INTERNATIONAL JOURNAL OF SCIENTIFIC & TECHNOLOGY RESEARCH VOLUME 9, ISSUE 03, MARCH 2020 ISSN 2277-8616

Seismic Strengthening of R.C Beam-Column

Joint Using Post Installed Headed Anchors
K. Pdmanabham, K. Rambabu
Abstract: Post Installation of Headed Anchors (PIHA) is an advanced technique proposed in this study for structural strengthening of R.C beam column
joints (BCJ). Previous research work on seismic damage of joints are widely correlated with shear deformation and bond slip of anchored reinforcement
in joint core. To mitigate complex issues of reinforcement congestion, anchorage, fabrication and placement of reinforcement in congested geometry of
BCJ, this novel technique of ―Post Installation by Headed Anchors‖ (PIHA) is proposed in this paper. It is an effective measure contributing to enhance
implicit properties of shear, stiffness, confinement and ductility of joint core. This method gives viable solutions to conventional design practice of Precast
and Cast-in place joints .Headed anchors provides good supplement to hooked anchorage system for improved shear, anchorage and ductile properties
by delay the ultimate failure of joint. The state of headed-bars considered in this study is of bonded and un-bonded conditions with concrete. This paper
focused on analytical aspects of proposed PIHA system so as to evaluate its strength and parametric influence against seismic shear strengthening of
BCJ. Principle observations made in this study are ―Theory behind post installation, Fastening techniques, Force transfer mechanism, Failure modes,
Seismic suitability of anchors, and Implicit strengthening of joint core.

Index Terms: Beam-Column joint, Fastening Techniques, Headed Anchors, Implicit Strengthening, Post Installation, Seismic behavior.
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1 INTRODUCTION The observations expressed that seismic joints with straight or

The current research on Seismic Beam-column joints (BCJ) hooked bars of stirrup confinement does not exhibit good
are based on explicit strengthening techniques such as seismic response and results large shear deformation during
Section enhancement, Fiber-wrapping, Metal stripping etc. It is cyclic loads. Experimental studies by Metwally E.L,Chen-1988
more significant that during seismic action, the implicit [1]. Allath, Kunnath.1995 [2] expressed that shear deformation
strengthening of joint enhance shear and ductile properties of and reinforcement anchorage in BCJ are identified problems
joint core. Inspiteof this, the current seismic practice of BCJ which associated with geometric configuration and detailing
believed to be more conservative and accordingly due aspects of reinforcement. Studies conducted by Kaung J.S-
considerations are assigned by design codes for discrete joint 2006 [3] Hayashi-1994 [4] expressed that anchorage of beam
conditions of external BCJ as it possess more vulnerability of reinforcement significantly influence shear performance of
high seismic shear. In this process, joints are subjected to joint. In this context, studies made by Park and Millburn-1983
complex force transfer mechanism .Most of the critical failures [5], Kitayama , Otani and Aoyamu-1987 [6], Park and Ruitong -
in joints are happened by shear deformation and bond lose. In 1988[7], Joh-Goto-Shibata-1992[7] discussed on size
this context, the current seismic codes are unable to establish limitations of anchored reinforcement, during relocation of
the real time behavior of joint failures .This is due to most of plastic hinge away from joint region . Wallance 1998 [8]
design codes are established on strength based system and is expressed that headed bars in joint core was well supported
far away to establish joint on performance based system as it the strut and tie mechanism. It facilitate ease of fabrication,
suggested to impeded good energy dissipation (MJN Priestly- concrete placement and enhance the performance compared
1975). And most of the designers intended to validate strength to hooked bars. But most of this studies are unable to
based joint performance by displacement approach .This is establish rigid condition of joint under seismic action, as its
quite contradictive approach and hindered the prospects of design considerations are based on (Bernoulli) flexural beam
joint design. The constructability of joints is another important theory of strength based system under inelastic deformation.
issue where the engineers often failed to establish correct In this scenario, ductile detailing of hooked anchorage system
detailing aspects of steel in joint core due to its confined gives deviated results of real design practice. anchors and
geometry. In this context the effectiveness of conventional detailing aspects to mitigate design issues of BCJ
hooked anchorage system is unable to restrict the fracture
mechanism of joint core .These flaws need to address by 1.2Current Research
theoretical means and modeled for real time failures of joints. Eligehausen.R-2006 [9] conducted a wide range of pullout
Park. R & Pauley. T [1975], recommend usage of mechanical tests on headed anchorage system to study fracture failures of
cast in-place R.C joints. He concluded that headed bars shows
1.1Previous Research good convergence with Strut-Tie mechanism and prevent or
Research community found the significance of beam delays the premature failure of joints. Since headed anchors
reinforcement and its anchorage mechanism in joint core. posses good seismic energy dissipation and stable CCT
(compression-compression tension) node conditions its
strength is more than hooked anchors (Fig.4). Hence use of
headed anchors are more vigilant at discrete joint conditions
• K.PADMANABHAM is a Research scholar, currently pursuing PhD in (External joints). ACI 374.1-0515 suggested on substitution of
Civil engineering, Andhra University, Visakhapatnam, India. E-mail: hooked anchors by mechanical (headed) anchorage system
kuncha2001@gvpce.ac.in gives an appropriate or better results of joint performance.
• K. RAMBABU is a Professor of Civil engineering and Research guide
with Doctorate in Civil Engineering(Structures),currently working at
Park and Pauley-1989 stated that the functionality of shear
College of Engineering, Andhra University, Visakhapatnam, India. reinforcement is more crucial than confinement under pure
shear conditions, of joint core. Walker- 2002 [10] expressed
that during low deformation of BCJ (drift < 1.5%) the strength
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and stiffness degradation of joint is nominal, and shear

strength of joint may consider as less than 10(fck) ½ (Psi).
Noguchi, Kashiwa Zaki -1992 [11] expressed that during large
deformation of joint (drift > 2%) confinement is more
significant. Fujii, Morita-1992 [12] noted that degradation of
shear rigidity is accelerated when the shear strains of joint
reaches 0.5%. Oka and Shiohara-1992 [13] stated that shear
strength of joint does not possess linear relation with concrete
compressive strength .Hayashi, Teraoka -1994 [4] expressed
about post failure conditions of joint significantly influenced by
amount of transverse reinforcement provided in joint core.
Krishna. P & Ghimire -2019 [14],[15] proposed an expression
for calculation of shear strength of joint by headed anchorage
system .
The participation of compression strut (by concrete) and
The results shows that anchorage strength of headed bars are
tension tie (by steel) is more crucial in STM formation, as it
proportional to [compressive strength of concrete]0.24 and the
significantly influence the strength and performance of joint
contribution of confinement is proportional to area of
core. It is further depends on load history ,boundary
confinement steel arranged parallel to headed reinforcement.
conditions, state of concrete (cracked or un-cracked) and
Sung Chul Chun,-2019 [16] proposed a beam-column joint
detailing aspects of steel in joint core .The strength of
model for anchorage strength of headed bar. The results
compression strut, steel ties and node junction shows (Fig.4).
shows that contribution of head bearing is not related to
During force transfer mechanism the detailing aspects of
embedment depth of headed bar. Experimental studies of
reinforcement should meet the strain compatibility of joint
Chutarat, Aboutaha-2003 [17] expressed the effect of cyclic
concrete within joint core. To meet external force action,
loads on headed fasteners and concludes that headed bars
headed anchors develop implicit shear resistance by head
are more effective than hooked anchors, and reduce the bond
bearing and bond resistance of bar which depends on type of
length as is possess both bearing resistance of head and bond
fastening and installation methods. It is comprised by (i)
resistance of bar both mechanisms helps to relocate potential
plastic hinge formation away from joint core. Experimental
studies by Vaibhav. R. Pawar -2017 [18] concludes that
headed anchorage shows satisfactory seismic performance
against shear deformation. Further the study observed that no
brittle failure of concrete occurred in joints of headed bars , if
the ratio of head size and bar diameter (Abrg/Ab) is at least 2.5
and the minimum embedment depth of bar is 11db (db: Bar
diameter).Also large head size bars (Abrg /Ab > 4.2) exhibit
higher anchorage strength than small heads(Abrg/Ab=2.6-2.9).

2 OBJECTIVES

Objectives are configured to verify the analytical aspects of mechanical, (ii) friction, (iii) bonded anchorage system
headed anchorage system under Post installation of
fastening techniques. The emphasis is based on physics Mechanical anchorage (Fig.2a) of headed bars comprise
related to (i) Force transfer mechanism, (ii) Failure significant role in force-transfer mechanism within joint core.
conditions of joint, (iii) Seismic suitability of headed The force transfer mechanism constituted by interlock action
anchors, (iv)Implicit strengthening . of bearing between the headed fastener and concrete in the
anchorage system. This system is useful for both Cast-in-situ
3 FORCE TRANSFER MECHANISM AND (headed studs, anchor bolts and anchor channels) and
FAILURE MODES Precast concrete where the fastening system proceed by
screw anchors or undercut anchors. Frictional anchorage
(Fig.2b) results by generation of expansion forces, that gives
During seismic action, headed anchorage system of Beam- frictional resistance at interface of anchor and concrete. During
column joints are subjected to lateral action of cyclic forces this process, expansion forces generate the frictional
(tension or compression), transverse shear, or combination of resistance between anchor and surface of hole. The generated
both (Fig.1).Transfer of this explained by Strut-Tie Mechanism frictional resistance forces are in equilibrium conditions with
(STM) of discrete conditions shown in external beam-column the applied tensile force. Chemical bonded anchorage (Fig.2c)
joint.(Fig.4). is the most conventional method of Post-installed fastening
system. It is also termed as bonded or adhesive anchoring
which refers the anchorage system comprised by bond action
between steel element (threaded or deformed bar) which was
installed in drilled hole and developed bond between steel and
concrete.

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4 PARAMETRIC INFLUENCE ON HEADED than five times the least cover dimension of anchored bar
ANCHORS (Fig.5).
The performance evaluation of seismic joints is based on the
development of interaction mechanism between inelastic
behavior of beam and elastic behavior column at joint core. In
this context, Park.R& Pauley.T concluded that unless
significant axial load (P) act on column[P <(0.10-0.30)fck.Ac] the
design of seismic BCJ should based on assumptions that no
shear force is resisted by concrete and shear the transfer
through diagonal compression strut in joint core is obviated.
Provision of steel reinforcement is considered in shear
resistance mechanism. But this argument is deviated as the
research studies of joint failures expressed on significant role
by compression strut formation in concrete. During high
seismic conditions, the behavior of headed anchors in beam-
column joints are influenced by following parameters The shallow anchorage system is one in which the anchorage
is less than five times of least cover of anchored bar. In the
context of headed anchors, more bearing strength is provided
by use of greater embedment depth due to good confinement
effect produced by concrete during diagonal compressive strut
formation. Similarly in the shallow anchorage system, less
confinement effect was produced by concrete and results less
strength of joint. Failures of headed anchorage system may
classified under shallow and deep anchorage. In shallow
anchorage system (anchor depth < 20 bar diameter) the failure
is attributed to concrete cone breakout failure and in deep
anchorage system (anchor depth > 20 bar diameter) the failure
is due to side face blow out of concrete. Wallace, and Chun et
al.,(2009) suggested that the minimum embedment of headed
Strut-Tie mechanism anchors should be more than 12ø and relative head are ratio
Based on STM approach headed anchorage system provides (ρ) should between 3to4 .Experimental findings of Thompson
static equilibrium conditions at nodal points (Nodes) through et al[20] expressed that the optimum head bearing strength of
appropriate force transfer mechanism . Typical compression- effective concrete strength achieved by deep anchorage
compression-tension (CCT) node formations are shown in system when the anchor embedment reach 0.7L (Fig:6)
Fig.4, where the presence of headed anchors classified by (i) Experimental studies of Sung chul chun -2009 [16] discussed
External and (ii) Internal formation of CCT nodes. Since the on failure patterns of headed bars in shallow, moderate and
discrete joint conditions of external beams are exclusively deep anchorage system. In shallow anchorage system
correlated with truss mechanism of force transfer system, (Embedment depth Ld < 50% of column depth L) the cone
formation of CCT node plays an important role in shear shaped concrete failures are generally happened. The bearing
resistance mechanism of joint . The formation of Strut, Tie and stress of head is not fully develop in this system and joint strut
Node junction decides the strength of joint. As described is not confined by head. In moderate depth of anchorage
above, the formation of external nodes (Fig.4a&4b) gives more system (embedment depth Ld= 50%-70% of column depth L)
strength than internal nodes (Fig.4c&4d). In the conventional the concrete break-out failures are generated by radiating of
design system of joints, formation of sallow and deep strut cracks from both sides of head. Here the head bearing is
conditions are developed upon the effective depth of beam partially participated to bond conditions for shear resistance of
and column width. Deep strut conditions are more vulnerable anchored bar. The deep anchorage system (Ld > 0.70L)
than shallow struts, as concrete exhibit crushing or buckling comprise the diagonal shear cracks initiated at head and
failure due to internal stresses. Hence deep strut conditions propagated towards compression zone of beam. Both head
of joints should verified by tensile strength of concrete. bearing and bond stress are fully contributed to develop shear
resistance mechanism of anchors. And the side face blowout
Anchorage Depth of concrete is a susceptible failure in deep anchorage.
The failure of anchors crucially depends on its embedment
depth, and grade of concrete and confinement factor of joint
core. Since the anchorage depth significantly influence the
force transfer mechanism and strength of joint, its
embedment depth is more crucial during failure
assessment of a joint. In this context, studies conducted by
De.Vries R.A [19] ,Thompson M.K [20] used a simple
definition on shallow and deep embedment depth of
headed anchorage system (Fig.6). Studies of Hung Jen
Lee-2009 [21] mentioned another definition on embedment
depth of anchors in BCJ. Accordingly deep anchorage
system is one which possess embedment depth greater
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As per ACI 352R-02 code, any type of hooked or headed bars under elastic un-cracked and elastic cracked sections of
that may satisfy ASTM-A970 specifications can use for seismic concrete. The stress distribution at various phases of joint
anchorage of beam-column joint and the embedment depth is concrete is shown in Fig.7
more than 8φ (φ: diameter of anchor). Research findings of
Hung Jen lee-2009 [21] expressed the usage of multi headed
anchors in joint core as it effectively enhance shear capacity of
joint during cyclic loads producing high drift conditions.

Condition of joint concrete

In the design of headed anchorage system, the state of
concrete should consider by un-cracked or cracked conditions.
The un-cracked joint conditions is one which is no cracking of
concrete occurs in the embedded length of bar during service
life and the failure mode intends to only fracture failure of steel
reinforcement in joint core. Studies pertaining to cracked
concrete conditions are more significant in the seismic design The mechanical property of concrete shows significant
of anchors as it exhibit the strength of pre and post damage influence on bond development between concrete and
conditions by connecting elements. It is a recommended reinforcement. Fig.7 explains different phases of concrete
practice that design of seismic joints must proceed under conditions and crack pattern against its condition of damage.
cracked conditions of concrete, since the bond strength Elastic-un cracked concrete (Fig.7a) is based on the principle
influenced by development of tensile strains in joint concrete. that once joint concrete reached its tensile capacity, splitting
In this process, the seismic design considerations of joint must cracks formed and bond failure mandatory. In this context the
study the effect of concrete softening before crack formation bond capacity is limited under pure elastic conditions of
and tension stiffening effect after crack formation of joints as to concrete. As shown in Fig.7b Elastic-Cracked section of joint
apply suitable confinement measures in joint core. Modeling of core gives slightly higher bond capacity and allowing crack
cracked concrete is quite essential to evaluate performance zone around the reinforcement of elastic behavior outside the
and strengthening measures of joint. In this context design of zone. Tensile stress are not allowed in the cracked zone. The
beam column joints proceed under a).Smeared crack plastic elastic-cracked concrete gives higher capacity than elastic
model (Degraded stiffness model), b).Damage plasticity concrete. It allows the regions of high tensile stress to get
model. c). Bond slip model (by ABAQUS). Experimental results away from reinforcement surface up to distance where
of previous studies on seismic joints expressed that the stresses act over large area. The elastic-cohesive concrete
cracked concrete reduce its tensile strength considerably (Fig.7c) allows generation of tensile stresses within cracked
(approx 25%) of headed anchors (approximately compared zone. This formation is based on cohesive theory of material
with un-cracked conditions. mentioned in fracture mechanics. In this model, the tensile
behavior of concrete was derived by using principles of
4.3 Joint confinement fracture mechanics. This model is based on tensile behavior of
Two types of confinement effects are influencing the joint concrete which was derived from principles of fracture
behavior of. They are (i) External confinement of joint by mechanics. Plastic concrete (Fig.7d) referred as optimum
holistic action of members (ii) Internal confinement of joint by distribution of tensile stress in concrete and gives highest
implicit strengthening process of joint core. The design codes capacity of concrete. The splitting mode of failure is based on
address empirical solutions for external confinement of joint tensile strength of concrete. It is most economical process and
only. Internal confinement significantly influence anchorage produce uniform stresses distribution in concrete. Ultimate
mechanism of joint through Active and Passive Confinement failure strength of concrete is higher than rest of the modeled
process. Active confinement is one the stress field developed concrete.
in the joint core due to action of superimposed loads (Dead
load, Live load, Pre-stressing Load). The provision of 5 POST INSTALLATION TECHNIQUE OF
unbounded headed anchorage system comes under Active HEADED ANCHORS
confinement effect.(Ref.Fig.9) Passive confinement is referred
as stress field generated due to forces in the reinforcement
Use of headed anchor is considerably increased during the
detailed in anchorage zone (Stirrups, Headed studs, and
recent past as it possess quick and easy installation followed
Helical reinforcement). Since the confinement steel do not play
by economic viability. It is in the form of mechanical, friction
any intermediate role against resistance of splitting tensile
and adhesive anchoring system for structural strengthening of
stresses of concrete until cracks appeared an intersect the
concrete. The Headed fasteners are designed to exhibit
confinement reinforcement, it is termed ―Passive confinement‖
sufficient load carry capacity and allowable deformation in
system. (Ref. Fig. 8).The splitting action resisted by confining
joint. The anchorage system is defined by way of its
reinforcement depends on width of splitting cracks, which is
installation, and is defined by (i) Direct installation, (ii) Drilled
tapered through the length from anchored bar .The
installation and (iii) Cast in- place installation system. For all
confinement reinforcement is more effective when it placed
these conditions, the detailing aspects of headed fasteners are
close to the surface of headed bar. Most of the previous
more significant for shallow and deep strut conditions.
experimental studies are based on passive confinement
conditions and limited studies are conducted for active
confinement conditions of joint core. During seismic action the
behavior of post installed headed anchors are characterized
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TABLE .1
POST INSTALLATION OF HEADEDANCHORS

In the Torque controlled expansion method, the transfer of

external tensile force to base material through friction and
mechanical interlocking with base material. It may generates
pre-stressing force in bolt and clamps the fastened anchor
5.1Post Installation process against the surface of base material. This pre-stressing force
Studies conducted by Higgins-1994 , Klingner-1998 , Cook, diminished after installation of anchor due to relaxation of
Konz-2001, Fujikake-2003 addressed on installation conditions localized stresses in concrete .Torque controlled anchors may
of anchors (direction of installation, drilling of holes , cleaning, further classified under sleeved and bolted type. The sleeve
and moisture during installation), and loading conditions on type anchors consists of threaded bolt, nut and washer with
anchor (i.e., short-term or long-term loading) at service expansion sleeve deformations that provided to prevent
conditions. The installation process of anchors are classified spinning of the anchor in the hole. The bolt type anchor
as follows. (Ref. Fig.8) (i) Pre-position installation, (ii) In-place typically consists of bolt, the end of which was swaged or
installation (iii) Stand-off installation machined into conical shape. Installation of Torque controlled
mechanical anchors are generally carried out by inserting the
anchor in drilled hole and apply specified torque on bolt or nut
with torque wrench. Once the bolt or nut receive bearing
against the base material , then the further application of
torque draws the cone at the embedded end of the anchor up
into the expansive sleeve ,thereby expanding the expansion
elements against the sided of the drilled hole. To ensuing
sufficient frictional resistance in torque controlled bolts should
keeps the bolt in tension. If the torque controlled expansion
anchor not set correctly, then it will rotate before achieve the
prescribed torque. This type of anchors installed through use
of drilling machine and specified tolerance allowed during
As explained in Fig8, three types of anchor installation preparation of hole size in concrete.
configurations are commonly used during fastenings of direct
installation. It is mentioned by (i) Pre-position drilled
installation (Grouted anchors) (ii) In-situ placed installation (iii)
Stand-off installation. Pre-position drilled installation (Fig.8a)
involves making a drilled hole with suitable clearance in
hardened concrete and insert the fastened anchor and fix it
with high strength epoxy grout. The size of drilled hole must be
large enough to develop bond resistance between the anchor
and constituent grouted material. Post installation of bonded
anchors is an example of this method. In the In-place
installation of anchor installation (Fig.8b) anchor is fixed in
position and monolithically casted with concrete during joint
construction. Hence the embedded anchor is in direct contact As shown in Fig.10, the displacement controlled expansion
with concrete surface and produce shear resistance against anchors (drop-in anchors) consist use of expansion sleeve and
bond.(Fig.8b). The stand-off installation is a post installation plug. The sleeve is internally threaded so as to accept
method, where the anchor is fastened through inserted made threaded element. The displacement anchor transfer the
in hardened concrete through torque controlled mechanical tension load to base material by friction and in the localized
anchorage system (using screws, bolts etc.)(Fig.8c) .The deformation through mechanical interlocking. Magnitude of
drilled anchor is able to take compression, tension through expansion force depends on sleeve size, expansion,
mechanical interlocking between steel threads and deformation resistance against concrete, and gap between
surrounding concrete. This type of installation further classified sides of drilled hole and anchor. The initial expansion force
by (i) Torque controlled method (Fig.9) (ii) Displacement produced by anchor is more than torque controlled expansion
controlled method. (Fig.10) anchor, but the high expansion stresses induced is reduced
later stages by relaxing stress of concrete.
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Post Installation process 5.3 Un-Bonded or confined anchorage system

The post installation of headed anchor in joint concrete is As referred in Fig.12 the Un-bounded headed anchorage
designated by two methods of fastening systems. Each system is produced active confinement effect on joint core.
method constitutes its own merits in strengthening process of This method is similar to pre-tension effect through
joint core. This methods are classified by confinement of joint core and adoptable when the joint
(i) Bonded anchorage (ii) Un-bonded anchorage. Bonded subjected to low or moderate shear strength. In this process
anchorage system provides passive confinement effect in joint headed bars are passing through an existing opening (sleeves
core through stress field generation and creating internal or conduits) of joint and the tail end of anchor is embedded in
forces of reinforcement placed around anchorage. Provision of beam concrete to develop sufficient bond strength against pull
un- bounded anchorage system comes considered by active out failure of joint. This process is intended to produce tensile
confinement mechanism as detailed below. Mechanism of both resistance of headed bar by head bearing system only and no
systems are shown in Fig.11 & Fig.12 bond stress of stem participated during force transfer
mechanism. In this possess headed anchor in joint core is not
5.2Bonded or Adhesive anchorage system in direct contact with concrete. This process contributed to
As shown in Fig.11 the anchored reinforcement is in direct induce implicit pre-tensioning force in anchored steel so as to
contact with joint concrete .Adhesive bonded anchors are produce confinement effect on joint core. The pre-tension
sensitive against type of loading and direction of installation. forces applied by torque controlled screwed anchors arranged
During its installation hole is drilled across the joint with at tail of headed bar. The un-bonded anchoring system can
tolerance and produced to intercept required depth of also proceeded by using undercut expansion anchors. This
connected beam. Later the drilled hole is filled with epoxy system is very suitable for moderate seismic conditions, where
grouted material which interfaces both reinforcement and the joints are inhibited by low shear conditions.
concrete. Another method of bonded anchorage is provided by
inserting screwed anchors through joint concrete and later 6.SEISMIC QUALIFICATION OF HEADED
filled the drilled hole by grouted material. Hence the resistance
ANCHORS
is provided by head bearing and frictional bond resistance of
Bonded expansion or undercut anchors are suitable to use in
stem through anchorage of reinforcement. Analysis of this
cracked concrete. In absence of steel rupture, bonded
method is proceed by pull-out tests. In this process, anchored
undercut anchors exhibit concrete cone breakout failure when
bar is in direct contact with surface of concrete and contributed
the load in tension reaches its ultimate state. The bonded
to develop stable CCT (Fig 4) node conditions during implicit
anchor constitute grouting materials such as polymer resins,
strengthening process of joint core. The bond between headed
cementecious material of epoxy grout or combination of the
anchor-grout and grout-concrete is more crucial during shear
above. The bonded anchorage system is in the form of (i)
resistance mechanism. The bonded system is similar to cast-in
Capsule anchorage system, (ii) Injection system. In the
place joint connection by headed anchors. This technique is
capsule anchoring , threaded rod equipped with 45o chisel or
suitable to meet seismic requirements of both undamaged and
wedge shaped tip with hexagon nut and washer that was in
damaged state of concrete in joint core and preferred to use
conjunction with foil pouch filled with constituent bonding
for moderate or high strength concrete . The bond force
material. The required embedment is marked on the threaded
between anchor surface and drilled concrete hole produce
bar and filled by polymer resin, hardener, and quartz
adhesive bond resistance against applied tension and bond
aggregate at definite proportion .The capsule pouch placed in
force in equilibrium conditions [Ref. Cook @al.-1998] [29] . If
hole from which drilled dust has been removed. The threaded
adhesive bond between anchor and concrete tends to brake,
rod driven into the capsule until the embedment depth marked
then force transfer is provided by friction action. Use of
by percussion and rotary drilling method. When driving the rod
polymer modified cement concrete is one of the suggestive
into the hole, the glass capsule is broken and fragmented into
grouted material to develop bond between concrete and
pieces and the resin, hardener and fragmented pieces are
anchored steel of joint core. During seismic action the bond
mixed with sufficient energy input to induce rapid curing and
strength of headed anchor significantly influenced by
the annular gap around the threaded rod filled with polymer
Poisson’s effect and larger bars intends to greater volume
matrix. In the injection anchorage system, the drilled hole is
change (tensile force) and results high reduction of mechanical
mechanically cleaned by stiff brush and compressed blow air.
interlocking or frictional resistance.
Due considerations are required in PIHA during nonlinear
cyclic action of seismic forces. Most of the designs considered
static load capacity of anchors with multiple factor while
assessing seismic capacity of anchors . The seismic behavior
of post installed headed anchors depends on prevailing
conditions of concrete core, embedment depth, type and
sequence of loads acting on joint. The seismic action of
anchors may subjected to combination of tension, and shear
loads while performing inelastic response cracked concrete
conditions under varying crack width. Expansion anchors are
intend to produce expansive force on concrete and preferred
to locate at far distance from edge of concrete with sufficient
spacing between the anchors. The distance between the
anchors is a function of anchor diameter (ø) that is anchor with
larger diameter must place far away from edge of concrete.
Provision of multi headed anchors in joint core may effectively
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transfer the compressive forces into the diagonal strut of joint stress at anchor plate coincide with each other. As a result the
core and establish good seismic absorption of joint during high capacity of mechanical anchor increased by bearing plate
seismic conditions. In the context of above observations, located within the concrete strut area. Use of supplementary
bonded anchorage system recommended in low drift shear reinforcement is used to enhance tensile capacity of
conditions of joint (drift<1.5%) as the joint sustain with concrete and to avoid cone of fracture. Subsequently it
considerable strength and stiffness of seismic loads. provides good confinement in joint core. (Fig.13c & Fig.13d)
Subsequently, the un-bonded anchorage system preferred in
high drift conditions (drift>2%) conditions, where the joint 8.SUGGESTIVE MEASURES
subjected to considerable degradation of strength and Design codes of ACI 349-01,352-02R, NZS3101, and FIB-
stiffness. Hence post confinement effect is more significant 2000 are presented confined discussion on PIHA technique
during high drift conditions. Experimental findings of Hung-Jen during mechanical anchorage of R.C foundations. But no
Lee -2009 [21] addressed the usage of double headed specific guidelines addressed for its adoptability in seismic
anchors in joint core for enhance anchorage capacity and
BCJ except few design limitations. Most of the codes follows
cyclic behavior of joint in high seismic conditions (drift >4%). seismic compliance of joints as per strength of concrete rather
The findings concluded that use of single headed anchors may than shear reinforcement provisions .Codes are widely
limit to low drift conditions (drift <3.5%). The use of multi contradicted on parametric influence of joint against shear
headed anchors may delay the reduction of shear strength in resistance mechanism, which include detailing aspects of
joint core. shear reinforcement. Strength reduction factors of cracked
concrete are normalized in concrete under cone of failure
7. IMPLICIT SHEAR STRENGTHENING OF (0.65), side face blowout failure (0.55), and pull out or pry out
JOINT CORE failure (0.45) which are defined in post installation of anchors
Implicit shear strengthening of joint core is a mechanism by direct tension (absence of supplementary reinforcement).
achieved by induced confinement effect implicitly so as to For cracked concrete section the strength reduction factor
reduction the tensile stresses in joint core. In the headed (0.70) during face blowout failure is need to consider during
anchorage system, the active confinement effect of post installation of anchor. The concrete mode of failure is not
unbounded anchorage system and passive confinement effect acceptable in the design of headed anchorage system. The
of bonded anchorage system within joint core are significantly failure of steel is acceptable due to possessing ductility. Use of
influence the efficient stress transfer mechanism by Strut and supplementary reinforcement in with headed bars will improve
Tie method. The contribution of concrete strength under strut the ductility of joint during failure. To meet this requirement,
action is accompanied by head bearing and bond resistance of supplementary steel should satisfy displacement compatibility
headed anchor. The pure shear conditions of joint inhibit such as developing appropriate tensile force prior to peak
development of principal stresses in joint core. In this process, failure of concrete.
concrete failure is attributed to development of excess
compressive stresses or tensile strain in major principal 9. CONCLUSIONS
planes. Fig.12 shows the state of stress conditions in hooked This paper reports the analytical aspects of Post Installed
and headed anchorage system of external beam column joint. Headed Anchorage (PIHA) system used for strengthening of
The anchorage capacity of hooked bar is same as regardless external beam-column joints. PIHA system is based on the
the direction of bent of hooked bar and the hook extension is
principle of ―Developing Implicit Strengthening Mechanism‖ of
placed towards joint and the hook possess poor shear
resistance mechanism when it bent outward direction as the joint core. A wide range of advantages are featured in this
minimum steel contributed in concrete strength. Hence joint system against seismic strengthening and constructability. It is
core with hooked anchorage shows poor cyclic response most adoptive technique for precast and cast–in place joints
and useful to strengthen BCJ at moderate, high seismic
conditions. and rehabilitation of damaged joints. The headed
bars used in this system are verified at bonded or un-bonded
conditions of concrete. Salient features are follows.
 Post-Installed Headed Anchorage (PIHA) system provides
implicit enhancement of shear resistance in beam-column
joint by confinement and bond resistance. PIHA restricts
brittle failure and shear deformation of joints and enhance
elastic stiffness and ductility of joint core.
 Use of headed bars in PIHA is an added advantage of
strengthening and delay the fracture failure of joint. It is good
means to provide stable CCT node conditions and improves
joint shear resistance.
The formation of single strut mechanism (Fig.13a, Fig.13b)
 Provision of headed bars at bonded phase of PIHA is
results unbalanced equilibrium conditions of forces and results
recommended when good concrete conditions exists in joint
poor performance of joint. During cyclic conditions the headed
core (undamaged conditions). During this process PIHA
anchorage system provided efficient stress flow since the
provides shear resistance mechanism through passive
direction of concrete strut (hatched area) and local bearing
confinement effect and establish a bond between steel and
concrete through friction and bearing resistance of head.

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 Provision of headed bars in un-bonded conditions of PIHA [9]. Eligehausen, R., Mallée, R., and Silva, J. F. (2006).
is suitable during poor conditions of joint concrete ―Anchorage in concrete construction‖, Ernst & Sohn,
(preferably damaged).This system gives shear resistance Berlin, 378
mechanism by active confinement effect of joint by induce [10]. Walker, S., Yeargin, C., Lehman D., and Stanton, J.
pretension forces by confined anchorage system. Anchor (2002) "Performance-based Seismic Evaluation of
heads pays key role in shear resistance mechanism. Existing Joints" Proceedings of the Seventh U. S.
National Conference on Earthquake Engineering,
 PIHA restricts the entry of heavy reinforcement from
Paper # 673, May 2002.
beams to joint core. The additional requirements for
[11]. Noguchi H, Kashiwazaki T ―Experimental studies on
anchorage and bond strength of beam can be substituted
shear performances of RC interior column–beam
by PIHA technique.
joints with high-strength materials‖. In’ Proceedings of
the 10th world conference on Earthquake
10. RECOMMENDATIONS engineering, July 1992, Spain
This study recommends usage of Post Installed Headed [12]. Fujii.S, Morita.S ―Comparison between interior and
Anchorage (PIHA) system during seismic strengthening of exterior beam column joint behavior and design for
external R.C beam-column joints. Analytical studies explain seismic resistance‖ ACI, special publication.SP-
the implicit strengthening mechanism of joint by PIHA. It 123,PP145-165
provides rapid and assured process to mitigate the [13]. Oka, K. and Shiohara, H. (1992). ―Tests on high-
construction problems and rehabilitate the joints in beam- strength concrete interior beam-column joint sub-
column joints such as reinforcement congestion, anchorage , assemblages‖Tenth World Conference on Earthquake
fabrication , and rehabilitation etc This method has good Engineering, Madrid, Spain, pp.3211–3217.
adoptability for precast and cast in-situ R.C joints. [14]. Ghimire, Krishna P. Shao, Yun , Darwin, David ,
O'Reilly, Matthew ―Conventional and High-Strength
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