chapter 4 experimental investigations on exterior beam...
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51
CHAPTER 4
EXPERIMENTAL INVESTIGATIONS ON EXTERIOR
BEAM-COLUMN JOINTS
4.1 GENERAL
Buildings subjected to seismic forces may undergo several
reversals of stresses in the course of an earthquake. Beam-column joints are
the critical region in moment resisting frames. The failure of reinforced
concrete structures in recent earthquakes caused concern about the
performance of beam-column joints. Due to the restriction of space available
in the joint block, detailing of reinforcement assumes great significance.
Confinement of joint is one of the ways to improve the performance of beam-
column joints during earthquakes. This chapter describes an experimental
study of exterior beam-column joints with two non-conventional
reinforcement arrangements. One exterior beam-column joint of a six storey
building in seismic zone III of India was designed for earthquake loading
(Ingle and Jain 2005). The transverse reinforcement of the joint assemblages
were detailed as per IS 13920:1993 and IS 456:2000 respectively. The
proposed non-conventional reinforcement was provided in the form of
diagonal reinforcement on the faces of the joint, as a replacement of ties in the
joint region for joints detailed as per IS 13920 and as additional reinforcement
for joints detailed as per IS 456. These newly proposed detailing have the
basic advantage of reducing the reinforcement congestion at the joint region.
In order to study and compare the performance of joints with different
detailing, four types of one-third scale specimens were cast (two numbers in
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each type). The main objective of the present study is to investigate the
effectiveness of the proposed reinforcement detailing. All the specimens were
tested under reversed cyclic loading, with appropriate axial load.
One of the basic assumptions of frame analysis is that the joints are
strong enough to sustain the forces (moments, axial and shear forces)
generated by the loading, and to transfer the forces from one structural
element to another (beam to column in most of the cases) (Subramanian and
Rao 2003). The assumption of rigidity of joints often ignores the effects of
high shear forces developed within them. Actually, the shear failure is always
brittle in nature and cannot be deemed as an acceptable structural performance
especially during earthquakes. Thus, a proper understanding of the joint
behaviour is imperative in designing earthquake-resistant joints.
In Indian Code of practice for plain and reinforced concrete (IS 456
: 2000), the joints are usually neglected for specific design and attention is
restricted to the provision of sufficient anchorage for beam longitudinal
reinforcement. This may be acceptable when the frame is not subjected to
earthquake loads. Often, the poor design of beam column joints is
compounded by the high demand imposed by the adjoining flexural members
(beams and columns) in the event of mobilising their inelastic capacities to
dissipate seismic energy. Unsafe design and detailing within the joint region
jeopardises the entire structure, even if other structural members conform to
the design requirements. Among the Indian codes, IS 13920 : 1993 deals with
the ductile detailing of reinforced concrete structures subjected to seismic
forces. However, despite the significance of the joints in sustaining large
deformations and forces during earthquakes, specific guidelines or
recommendations on beam-column joints are not included in the Indian code of
practice (IS 456 : 2000).
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According to the capacity design philosophy, beam hinging (while
avoiding column hinging and joint shear failure) is the most desirable mode of
failure to guarantee high energy dissipation during earthquakes, through
significant inelastic deformation and without overall reduction of strength.
The implementation of design philosophy presents serious problems
particularly in quantifying the different types of damage (structural and non-
structural) and what constitutes frequently minor, sometimes moderate, and
rarely major earthquakes. Plastic hinges are “expected” at locations where
structural damage can be allowed to occur due to inelastic actions involving
large deformations. Hence, in seismic design, damage in the form of plastic
hinges are accepted to form in beams rather than in columns (Uma and
Meher Prasad 2006). Proper confinement of joints can be done to rectify the
column hinging or joint shear failure. To satisfy the strong column-weak
beam collapse mechanism, it is important to understand the progress of
damage and failure pattern in joints under seismic loads. This can be obtained
by the reversed cyclic loading test.
In the past three decades extensive research has been carried out for
studying the behavior of joints under seismic loading (Jisra 1991). Various
international codes (ACI 352 R, Euro Code 8, NZS 3101 etc.) have been
subjected to periodic revisions. The role of transverse reinforcement and
mechanism of shear transfer in the joints for seismic resistance is the subject
of much debate (Hwang et al 2005). Experimental results on performance of
beam-column sub assemblages of modern structures indicate that current
design procedures could sometimes lead to excessive damage of the joint
regions (Tsonos 2007). IS 13920 : 1993 assumes that the role of hoops is to
confine the joint core. The real function of hoops may be both to confine the
joint core and to carry shear as tension tie and hence to reduce the width of
the crack. The special confining reinforcement (IS 13920 : 1993) serves three
purposes. It provides shear resistance to the member. It also confines the
concrete core and thereby increases the ultimate strain of concrete, which
54
gives greater ductility to the concrete cross section and enables it to undergo
large deformations. Again it provides lateral restraint against buckling of the
compression reinforcement. Experimental studies reveal that the usage of the
rectangular spiral reinforcement significantly improves the seismic capacity
of external beam-column connections (Karayannis et al 2005).
The present study aims at developing an innovative methodology to
confine the joint region. The confinement of joints is achieved by the
inclusion of inclined bars at the joint region. An external beam-column joint
from an actual six storey building has been considered for the present
investigation. The details regarding the design of joint region and
experimental studies on the same are discussed in the following sessions.
4.2 ANALYSIS AND DESIGN OF RC BEAM-COLUMN JOINT
A six storey RC building located at Chennai, India (in seismic zone
III as per IS 1893 (Part 1): 2002) on medium soil was analysed. The joint of
column marked “A” in Figure 4.1 was considered for design. The length of
the column was 3 m and the cross section was 450 mm deep by 300 mm wide.
The longitudinal and transverse beams were 450 mm deep by 300 mm wide.
A live load of 3 kN/m2 and floor finish of 1 kN/m2 were taken for analysis.
The thicknesses of peripheral and internal walls were taken as 250 mm and
150 mm respectively. M30 grade concrete and Fe 415 grade steel were used
for design. The plane frames constituting the joint regions were analysed. The
design was carried out based on the proposed amendments to IS 1893 and IS
13920 (Ingle and Jain 2005). Four types of detailing for beam-column joints
were considered for the present study, such as (i) joint detailed as per IS
13920 : 1993, (ii) joint detailed as per IS 13920 : 1993 with the proposed
non-conventional detailing. The non-conventional detailing comprises of two
cross inclined bars as a replacement of ties in the joint
55
(a) Plan of the building
(b) Elevation of longitudinal frame (c) Elevation of transverse frame
Figure 4.1 Details of the frames analysed
3m
3m
3m
3m
3m
3m
A A B
Exterior joint
3.6 m 5.1 m 2.7 m 5.1 m 3.6 m
A
B
Exterior joint
3.6 m 5.1 m 2.7 m 5.1 m 3.6 m
A
B 3.
6 m
3.
6 m
5.
1 m
X
Y
56
region. (iii) joint detailed as per IS 456 : 2000 with additional U bars as per
SP : 34(S & T)-1987, (iv) joint detailed as per IS 456 : 2000 with additional
U bars and the proposed non-conventional detailing. The non-conventional
detailing comprises of two cross inclined bars as additional confining
reinforcement in the joint region.
The shear forces, bending moments and axial forces around the
exterior beam-column joints due to the induced earthquake loading were
estimated. Seismic analysis was performed using equivalent lateral force
method given in IS 1893: 2002 and its proposed amendments (Jain and Murty
2005a).
4.2.1 Design of Beam
Plane frames were analysed and the force resultants for various
load cases such as dead load, live load and earthquake load in beam AB of
short frame were estimated and are shown in Table 4.1.The force resultants
for different load combinations considering earthquake load in Y direction
which is predominant, is shown in Table 4.2. The beam region of the joint
was checked for axial stress, member size, minimum and maximum
reinforcement to design as a flexural member. The design moment and shear
force from the critical load combinations for the beam AB were 160.05 kNm
and 112.8 kN respectively. Longitudinal reinforcement details of transverse
beam and longitudinal beams near the exterior joint are shown in Table 4.3,
Figure 4.2(a) and Figure 4.2(b). Spacing of ties on beam AB near joint was
calculated as per IS 456 : 2000 and as per IS 13920 : 1993. Two legged ties of
8 mm diameter were provided at a spacing of 100 mm centre to centre near
the joint and at 150 mm centre to centre near the mid span. The beam bars
were provided with adequate development length to take care of the pull out
force under cyclic loading.
57
Table 4.1 Force Resultants in Beam of Short Frame for Various Load
Cases
Load Case
Left end(A) Centre Right end (B)
Shear (kN)
Moment (kNm)
Shear (kN)
Moment (kNm)
Shear (kN)
Moment (kNm)
Dead Load 24.90 -12.70 -3.38 9.69 31.60 24.70
Live Load 8.10 -4.70 -1.62 4.04 11.30 10.50
Earthquake Load EQY
-50.30 94.00 -50.30 3.46 50.30 87.00
Table 4.2 Force Resultants in Transverse Beam for Different Load
Combinations
Sl. No.
Load Combinations
Left end Centre Right end
Shear (kN)
Moment (kNm)
Shear (kN)
Moment (kNm)
Shear (kN)
Moment (kNm)
1 1.5DL+1.5LL 49.50 -26.10 -7.51 20.62 64.35 52.80
2 1.2(DL+0.25*L
L+EQY) -28.05 96.15 -64.91 17.00 101.67 137.19
3 1.2(DL+0.25*L
L-EQY) 92.67 -129.45 55.81 8.70 -19.05 -71.61
4 1.5(DL+EQY) -38.1 121.95 -80.53 19.74 122.85 167.55
5 1.5(DL-EQY) 112.8 -160.05 70.37 9.36 -28.05 -93.45
6 .9DL+1.5EQY -53.04 129.57 -78.50 13.92 103.89 152.73
7 .9DL-1.5EQY 97.86 -152.43 72.40 3.54 -47.01 -108.27
58
Table 4.3 Details of Main Reinforcement in Beams Near the Joint A
Beam Top Main reinforcement
Bottom Main Reinforcement
Transverse beam near joint A 4- 20 Φ bars 4- 20 Φbars
Longitudinal beam left to joint A 3- 20 Φ bars 2- 20 Φbars
Longitudinal beam right to joint A 3- 20 Φ bars 2- 20 Φbars
Figure 4.2(a) Reinforcement Details of Transverse Beam Near the
Exterior Joint
59
Figure 4.2(b) Reinforcement Details of Longitudinal Beams Near the
Exterior Joint
4.2.2 Design of Column
The column was designed for earthquake loading in both X-direction
and Y-direction. Hence, all 13 load combinations as shown in Table 4.4 were
considered and critical combination is selected. Since the joints are subjected
to large shear force during earthquakes; joint shear strength should be
checked. Hence adequacies of depth and width for the column are checked as
per the proposed provisions for beam-column joints. The column is checked
for flexural member considering the lower axial force.
60
Table 4.4 Forces in Column for different Load Combinations
Col
umn
end
Forc
e/M
omen
t (k
N/k
Nm
)
1.5(
DL
+LL
)
1.2(
DL
+LL
+EQ
X)
1.2(
DL
+LL-
EQX
)
1.2(
DL
+LL
+EQ
Y)
1.2(
DL
+LL-
EQY
)
1.5(
DL
+EQ
X)
1.5(
DL
-EQ
X)
1.5(
DL
+EQ
Y)
1.5(
DL
-EQ
Y)
(0.9
DL
+1.5
EQ
X)
(0.9
DL
+1.5
EQ
X)
(0.9
DL
+1.5
EQ
Y)
(0.9
DL
+1.5
EQ
Y)
AB
Axi
al L
oad
-839 -748 -594 -458 -885 -782 -590 -419 -953 -508 -315 -145 -678
AT 647.9 572.4 464.2 369.1 667.4 596.4 461.1 342.3 715.2 384.9 249.6 130.8 503.7
BB -648 -572 -464 -369 -667 -596 -461 -342 -715 -385 -250 -131 -504
BT 456.6 398.3 332.3 276.5 454.1 412.8 330.3 260.6 482.6 264.2 181.7 111.9 333.9
AB
Mom
ent M
x
-14.6 46.68 -70 -11.6 -11.6 61.5 -84.3 -11.4 -11.4 66.06 -79.7 -6.84 -6.84
AT -12.9 44.76 -65.4 -10.3 -10.3 58.95 -78.8 -9.9 -9.9 62.91 -74.8 -5.94 -5.94
BB -14 45.96 -68.3 -11.2 -11.2 60.45 -82.4 -11 -11 64.83 -78 -6.57 -6.57
BT -14.1 36 -58.6 -11.3 -11.3 48 -70.2 -11.1 -11.1 52.44 -65.8 -6.66 -6.66
AB
Mom
ent M
y
-13.5 -10.8 -10.8 50.28 -71.9 -9.9 -9.9 66.45 -86.3 -5.94 -5.94 70.41 -82.3
AT -12.9 -10.3 -10.3 48.72 -69.4 -9.45 -9.45 64.35 -83.3 -5.67 -5.67 68.13 -79.5
BB -12.9 -10.3 -10.3 51.72 -72.4 -9.45 -9.45 68.1 -87 -5.67 -5.67 71.88 -83.2
BT -13.1 -10.4 -10.4 40.32 -61.2 -9.6 -9.6 53.85 -73.1 -5.76 -5.76 57.69 -69.2
AB
Shea
r Fo
rce
F y 9.3 7.44 7.44 -34.3 49.2 6.75 6.75 -45.5 58.95 4.05 4.05 -48.2 56.25
AT -8.7 7.44 -6.96 33.36 -47.3 -6.3 -6.3 44.1 -56.7 -3.78 -3.78 46.62 -54.2
BB 8.7 6.96 6.96 -33.4 47.28 6.3 6.3 -44.1 56.7 3.78 3.78 -46.6 54.18
BT -8.7 -6.96 -6.96 29.88 -43.8 -6.3 -6.3 39.75 -52.4 -3.78 -3.78 42.27 -49.8
AB
Shea
r Fo
rce
F x 9.9 -31.6 47.4 7.92 7.92 -41.6 57.15 7.8 7.8 -44.7 54.03 4.68 4.68
AT -9.3 30 -44.9 -7.44 -7.44 39.45 -54.2 -7.35 -7.35 42.39 -51.2 -4.41 -4.41
BB 9.3 -30 44.88 7.44 7.44 -39.5 54.15 7.35 7.35 -42.4 51.21 4.41 4.41
BT -9.3 25.44 -40.3 -7.44 -7.44 33.75 -48.5 -7.35 -7.35 36.69 -45.5 -4.41 -4.41
The exterior column was designed for an axial load of 953 kN and a
moment of 86.3 kNm, which were the critical values obtained from the
thirteen different load combinations. Eight numbers of 20 Φ bars were
provided with bars distributed equally on all faces of column. The joint was
61
designed for strong column-weak beam conditions for earthquake in
Y direction and earthquake in X direction as per the draft revision of IS 13920
(Jain and Murty 2005b). Special confining reinforcement were provided in the
joint region for detailing as per IS13920 : 1993. In the detailing as per Indian
code of practice for plain and reinforced concrete IS 456 : 2000, joints were
not provided with tie, but U bars were provided for confining the joint core as
given by SP 34 : 1987.
4.2.3 Design of Exterior Joint
The joint shear strength and strong column weak beam conditions
for earthquake in Y direction and earthquake in X directions were checked as
per draft revision of IS 13920 and found satisfied. The joint shear in exterior
joint while earthquake in Y direction and in X direction are shown in Figure 4.3.
Since equal reinforcement were provided at the top and bottom of
transverse beam, the column shear for sway to right or to left during
earthquake loading in Y direction, colV was obtained as 74.50kN. The force
developed in the top reinforcement of beam 1T was 651.55kN. Thus the joint
shear force, intjoV is obtained from Equation (4.1) as 577.04 kN.
colV1TjointV (4.1)
For the earthquake loading in X direction, the column shear for
sway to right or to left, colV was obtained as 98.50 kN. The force developed in
the top reinforcement of right longitudinal beam 1T was 488.66 kN. The force
developed in the bottom reinforcement of left longitudinal beam 2T was
62
325.78 kN. Thus the joint shear force intjoV is obtained from Equation (4.2) as
715.94 kN.
jo int 1 2 colV T C V (4.2)
where 2C is the compressive force in the left beam which is equal to the
tensile force 2T developed in the left beam.
(a) Earthquake in Y direction (b) Earthquake in X direction
Figure 4.3 Joint Shear in Exterior Beam- Column Joint
The effective width of joint ( jb ) as per draft revision of IS 13920,
is lesser of (i) cbj 0.5hbb and (ii) cbb j where, bb is the width of beam;
ch is the depth of column in the considered direction of shear; cb is the width
of column. Effective shear area of joint jjej hbA : where jh is the depth of
joint which can be taken as depth of column.
Shear strength of the joint ( ckej fA0.1 ) was obtained as 739.43 kN;
which is higher than the joint shear force for the earthquake loading in
Y direction and in X direction. Hence, the joint has adequate shear strength in
63
both directions as per proposed revision. The flexural strength ratio of column
to beam for earthquake in Y direction and earthquake in X direction were
obtained as 1.84 and 1.39 respectively. Hence, the requirement of strong
column-weak beam condition was satisfied for the earthquake loading in both
Y and X directions.
Thus the joint sub assemblage considered for the experimental
study was checked for shear strength, and strong column-weak beam theory.
The only difference between the sub assemblages considered for the present
study was in the detailing of transverse reinforcement. The special confining
reinforcement is continued in the joint region for detailing as per IS
13920:1993. Out of the two newly proposed detailing patterns, the first were
detailed as per IS 13920:1993 with the non-conventional confining
reinforcement pattern at the joint area. Here the transverse reinforcement at
the joint region was replaced by two pairs of the X-type confining
reinforcement. One additional tie was provided at the middle of the joint to
avoid buckling of the longitudinal reinforcement. In the second proposed
detailing pattern, the detailing is as per IS 456:2000 including the non-
conventional confining reinforcement pattern at the joint area. The joints with
transverse reinforcement detailing as per the Indian code of practice for plain
and reinforced concrete IS 456:2000, joints are not provided with tie, but the
U bars are provided for confining the joint core as given in SP 34: 1987.
4.3 DESCRIPTION OF SPECIMENS
The specimens were cast as one-third scale models and
classified into four groups with two numbers in each group. The Type 1
specimens (A1-13920 and A2-13920) were cast with transverse reinforcement
detailing as per IS 13920: 1993. The Type 2 specimens (X1-13920 and X2-
13920) were detailed as per IS 13920:1993 and with the newly proposed non-
64
conventional reinforcement. One additional tie was provided at the middle of
the joint to avoid the buckling of longitudinal reinforcement in Type 2
specimens. The Type 3 specimens (A1-456 and A2-456) were detailed as per
IS 456: 2000. The Type 4 specimens (X1-456 and X2-456) were cast with the
detailing as per IS 456:2000 and with the newly proposed non-conventional
reinforcement.
The non-conventional reinforcement was provided in the form of two
inclined bars on both faces of the joint as shown in Figure 4.4. The percentage
of diagonal bars provided was the same as that of confining reinforcement
required at the joint as per IS 13920: 1993. Care was taken to provide the
adequate development length as per the code requirement. All the eight
specimens were tested under constant axial load with cyclic load at the end of
the beam. One of the specimens from each group was subjected to an axial
load of three per cent of column axial load capacity and the other specimen
was subjected to an axial load of ten per cent of column axial load capacity
(Karayannis et al 2008). The value of axial load on the second specimen was
arrived from the axial force on the upper column of the assemblage at the
critical load combination selected for design.
Figure 4.4 Pattern Showing the Cross Inclined Confining Bars
65
The dimensions of the joints, diameter of reinforcement and
alignment of longitudinal reinforcement of column were in accordance with
the proposed amendments in IS 13920 (Jain and Murty 2005b). The
longitudinal reinforcement at the column region and beam region of the
specimens were checked for strong column-weak beam theory. High yield
strength deformed bars were used for the longitudinal reinforcement and high
yield strength bars were used for stirrups. The column was rectangular in
shape with dimensions 100 mm x 150 mm and the beam with dimensions
100 mm x 150 mm (Abrams 1987b). The details of the joint assemblage
specimens are shown in Figure 4.5 to Figure 4.8 and in Table 4.5. The photographs
of reinforcement cages are shown in Figure 4.9 and Figure 4.10.
4.4 CASTING OF SPECIMENS
The specimens were cast using ordinary Portland cement (53 grade)
conforming to IS 12269 : 1987. River sand (medium type) passing through
4.75 mm IS sieve and having a fineness modulus of 2.77 was used as fine
aggregate. Crushed granite stone of maximum size not exceeding 8 mm and
having a fineness modulus of 3.58 was used as coarse aggregate. The mix
design was carried out and the proportion was arrived as 1: 0.87: 1.32 by
weight and the water-cement ratio was kept as 0.48. The 28-day average
compressive strength from 150 mm cube test was 44.22 N/mm2. The yield
stress of reinforcement was 432 N/mm2. All the specimens were cast in
horizontal position inside a steel mould on the same day. Specimens were
demoulded after 24 hours and wet cured for 28 days.
66
Figure 4.5 Reinforcement Details of the Beam-Column Joint Specimen
as per IS 13920 (Type 1)
67
Figure 4.6 Reinforcement Details of the Beam-Column Joint Specimen
as per IS 13920 with Diagonal Confining Bars (Type 2)
68
Figure 4.7 Reinforcement Details of the Beam-Column Joint Specimen
as per IS 456 (Type 3)
69
Figure 4.8 Reinforcement Details of the Beam-Column Joint Specimen
as per IS 456 with Diagonal Confining Bars (Type 4)
70
Table 4.5 Reinforcement details of test specimens
Specimen designation
Column Beam Joint
Remarks Longitudinal Rein-
forcement
Transverse Reinforcement
Longitudinal Reinforcement
Transverse Reinforcement
Transverse Reinforcement
A1-13920 and
A2-13920
4 Φ8 and 4 Φ6
Φ3 at 25 mm c/c for a distance of 230
mm at either side of joint and 50 mm c/c
for remaining portion
2 Φ8 and 2 Φ6 (top and
bottom)
Φ3 at 35 mm c/c for a distance of 270 mm from joint and 50 mm c/c for
remaining length
Φ3 at 25 mm c/c Confining reinforcement at joint is as per IS 13920:1993
X1-13920 and
X2-13920
4 Φ8 and 4 Φ6
Φ3 at 25 mm c/c for a distance of 230 mm at either side of joint and 50 mm c/c
for remaining portion
2 Φ8 and 2 Φ6 (top and bottom)
Φ3 at 35 mm c/c for a distance of 270 mm from joint and 50 mm c/c for
remaining length
1 Φ3 lateral and 2 Φ6 diagonally on two faces
with development length in tension
extended to upper and lower column
Confining reinforcement at joint is as per minimum
requirement in column and additional diagonal bars on
two faces
A1-456 and A2-456
4 Φ8 and 4 Φ6 Φ3 at 100 mm c/c
2 Φ8 and 2 Φ6 (top and bottom)
Φ3 at 35 mm c/c for a distance of 270 mm from joint and 50 mm c/c for
remaining length
Two U bars, Φ3 are provided with
development length in tension extended to beam
No ties at joint. But two U bars (hairclip-type bend) used
for confinement as per IS: 456:2000 & SP34:1987
X1-456 and X2-456
4 Φ8 and 4 Φ6 Φ3 at 100 mm c/c
2 Φ8 and 2 Φ6 (top and bottom)
Φ3 at 35 mm c/c for a distance of 270 mm from joint and 50 mm c/c for
remaining length
Two U bar , Φ3 as per IS 456 and 2 Φ6
diagonally on two faces with development length in tension
extended to upper and lower column
No ties at joint. But two U bar (hairclip-type bend) used for
confinement as per IS: 456:2000 & SP34:1987 and additional diagonal bars on
two faces
71
(a) Conventional (Type 1) (b) Non-conventional (Type 2)
Figure 4.9 Photographs showing detailing as per IS 13920
(a) Conventional (Type 3) (b) Non-conventional (Type 4)
Figure 4.10 Photographs showing detailing as per IS 456
72
4.5 EXPERIMENTAL SET UP
The joint assemblages were subjected to axial load and reversed
cyclic load. The specimens were tested in an upright position and static
reversed cyclic loading was applied at the end of the beam. A constant
column axial load was applied by means of 392.4 kN (40 t) hydraulic jack
mounted vertically to the 981 kN (100 t) loading frame to simulate the gravity
load on the column. The axial load was selected as three and ten per cent of
the column axial load capacities for the first and second series respectively.
Axial load for the first series specimen was 15.92 kN (1.62 t) and for the
second series was 53.06 kN (5.41 t). One end of the column was given an
external hinge support, which was fastened to the strong reaction floor and the
other end was laterally restrained by a roller support to allow moment-free
rotation at both ends. A schematic drawing of the setup is shown in Figure
4.11. Cyclic loading was applied by two 196.2 kN (20 t) hydraulic jacks, one
kept fixed to the loading frame and the other to the strong reaction floor.
Reversed cyclic load was applied at 50 mm from the free end of the beam
portion. The test was load-controlled and the specimen was subjected to an
increasing cyclic load up to failure with the load increment of 1.962 kN (200
kg).
Figure 4.12 shows the loading sequence of the test assemblages.
Load cells with least count 0.0981 kN were used. The specimens were
instrumented with Linear Variable Differential Transformer (LVDT) having
least count 0.1 mm to measure the deflection at the loading point. An
inclinometer was fixed on top of the beam element of specimen to measure
the rotation near the interface during loading. Electrical resistance strain
gauges were used to measure the strain in the reinforcement during testing.
The experimental setup at laboratory is shown in Figure 4.13.
73
Figure 4.11 Schematic Diagram of Test Set-Up
Figure 4.12 Sequence of Cyclic Loading for Exterior Beam-Column Joint
74
Figure 4.13 Test Setup in the Laboratory
4.6 RESULTS AND DISCUSSION
In this section, the test results are presented in the form of load-
deformation hysteretic curves, energy dissipation curves, ductility charts,
moment - rotation envelope curves, stiffness degradation curves and the force
- reinforcement strain envelopes. The observations during the test are briefly
described here.
4.6.1 Cracking pattern and Failure Mode
In all the specimens in the first series (with axial load 15.92 kN),
initial diagonal hairline crack in the joint occurred at the second loading cycle
when the load reached 3.924 kN in both positive and negative cycle of
loading. But for the second series (with axial load 53.06 kN), the first crack
appeared only in the third cycle for a load of 5.886 kN in all specimens. The
75
yield and ultimate load for the test specimens are shown in Table 4.6.
The cracking patterns of test specimens in the first and second series are
shown in Figure 4.14 and Figure 4.15. In almost all specimens tensile cracks
were developed at the interface between the column and beam. Extensive
damage at the joint region can be noticed in specimen A1-13920 (Figure
4.14(a)), but when axial load increases the damage gets reduced as seen in
A2-13920 (Figure 4.15 (a)). The specimens failed due to the advancement of
crack width at the interface between beam and column. There was a clear
vertical cleavage formed at the junction of all the specimens. In addition, for
the first series specimens, A1-13920 and A1-456, cracks were developed in
the joint region also (Figure 4.14 (a) and Figure 4.14(c)). But, in the case of
X1-13920 and X1-456, the cracks were concentrated in the beam region only
(Figure 4.14(b) and Figure 4.14(d)). Among the specimens, X1-456 exhibited
the best performance. The second series specimens were tested under
increased axial load. The performance was improved due to the increased
axial load. Figure 4.15 shows the specimen in the second series. Like in the
case of the first series, the specimens had failed due to vertical cleavage at
beam-column joint interface.
Table 4.6 Yield and Ultimate Load of Specimens from Experiment
Designation of specimen
Experimental Yield Load (kN)
Experimental Ultimate Load (kN)
Downward direction
Upward direction
Average (Pye)
Downward direction
Upward direction
Average (Pue)
A1-13920 11.77 11.77 11.77 16.18 15.69 15.93 X1-13920 13.73 13.73 13.73 18.63 17.65 18.14
A1-456 15.69 13.73 14.71 16.67 14.71 15.69 X1-456 13.73 13.73 13.73 19.62 19.62 19.62
A2-13920 15.7 15.7 15.7 17.65 19.62 18.64 X2-13920 13.73 13.73 13.73 18.64 18.64 18.64
A2-456 15.7 15.7 15.7 18.64 18.64 18.64 X2-456 15.7 13.73 14.71 19.62 19.62 19.62
76
(a) Specimen A1-13920
(b) Specimen X1-13920
(c) Specimen A1-456
Figure 4.14 Crack pattern in the Specimens of First Series
77
(d) Specimen X1-456
Figure 4.14 (Continued)
(a) Specimen A2-13920
(b) Specimen X2-13920
Figure 4.15 Crack Pattern in the Specimens of Second Series
78
(c) Specimen A2-456
(d) Specimen X2-456
Figure 4.15 (Continued)
A major flexural crack was developed at the beam-column interface
in all specimens. For the specimens with inclined bars (with non-conventional
detailing) no major cracks were noticed at the joint and the joint remained
intact throughout the test. Here the failure was dominated by tensile failure at
the interface than at the joint failure. The improvement of performance by
developing further cracks away from the joint face to the beam region can be
noticed for the specimens with U-bars and inclined bars (X1-456 and X2-
456). For specimen X2-456 the crack width is also less compared to other
specimens.
79
-20
-15
-10
-5
0
5
10
15
20
-35 -30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30 35 40 45
Beam end displacement (mm)
Loa
d (k
N)
4.6.2 Hysteretic Loops
The force - displacement hysteretic loops for all specimens are as
shown in Figure 4.16 to Figure 4.23. From Table 4.6 it can be seen that the
ultimate load carrying capacity is higher for specimens detailed as per IS 456
with U bars and inclined bars. In this type of non-conventional detailing, the
stiffness increases with loading up to seventh cycle. Here the ductility and
energy dissipation capacities increased without compromising the stiffness. In
general, specimens with inclined bars perform better than conventionally
detailed counterparts. A relative comparison of overall force-deformation
behavior of all specimens in series 1 and series 2 are as shown in Figure 4.24
and Figure 4.25 respectively. It is clear that specimen X1-456 and X2-456
attained maximum loads with less strength degradation.
Figure 4.16 Load Versus Displacement Curve of Specimen A1-13920
80
-20
-15
-10
-5
0
5
10
15
20
-35 -30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30 35 40 45Beam end displacement (mm)
Loa
d (k
N)
Figure 4.17 Load versus Displacement Curve of Specimen X1-13920
-20
-15
-10
-5
0
5
10
15
20
-35 -30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30 35 40 45Beam end displacement (mm)
Loa
d (k
N)
Figure 4.18 Load versus Displacement Curve of Specimen A1-456
81
-20
-15
-10
-5
0
5
10
15
20
-35 -30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30 35 40 45Beam end displacement (mm)
Loa
d (k
N)
Figure 4.19 Load versus Displacement Curve of Specimen X1-456
-20
-15
-10
-5
0
5
10
15
20
-35 -30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30 35 40 45Beam end displacement (mm)
Loa
d (k
N)
Figure 4.20 Load versus Displacement Curve of Specimen A2-13920
82
-20
-15
-10
-5
0
5
10
15
20
-35 -30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30 35 40 45Beam end displacement ( mm )
Loa
d ( k
N )
Figure 4.21 Load versus Displacement Curve of Specimen X2-13920
-20
-15
-10
-5
0
5
10
15
20
-35 -30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30 35 40 45
Beam end displacement (mm)
Loa
d (k
N)
Figure 4.22 Load versus Displacement Curve of Specimen A2-456
83
-20
-15
-10
-5
0
5
10
15
20
-35 -30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30 35 40 45
Beam end displacement (mm)
Loa
d (k
N)
Figure 4.23 Load versus Displacement Curve of Specimen X2-456
-20
-15
-10
-5
0
5
10
15
20
-35 -30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30 35 40 45
Displacement (mm)
Loa
d (k
N)
A1-13920 X1-13920 A1-456 X1-456
Figure 4.24 Load-Displacement Envelopes of Specimens in First series
84
-20
-15
-10
-5
0
5
10
15
20
-35 -30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30 35 40 45
Displacement (mm)
Loa
d (k
N)
A2-13920 X2-13920 A2-456 X2-456
Figure 4.25 Load-Displacement Envelopes of Specimens in Second Series
0
250
500
750
1000
1250
1500
1750
2000
2250
2500
0 10 20 30 40 50
Cumulative upward displacement (mm)
Cum
ulat
ive
ener
gy d
issi
pate
d (k
Nm
m)
A1-13920X1-13920A1-456X1-456A2-13920X2-13920A2-456X2-456
Figure 4.26 Cumulative Energy Dissipation curves of Specimens
85
4.6.3 Energy Dissipation
Structures with higher energy dissipation characteristics are able to
undergo stronger shaking and exhibit better seismic response. The area
enclosed by a hysteretic loop at a given cycle represents the energy dissipated
by the specimen during that cycle (El-Amoury and Ghobarah 2002). Figure
4.26 shows the cumulative energy dissipated versus cumulative displacement
curve of all specimens. It was observed that specimen X2-456 has the highest
value of energy dissipation of 2478.74 kN mm. The energy dissipation for all
the specimens is shown in Figure 4.27. It was observed that the increase in
axial load is beneficial for improving the energy dissipation characteristics of
specimens with inclined bars and U-bars. The energy dissipated in each cycle
for the test specimens are shown in Figure 4.28 to Figure 4.31. It is clearly
observed that specimens detailed with inclined bars (the non-conventional
detailing) have more energy dissipation than that of conventionally detailed
specimens.
995.54
1295.91376.68
804.86547.21
1438.29
735.65
2478.74
0250500750
1000125015001750200022502500
A-13920 X-13920 A-456 X-456Specimen
Cum
ulat
ive
ener
gy d
issi
pate
d (k
Nm
m)
series-1 series-2
Figure 4.27 Energy Dissipation Chart
86
0
200
400
600
800
1000
1200
C-1 C-2 C-3 C-4 C-5 C-6 C-7 C-8 C-9Cycle Number
Ene
rgy
Diss
ipat
ed (k
Nm
m)
A1-13920, DETAILED AS PER IS 13920, AXIAL LOAD 15.92 kNX1-13920, DETAILED AS PER IS 13920 WITH DIAGONAL CONFINING BARS, AXIAL LOAD 15.92 kN
Figure 4.28 Energy Dissipation Chart in Each Cycle for Specimens
A1-13920 and X1-13920
0
200
400
600
800
1000
1200
C-1 C-2 C-3 C-4 C-5 C-6 C-7 C-8 C-9 C-10 C-11Cycle Number
Ene
rgy
Diss
ipat
ed (k
Nm
m)
A1-456, DETAILED AS PER IS 456, AXIAL LOAD 15.92 kNX1-456, DETAILED AS PER IS 456 WIT H DIAGONAL CONFINING BARS, AXIAL LOAD 15.92 kN
Figure 4.29 Energy Dissipation Chart in Each Cycle for Specimens
A1-456 and X1-456
87
0
200
400
600
800
1000
1200
C-1 C-2 C-3 C-4 C-5 C-6 C-7 C-8 C-9 C-10Cycle Number
Ene
rgy
Diss
ipat
ed (k
Nm
m)
A2-13920, DETAILED AS PER IS 13920, AXIAL LOAD 53.06 kNX2-13920, DETAILED AS PER IS 13920 WITH DIAGONAL CONFINING BARS, AXIAL LOAD 53.06 kN
Figure 4.30 Energy Dissipation Chart in Each Cycle for Specimens
A2-13920 and X2-13920
0
200
400
600
800
1000
1200
C-1 C-2 C-3 C-4 C-5 C-6 C-7 C-8 C-9 C-10Cycle Number
Ene
rgy
Diss
ipat
ed (k
Nm
m)
A2-456, DETAILED AS PER IS 456, AXIAL LOAD 53.06 kNX2-456, DETAILED AS PER IS 456 WIT H DIAGONAL CONFINING BARS, AXIAL LOAD 53.06 kN
Figure 4.31 Energy Dissipation Chart in Each Cycle for Specimens
A2-456 and X2-456
88
4.6.4 Displacement Ductility
Ductility is the capacity of the structure/member to undergo deformation beyond yield without loosing much of the load carrying capacity. In earthquake resistant design of structures, all the critical regions should be designed and detailed with large ductility and stable hysteretic behaviour. The ductility is generally measured in terms of displacement ductility, which is the ratio of the maximum deformation that a structure or element can undergo without significant loss of initial yielding resistance to the initial yield deformation (Park and Paulay 1975). The displacement ductility for all specimens are presented in Table 4.7. The effect of axial load on the displacement ductility can be seen from the ductility bar chart shown in
Figure 4.32. The series 1 specimen with '0.03f Ac g column axial load (where
Ag is the gross cross sectional area of column) had higher ductility than the
series 2 specimens with gA'c0.1f column axial load. The displacement
ductility of specimens with inclined bars was found to be higher than their conventionally detailed counter parts. The specimens X1-456 and X2-456 (with inclined bars and U bars) exhibited high displacement ductility.
Table 4.7 Displacement Ductility of Specimens
Specimen
Displacement (mm) Displacement ductility Average
ductility
% increase of Ductility
for Non conventional
specimens
Yield Ultimate
Downward direction
Upward direction
Downward direction
Upward direction
Downward direction
Upward direction
A1-13920 4.2 3.8 36.5 18.9 8.69 4.97 6.83 -----
X113920 4 3 43.7 18 10.93 6 8.46 23.86
A1-456 4.4 5.5 22.6 22.1 5.13 4.01 4.57 -----
X1-456 3.9 1.4 29.9 14.5 7.67 10.35 9.01 97.15
A2-13920 2.6 1.8 18.2 11.8 7 6.56 6.78 -----
X2-13920 5.1 1.9 35 14.6 6.86 7.68 7.27 7.23
A2-456 5 5 22.8 14 4.56 2.8 3.68 -----
X2-456 4.1 2.9 40.8 22.6 9.95 7.79 8.87 141
89
Figure 4.32 Comparison of Ductility of Beam-Column Joint Specimens
4.6.5 Joint Shear Stress
The horizontal shear stress of the exterior joint sub assemblage can
be given by Equation (4.3) (Murty et al 2003).
cLcD0.5bL
bdbL
hcoreAP
jhτ (4.3)
where P is the imposed cyclic load at the end of the beam, bL and cL are
lengths of beam and column, respectively, cD is total depth of column, bd is
effective depth of beam and hcoreA is the horizontal cross-sectional area of
the joint core resisting the horizontal shear force. It is necessary to limit the
magnitude of horizontal joint shear stress to protect the joint against diagonal
crushing. The ACI-318 standard limits the horizontal joint shear stress as
'cf0.083γ MPa, where '
cf is the cylinder compressive strength in MPa. The
0
1
2
3
4
5
6
7
8
9
10
A-13920 X-13920 A-456 X-456
Specimen
Disp
lace
men
t Duc
tility
Series-1Series-2
90
factor γ depends on the confinement provided by the members framing in to
the joint; is taken as 20, 15 and 12 for interior, exterior, and corner joints
respectively.
Table 4.8 Comparison of Ultimate Joint Shear Stress with ACI Code
Prescribed Limiting Values
Designation of specimen
Downward ultimate
load
uP kN
hjτ
MPa
% increase for X-
specimen ACI
hj
ττ
Upward Ultimate
Load
uP kN
hjτ MPa
% increase for X-
specimen ACI
hj
ττ
A1-13920 16.18 6.02 --- 1.01 15.69 5.84 --- 0.98
X1-13920 18.63 6.94 15.28 1.17 17.65 6.57 12.5 1.10
A1-456 16.67 6.20 --- 1.04 14.71 5.47 --- 0.92
X1-456 19.62 7.31 17.90 1.23 19.62 7.31 33.64 1.23
A2-13920 17.65 6.57 --- 1.10 19.62 7.31 --- 1.23
X2-13920 18.64 6.94 5.63 1.17 18.64 6.94 --- 1.17
A2-456 18.64 6.94 --- 1.17 18.64 6.94 1.17
X2-456 19.62 7.31 5.33 1.23 19.62 7.31 5.33 1.23
In Table 4.8, 'cf is 35.4 MPa and maximum permissible shear stress
as per ACI is 5.92 MPa.
The ultimate value of joint horizontal shear stress induced in the
joints are almost equal or little higher than the ACI recommended values.
It can be seen from Table 4.8 that the shear resisting capacity is more for non-
conventionally detailed specimens than the conventional specimens. The
increase in axial load also improves the shear capacity of joints.
91
4.6.6 Moment Rotation Relation
The envelope curve for the moment at beam near interface of the
sub assemblage and the rotation of the beam element during downward
loading in each cycle are shown in Figure 4.33 and Figure 4.34. Rotation was
higher for the first series specimens than the second series. The rotation is
higher for X1-456 which attained maximum rotation of 0.158 radians.
0100020003000400050006000700080009000
10000
0 0.05 0.1 0.15 0.2
Mom
ent (
kNm
m)
Rotation at joint interface (radians)
A1-456A1-13920X1-456X1-13920
Figure 4.33 Moment- Rotation Relation of Beam Element of Specimens
in the First Series
92
0100020003000400050006000700080009000
10000
0 0.05 0.1 0.15 0.2
Mom
ent (
kNm
m)
Rotation at joint interface(radians)
A2-456A2-13920X2-456X2-13920
Figure 4.34 Moment - Rotation Relation of Beam Element of Specimens
in the Second Series
4.6.7 Stiffness
The stiffness of the beam-column joint was approximated as slope
of the peak to peak line in each loading cycle. The variations of stiffness in
each cycle corresponding to maximum displacement are calculated for the
first and second series specimens and are as shown in Figure 4.35 and Figure
4.36 respectively. Stiffness was higher for specimen X1-456 in the first
series, but in second series initially the stiffness was higher for specimen
A2-13920, but the specimens X2-13920 and X2-456 showed considerable
displacement without sudden loss in load carrying capacity.
93
0
5
10
15
20
25
30
0 5 10 15 20 25 30 35 40 45
Sti
ffn
ess(
kN/m
m)
Displacement (mm)
A1-13920X1-13920A1-456X1-456
Figure 4.35 Stiffness Degradation of Specimens in the First Series
0
5
10
15
20
25
30
0 5 10 15 20 25 30 35 40 45
Sti
ffn
ess
(kN
/mm
)
Displacement (mm)
A2-13920X2-13920A2-456X2-456
Figure 4.36 Stiffness Degradation of Specimens in the Second Series
94
4.6.8 Reinforcement Strain
In order to study the effectiveness of non-conventional detailing,
electrical resistance strain gauge data for the first series of specimens were
measured. The locations of strain gauges are shown in Figure 4.37. The
envelopes of beam end force - reinforcement strain curve are shown in Figure
4.38 to Figure 4.41. It can be seen that the column bars of the conventionally
detailed specimens strained to some extent. But for the non-conventional
specimen the strain in column main bars were negligible. The main
reinforcement in the outer face of columns was free from strains. This
indicates that shear damage in the joint region was reduced for these
specimens. Strain readings of gauges confirmed that all column longitudinal
bars remained elastic during testing. The inclined bars remained active during
the entire cycle and resisted the shear stresses. The strains in the beam bars
were higher for all the specimens which show the beam mode failure.
Figure 4.37 Locations of Strain Gauges
95
-20
-15
-10
-5
0
5
10
15
20
-0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5
For
ce (k
N)
strain(%)1-Beam top
2-Beam bottom3-Column near4-Column far
5-Lateral
Figure 4.38 Force - Reinforcement Strains Envelope of Specimen A1-13920
-20
-15
-10
-5
0
5
10
15
20
-0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5
For
ce (k
N)
Strain (%)1-Beam Top2-Beam bottom3-Column near4-Column far5-Lateral6-Inclined bar(1)7-Inclined bar(2)
Figure 4.39 Force - Reinforcement Strains Envelope of Specimen X1-13920
96
-20
-15
-10
-5
0
5
10
15
20
-0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5
For
ce (k
N)
Strain (%)
1-Beam top2-Beam bottom3-Column near4-Column far5-U-bar
Figure 4.40 Force - Reinforcement Strains Envelope of Specimen A1-456
-20
-15
-10
-5
0
5
10
15
20
-0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5
For
ce (k
N)
Strain (%) 1-Beam Top
2-Beam Bottom
3-Column near
4-Column fa r
5-U-bar
6-Inclined bar(1)
7-Inclined bar (2)
Figure 4.41 Force- Reinforcement Strains Envelope of Specimen X1-456
97
4.7 SUMMARY
The experimental study focused on developing non-conventional
reinforcement detailing of the exterior beam-column joint in order to reduce
the congestion in the joint region. Eight specimens were cast and tested. The
confining reinforcement was detailed as per i) IS 13920 (Type 1), ii) IS 13920
with non-conventional detailing (Type 2), iii) IS 456 (Type 3), and iv) IS 456
with non-conventional detailing (Type 4). All the specimens were tested
under reversed cyclic loading, with appropriate axial load. The experimental
investigation shows that the beam-column joints designed as per IS 456 with
non-conventional detailing performed well. It is observed that the non-
conventionally detailed specimens (Type 2 and Type 4) have an improvement
in average ductility of 16% and 119% than their conventionally detailed
counter parts (Type1 and Type 3). Further, the joint shear capacity of the
Type 2 and Type 4 specimens are improved by 8.4% and 15.6% than the
corresponding specimens of Type 1 and Type 3 respectively. The proposed
non-conventional detailing is simple without any congestion compared to the
detailing as per IS: 13920. It is suggested that the joint detailing of Type 4
specimen i.e., providing inclined bars and hairpin bends as laterals can be
used in the exterior joints of low rise moment resisting frames in low to
moderate seismic risk region.