Anchorage behavior of 90-degree hooked beam bars in reinforced concrete knee joints

Transcription

1 Anchorage behavior of 9-degree hooked beam bars in reinforced concrete knee joints O. Joh& Y. Goto Division of Social and Geotechnical Engineering Graduate School of Engineering, Hokkaido University Kita-ku Kita 13nisi 8 Sapporo , Japan joh@eng. hokudai. ac.jp Abstract Beam bars of reinforced concrete structures are usually anchored with 9-degree hooks in exterior beam-column joints. However, there have been far fewer studies on the anchorage behavior of 9-degree hooked-bar than on straight-bar anchorage. In our previous paper, we divided the anchorage failure of 9-degree hooked bars in exterior beam-column joints in a middle storey in a building into three modes: side split failure; local compression failure; and raking-out failure, in which a concrete block, approximating the inside dimensions of the hooked bar in size, is raked out toward the beam side of the column due to thepresence of many beam bars and/or to short development length within the joint The purpose of this paper was to clarify the anchorage performances on the rakingout failure mode of beam top-bars with 9-degree hooks arranged in an exterior beam-column joint in the roof story of a building. Fifteen specimens of knee joints with various arrangements of L-shaped beam bar anchorage and various material properties, were subjected to pull-out loading on the beam top-bars. From the experimental results, we were able to conclude that: (1)the anchorage mechanism depends on stress transmission from the tail portion of the beam bar hooks to the adjacent column bars in a joint and that the anchorage mechanism in a knee joint was quite different from that in a exterior beam-column joint in a middle story; (2) the main factors in influencing anchorage strength are tail length, horizontal distance between tail and column bars and between tail bars and the beam end, lateral reinforcement ratio within the joint and concrete strength; and (3) accurate estimation of anchorage strength can be obtained by taking into account the influence of the above mentioned factors.

2 34 Earthquake Resistant Engineering Structures I. Introduction When the main bars of a reinforced concrete member are anchored to an adjacent member, in case in which the adjacent member is shallow, such as in the connection of girders to an exterior column, of a beam to an exterior girder, of a slab to an exterior wall, and of a wall to a transverse wall, the bar ends are usually arranged with a 9-degree hook. It is generally thought that pull-out resistance of a bar anchorage with a 9-degree hook is shared amonge the horizontal straight portion, the bent portion and the tail of the bar. Some design codes for reinforced concrete structures, including that of Japan, demand that the total length of such a hook in an exterior beam-column joint in a middle story of a building is the same development length as that required for a non-hooked straight-bar anchorage. However, in a previous paper [Ref.l], we demonstrated the tail portion of a 9-degree hook provides little pull-out resistance. On the other hand, the above mentioned design codes also demand that in an exterior beam-column joint in an roof story the tail portion alone of 9-degree hook is the same development length as that required for a non-hooked straight-bar anchorage. This requirement is based on the assumption that the horizontal straight portion of a hook is not confined by the above column, thus no bond resistance can be expected from this portion. Therefore, the design requirements for hooked anchorages differ between beam-column joints in middle and roof stories. There have been far fewer studies on anchorage behavior of 9-degree hooked bars, especially on hooked bar anchorage in knee joints (the connection of an exterior column to a beam in a roof story), than on straight bar anchorage. The purpose of this study was to clarify the anchorage performances of beam top-bars with a 9-degree hook in reinforced concrete knee joints. 2. Experiment 2.1 Test Specimens Columns, about half normal size and with exterior beam-column joints at either ends, were used as specimens in this study. As shown in Figure 1, no beam concrete or compressive beam bar were attached in order to simplify production of the specimens. Two series of specimens were tested: the TA series, with a horizontal development length L^ (the distance the from beam end to the tail center) of34mm; and the TB series, with an L^ of 27mm. Specimens TA19-2 and TB19-2 were standard specimens from each series and the other specimens differed from theses standard specimens in only one of test variables. The main variable, vertical development length L^ (distance from tail end to horizontal bar center), was tested at values of 8d, and 24d, where d is the nominal diameter of beam bar. These values are denoted by -1, -2 and -3, respectively, being added to the specimen number. Other variables tested are shown in Table 1. The dimensions of the beam longitudinal bars were identical: four bars were arranged in a single layer; the spacing between the center of each bar was

3 Table 1: Variations of specimens Ldh Specimen (mm) TA TA TA TA19-2CL 34 TA19-2CH 34 TA19-2A 34 TA19-2S 34 TA TA TB TB TB TB19-2P2 27 TB19-2P3 27 TB19L-2 27 TB19R-2 27 LA> (mm) 8d I6d 24d I6d 8d 8d 24d (Note) *B.B.: Beam bar *d: Diameter of B.B. ^Standard specimen: fc=3mpa,/7w=.21%, Dc=4mm, bc=3mm, r=3d,jb=328mm, Column bar 18 Hook, Beam bar: 4-D19/SD685 *P:Plate anchorage of column bar *L:9-degree hooked anchorage of column bar Earthquake Resistant Engineering Structures 35 Table 2: Measured properties CONCRETE 1/3 2/3 emax Variations (example) (GPa)(GPa) (u) (TB=26.3MPa (Standard) <7B=34.2MPa CB=.7MPa fc=2mpa fc=45mpa PL B.B.: 6-D13 B.B.: 6-D13 MStandard) B.B*: SD345 STEEL OV Sy CTmax ts D19(SD685) D19(SD345) D16(SD685) D13(SD685) j)(SR345) cr:mpa,e:( Horizonal debelopment length <U u. - r A^dh --H (^ O C =1 2-1 'jj8 I'l'S Horaizontal straight bar/portion compression^ < corner/zone I -»-* _t TImaginary S beam g beam side Z^=34(TA series) =27(TB s( me ^ fo >p 1... "\. u i- "~ ^r Rl ' Figure 1: Details of specimens (

4 36 Earthquake Resistant Engineering Structures equiverent to 3d, or 57 mm; and the bars were covered with 2d, or 36 mm, of concrete. The column specimens were all 3 mm in width and 4 mm in depth. The depth of the imaginary beam was 4 mm, with a moment arm of 328 mm. The values of variables in the standard specimens were L^ =, ob (concrete compression strength) = 33 MPa (in design strength) and p^ (lateral reinforcement ratio within the joint) =.21%. The reinforcement bars of the columns were hooked 18-degrees. The beam bars were high-strength threaded deformed bar of 19 mm in diameter. The inside radius of the beam bar 9-degree hook was 57 mm, or 3d. The mechanical properties of the steel bars and typical concrete used in the specimens are shown in Table 2. The aggregate used was crushed stone with a maximum size of 13 mm matched to the scale of specimens. The beam bars used were a high-strength threaded and normal deformed bars of 19 mm and 13 mm in diameter, respectively. 2.2 Instrumentation and Loading Tensile load P was applied horizontally to the beam bars by a 2kN oil-jack, as shown in Figure 1. Reaction Rl was applied at the compression zone of the imaginary beam cross section by a steel plate with a height of one-fifth beam depth, and reaction R2 was applied to the mid-point of the out-side of the column. The four beam bars were controlled so as to distribute the pull-out displacement equally among the bars in order to simulate actual beam bar conditions. Thus the tensile loads on each bar varied slightly. 3. Experimental Results and Discussion 3.1 Behavior of Cracks and Failure Figure 2 is a schema showing a typical crack pattern that appeared on the side of specimens at the final loading stage and showing the mark of each crack. The cracking process occurred as follows: (1) a flexural crack F appeared at the bottom of the joint in the tensile region but did not open widly; (2) the crack SF appeared in the end of the column near the mid-point of the horizontal straight portionof the bar in the joint and extended to the compression corner reducing the stiffness of the joint; (3) the crack S appeared at the bend of the bar, extended to the compression corner severely reducing the stiffness of the joint and led usually specimens of the TA series to the ultimate stage; (4) the crack V started along the bar bend, extended along the tail of bar and led usually specimens of the TB series tothe ultimate stage; and (5) the crack SH branched out from the crack V and extended towards the compression zone. 3.2 Failure Modes Figure 3 is a schema showing typical failure modes obtained from this test. Photographs of some specimens taken after the loading test are shown in Figure 4. The failure modes were divided into four classes according to guidlines described in our previous paper.

5 Earthquake Resistant Engineering Structures 37 Figure 2: Typical cracks RO SS JS Figure 3: Failure modes in beam-column joint TA19-1 TA19-2 TA19-3 TA19-2CH high strength cone. TB19-1 TB19-2 TB19-2P3 TB19L-2 L^=8j Lfr=16j liigh reinf. ratio nonnal strengtli bar Figure 4: Examples of crack patterns after loading test (1) Raking-out anchorage failure (RO). A concrete block, approximating the inside dimensions of the hooked bar in size, is raked out toward the beam side of the column while rotating on the compression corner. The ultimate stage of this failure mode was led by opening crack S for TA series and crack V for TB series specimens and was due to bond failure of the tail of the bars. (2) Side-split anchorage failure (SS). The concrete covering the bent portion of the beam or column bars in a joint peels away leaving a dish-shaped depressions individually on both sides of the joint. This type of failure is due to split stress arround the inside of the bend portion of the bars. Except for TA19-1, the column (not beam) bar hooks in TA series specimens failed in side split of concrete, simultaneously with raking-out failure. (3) Fracture of the column bar hook (FR). The bend portions of the column bars are broken by bending reversal resulting from an increase in deformation.

6 38 Earthquake Resistant Engineering Structures (4) Shear failure of joint (JS). The concrete around the compression comer in a joint is crushed after the development of a crack S. No specimen in this study demonstrated evidently this type of failure. 3.3 Load vs. Displacement Relationship Some examples of Load vs. displacement curves are shown in Figure 5. The displacement was measured as relative displacement between a point on beam bars on the column face and the mid point of the column depth. In many specimens, the stiffness of the joint gradually declined due to an S-crack and/or V-crack and load resistance after maximum load also deteriorated gradually. However, the low L^ and L& values of TB19-1 and TB19-2 led to a severe deterioration in load resistance soon after the onset of a decrease in the stiffiiess of the joints. 7\,(kN) TA19-1 -TA13- TA13-2 TA19-3 TB19-3 TB19-1 TB Bar Displacement,5(mm) Figure 5: e*pt vs. 6 relation-curves Figure 6: Relationship of «,!T«and Ld* 3.4 Influence of Vari ables on Anchorage Strength The experimental and calculated values of ultimate stage are shown in Table 3, and the relationship betwen the experimental ultimate strength ^7^ and the vertical development length L^ is plotted in Figure 6. Test results indicate that acptu increased linearly with increases in L& in TA series specimens. This means that tensile stress applied to the tails of the beam bars was transmitted to the tensile column bars in a joint accoring to the lapped bar-joint stress transmission mechanism. Stress transmission in TA19-2L, with 9-degree column bar hooks, was higher than that in TA19-2, which had 18-degree hooks, and that in TA19-2A, without hooked column bars, was lower than that in TA19-2. The ^7^ in the TA series specimens was found to be larger than that in the TB series specimens. The reasons for these larger ^7^ values are thought to be: 1) tail stress 7} in the TA specimens was larger because of the short lateral distance between the tails and the column bars, and 2) the resistant moment on the reaction point expressed

7 Earthquake Resistant Engineering Structures 39 by L^ x Tt was larger in the TA specimens than in the TB specimens because of their longer L^. Test results also indicated that the ^7% in the TB specimens with an L& less than the moment arm of the beam (/& = 328mm) was low because the column hooks were situated further from the beam bars so that a premature crack along the tail of the hooked bar (V-crack in Figure 2) prevented the transmission of the tail stress to the column bars. The ^7^ value for TB19-3, which had an L*> longer than the moment arm of the beam, was large because the V-crack along the tail could not expand into the horizontally compressive zone in the column. 3.5 Evaluation of Anchorage Strength with Raking-out Failure In the previous paper(ref.2), we proposed the anchorage strength with rakingout failure in exterior beam-column joints in a middle story of a building to be where TC (horizontal resistance of concrete) = 2L^ b^ a, s TW (horizontal resistance of lateral reinforcement) = (1) L^ (horizontal development length of beam bar) = L + r + d/2, a» is the total sectional area of lateral reinforcement crossing failure planes, b^ (effective joint width) = b^ k» (coefficient of effective lateral reinforcement) =.7, k^ (axial stress modification factor) = 1 +.2cr^, cr, (concrete tensile strength) = t/"e%, (JQ is the column axial stress and not larger than.$ e%, cr^ is the yield stress of lateral reinforcement, and 6 is the strut angle. The concept of this resistance mechanizm and the dimensional variables are explained schematically in Figure 7. Bar tail Radius of bar bend Inclined failure plane Vertical failure plane tr Vertical failure plane when 2C=<6cg when where - sin y (=336mm) L^;develop.length(=2mm) Figure 7: Previously proposed model on failure mechanism The average of A, which is the ratio of experimental ultimate strength ^7% to calculated one cai? RE, for all specimens was.74 and the standard deviation was.24. This unfavorable result was due to the fact that the equation

8 4 Earthquake Resistant Engineering Structures disregarded the peculiar characteristics of knee joints; i.e., the ultimate strength depended on tail length and the knee joints had no column on the top. Therefore, in the present study we propose to evaluate the anchorage strength of knee joints with raking-out failure taking into consideration these characteristics. In the new equation, the horizontal resistance of concrete 7^ is modified, while the horizontal resistance of hoops T# remains the same and the new componemt 7^ (horizontal resistance of tails) is added. There is no need for the axial stress modification factor k^ because there is no axial load exerted on a knee joint. Thus, from the equation (1) we obtain,,,^=rc+^+7^. (2) 1) Consideration for the lack of an upper column It is assumed in equation (1) that the concrete tension stress on a pair of failure planes inclined 45-degree above and below the beam bars provides the lateral resistance of concrete. However, in a knee joint, the failure plane was found to open vertically above the beam bars, not along the inclined failure plane, and this vertical crack provided any lateral resistance as it developed prematurely and ultimately opened for too wide. Therefor, the component TC from equation (1) was modified to 7^ =^-6,,.cr, /,W. (3) 2) Consideration for the effect of beam bar tails on horizontal resistance The tail portions of beam bars in an exterior beam-column joint in a middle story play scarcely any role because a part of diagonal compression stress is the sum of horizontal beam force and vertical column force which was supplied by bending moment of upper column. The tail portions of beam bars in a knee joint however need to share the vertical force instead of the upper column. This vertical force shared by the tails is transmitted first to the outside tensile column bars in the joint and, through them, finally to a lower column. As the amount of transmitted force grew in accordance with the tail length, and as the force was effected by the anchorage type of the column bars, the horizontal resistance of the tail T^ was defined by the following equation: TH=T,-L*/h, (4) where 7) (resistance force of tail) = 7 T^ Zy (L^ -r-d / 2). y is the effective factor for bond strength of tail calculated by using the bond strength distribution shown in Figure 8, T^ (bond strength of tail bar) =.56-/~c%, and ly/is the total perimeter of the tail bars. The effective factor y is the ratio of assumed to full z^ distribution areas, which were defined as the bond strength at point B of TA series was ^ for 9-degree hooks of column bars, rjl for 18- degree hooks and zero for steel-plate anchorage, and as the bond strength between B-C zone was zero. The calculated anchorage strengths cai? RK and their components are shown in Table 3, and the relation between these values and the experimental values is

9 Earthquake Resistant Engineering Structures 41 plotted in Figure 9. The average of A, which is the ratio of experimental ultimate strength ^7^ to calculated ultimated strength,,,7^, for all specimens was 1.12 and the standard deviation was.85. Except for TA19-2CL, which had low concrete strength, the variance of A was less than about ± 1 %, thus it can be said that the proposed equation is highly accurate for evaluating anchorage strength. Expressing the effect of differences in column hook type on anchorage strength by the effective bond strength at point B, was particulary helpful in estimating anchorage strength. In doing this, the value for the bond strength between zone B-C in the TB series specimens, in which ISO-degree hooks of column bars were situated at a distance from the tail bars, was taken to be. However, this value for bond strength can be adjusted to allow for column hooks Table 3: Observed and calculated anchorage strengths at ultimate stage Specimen (MPa: TA TA TA TA19-2CH.7 TA19-2CL 26.3 TA19-2A 38.5 TA19-2S 38.5 TA TA TB TB TB TB19-2P TB19-2P3 23. TB19L TB19R (kn) Tc J. ttr f (kn) (kn) (kn) exp cal caltrk Tc Tw Ta (kn) (kn) (kn) , (kn) exp cal Figure 8: Distribution of bond strength Figure 9: Relationship of **ptu and caitu

10 42 Earthquake Resistant Engineering Structures that cross the tail bars. The effect of concrete strength on the anchorage strength has been discussed in our previous study on exterior beam-column joints in a middle story. Further, in the present study, we found that the difference expressed by /I were only 4 % for specimens TB19-2P3 and TB19R-2, which had the lowest concrete strength and did not depend on the bond strength of tail. Therefore the difference between the calculated and experimental results is certainly due to the problem of bond strength estimation. 4. Conclusion We examined the anchorage behavior of beam-column knee joints using columns with 9-degree hooked beam bars as study specimens. The test variables of the specimens were horizontal and vertical development lengths, concrete compressive strength, hoop reinforcement ratio, beam bar diameter, hook type of column bar, etc. The conclusions obtained from the experimental results are as follow: 1) Anchorage mechanism of hooked beam bars in knee joints located in a top story was quite different from that in exterior beam-column joints located in a middle story. 2) Anchorage strength depended on stress transmission from tail bars to column bars within the joint; not on the on bond behavior along the horizontal development portion of the beam bars. 3) Stress transmission increased as the tail length of beam bars increased, and was effected by the positioning of the column hooks to beam bars. 4) A modified equation for estimating the anchorage strength of 9-degree hooked bars in kneejoints was derived from our previous equation used for beam-column joints in middle stories. Our experimental results comfirmed the high accuracy of this modifided equation. Acknowledgement The authors gratefully acknowledge thefinancialsupport by the Building Center of Japan (Grant-in-Aid for Scientific Research, 1997) and the experimental works by J.Iwanami and Y. Yamazaki who were Master course students of Hokkaido University. References 1. Joh,O., Goto, Y. & Shibata, T., Anchorage of Beam Bars with 9-Degree Bend in Reinforced Concrete Beam-Column Joints, Proc. of the Tom Paulay Symposium "Resent Development Lateral Force Transfer in Building," American Concrete Institute, SP-157, pp , Joh,O., & Shibata, T., Anchorage Behavior of 9-Degree Hooked Beam Bars in Reinforced Concrete Beam-Column Joints, Proc. of the 12th World Conference on Earthquake Engineering, Acapulco, CD-ROM-3, 1996