N. Subramanian and D.S. Prakash Rao

Seismic design of joints in RCstructures A review

The failure of reinforced concrete structures in recentearthquakes in several countries has caused concern about theperformance of beam-column joints. IS codes do not includerecommendations on beam-column joints explicitly. Thereversal of forces in beam-column joints during earthquakesmay cause distress and often failure, when not designed anddetailed properly. The fifth revision of IS 1893 has broughtmore than 50 percent of the country under moderate and severeseismic zones. Under these circumstances, the detailing ofjoints assumes more importance. The behaviour and design oftwo-, three- and four-member beam — column joints in framedstructures are discussed; obtuse and acute angle joints areincluded. Detailing of the joints based on experimentalinvestigations is also explained. The specifications ofAmerican, New Zealand and Indian codes of practice areappraised. An equation for calculating the area of jointtransverse reinforcement has been proposed for the Indiancode, based on recent research.

The performance of framed structures depends not onlyupon the individual structural elements but also upon theintegrity of the joints. In most of the cases, joints of framedstructures are subjected to the most critical loading underseismic conditions. The failure of several reinforced concrete(RC) structures during the recent earthquakes in India aswell as other countries causes concern about the performanceof the beam-column joints 1, 2 .

However, despite the significance of the joints insustaining large deformations and forces duringearthquakes, specific guidelines are not explicitly included in

Indian codes of practice (IS 456: 2000 and IS 13920: 1993).While considerable attention is devoted to the design ofindividual elements (slabs, beams and columns), no consciousefforts are made to design joints in the absence of suitableguidelines. It appears that the integrity and strength of suchjoints are assumed to be satisfied by lapping the reinforcement.The design and detailing of joints play a crucial role inproviding ductility and strength required to sustain largedeformations and reversed stresses during earthquakes.

One of the basic assumptions of the frame analysis is thatthe joints are strong enough to sustain the forces (moments,axial and shear forces) generated by the loading, and totransfer the forces from one structural member to another(beams to columns, in most of the cases). It is also assumedthat all the joints are rigid, and the members meeting at ajoint deform (rotate) by the same angle. Hence, it is clearthat unless the joints are designed to sustain these forces anddeformations, the performance of structures will not besatisfactory under all the loading conditions, especially underseismic conditions. Post-earthquake analyses of structures,accidental loading or laboratory tests show that the distressin the joint region is the most frequent cause of failure, ratherthan the failure of the connected elements".

Considerable research and test results are reported onthe strength and behaviour of beam-column joints'"".Recommendations on the desip and detailing are alsoavailable, based on these results -2C . The behaviour of thebeam-column joints normally encountered in residential andcommercial multistorey buildings is discussed in this paper.Acute and obtuse angled joints are also included. The designand detailing procedures are summarised to provide

1

February 2003 * The Indian Concrete Journal 883

884

•

•Fig 1 Typical beam-column joint withample length of anchorage for beamreinforcement in the column but withincorrect placement

Fig 3 Poor quality concrete at thecritical region of the joint, obviouslydue to poor quality formworkcoupled with inadequate compaction

Fig 4 Poor awareness about jointdetailing; main reinforcement in thecolumn is bent to allow for change inthe cross section

Fig 2 The beam reinforcement is not anchored properly inthe structure

guidelines for satisfactory performance of joints under criticalloading conditions.

The approaches of the ACI and New Zealand differ indesigning the joints"' Is ' '7' '8 . The former does not considerthe joint stirrups while estimating the joint shear capacity,while the capacity of joint stirrups is accounted in the latterapproach.

Some of the design and construction practices peculiar toIndia are also discussed. The revised IS 1893: 2002 hasenhanced the lateral forces on structures considerablycompared to the previous version, which makes the designof joints imperative26 . As per this code, more than 50 percentarea of India falls under moderate and severe earthquakezones, thus signifying the importance of earthquake detailingof joints.

Requirements of beam-column jointsThe essential requirements for the satisfactory performanceof a joint in an RC structure can be summarised as follows 6'".

(i) A joint should exhibit a service load performance equalto or greater than that of the members it joins; that is,the failure should not occur within the joints. If at all,failure due to overloading should occur in beamsthrough large flexural cracking and plastic hingeformation, and not in columns.

(ii) A joint should possess a strength that corresponds atleast to the most adverse load combinations that theadjoining members could possibly sustain repeatedlyseveral times, if possible.

(iii)The strength of the joint should not normally governthe strength of the structure, and its behaviour shouldnot hinder the development of the full strength of theadjoining members.

(iv) Ease of fabrication and good access for placing andcompacting concrete are the other significantparameters of joint design.

Design and detailing of jointsThe problems of detailing and construction of beam-columnjoints are often not appreciated by designers. Because of therestricted space available in the joint block, the detailing ofreinforcement assumes more significance than anywhere else.Indeed, the conflict between the small size bar requirementfor good performance, and large size bars required for easeof placement and concreting is more obvious at the jointsthan anywhere else. This is particularly true at internal joints,

•

•

•The Indian Concrete Journal * February 2003

where the beams intersect in both the horizontal directions,or where large moments are to be sustained by theconnections. In the absence of specifications from thedesigners, site engineers often adopt expedient proceduresfor detailing, which are not always conducive for satisfactorystructural performance.

Some of the incorrect detailing practices adopted by siteengineers in India are illustrated in Figs 1 to 4. Fig 1 shows atypical beam-column joint with ample length of anchoragefor beam reinforcement in the column but with incorrectplacement. The beam bars are bent upwards instead ofdownwards; such disposition of reinforcement may causediagonal cracking, leading to shear failure of the joint'''. Thebeam reinforcement is not anchored properly in the structureshown in Fig 2; while the negative steel in the beam may beadequate to sustain the forces under the applied loads,inadequate anchorage may render the bars ineffective undercritical loading conditions (seismic loading, for instance). Fig3 shows poor quality concrete at the critical region of thejoint, obviously due to poor quality formwork coupled withinadequate compaction. Possibly, the site engineer was notaware of the significance of the joint, while allowing columnbars to be bent in the structure shown in Fig 4. The bendingof bars has damaged concrete at the joint, and destroyed thebond between steel bars and concrete. The crooked bars atthe column base may cause excessive stresses in the column,and lead to early distress, especially under lateral forcesinduced by earthquakes.

In all these cases, the joints do not appear to have beendesigned or checked for their performance; it is unlikely thatthey will behave satisfactorily under the critical forces inducedby earthquakes. It may also be noted that shear reinforcementis not usually provided in these joints; the column ties arealso not to the codal specifications 22 . Further, some of thebeam reinforcement bars lie outside the column bars, Figs 1and 2, which is not a correct practice; the beam reinforcementshould lie within the column reinforcement.

Because the joint block area is small relative to the membersizes, it is essential to consider localised stress distributionwithin the joints. A simplified force system may be adoptedin designing beam to column connections. The quantity ofsteel required is calculated on the assumption that steel reachesthe design yield stress, and the concrete its design compressivestress. Where local bearing or bond failure is expected, thelower of the two capacities for flexural and local failure shouldbe adopted based on experimental results. It is essential toprevent bond and anchorage failure within the joints,especially at the external joints, through proper design anddetailing practice.

17177

(a) Lateral loading (b) Gravity loading

Fig 5 Opening joints in a frame

Fig 6 Forces on an interior beam -column joints"

closing corners. The internal joints tend to open on one side,and close on the other depending upon the load applied.

The joints will be subjected to alternate opening andclosing forces under seismic loading. For closing corners, testshave shown that the usual detailing will be satisfactory.However, for an opening joint with the same details, theflexural efficiency may be only about 25 percent of thestrength of the members meeting at the joint"' In . Theefficiency of connections and joints is usually defined as theratio of failure moment of the connection or joint to thecapacity of adjoining members 12' 13 .

Typical corner joints in plane frames comprise twomembers, Fig 5, whilst in a three-dimensional frame,additional members, inclined to the vertical plane, may bepresent.

Internal jointsFor a four-member connection, Fig 6, if the two beammoments are in equilibrium with one another then noadditional reinforcement is required. Fig 6 (a) indicates theforce distribution in a frame with lateral loading (wind orearthquake loads) with two opening corners at locations B,and two closing corners at locations A. Diagonal cracks areliable to form along A-A unless stirrups are provided alongB-B, or the principal tensile stress is restricted. The stressesinduced on an internal beam-column joint are shown inFig 6 (b). The principal mechanisms of failure of such a jointare:

• shear failure within the joint• anchorage failure of bars, if anchored within the joint,

and• bond failure of beam or column bars passing through

the joint.

The joint is designed based on the fundamental conceptthat failure should not occur within the joint; that is, it isstrong enough to withstand the yielding of connecting beams(usually) or columns.

Seismic shear resistanceHorizontal shear force of resistance within the joint, Fig 6,can be calculated by 17'

V,, = (11, 7 + A,2) .fs,:+ C' — V,e, (1)

Corner jointsThe external joints(corner joints) of aframe can bebroadly classified v2into opening and AB2

closing corners.The corners thattend to open(increase theincluded angle) areshown circled inFig 5, while thoseat the right endstend to decreasethe included angle,and are termed

Potential failure planeA

(a)

As2f,

February 2003 * The Indian Concrete Journal 885

Tensile forcedeveloped in stirrups V,

where,A s, = area of tension steel in beam 1A,2 = area of compression steel in beam 2C' = compressive force in concrete = k f.k b xf, = factored yield strength which allows for over

strength (the value of fs, may be taken tobel .25 times fs„ the characteristic strength ofsteel)

b = breadth of the beamk, x = stress block parametersfa = characteristic strength of concrete

V „, = net column shear forceThis shear V,, is resisted by compressive strut action in the

concrete and the horizontal stirrups as shown in Fig 7.Conservatively, the area of horizontal stirrups can becalculated for steel of design strength f, (= fy/y ; y being thepartial safety factor of steel) as

= (2)

ACI 318RM-02 recommends that the cross-sectional areaof horizontal stirrups should not be less than either"

A,h = 0.3 (s hcf' / f,,h)[(Ag / Ac) - 1] (3)or

= 0.09 ski, / f,,, (3a)

In the case of circular columns, the volumetric ratio ofspiral or circular hoop reinforcement, p„ should not be lessthan

0.12 f'„/

(4a)

or0.45 f',/ fo [(A,/ Ac) -1]

(4b)

Compressive strutin concrete

Failure plane

LFig 7 Horizontal shear resistance In a beam -column joInt' 7

f,h = yield strength of the stirrups".

The American code also stipulates that the spacing of thetransverse reinforcement should not exceed the least of onequarter of the minimum column dimension, eight times thediameter of the smallest longitudinal bars to be restrained or300 mm.

Recently Saatcioglu and Razvi modified this equation totake into account the allowable storey drift ratio of 2.5 percentas

P, 0.35x ', Ag 1 1 P,

• fy, Ac X41)P0

where,= / (h, s) ], the area ratio of transverse

confinement reinforcement= Capacity reduction factor ( = 0.90 )

Po = Nominal concentric compressive capacity ofcolumnMaximum axial load on column duringearthquake

= Confinement efficiency parameter

1.= 0.15.1kb, /(ss, )

b, = Core dimension, centre to centre of perimeterties

s = spacing of transverse reinforcement along thecolumn height

s, spacing of longitudinal reinforcement,laterally supported by corner of hoop or hookof cross tie.

The amount of transverse steel required by equations (3)and (4) is linearly related to the compressive strength ofconcrete, fc,„ This may result in very large amount oftransverse steel when high strength concrete (for example100 N / mm 2) is used 20 .

Stirrups must cross the failure plane shown in Fig 7, andbe anchored at a distance that is not less than one-third of theappropriate column dimension on each side of it' 7 . Themaximum spacing of stirrups should not exceed thatappropriate to the adjacent column.

Often the reinforcement in a beam-column joint isseverely congested due to too many bars converging withina limited space of the joint (especially when the joint isconfined on all the four sides by connecting beams of equalsize). The beam and column sizes must be adopted carefully.In case the sizes of beam and column are the same, it will bevery difficult to place the reinforcement within the joint.Usually the bars are cranked in plan as shown in Fig 8 or inelevation, or in both in plan and elevation, which is not adesirable arrangement. Another undesirable practice is toplace the beam reinforcement outside the column bars Figs Iand 2. It is possible to avoid congestion of steel and theabove arrangements by selecting a little larger section forthe column, thereby reducing reinforcement fraction.

P,

P S

where,A,,, = area of stirrups and cross-ties,

= stirrup spacing,h, = dimension of the concrete core perpendicular

to transverse reinforcement underconsideration (centre-to-centre of perimeterreinforcement),

A, = gross area of the concrete section,A, = area of the concrete core (to the outside of

the stirrups),= concrete cylinder compressive strength,

(5)

886 The Indian Concrete Journal * February 2003

100

(a) (b) 40•

Beam Main beam bars

B i Stirrups

(a) Plan

M

(a) Opening corner(b) Elevation (b) Closing corner

Fig 8 Undesirable detail at beam-column joint

Bond stresses

High bond stresses may develop at interior beam-columnjoints, where bars may have a high tensile stress at one faceand a high compressive stress on the other face. Further, thepenetration of yield zone from the plastic hinge in the beammay reduce the effective bond. The loss of bond at the columnface results in high bearing stresses in concrete, which can besustained only if the concrete within the joint region is wellcompacted. The New Zealand code limits the bar diameterto 1 / 25 of the column depth for grade 275 steel (f r = 275 N/mm 2 ) or 1 / 35 for grade 380 steel (fs,. = 380 N/mm 2) to avoidbond failure within the joint' s .

ACI 318 RM-02 recommends that when the beamreinforcement extends through a beam-column joint, thecolumn dimension parallel to the beam reinforcement shouldnot be less than 20$ (4) = diameter of the largest column bar)for normal weight concrete, and 26$ for lightweight concrete".

Shear stressesACI 318-RM-02 restricts the shear stresses (in MPa) for jointsin order to avoid the possibility of failure in the compressionstruts to the values indicated below 34 :

(i).,joints confined on all the four faces : 1.7 Ai 41, (6)

(Ai) . joints confined on three faces or on two oppositefaces: 1.25 A, -Ni f ,( 7)

(iii) others : 1.0 A, .Ni f , (8)

where,

A i = Effective cross-sectional area of the joint, in aplane parallel to the plane of reinforcementgenerating shear in thejoint in m". The joint depthis the overall depth of thecolumn.

Fig 9 Forces in a two member connection I-joints 8

A joint is regarded as confined if members frame intoeach face of the joint, each member covering at least three-quarters of the joint face area".

External beam-column jointsTwo-member beam to column joints, which occur in portalframes and at the top of building frames, are probably themost difficult joints to design, particularly when appliedmoments tend to 'open' the joint. Opening joints also occurin corners of tank walls and in wing wall-to-abutmentjunctions in bridges.

The simplified force system for an opening corner joint,shown in Fig 9 (a), illustrates how both compressive (C) andtensile forces (T) must change direction by 90" as they 'goround the corner'. Consequently, and in the absence ofdiagonal tie member (D), the tensile forces (T) will tend tostraighten the reinforcement bars, which will then pull out ofthe corner at B. There will also be a tendency for thecompressive forces (C) to "push off" the concrete at cornerA.

The directions of all the forces are reversed for a closingcorner as indicated in Fig 9 (b), and consequently the diagonalmember D will be in compression. The resulting diagonalcompressive force can be resisted by the concrete withoutreinforcement, and hence the joint strength is more readilyachieved than with an opening corner.

A number of reinforcement layouts have been suggestedfor the two-member beam - column joints. The efficienciesof four such arrangements of reinforcement, shown in Fig 10for opening corners are shown graphically in Fig 11 8". Theefficiencies of all the joints are not satisfactory when acting as

60 (d))

(c)(b)(a))

0.5 1.0 1.5 2.0 2.5 30

Tension steel. percent

Where a beam frames into a supportof larger width, the effective width of thejoint should not exceed the smaller of:

(i) beam width plus the joint depth,

(ii) twice the smaller perpendiculardistance from the longitudinal axisof the beam to the column side.

is

20

Type ofdetail

(Fig 10)

0

(c)

Fig 10 Basic reinforcement detailsfor two-member connections 8

(d)

Fig 11 Efficiency of opening corners"

February 20(13 * The Indian Concrete Journal 887

T

(a) Stirrups along AB

T

C

(b) Stirrups normal to AB

C=T

= 25 - 40percentwith diagonalstirrups ,1 = 75percent

PotentialFailure crack

Spiral idealisedinto 3horizontallegs + 3verticallegs

a) Loop detailCrack cutslooped/hookedbars normally at1 and 2

(b) Two U - c) Vertical (d) Cross diagonal spiralhooks stirrups As) =::14 (o)' x cos45 xAsj =n/4 (0)' x 2 Asj =■■/4 (0 2 x (3+3) legsbars x 2 cuts cos45 x 3stirrups

x 2 legs

0

Fig 12 Diagonal stirrups to be used with basicreinforcement details^'°

opening corners, though they proved to be satisfactory forclosing corners8 . However, it has been found that the jointefficiency can be enhanced by including diagonal stirrupsalong AB or perpendicular to AB as shown in Fig 12 "I ' m . Thediagonal stirrup along AB as shown in Fig 12 (a) has thegreatest effect on joint strength. Although the stirrupperpendicular to AB, Fig 12 (b) has less effect on joint strength,its omission may lead to unacceptably wide cracks 8 .

Half the area of main reinforcement should be providedas inclined bars, for a main steel ratio up to 1.0 percent andequal to the main steel for 1.0 to 1.5 percent ". If the mainreinforcement is less than 0.4 percent, the inclined bars maybe omitted altogether. The diagonal stirrups may be includedin a corner chamfer, if provided. It is also desirable to restrictthe main reinforcement percentage to about 0.8 to 1.2 percentin order to avoid failure in the corner 7,10 .

The area of radial hoops required is approximatelygiven by

[(fs• / +(h 1 I It, yi(A,,/,i) (9)

= yield strength of the radial hoops with 'n' legs,

rack

1 = 85 - 130percent

f, = yield strength of the main steel,and h2 = beam depth and column width respectively,

and= the area of tension steel in the beam.

It is also suggested that the radial hoops are to be providedonly when the flexural steel content exceeds 0.5 percent.

It may be noted that the problem is not so severe in obtuseangled corners, since the internal forces are not turnedthrough such a large angle.

Desayi and Kumar concluded from the tests on six typesof L-joints shown in Fig 13 that details B, C and E developedthe full strength of the members (efficiency — 100 percent) 74 .They found that details D and F displayed about 62 percentaverage efficiency ( ), while detail A was only about 40percent efficient. They also observed that detail C developedthe minimum crack width (about 0.2 mm) at working loads.

Desai and Kumar proposed the following equation forpredicting the ultimate moment capacity of the joint, basedon the area of steel resolved normal to the direction of thepotential crack, A, (A,, may be calculated as shown in Fig 14)

= 0.607f,, bd2 + 0.566f, A, d

(10)

wheref.,, = split cylinder tensile strength of concrete,b = breadth of the beam at the jointd = effective depth at the joint

A,1 = area of steel across the crack in the jointfy = yield stress of the joint steel

The above equation was found to give reasonablyaccurate values when compared with their experimentalresult?.

T-jointsT-joints in cast-in-situ reinforced concrete may be classifiedinto two main groups:

(i) connections between members of small width, forexample between a column and a beam,

(ii) connection between walls and slabs (between bridgeslab and the supporting pier), where the joints havelarge longitudinal dimensions.

•a,,where,

fei

= 75 - 85percent

0

(a) Detail A simple detail (b) Detail B Loop detail (c) Detail C Two u- hooks detail

(i) (ii) (iii) f\

(Ii

(d) Detail D Vertical

e ) Detail E Simple detail (f) Detail F Cross diagonalstir ups details spiral detail

(i) 2-legged stirrups: 2 in D1 3 in 02. 4 in D3 and E

002 O 6 mm hanger bars(iii) Circular spiral; pitch = 100.0 mm and diameter = 75.0mm

11 = 60percent 11 100percent = 70percent

Fig 13 Six types of L-joints considered in Reference 24

Fig 14 Typical calculations for A si24

888 The Indian Concrete Journal a February 2003

101 A„. 0.5 A,

11=riarog kmmitimm

U - type bars .1.11

Em. - V'

(a)=NNE

(b)

a) Detail with L -bars (b) Detail with U-bars

Ld

A,l,

1

e;\." Crack T

t, C

T = C

(a) Member forces

(b) Forces In joint block

Fig 15 Forces in a three member T-joint 6

In the latter case, due to constructional difficulties, stirrupsshould be avoided.

Such joints comprise closing and opening corners on eitherside of the joint. Similar to two member joints, here also theclosing corner is stronger than opening corner. The forcesacting on a T-joint are shown in Fig 15. The bent tensile barinduces contact pressure under the bend, which is balanced bya diagonal compression strut with the compressive force F,.Diagonal tensile stress F, in the joint may cause a diagonalcrack as shown in Fig 15 (a).

Two details, one comprising L - bars from the beam tolower column, and the other with U-bars, are shown inFig 16. The L- and U - bars should have sufficient overlapwith main steel to develop full anchorage force. If the depthof the beam is greater than 600 mm, SP 34 recommends theuse of U - bars as shown in Fig 16 (b) 13 . Experimental evidenceindicates that such details develop the ultimate moment forbeam reinforcement percentage up to 1.2 percent, unlessdiagonal ties in the form of stirrups (as shown in Fig 12) areprovided b . Inclusion of such diagonal ties will result incongestion of reinforcement. When they are not includedand where beam reinforcement percentages exceed 1.2percent, the following design procedure is recommended inorder to avoid unacceptably wide diagonal cracks in the joint s .It has also been found that the geometry of the joint has aprofound effect on its behaviour s . For example, deeper beamsframing into shallow columns do not perform satisfactorily.

The Maximum shear stress on the joint block is taken as s

= 1.5 T / A, (11)

where,T = tensile force in the beam reinforcement,

A, = area of the joint block.

The shear stress (Equation 11) should not exceed themaximum permissible shear stress given by

qa = 11.1: ff (12)

wheref, = tensile strength of concrete,f = compressive stress in the column.

The values corresponding to the design procedure areadopted in the above equation (limit state or working stress).

This limitation is in effect, a limitation on the beam moment,which can be safely transmitted by the joint block.Alternatively, the principal tensile stress as given below couldalso be checked'.

Principal tensile stress = f. / 2 +.11(1 / 4)+ q 2 „,..x (13)

The value given above should not exceed the design tensilestrength of the concrete C at the serviceability limit state. Inclinedreinforcement should be provided at the upper corner in orderto reduce cracking at the junction. Only the ties that are situatedin the outer two-thirds length of potential diagonal failure crack,which runs from corner to corner of the joint, should beconsidered to be effective, Fig16 (b). Thus, if V s is the joint shearto be carried by the ties,

A, = [0.5 Vs s/(d f,)] (14)

where,A, = total area of the tie legs in a set making up

one layer of shear reinforcement, andd = effective depth of the beam.

Anchorage of bars at jointsFrequently, joints are the weakest links in a frame due to lackof adequate anchorage for bars extending into the joints fromthe columns and beams. Unless special measures are takento move the plastic hinge region away from the face of thecolumn, the onset of yielding in the beam will penetrate thecolumn area. For this reason, Key 17 suggests that theanchorage length of beam bars, anchored within the columnarea in external joints, should be reduced by the lesser of halfthe column depth or 10 times the bar diameter as illustratedin Fig 17. Park and Paulay suggest that for earthquakeresistant structures the development length of the beamreinforcement should be computed from the beginning of90° bend, rather than from the face of the column ° . In deepcolumns and whenever straight beam bars are preferred,mechanical anchors could be used°, Fig 17(b). The top bars ina beam, passing through holes in a bearing plate may bewelded to a steel plate° . One solution to the difficult problemof anchoring beam bars in external joints (especially forshallow columns) is the use of projecting beam stubs as shownin Fig 18. This type of detailing has been adopted in

Fig 16 Reinforcement for three member connections 4 . 23."

1 I

February 2003 * The Indian Concrete Journal 889

Not less thanh,/2 or 10d Mechanical

anchorage

Bar anchoragemeasured fromthis line

(a) (b)

Fig 17 Anchorage of beam bars at an external joint"

_Emmimml■••

Christchurch, New Zealand 174 . However, architects mayobject to this solution on aesthetic grounds.

Indian code provisonsThe Indian code on plain and reinforced concrete, IS 456:2000,does not contain explicitly any provision for the design ofbeam-column joints . However, the Indian Code for ductiledetailing of reinforced concrete structures subjected toseismic forces suggests that the transverse reinforcement asrequired at the end of column shall be provided throughoutthe connection22 . The area of reinforcement at the end ofrectangular column is given by22 .

f A,=0.18sh —d-(-= —1)

f,„

For circular column this equation is given by

A th = 0.09sD, —1)f A

(16)

where= longer dimension of the rectangular

confining loop measured to its outer faces;but not to exceed 300 mdiameter of core measured to the outside ofthe spiral or loop.

It is to be noted that these equations are based on the ACICode provisions given in Equations (3) and (4). In fact, theACI Code correctly assumes that the spiral hoops in circularconcrete columns provide better confinement than therectangular ties of rectangular columns and gives a coefficientof 0.45 for circular columns as compared to the value 0.3given in Equation 3. But the Indian code wrongly gives alesser value for the coefficient for circular column as comparedto the rectangular column and gives arbitrary values for thesecoefficients.

It is of interest to note that IS 13920 also specifies that thespacing of hoops shall not exceed one fourth of minimummember dimension but need not be less than 75 mm normore than 100 mm. The code also suggests that if the connec-tion is confined by beams from all four sides and if eachbeam width is at least 75 percent of column width, the amountof transverse reinforcement may be reduced by 50 percent22 .In this case, the spacing of hoops shall not exceed 150 mm. In

addition IS 1392Ustipulates that thebeam reinforce-ment in an externaljoint should bewell anchored(with an anchor-age length, be-yond the inner faceof the column,equal to the devel-opment length intension plus 10 Fig 18

times the bar di- external joint with stub beam"Reinforcement detail at an

ameter minus theallowance for 90° bend(s)). In an internal joint both face barsof the beam should be taken continuously though the col-umn.

SP 34 recommends that if the area of reinforcementexceeds 1.0 percent, diagonal stirrups as shown in Fig 12 (a)as well as splay steel as shown in Fig 12 (b) should be providedalong with a corner haunch 23 . These recommendations are inconformity with international practices.

Bearing stresses inside bendsIS 456: 2000 recommends that the high bearing stressesinduced in the concrete on the inside of bends can be estimatedas 27

= F,,,! (r 4)) (17)

whereF,,, = tensile force in the bar at ultimate loads,r = internal radius of the bend,0 = bar diameter.

The Indian code also states that this bearing stress shouldnot exceed 1.51: k / [ 1 + (2 4) lab)] at the limit state 27 .

Where a l, = centre-to-centre distance between the barsperpendicular to the plane of the bend; for a bar adjacent tothe face of a member, a,, should be taken as the cover plus bardiameter (1).

High strength concrete columnsRecent research on high-strength concrete column indicatesthat the strength gain due to confinement is independent ofconcrete strength and percentage of strength gain is lowerfor high strength concretes'''. Hence high strength concretecolumns may require proportionally more confinement toattain prescribed deformabilities. It has also been found thathigh strength reinforcement (with yield strength of 600 MPa)confines high strength concrete columns effectively'''.

Deficiency of ACI and IS code formulaeIt has been shown recently that the formula suggested bythe American code Equation (3) and (4) do not provide goodcorrelation with experimental data, producing over-conservative quantities of transverse reinforcement forspirally reinforced circular columns". Similarly forrectangular and square columns also the correlation withexperimental results was poor". The columns with the same

(15)

Secondary bars

890 The Indian Concrete .Journals February 2003

(A,„) inclinedreinforcement

Main reinforcement U bars

(a)

amount and spacing of confinementreinforcement showed significantlydifferent strength and deformability whenconfined by different arrangements oftransverse reinforcement'''. While a squarecolumn with four corner bars and tied withperimeter hoops showed the worstbehaviour, columns confined with well

M

distributed longitudinal reinforcement,laterally supported by cross-ties, over- Figlapping hoops, or both, showed significantlyimproved performance.

Based on these observation, Saatchioglu and Razvi' 9proposed the following equation which takes into account thetie spacings as well as the spacing of cross-reinforcement in thecross-section plane, Si

A,, 0.2sh,[f ,./(f,.„k,)

where,

k2 = 0.1511h! /(ss, ) I

with [(AR / A, ) —1 .1 � 0.3where,

b, core dimension, centre-centre of perimeterties

s = centre to centre spacing of transversereinforcement along column height

s, = centre-to centre spacing of longitudinalreinforcement, laterally supported by cornerof hoop or hook of cross-tie

The above formula is found to correlate with test resultsof both high strength and normal strength concrete circularcolumns as well as square columns. Hence the authors proposethis equation for the Indian Code.

Note that for a displacement based design, the effect ofaxial force should also be considered as given in Equation 5.

Obtuse angled and acute angled cornersCorners with obtuse and acute angles occur in bridgeabutments between the wing walls and the front wall and infolded plate roof. Tests on V shaped beams with 135° to 145°corners have been conducted by Nilsson and Losberg" andWahab and Ali 12 for various reinforcement details as shownin Fig 19. Based on these investigations they concluded thefollowing.

(i) The efficiency of the joint detail is improved wheninclined bars are added to take up the tensile force atthe inner corner. Loops with inclined bars Fig 19 (d)are preferable for continuous corners between lightlyreinforced slabs (since their efficiency is of the orderof 80 to 130 percent).

(ii) The efficiency of corners improved significantly whenthe thickness of the adjoining members were different.(The efficiency increased to 197 percent when the

Ali12

thickness of one leg was increased from 100 to 300mm). Further, the mode of failure changed fromdiagonal tensile failure to flexural failure.

(iii) The efficiency of the corner increased by about 32 percentwhen the length ratio of the two legs was changed from1 to 2.

Similar conclusions are reported by Martin'. The mainreinforcement should be restricted to about 0.65 to 1.0 percentof the section in order to avoid brittle failure of the corner'''.If the area of inclined reinforcement, A,, is the same as that ofmain reinforcement, the main reinforcement percentage mayhe increased to about 0.8 to 1.2 percent. If the reinforcementpercentage is higher than this, the corner must be providedwith a reinforced haunch and stirrups".

Nilsson and Losberg also tested 60° acute angle specimensand observed that the failure had the same characteristics asin the tests with 90° and 135 ° corners". Based on their tests,they suggested a reinforcement layout shown in Fig 20. Theinclined reinforcement in acute angled corner is laid in ahaunch. The length of this haunch is at least one half thethickness of the wing wall and the reinforcement is less than0.5 to 0.75 percent, and at least.equal to the thickness of wingwall when it is about 0.8 to 1.2 percent. The bars must not bespliced in the corner region. Further, recesses or openingsshould not be made at or in the immediate vicinity of cornersor joints, since they reduce the strength and stiffness of theconnection considerably.

Summary and conclusionsStructural joints in a rigid frame should be capable of sustainingforces higher than those of the connecting members.However, while beams and columns are designed and detailedwith considerable care, the same cannot be said about thejoints in RC rigid frames. The structural behaviour will be

Fig 20 Reinforcement details for obtuse and acutecorners"

A jA

Efficiency

M 85percent

M 90percent

M 102percent

Efficiency

M 139percent

M 120percent

19 Reinforcement details tested by Wahab and

February 2003 * The Indian Concrete Journal 89I

different from that assumed in the analysis and design, if thejoints are incapable of sustaining the forces and deformationsinduced due to the transfer of forces among the membersmeeting at the joint.

Some of the common incorrect design and constructionpractices of beam-column joints are explained along withpossible remedies. Various types of joints, their behaviourunder lateral forces are disctissed; experimental results arealso appraised. Structural joints in frames and slabs areincluded.

Especially, opening of joints has to be considered properlysince it will result in diagonal cracking of the joint. Suchopening of joints occurs in multi-storeyed structures due tolateral loads. The discussions presented pertain to seismicforces, but are of general nature and can be applied tostructures subjected to lateral forces. The behaviour of varioustypes of joints is discussed in this paper, along with designand detailing aspects. An equation for calculating the area oftransverse reinforcement in the joint has been proposed basedon recent research. This equation is applicable to both normalstrength and high strength concretes. A comparison of thecodal equations show that the values determined by Indiancodes are unsafe and there is an urgent need to modify theIndian code provisions.

References1. SAA It.R.X;LU, M., et al, 1999 Turkey Earthquake: Performance of RC Structures,

Concrete International, March, 2001, Vol. 23, No. 3, pp. 47 - 56.

2. RAI, D. C. and SETH, A. E-conference on Indian seismic codes, The Indian

Concrete Journal, June 2002, Vol. 76, No. 6, pp 376-378.

3. MARTIN, 1. Full scale load test of a prestressed folded plate unit, Journal of the

American Concrete Institute, December 1971, Vol. 68, No. 12, pp. 937-944;Discussions, Vol. 69, No. 6, June 1972, pp. 349-356.

4. SOMERVII LE, G. and TAYI OR, H.P.J. The influence of reinforcement detailing onthe strength of concrete structures, The Structural Engineer, January 1972,Vol.50, No.1, pp. 7-19; Discussions, Vol. 50, No. 8, Aug. 1972, pp. 309-321.

5. BENNET, E.F.P. and TRENRERTH, R.J. Column load influence on reinforced concretebeam-column connections, Journal of the American Concrete Institute, Vol.69, No. 2, February 1974, pp. 101 - 1(19.

6. PARK R., and PAULAY, T. Reinforced Concrete Structures, John Wiley Sr Sons,New York, 1975.

7. PRAKASII RAO, U.S., Detailing of reinforcement in concrete structures, The

Indian Concrete Journal. December1984, Vol.58, No.12, pp 348-353, January1985, Vol.59, No.1, pp 22-25.

8. HoLmEs, M. and MARTIN, L.H. Analysis and Design of Structural Connections:Reinforced Concrete and Steel, Ellis Horwood Ltd., and John Wiley Sr Sons,Chichester, 1983.

9. BHATIACHARYA, S. and SENGUYIA, B. A study on core concrete within the beam-column junction, The Indian Concrete Journal, August 1990, Vol.64, No.8, pp.380-384

10. MAYFIELD, B., KONG, K.F. and BENNISON, A. Corner joint details in structurallight weight concrete, Journal of the American Concrete Institute. May 1971,Vol. 65, No.5, pp. 366-372

11. NassoN, I.H.E., and LOSIIERG, R. Reinforced concrete comers and joints subjectedto bending moment, Journal of Structural Division, American Society of CivilEngineering, June 1976, Vol. 102, No.ST6, pp.1229-1253

12. Atiout,-WAI mu, H.M.S., and AI.I, W.M. Strength and behaviour of reinforcedconcrete obtuse corners under opening bending moments, ACI StructuralJournal, November - December 1989, Vol. 86, No.6, pp. 679-685.

13. Skerrace, E., Srtosso, J. ANDERSEN, N.1-1., and NIELSEN, T.B. Concrete framecorners, Journal of the American Concrete Institute, November - December 1984,

Vol. 81, No.6, pp. 587 - 593.

14. Building code requirements for structural concrete an.i '11■Nell tory, ACI318RM-02, American Concrete Institute, Farmington Hills, Michigan, 2002.

15. _Recommendations for design of beam-column joints in monolithic reinforcedconcrete structures, American Concrete Institute, ACI 352 R - 85, AmericanConcrete Institute - American Society of Civil Engineers, Committee 352,Detroit, 1985.

16. Standard method of detailing structural concrete, Institution of StructuralEngineers / Concrete Society, London, August 1985.

17. Kay, D., Earthquake Design Practice for Buildings, Thomas Telford, London,1988,p. 218.

18. Concrete structures standard, Part 1 and 2, Code and commentary on thedesign of concrete structures, NZS 3101- New Zealand Standard, 1995, NewZealand, 1995.

19. SAArcioca.u, M., and RAZVI, S. R., Displacement based design of reinforcedconcrete columns for confinement, ACI Structural Journal, January-February2002, V. 99, No.1, pp. 3 -11.

20. ALAMEDDINE, F., and Et 'SAKI, M., R. High strength RC connection subjected toinelastic cyclic loading, Journal of Structural Engineering, ASCE, March 1991,Vol. 117, No.3, pp. 829-850.

21. Indian Standard code of practice for earthquake resistant design andconstruction of buildings, IS : 4326-1993, Bureau of Indian Standards, NewDelhi, 1993.

22. _Indian Standard code of practice for ductile detailing of concrete structuressubjected to seismic forces, IS : 13920 -1993, Bureau of Indian Standards, NewDelhi, 1993.

23. Handbook on concrete reinforcement and detailing, SP: 34 (S&T) -1987, Bureau of Indian Standards, New Delhi, 1987.

24. DESAYI, P. and KUMAR, A. Detailing of opening L joints in RC frames, Proceedingsof the National Seminar on Detailing of RC and Steel Structures, Association ofConsulting Civil Engineers, Madras, 22-23, December 1989.

25. TAYLOR, H.P.J., and CI ARKE, J.L., Some detailing problems in concrete framedstructures, The Structural Engineer, Jan. 76, Vol. 54, No.1, pp. 19-32.

26. Indian Standard criteria for earthquake resistant design of structures, Part1 General provisions and buildings, IS 1893 : 2002, Fifth revision, Bureau ofIndian Standards, New Delhi, 2002.

27. Indian standard code of practice for plain and reinforced concrete, IS 456: 2000, Fourth Revision, Bureau of Indian Standards, New Delhi, 2000.

Dr N Subramanian is the chief executive ofComputer Design Consultants, Chennai. Thehighlights of his professional career of 20 years includedesigning multi-storey concrete buildings, steeltowers, industrial buildings and space frames. Hehas published more than 150 technical papers inseminars and journals, and published 17 wide sellingbooks. He is a life fellow of several professional

bodies, including the American Society of Civil Engineers. He isthe former vice-president of the Indian Concrete Institute and theAssociation of Consulting Civil Engineers (India). He is therecipient of the ACCE-Nagadi Award (for his book on spacestructures) and Tamil Nadu Scientist Award.

Prof D.S. Prakash Rao is presently working in theUniversity College. of Engineering (Autonomous),Osmania University, Hyderabad. He has over 30years experience in research, design and teaching,and worked earlier with the Structural EngineeringResearch Centre, India, and a few educationalinstitutions in India and overseas. He also worked

with the Institut fuer Massivbau, University of Stuttgart, Germanyas a DAAD fellow. He has published over 150 research papersand a few books. His current research areas include seismicanalysis, expert systems and theory of reliability.

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892 The Indian Concrete Journal * February 2003