i. introduction...anchorage behavior of 90-degree hooked beam bars in reinforced concrete knee...

Anchorage behavior of 90-degree hooked beam

bars in reinforced concrete knee joints

O. Joh& Y. Goto

Division of Social and Geotechnical EngineeringGraduate School of Engineering, Hokkaido UniversityKita-ku Kita 13nisi 8 Sapporo 060-8628, JapanEmail: joh@eng. hokudai. ac.jp

Abstract

Beam bars of reinforced concrete structures are usually anchored with 90-degreehooks in exterior beam-column joints. However, there have been far fewerstudies on the anchorage behavior of 90-degree hooked-bar than on straight-baranchorage. In our previous paper, we divided the anchorage failure of 90-degreehooked bars in exterior beam-column joints in a middle storey in a building intothree modes: side split failure; local compression failure; and raking-out failure,in which a concrete block, approximating the inside dimensions of the hookedbar in size, is raked out toward the beam side of the column due to the presenceof many beam bars and/or to short development length within the joint Thepurpose of this paper was to clarify the anchorage performances on the raking-out failure mode of beam top-bars with 90-degree hooks arranged in an exteriorbeam-column joint in the roof story of a building. Fifteen specimens of kneejoints with various arrangements of L-shaped beam bar anchorage and variousmaterial properties, were subjected to pull-out loading on the beam top-bars.From the experimental results, we were able to conclude that: (1) the anchoragemechanism depends on stress transmission from the tail portion of the beam barhooks to the adjacent column bars in a joint and that the anchorage mechanism ina knee joint was quite different from that in a exterior beam-column joint in amiddle story; (2) the main factors in influencing anchorage strength are taillength, horizontal distance between tail and column bars and between tail barsand the beam end, lateral reinforcement ratio within the joint and concretestrength; and (3) accurate estimation of anchorage strength can be obtained bytaking into account the influence of the above mentioned factors.

Transactions on the Built Environment vol 38 © 1999 WIT Press, www.witpress.com, ISSN 1743-3509

34 Earthquake Resistant Engineering Structures

I. Introduction

When the main bars of a reinforced concrete member are anchored to an adjacentmember, in case in which the adjacent member is shallow, such as in theconnection of girders to an exterior column, of a beam to an exterior girder, of aslab to an exterior wall, and of a wall to a transverse wall, the bar ends areusually arranged with a 90-degree hook. It is generally thought that pull-outresistance of a bar anchorage with a 90-degree hook is shared amonge thehorizontal straight portion, the bent portion and the tail of the bar. Some designcodes for reinforced concrete structures, including that of Japan, demand that thetotal length of such a hook in an exterior beam-column joint in a middle story ofa building is the same development length as that required for a non-hookedstraight-bar anchorage. However, in a previous paper [Ref.l], we demonstratedthe tail portion of a 90-degree hook provides little pull-out resistance. On theother hand, the above mentioned design codes also demand that in an exteriorbeam-column joint in an roof story the tail portion alone of 90-degree hook is thesame development length as that required for a non-hooked straight-baranchorage. This requirement is based on the assumption that the horizontalstraight portion of a hook is not confined by the above column, thus no bondresistance can be expected from this portion. Therefore, the design requirementsfor hooked anchorages differ between beam-column joints in middle and roofstories. There have been far fewer studies on anchorage behavior of 90-degreehooked bars, especially on hooked bar anchorage in knee joints (the connectionof 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 beamtop-bars with a 90-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 eitherends, were used as specimens in this study. As shown in Figure 1, no beamconcrete or compressive beam bar were attached in order to simplify productionof the specimens. Two series of specimens were tested: the TA series, with ahorizontal development length L (the distance the from beam end to the tailcenter) of 340mm; and the TB series, with an L of 270mm. Specimens TA19-2and TB19-2 were standard specimens from each series and the other specimensdiffered from theses standard specimens in only one of test variables. The mainvariable, vertical development length L (distance from tail end to horizontal barcenter), was tested at values of 8d, 16d and 24d, where d is the nominal diameterof beam bar. These values are denoted by -1, -2 and -3, respectively, being addedto the specimen number. Other variables tested are shown in Table 1.

The dimensions of the beam longitudinal bars were identical: four bars werearranged in a single layer; the spacing between the center of each bar was


Earthquake Resistant Engineering Structures 35

Table 1: Variations of specimens Table 2: Measured properties

Specimen

TA19-1TA19-2TA19-3TA19-2CLTA19-2CHTA19-2ATA19-2STA13-1TA13-2TB19-1TB19-2TB19-3TB19-2P2TB19-2P3TB19L-2TB19R-2

Ldh

(mm)340340340340340340340340340270270270270270270270

LA>

(mm)8dI6d24d16d16d16dI6d8d16d8d16d24d16d16d16d16d

Variations

| (Standard)

fc=20MPafc=45MPa

PL

B.B.: 6-D13B.B.: 6-D13

MStandard)

B.B*: SD345

• CONCRETE £1/3 £2/3 emax(example) (GPa)(GPa) (u)(TB=26.3MPa 21.8 18.8 2760<7B=34.2MPa 25.5 21.4 2830CB=44.7MPa 26.8 23.8 2920

STEEL OV Sy CTmax tsD19(SD685) 667 3560 892 187D19(SD345) 386 2140 582 192D16(SD685) 795 4250 911 187D13(SD685) 728 3590 927 1996j)(SR345) 369 1720 486 215cr:MPa,E:(

(Note)*B.B.: Beam bar*d: Diameter of B.B.Standard specimen:fc=30MPa,/7w=0.21%,Dc=400mm, bc=300mm,r=3d,jb=328mm,Column bar 180°Hook,Beam bar: 4-D19/SD685*P:Plate anchorage

of column bar*L:90-degree hookedanchorage of column bar

Horizonal debelopment lengthu. r --H C<U | - A^dh (^ O

•=1| I'l'S2-1 |'jj8Horaizontal straight

bar/portion

compression <corner/zone | I

-»-* _t

TImaginary Sbeam g

beam side

Z =340(TA series)

1...

=270(TB s(

^ fO"\. u

"~00

r

me

i-

Rl '

(™

>p

Figure 1: Details of specimens



equiverent to 3d, or 57 mm; and the bars were covered with 2d, or 36 mm, ofconcrete. The column specimens were all 300 mm in width and 400 mm in depth.The depth of the imaginary beam was 400 mm, with a moment arm of 328 mm.The values of variables in the standard specimens were L = 16d, oB (concretecompression strength) = 33 MPa (in design strength) and p (lateralreinforcement ratio within the joint) = 0.21%. The reinforcement bars of thecolumns were hooked 180-degrees. The beam bars were high-strength threadeddeformed bar of 19 mm in diameter. The inside radius of the beam bar 90-degreehook was 57 mm, or 3d. The mechanical properties of the steel bars and typicalconcrete used in the specimens are shown in Table 2. The aggregate used wascrushed 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 of19 mm and 13 mm in diameter, respectively.

2.2 Instrumentation and Loading

Tensile load P was applied horizontally to the beam bars by a 2000kN oil-jack,as shown in Figure 1. Reaction Rl was applied at the compression zone of theimaginary beam cross section by a steel plate with a height of one-fifth beamdepth, 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 displacementequally among the bars in order to simulate actual beam bar conditions. Thus thetensile 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 ofspecimens at the final loading stage and showing the mark of each crack. Thecracking process occurred as follows: (1) a flexural crack F appeared at thebottom of the joint in the tensile region but did not open widly; (2) the crack SFappeared in the end of the column near the mid-point of the horizontal straightportionof the bar in the joint and extended to the compression corner reducingthe 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 jointand led usually specimens of the TA series to the ultimate stage; (4) the crack Vstarted along the bar bend, extended along the tail of bar and led usuallyspecimens of the TB series tothe ultimate stage; and (5) the crack SH branchedout 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 Figure4. The failure modes were divided into four classes according to guidlinesdescribed in our previous paper.



RO SS JSFigure 2: Typical cracks Figure 3: Failure modes in beam-column joint

TA19-1 TA19-2 TA19-3 TA19-2CHhigh strength cone.

TB19-1 TB19-2 TB19-2P3 TB19L-2L =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 theinside dimensions of the hooked bar in size, is raked out toward the beam side ofthe column while rotating on the compression corner. The ultimate stage of thisfailure mode was led by opening crack S for TA series and crack V for TB seriesspecimens and was due to bond failure of the tail of the bars.(2) Side-split anchorage failure (SS). The concrete covering the bent portion ofthe beam or column bars in a joint peels away leaving a dish-shaped depressionsindividually on both sides of the joint. This type of failure is due to split stressarround the inside of the bend portion of the bars. Except for TA19-1, thecolumn (not beam) bar hooks in TA series specimens failed in side split ofconcrete, simultaneously with raking-out failure.(3) Fracture of the column bar hook (FR). The bend portions of the column barsare broken by bending reversal resulting from an increase in deformation.



(4) Shear failure of joint (JS). The concrete around the compression comer in ajoint is crushed after the development of a crack S. No specimen in this studydemonstrated evidently this type of failure.

3.3 Load vs. Displacement Relationship

Some examples of Load vs. displacement curves are shown in Figure 5. Thedisplacement was measured as relative displacement between a point on beambars on the column face and the mid point of the column depth. In manyspecimens, the stiffness of the joint gradually declined due to an S-crack and/orV-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 severedeterioration in load resistance soon after the onset of a decrease in the stiffiiessof the joints.

7\,(kN)

TA19-3TA19-1

-TA13-

TA13-2

TB19-3

TB19-1 TB19-2

0 5 10Bar Displacement,5(mm)

15 200 400 600

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 thevertical development length L is plotted in Figure 6. Test results indicate thatacpTu increased linearly with increases in L& in TA series specimens. This meansthat tensile stress applied to the tails of the beam bars was transmitted to thetensile column bars in a joint accoring to the lapped bar-joint stress transmissionmechanism. Stress transmission in TA19-2L, with 90-degree column bar hooks,was higher than that in TA19-2, which had 180-degree hooks, and that in TA19-2A, without hooked column bars, was lower than that in TA19-2. The 7 in theTA 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 theTA specimens was larger because of the short lateral distance between the tailsand the column bars, and 2) the resistant moment on the reaction point expressed



by L^ x Tt was larger in the TA specimens than in the TB specimens because oftheir 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 columnhooks were situated further from the beam bars so that a premature crack alongthe tail of the hooked bar (V-crack in Figure 2) prevented the transmission of thetail 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 alongthe 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 raking-out failure in exterior beam-column joints in a middle story of a building to be

(1)

where TC (horizontal resistance of concrete) = 2L^ • b^ • a, s

TW (horizontal resistance of lateral reinforcement) =

L^ (horizontal development length of beam bar) = L + r + d/2, a» is the totalsectional area of lateral reinforcement crossing failure planes, b (effective jointwidth) = 5 + 0.6 b^ k» (coefficient of effective lateral reinforcement) = 0.7, k^(axial stress modification factor) = 1 + 0.020cr , cr, (concrete tensile strength) =t/"e%, (JQ is the column axial stress and not larger than 0.0$ e%, cr is the yieldstress of lateral reinforcement, and 6 is the strut angle. The concept of thisresistance mechanizm and the dimensional variables are explained schematicallyin Figure 7.

Bar tail

Radius ofbar bend Vertical

failure plane

tr

Inclinedfailure plane

Verticalfailure plane

when 2C=<6cg whenwhere - sin y (=336mm)

L ;develop.length(=200mm)

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 0.74 and the standard deviationwas 0.24. This unfavorable result was due to the fact that the equation



disregarded the peculiar characteristics of knee joints; i.e., the ultimate strengthdepended 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 jointswith raking-out failure taking into consideration these characteristics. In the newequation, the horizontal resistance of concrete 7 is modified, while thehorizontal 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 stressmodification 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 columnIt is assumed in equation (1) that the concrete tension stress on a pair of failureplanes inclined 45-degree above and below the beam bars provides the lateralresistance of concrete. However, in a knee joint, the failure plane was found toopen vertically above the beam bars, not along the inclined failure plane, and thisvertical crack provided any lateral resistance as it developed prematurely andultimately 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 resistanceThe tail portions of beam bars in an exterior beam-column joint in a middle storyplay scarcely any role because a part of diagonal compression stress is the sumof horizontal beam force and vertical column force which was supplied bybending moment of upper column. The tail portions of beam bars in a knee jointhowever need to share the vertical force instead of the upper column. Thisvertical force shared by the tails is transmitted first to the outside tensile columnbars in the joint and, through them, finally to a lower column. As the amount oftransmitted force grew in accordance with the tail length, and as the force waseffected by the anchorage type of the column bars, the horizontal resistance ofthe 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 bondstrength distribution shown in Figure 8, T (bond strength of tail bar) = 0.56-/~c%,and ly/is the total perimeter of the tail bars. The effective factor y is the ratio ofassumed to full z distribution areas, which were defined as the bond strength atpoint B of TA series was ^ for 90-degree hooks of column bars, rjl for 180-degree hooks and zero for steel-plate anchorage, and as the bond strengthbetween B-C zone was zero.

The calculated anchorage strengths cai? RK and their components are shownin Table 3, and the relation between these values and the experimental values is



plotted in Figure 9. The average of A, which is the ratio of experimental ultimatestrength 7 to calculated ultimated strength ,,,7, for all specimens was 1.012and the standard deviation was 0.085. Except for TA19-2CL, which had lowconcrete strength, the variance of A was less than about ± 10 %, thus it can besaid that the proposed equation is highly accurate for evaluating anchoragestrength. Expressing the effect of differences in column hook type on anchoragestrength by the effective bond strength at point B, was particulary helpful inestimating anchorage strength. In doing this, the value for the bond strengthbetween zone B-C in the TB series specimens, in which ISO-degree hooks ofcolumn bars were situated at a distance from the tail bars, was taken to be 0.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:

TA19-1 34.2TA19-2 34.2TA19-3 35.7TA19-2CH 44.7TA19-2CL 26.3TA19-2A 38.5TA19-2S 38.5TA13-1 35.8TA13-2 35.8TB19-1 39.1TB19-2 39.1TB19-3 35.7TB19-2P2 38.1TB19-2P3 23.0TB19L-2 36.9TB19R-2 23.0

(kN)299412516490431332468267364196199326284265207169

Tc(kN)449449455507391468468476476337337325336262329262

J. ttr f

(kN)393939393939393939262626661052626

(kN)488488495547431508508516516364364351402367356288

exp

cal0.610.8410.400.901.000.660.920.520.710.550.540.930.710.720.580.59

Tc(kN)212212217243186225225227227168168161166129163129

Tw(kN)444444444444444444444444881464444

Ta(kN)4114127016012493205288800840000

7 ,

0.5880.7300.8330.7300.7300.4561.0000.5690.6480.0000.0000.3290.0000.0000.0000.000

(kN)297397531447354362474299359212212289254275207173

exp

cal1.011.040.971.101.220.920.990.891.010.930.941.131.120.961.000.98

calTRK

100 200 300 400 500 600

Figure 8: Distribution of bond strength Figure 9: Relationship of **pTu and caiTu



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 wereonly 4 % for specimens TB19-2P3 and TB19R-2, which had the lowest concretestrength and did not depend on the bond strength of tail. Therefore the differencebetween the calculated and experimental results is certainly due to the problemof bond strength estimation.

4. Conclusion

We examined the anchorage behavior of beam-column knee joints usingcolumns with 90-degree hooked beam bars as study specimens. The testvariables 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 experimentalresults are as follow:

1) Anchorage mechanism of hooked beam bars in knee joints located in a topstory was quite different from that in exterior beam-column joints located in amiddle story.

2) Anchorage strength depended on stress transmission from tail bars tocolumn bars within the joint; not on the on bond behavior along the horizontaldevelopment portion of the beam bars.

3) Stress transmission increased as the tail length of beam bars increased, andwas effected by the positioning of the column hooks to beam bars.4) A modified equation for estimating the anchorage strength of 90-degree

hooked bars in kneejoints was derived from our previous equation used forbeam-column joints in middle stories. Our experimental results comfirmed thehigh accuracy of this modifided equation.

Acknowledgement

The authors gratefully acknowledge the financial support by the Building Centerof Japan (Grant-in-Aid for Scientific Research, 1997) and the experimentalworks by J.Iwanami and Y. Yamazaki who were Master course students ofHokkaido University.

References

1. Joh,O., Goto, Y. & Shibata, T., Anchorage of Beam Bars with 90-DegreeBend in Reinforced Concrete Beam-Column Joints, Proc. of the Tom PaulaySymposium "Resent Development Lateral Force Transfer in Building,"American Concrete Institute, SP-157, pp.97-116, 1995

2. Joh,O., & Shibata, T., Anchorage Behavior of 90-Degree Hooked Beam Barsin Reinforced Concrete Beam-Column Joints, Proc. of the 12th WorldConference on Earthquake Engineering, Acapulco, CD-ROM-3, 1996


i. introduction...anchorage behavior of 90-degree hooked beam bars in reinforced concrete knee...

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