1979, 1982, Architectural Institute of Japan, Design Guidelines for Earthquake Resistant Reinforced Concrete Buildings ased on Ultimate Strength Concept (draft) and Commentary (in Japanese), Octoer, 1988, 337 pp. 3. Paulay, T., R. Park, and M.J.N. Priestley, Reinforced Concrete Beam-column Joints under Seismic Actions, Journal, ACI, Vol. 75, No. 11, Novemer, 1978, pp Kaku, T., Resistance and Ductility of Exterior Reinforced Concrete Beam-column Joint (in Japanese), JCI Colloquium on Ductility of Concrete Structures and Its Evaluation, March, 1988, pp. II Fujiwara, M., Y. Nishimura, and K. Minami, Behavior of Three-dimensional Beam-column Su-assemlages under Bi-directional Loading (in Japanese), Proceedings, Japan Concrete Institute, Vol. 10, No. 3, pp Kusakari, T., O. Joh, and T. Shiata, Experimental Study on Failure of Three-dimensional Reinforced Concrete Beam-column Joints (in Japanese), Report, AIJ Hokkaido Branch Meeting, Vol. 57, March, Fujii, S., S. Goto, S. Morita, and G. Kondo, The Behavior of 90 Deg. Bent Bar Anchorage in Exterior Beam-column Joint, Part-2: Evaluation of Anchorage Capacity (in Japanese), Summary Report, AIJ Annual Meeting, Structural Division, Octoer, 1983, pp Kitayama, K. and H. Aoyama, Earthquake Resistance of Reinforced Concrete Beam-column Su-assemlages (in Japanese), Proceedings, Seventh Japan Earthquake Engineering Symposium , Decemer, 1986, pp

13 (a) Beam Yield Mechanism () Wall Yield Mechanism (c) Wall Up-lifting Mechanism Fig. 1: Total Yield Mechanism 13

14 Fig. 2: Partial Yield Mechanism Fig. 3: Design Load and Deformation 14

15 Fig. 4: Design Shear in Joint Fig. 5: Shear Resisting Mechanism 15

16 Fig. 6: Effective Joint Area for Shear Resistance Fig. 7: Shear Strength and Concrete Strength (Interior Joint) 16

17 Fig. 8: Lateral Reinforcement and Shear Strength (Interior Joint) Fig. 9: Shear Strength and Concrete Strength (Exterior Joint) 17

18 Fig. 10: Shear Strength Magnification y Orthogonal Beams Fig. 11: Bi-directional Shear Interaction 18

19 Fig. 12: Anchorage Mechanism of 90-degree Hook 19