CHAPTER 8 RESULTS AND DISCUSSIONshodhganga.inflibnet.ac.in/bitstream/10603/10094/13... · RESULTS AND DISCUSSION 8.1 GENERAL This section presents the experimental results of the

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CHAPTER 8

RESULTS AND DISCUSSION

8.1 GENERAL

This section presents the experimental results of the investigation

on the joint seismic behaviour of beam-column joints under cyclic and reverse

cyclic loading, on the addition of different proportions of hybrid fibre in the

joint regions cast using three different mixes. The results which were obtained

from various tests are discussed in this chapter.

8.2 LOAD DEFLECTION BEHAVIOUR OF BEAM COLUMN

JOINT SPECIMEN

8.2.1 General

An important figure that must be generated to evaluate the

structural seismic performance is the force-displacement hysteresis response.

The force-displacement hysteresis response indicates the energy dissipation

capacity of the structure by considering the area encompassed by the

hysteresis loops. In this study, the lateral load and displacement of the beam

in a beam column joint were measured for drawing the hysteresis loops. In

order to study load carrying capacity and ductility of all the specimens,

envelopes of load-displacement hysteresis curves for all the specimens were

plotted for both cyclic and reverse cyclic loading. Using these envelopes the

peak load, ultimate displacements and ductility of the specimens were

obtained.

104

8.2.2 Cyclic Loading

8.2.2.1 Beam-Column Joint Specimens Cast using M20 Concrete

From Figures 8.1 to 8.7 and from the Table 8.1 it is observed that

the specimen I F21 which was cast by using M20 concrete had maximum

ultimate load compared to all the specimens. It was 35% greater than the

specimen cast using ordinary concrete (I O1) and 3.4% greater than the

specimen cast by using steel fibre only (I F11).

Figure 8.1 Load Deflection Curve Figure 8.2 Load Deflection Curve

for Specimen I O1 for Specimen I S1


for Specimen I F11 for Specimen I F21

105



Figure 8.7 Load Deflection Curve For Specimen I F51

Table 8.1 Ultimate Load and Deflection of M20 Concrete Specimens

Subjected to Cyclic Loading

Sl.NoSpecimen

Id

Ultimate

load (pu)

kN

Ultimate

Deflection

(mm) ( u)

Percentage of

increase in

Ultimate load

1 I O1 6.7 17 -

2 I S1 7.5 20 11.94

3 I F11 8.8 20 31.34

4 I F21 9.1 20 35.82

5 I F31 7.9 20 17.91

6 I F41 7.7 20 14.93

7 I F51 4.2 17.2 -37.31

106


From Figures 8.8 to 8.14 and from Table 8.2 it is observed that the

specimen II F21 which was cast using M25 concrete had maximum ultimate

load compared to all the specimens. It was 31% greater than the specimen cast

by using ordinary concrete (II O1) and 3.3% greater than the specimen cast by

using steel fibre only (II F11).


for Specimen II O1 for Specimen II S1


for Specimen II F11 for Specimen II F 21

107


for Specimen II F 31 for Specimen II F41

Figure 8.14 Load Deflection Curve for Specimen II F51



Sl.NoSpecimen

Id

Ultimate

load (Pu)

kN

Ultimate

Deflection

(mm) ( u)

Percentage of

increase in

Ultimate load

1 II O1 7 20 -

2 II S1 7.8 20 11.43

3 II F11 8.9 20 27.14

4 II F 21 9.2 25 31.43

5 II F31 7.8 20 14.29

6 II F41 7.7 20 11.43

7 II F51 5.9 20 -15.71

108

8.2.3 Reverse Cyclic Loading



specimen I F12 which was cast by using M20 concrete had maximum ultimate

load compared to all other specimens. It is 68% greater than the specimen cast

by using ordinary concrete (I O2) and 11% greater than the specimen cast by

using 1.5% steel fibre and 0.2% of polypropylene fibre (I F22). The increase

in steel fibre increases the ultimate load carrying capacity and increases in

polypropylene fibre decreases the ultimate load carrying capacity.

Figure 8. 15 Load Deflection Curve Figure 8. 16 Load Deflection Curve

for Specimen I O2 for Specimen I S2



109



Figure 8. 21 Load Deflection Curve for Specimen I F52

Table 8.3 Ultimate Load and Deflection of M20 Concrete Subjected to

Reverse Cyclic Loading

Sl.

No

Specimen

Id

Ultimate load (Pu) kN Ultimate Deflection (mm) ( u)

Upward Downward Upward Downward

1 I O2 10.2 10 30 15

2 I S2 15 13.4 30 30

3 I F12 17.2 16.8 45 30

4 I F 22 15.6 15.4 45 45

5 I F32 11.6 13.4 45 30

6 I F 42 12.4 12.2 45 30

7 I F 52 11.6 11.4 30 30

110



specimen II F 22 which was cast by using M25 concrete had maximum

ultimate load compared to all other specimens. It is 32% greater than the

specimen cast by using ordinary concrete (II O2) and 3.5% greater than the

specimen cast using 1.5% steel fibre (II F 12).

Figure 8. 22 Load Deflection Curve Figure 8.23 Load Deflection Curve

for Specimen II O2 for Specimen II S2


for Specimen II F12 for Specimen II F22

111

Figure 8.26 Load Deflection Curve Figure 8. 27 Load Deflection Curve

for Specimen II F32 for Specimen II F42

Figure 8. 28 Load Deflection Curve for Specimen II F52


Subjected to Reverse Cyclic Loading.

Sl.

No

Specimen

Id

Ultimate load (Pu) kN Ultimate Deflection (mm) ( u)


1 II O2 10.6 13.4 30 15

2 II S2 12.2 15.4 30 30

3 II F12 16.2 17.2 40 30

4 II F 22 16.6 17.8 45 45

5 II F32 16 15.2 45 30

6 II F 42 14.5 16.2 45 30

7 II F 52 11.2 12.0 30 30

112



specimen III F22 which was cast by using M60 concrete with 1.5% of steel

fibre and 0.2% of polypropylene fibre had maximum ultimate load compared

to all other specimens. It was 70% greater than the specimen cast using

ordinary concrete (III O2) and 9.3% greater than the specimen cast by using

1.5% steel fibre only (III F12).The increase in polypropylene fibre decreases

the ultimate load carrying capacity.


for Specimen III O2 for Specimen III S2

Figure 8. 31 Load Deflection Curve Figure 8.32 Load Deflection Curve

for Specimen III F12 for Specimen III F22

113

Figure 8. 33 Load Deflection Curve For Specimen III F32


Subjected to Reverse Cyclic Loading.

Sl.NoSpecimen

Id

Ultimate load (Pu) kNUltimate Deflection

(mm) ( u)


1 III O2 22 21.2 30 30

2 III S2 23.4 26 45 30

3 III F12 30.6 34.4 45 30

4 III F 22 32.7 37.6 45 45

5 III F32 28.4 30.4 45 30

8.2.3.4 Summary

The ultimate load carrying capacity is optimum for the

polypropylene fibre content of 0.2% in addition to the constant 1.5% of steel

content. Further increase in polypropylene fibre was found to reduce the

strength of the joint

114

8.3 OVERALL LOAD-DISPLACEMENT CURVES

8.3.1 General

The beam end load versus beam end displacement plots for all the

specimens were drawn to know the ultimate load and ultimate displacement.

8.3.2 Overall Load-Displacement Curves for M20 Concrete Specimens

Figure 8.34 shows the overall load displacement curve for all the

specimens cast by using M20 concrete. From this figure it is noted that the

specimen I F12 cast using. M20 concrete with 1.5% of steel fibre has

maximum ultimate load of 17.2 kN and specimen I F22 has max positive and

negative ultimate displacement of 45 mm.

Figure 8.34 Overall Load-Displacement Curves for M20 Concrete

Specimens

115

8.3.3 Overall Load-Displacement Curves for M25 Concrete

Specimens


specimens cast by using M25 concrete. From this figure it is noted that the

specimen II F22 cast by using. M25 concrete with 1.5% of steel fibre and

0.2 % of polypropylene fibres has maximum ultimate load of 17.8 kN and

maximum positive and negative ultimate displacement of 45 mm.

Figure 8.35 Overall Load-Displacement Curves For M25 Concrete

8.3.4 Overall Load-Displacement Curves for M60 Concrete

Specimens


specimens cast using M60 concrete. From this Figure it is noted that the

specimen III F22 cast by using. M60 concrete with 1.5% of steel fibre and

0.2 % of polypropylene fibres has maximum ultimate load of 37.6 kN and

max positive and negative ultimate displacement of 45 mm.

116

Figure 8.36 Overall Load-Displacement Envelope Curves For M60

Concrete Specimens

8.4 ENERGY DISSIPATION BEHAVIOUR

8.4.1 General

As a measure of the dissipated energy of the specimens, the area

under the full load-displacement envelopes was computed and defined as the

energy that could dissipate by the specimens before the system loses its stability.

8.4.2 Calculation of Accumulated Hysteretic Energy

The energy dissipation capacity is the summation of area of

hysteresis loop of each cycle for a particular specimen by measuring the area

of the each cycle manually or some other means.

8.4.3 Cyclic Loading

8.4.3.1 M20 Concrete Specimens

Table 8.6 and Figure 8.37 shows the energy dissipation capacity of

all the specimens cast by using M20 concrete subjected to cyclic loading.

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From this it is observed that the specimen I F21 had maximum energy

dissipating capacity compared to all the specimens.

Table 8.6 Energy Dissipation Capacity of M20 Concrete Specimens


Sl.NoSpecimen

Id

Energy dissipation

capacity (Ecu)

kNmm

1 I O1 104.52

2 I S1 122.48

3 I F11 134.40

4 I F21 149.52

5 I F31 114.56

6 I F41 110.2

7 I F51 74.80

Figure 8.37 Energy Dissipation Capacity of M20 Concrete Specimens


118

The energy dissipating power was increased by 28 % by adding

only steel fibre and 43 % by adding hybrid fibre with the combination of

1.5% steel fibre and 0.2 % polypropylene fibre. Further increase in the

polypropylene fibre is found to reduce the energy absorbing capacity

gradually. Specimen II F51 which have only polypropylene fibre (1.5%) had

less energy absorbing capacity compared to other specimens.


The energy dissipation capacity of all the specimens cast using

M25 concrete subjected to cyclic loading are presented in Table 8.7 and

Figure 8.38. From this it is observed that the specimen II F21 had the

maximum energy dissipating capacity compared to all the specimens. The

energy dissipation power is increased by 78 % by adding only steel fibre and

143.5 % by adding hybrid fibre with the combination of 1.5% steel fibre and

0.2 % polypropylene fibre. Further increase in the polypropylene fibre is

found to reduce the energy dissipating capacity gradually. Specimen II F51

which have only polypropylene fibre (1.5%) had least energy dissipating

capacity compared to other specimens.



Sl.NoSpecimen

Id

Energy

dissipation (Ecu)

in kNmm

1 II O1 106.75

2 II S1 131.75

3 II F11 192.72

4 II F 21 259.96

5 II F31 190.2

6 II F41 117.2

7 II F51 97.8

119



8.4.4 Reverse Cyclic Loading



all the specimens cast using M20 concrete subjected to reverse cyclic loading.

From this it is observed that the specimen I F22 had the maximum energy

dissipating capacity compared to all other specimens. The energy absorption

power has an increase of 87 % by adding only steel fibre and 205 % by

adding hybrid fibre with the combination of 1.5% steel fibre and 0.2 %

polypropylene fibre. Further increase in the polypropylene fibre is found to

reduce the energy absorbing capacity gradually. Specimen I F52 which have

only polypropylene fibre (1.5%) had less energy absorbing capacity compared

to other specimens.

120


Subjected to Reverse Cyclic Loading

Sl.No Specimen Id

Energy

dissipation (Ecu)

in kNmm

1 I O2 247.6

2 I S2 282.6

3 I F12 464.4

4 I F 22 755.2

5 I F32 588.4

6 I F 42 560

7 I F 52 304.2





all the specimens cast using M25 concrete subjected to reverse cyclic loading.

From this it is observed that the specimen II F22 had maximum energy

dissipating capacity compared to all the specimens. The energy dissipation

121

capacity was increased by 161 % by adding only steel fibre and 233 % by

adding hybrid fibre with the combination of 1.5% steel fibre and 0.2 %

polypropylene fibre. Further increase in the polypropylene fibre is found to

reduce the energy dissipating capacity gradually. Specimen II F52 which

have only polypropylene fibre (1.5%) had less energy dissipating capacity

compared to other specimens.



Sl.NoSpecimen

Id

Energy

Dissipation

Capacity(Ecu) kNmm

1 II O2 264

2 II S2 337

3 II F12 689.2

4 II F 22 879

5 II F32 668

6 II F 42 606

7 II F 52 369.2



122


Table 8.10 and Figure 8.41 shows the energy dissipation capacity

of all the specimens cast by using M60 concrete subjected to reverse cyclic

loading. From this it is observed that the specimen III F22 (1.5% of steel fibre

and 0.2% of polypropylene fibre) had maximum energy dissipating capacity

compared to all the other specimens. The energy dissipation power was

increased by 178 % by adding only steel fibre and 241 % by adding hybrid

with the combination of 1.5% steel fibre and 0.2 % polypropylene fibre.

Further increase in the polypropylene fibre is found to reduce the energy

dissipating capacity gradually. Specimen III F52 which have only

polypropylene fibre (1.5%) had less energy dissipating capacity compared to

other specimens.


Subjected to Reversed Cyclic Loading

123



Sl.No Specimen IdEnergy Dissipation

Capacity in (Ecu) kNmm

1 III O2 522

2 III S2 866

3 III F12 1455

4 III F 22 1781

5 III F32 1207

8.4.5 Overall Comparison of Energy Dissipation Capacity

From above results and discussions it is found that, of all the

specimens tested, the energy dissipation capacity of specimens in the F2

series which were cast by using hybrid fibre combination of 1.5% steel fibre

and 0.2% of polypropylene fibre had more energy dissipation capacity under

cyclic and reverse cyclic loading in concretes of M20, M25 and M60 grades.

Figure 8.42 shows overall comparison of the energy dissipating capacity of all

the specimens subjected to reverse cyclic loading. This increase in energy

dissipation capacity is about 225% greater than the specimen cast by using

ordinary concrete and 25% greater than the specimen cast by using steel fibre

only. Hence the hybrid fibre combination consisting of 1.5% of steel fibre

and 0.2% of polypropylene fibre can be adopted in beam-column joints of

structures in earthquake prone areas.

Figure 8.42 Energy Dissipating Capacity of all the Specimens

124

8.5 DISPLACEMENT DUCTILITY FACTOR

8.5.1 General

The displacement ductility factor is defined as the ratio between

ultimate displacement and yield displacement.

= ( u / y) (8.1)

where u is the deflection corresponds to ultimate load and y the deflection

corresponding to yielding of steel.

8.5.2 Displacement Ductility Factors for all the Specimens

Figure 8.43 shows the ductility factor of all the specimens cast by

using M20, M25 and M60 concrete. Displacement ductility values for all the

specimens are presented in Table 8.11. From the table it is observed that the

specimen cast using normal concrete exhibited low displacement ductility values.

The polypropylene fibre in addition to the steel fibre increases the ductility factor

but at this % the ultimate load and energy dissipation capacity was found to be

reduced. Increase in polypropylene fibre increases the ductility factor.

Figure 8.43 Ductility Factor of all the Specimens

125

Table 8.11 Displacement Ductility Factor and Ultimate Stiffness

Concrete

Grade

Specimen

Id

Yield

Displacement

y (mm)

Ultimate

displacement

u (mm)

Displacement

Ductility

Factor( u / y)

Ultimate

Stiffness

kN/mm

M20

I O2 30 45 1.50 0.8

I S2 31 60 1.94 0.84

I F12 31 75 2.41 1.16

I F 22 34 90 2.68 1.04

I F32 26 75 2.90 0.9

I F 42 20 75 3.75 0.88

I F 52 40 60 1.50 0.6

M25

II O2 31 45 1.45 0.86

II S2 33 60 1.82 0.9

II F12 34 75 2.20 1.21

II F 22 35 90 2.56 1.18

II F32 27 75 2.82 1.02

II F 42 20 75 3.68 0.94

II F 52 43 60 1.40 0.62

M60

III O2 48 60 1.25 1.8

III S2 43 75 1.73 2

III F12 36 75 2.10 2.25

III F 22 36 90 2.48 2.04

III F32 27 75 2.76 1.85

8.6 STIFFNESS BEHAVIOUR

8.6.1 General

In the case of reinforced concrete beam-column joints, stiffness of

the joint gets reduced when the joint is subjected to cyclic/repeated/dynamic

loading. This reduction in stiffness is due to the following reasons. During

cyclic loading, the materials, viz. concrete and steel, are subjected to loading,

126

unloading and reloading processes. This will cause initiation of micro-cracks

inside the joint and will sometimes lead to the fatigue limit of the materials.

This, in turn, increases the deformations inside the joints, thus resulting in

reduction in the stiffness. Hence, it is necessary to evaluate degradation of

stiffness in the beam-column joints subjected to cyclic or repeated loading.

8.6.2 Stiffness degradation of all the Specimens

The values of the secant stiffness obtained for each cycle are

plotted for all the specimens. The degradation of the secant stiffness is

obtained by plotting the ultimate stiffness versus corresponding cycle number

for each specimen tested. Figure 8.44, 8.46 and 8.47 show the stiffness plots

for specimens cast using M20, M25, and M60 concrete respectively and Table

8.11 shows the ultimate stiffness of all the specimens. It is observed from

these figures that as the number of cycles increases, stiffness decreases.

However, as the number of cycles increases, the rate of degradation of

stiffness decreases in the case of specimens additionally reinforced with

fibres. Figure 8.45 shows the stiffness degradation of each specimen in each

cycle cast using M20 concrete. The above behaviour may be attributed to the

fact that at the first cycle, micro cracks would not have initiated and hence the

fibres were not effective in the absence of formation of cracks. As the number

of cycles increases, micro-cracks develop, and fibres, which are distributed at

random, intercept these cracks and bridge across these cracks. This action will

control further propagation of cracks and will result in higher energy demand

for bonding and pull-out of fibres in the vicinity of cracks. During this

process, stiffness of the joint with fibres will not undergo much reduction

when compared to that without fibres. Addition of polypropylene fibre

reduces the stiffness.

127

Figure 8. 44 Stiffness Behaviour of M20 Concrete Specimens

Figure 8. 45 Stiffness of M20 Concrete Specimens in Each Cycle

128

Figure 8. 46 Stiffness Behaviour of M25 Concrete Specimens

Figure 8.47 Stiffness Behaviour of M60 Concrete Specimens

129

8.7 JOINT SHEAR STRESS

8.7.1 General

The design requirements for a beam-column joint in earthquake –

resistant structures is that, the joint must not yield before the adjoining

members reach their capacities and must not deform excessively. The joint

region is subjected to excessive shear stresses when any of the adjoining

members reach its over strength moment capacity associated with the

hardened plastic hinge.

8.7.2 Joint Shear stress - Experimental

For the exterior beam-column joint the horizontal and vertical joint

shear stresses ( jh, jv) can be calculated using the following formula. (Murty

et al. 2003)

jh =u

h

P

A core

b b b

b c

L L 0.5D

d L and (8.2)

jv = u b c c b

v

c c

P L 0.5D L D1

A core L d(8.3)

where Lb and Lc are the length of beam and column respectively; Db and Dc

are the total depth of beam and column respectively; db and dc are the

effective depth of beam and column respectively; Ahcore and A

vcore are the

horizontal and vertical cross sectional areas of the joint core resisting the

horizontal and vertical joint shear forces, respectively. Pu is the ultimate

load.

Ahcore = Dc * Dc

Avcore = Db * Dc

130


Model calculation for calculating horizontal vertical joint shear

stress for specimens cast using M20 concrete.

Substituting the constant values such as Lb , Lc, Db, Dc, db ,dc, Ahcore

and Avcore in equations (8.2) and (8.3).

The value of u

h

P

A core

b b b

b c

L L 0.5D

d L 0.0006*Pu

and the value of u b c c buv

c c

P L 0.5D L D1 0.00053* P

A core L d

Table 8.12 shows the horizontal and vertical shear stresses induced

in the joint region for the specimens cast using M20 Concrete. From this table

it is observed that the specimen I F12 has maximum horizontal and vertical

shear stress compared to all the other specimens.

Table 8.12 Ultimate Shear capacity of the Joint using M20 Concrete

Sl.No

(1)

Specimen

Id

(2)

Ultimate

Load

kN

(3)

Horizontal

Shear stress jh, in

kN/mm2=0.0006*

Col( 3)

Vertical Shear stress

jv in kN/mm2

=0.00053*

Col (3)

1 I O2 10.2 6.12 5.42

2 I S2 15.6 9.36 8.3

3 I F12 17.2 10.32 9.15

4 I F 22 15.4 9.24 8.19

5 I F32 13.4 8.04 7.13

6 I F 42 12.4 7.44 6.59

7 I F 52 11.6 6.96 6.17

131



in the joint region for the specimens cast by using M25 Concrete. From this

table it is observed that the specimen II F22 has maximum horizontal and

vertical shear stresses compared to all the other specimens.

Table 8.13 Ultimate Shear Capacity of the Joint using M25 Concrete

Sl.

No

Specimen

Id

Ultimate

Load

kN

Horizontal

Shear stress

jh, in N/mm2

Vertical

Shear stress

jv in N/mm2

1 II O2 13.4 8.04 7.13

2 II S2 15.8 9.48 8.4

3 II F12 17.2 10.32 9.15

4 II F22 17.8 10.68 9.47

5 II F32 16.2 9.72 8.62

6 II F42 16 9.6 8.51

7 II F52 12 7.2 6.38



in the joint region for specimens cast using M60 Concrete. From this table it is

observed that the specimen III F22 has maximum horizontal and vertical

shear stresses compared to all the other specimens.

132

Table 8.14 Ultimate Shear Capacity of the Joint using M60 Concrete

Sl.

No

Specimen

Id

Ultimate

Load

kN

Horizontal

Shear stress

jh, in

kN/mm2

Vertical

Shear stress

jv in kN/mm2

1 III O2 22 13.2 11.70

2 III S2 26 15.6 13.83

3 III F12 34.4 20.64 18.29

4 III F 22 37.6 22.56 20.00

5 III F32 30.4 18.24 16.17

8.7.3 Comparison of the Experimental Shear Stress with the

Previously Developed Model

8.7.3.1 General

An attempt has been made to compare the shear strength of joints

using the models available in literature for fibre reinforced joint. The details

of these models are presented in literature (Jiuru et al. 1992, Ganesan

et al. 2007b).

8.7.3.2 Model Developed by Jiuru et al. (1992)

A model for predicting the ultimate shear strength of the fibre

reinforced joints was developed based on the assumption that even after

cracking, considerable tensile stress remains in the concrete until the fibres are

pulled out from the matrix. Accordingly the ultimate shear stress is given by

c fib sV=V +V +V (8.4)

where Vc is the shear carried by the concrete, Vfib is the shear carried by the

fibres, and Vs is the shear carried by the joint stirrups. These are expressed as

133

c j j ac

c c ac

PV 0.1 1 b h f

b h f (8.5)

ffib f j j

f

lV 2 v b h

d (8.6)

shs ys

AV f d a

S (8.7)

lf = length of the steel fibre

df = diameter of the steel fibre

Af = aspect Ratio of the steel fibre

Vf = percentage of volume of steel fibre

P = axial load on the column

Vfib= shear resisted by fibre reinforced concrete

Vc = shear resisted by concrete

Vs = shear resisted by stirrups

fys = yield strength of transverse hoop reinforcement

8.7.3.3 Model Developed by Ganesan et al. (2007b)

For the effect of HPC in the model they carriedout, a regression

analysis. A parameter F (fibre factor) was introduced to account for the

combined effect of steel fibres, compressive strength of concrete, and

modulus of rupture, and was given by

cff f

f cr

flF 2 v b

d f (8.8)

Vpre= (th.)V ( 0.075F+1.6284) (8.9)

134

where (th.)V is given by Equation (8.4),

a = distance from extreme compressive fibre to the centroid of

compressive reinforcement

bf = bond efficiency factor (= 0.75)

bj (bc) = effective width of joint transverse to the direction

of shear

f ac = axial compressive strength of concrete

fc = compressive strength of concrete

fcr = modulus of rupture of concrete

P = axial compressive load of column

(exp) = experimental value of ultimate shear stress

(pre) = predicted value of ultimate shear stress

Vpre

= predicted value of ultimate shear force

(th.) = theoretical value of ultimate shear stress

(th.)V = theoretical value of ultimate shear force

h j = effective depth of joint parallel to the direction of

shear

(exp) =j j

exp

b h

V (8.10)

(th.) =j j

th

b h

V (8.11)

135

8.7.3.4 Comparison

An attempt has been made to compare the model proposed by Jiuru

et al. (1992) and Ganesan et al. (2007b) for shear strength with the current

experimental results.

Table 8.15 Comparison of Ultimate Shear Strength

Specimen(exp)

(N/mm2)

(1)

Calculated Values (th.)

(N/mm2) Ratio

(1)/(2)

Ratio

(1)/(3)Jiuru**

(2)

Ganesan N***

(3)

II O2 6.12 3.39 6.84 1.81 0.89

II S2 9.36 5.02 10.12 1.86 0.92

II F12 10.32 5.50 11.04 1.88 0.93

II F22 9.24 5.40 10.83 1.71 0.85

II F32 8.04 5.32 10.55 1.51 0.76

II F42 7.44 5.06 9.90 1.47 0.75

II F 52 6.96 2.39 4.62 2.91 1.51

II O2 8.04 3.54 7.28 2.27 1.10

II S2 9.48 5.17 10.62 1.83 0.89

II F12 10.32 5.59 11.50 1.85 0.90

II F22 10.68 5.65 11.71 1.89 0.91

II F32 9.72 5.30 10.64 1.83 0.91

II F42 9.6 5.12 10.14 1.88 0.95

II F52 6 2.72 5.19 2.21 1.16

III O2 13.2 5.45 8.87 2.42 1.49

III S2 15.6 7.07 11.51 2.21 1.36

III F12 20.64 7.65 17.32 2.70 1.19

III F22 22.56 7.55 17.34 2.99 1.30

III F32 18.24 7.34 16.63 2.49 1.10

AVERAGE 2.09 1.05

Coefficient of Variation 0.27 0.65

** Jiuru et al.. (1992);***

Ganesan et al. (2007b)

136

Details of comparison are given in Table 8.15. From the Table it is

observed that the average of the ratio of (exp) / (th.) is 2.09 in the case of

Jiuru et al. (1992) and 1.05 in the case of Ganesan et al. (2007b). Results

arrived based on the equations predicted by Ganesan et al. (2007b) gives the

satisfactory results for hybrid fibre reinforced joint also. From the table it is

observed that the ratio (exp) / (th.) have more variation in the case of ordinary

specimen and specimen reinforced with polypropylene fibre only. The model

predicted by Ganesan et al. (2007b) is also suitable for hybrid fibre reinforced

concrete joint, because the polypropylene fibre doesn’t take any load.

8.8 CURVATURE DUCTILITY FACTOR

8.8.1 Moment Curvature Behaviour

An attempt was made to study the moment curvature relationship

for all the specimens using the test results. The ductile behaviour of an

interior beam-column joint induces the formation of plastic hinges in the

beams near the column faces. To investigate the flexural behaviour of the

beams, various sections of the top and bottom reinforcement were

instrumented by strain gauges. The strains measured at 15mm below the

extreme tension fibre and 15mm above the extreme compression fibre have

been used to calculate the curvature, of the beam for every loading stage

using the relation

= t b

b

e e

d a (8.12)

where et = strain in the top reinforcement

eb = strain in the bottom reinforcement

db = effective depth of beam

a= compressive reinforcement cover

137

The values of moment M were calculated using the experimental

values of load and distance between the application of the load. Table 8.16

shows the moment, curvature ductility at peak load and yield load. These

values of M and were used to obtain moment-curvature plots for the joint.

8.8.2 Curvature Ductility Factor

The capacity of the member to deform beyond its initial yield

deformations with minimum loss of strength and stiffness depends upon the

ductility factor which is defined as the ratio of the ultimate deformation to its

yield deformation at first yield. Ductility may be defined easily in the case of

elastoplastic behaviour. Ductility factors in beam-column joint have been

defined in terms of curvature at critical section and is, (Ganesan et al. 2000)

Curvature ductility factor = u

y

(8.13)

where, u = curvature at peak load

y=curvature at yield = y

s b

f

E d x (8.14)

where, fy = the yield strength of reinforcement

Es =Modulus of elasticity of steel

db = the effective depth of b

x = depth of neutral axis

The curvature at peak load and curvature ductility factor thus

calculated for all M60 concrete specimens are given in Table 8.16. From the

table it is found that the hybrid fibre reinforced specimens have better values

of ductility factor than the other specimens. Increases in polypropylene fibre

increase the curvature ductility factor.

138

Table 8.16 Moment and Curvature Ductility Factor

Sl.

No

Specimen

Id

(Curvature

at peak load)

210X1

m

(Curvature at

yield load)

210X1

m

Curvature

Ductility

Factor

Moment at

Peak Load

kNmm

1 III O2 4.0927 3.0772 1.33 9900

2 III S2 5.4159 3.0772 1.76 11700

3 III F12 9.4373 3.0443 3.1 15480

4 III F22 10.828 3.0416 3.56 16920

5 III F32 11.472 3.0510 3.76 13680

8.9 JOINT DISTORTION

8.9.1 General

The stress conditions in the beam-column joint are indeed rather

complicated. The interior core of the joint bounded by the longitudinal bars in

the beam and column is in fact subjected to larger shear stresses and therefore

has more distortion compared with the entire joint. The two LVDTs mounted

diagonally on the rear face on the joint were used to measure the distortion of

the interior core of the joint. The joint distortion can be estimated as

Joint distortion = (((e1+e2) / 2) x (Dl / (hjxbj))) (8.15)

where

e1, e2 – Changes in length of the diagonal joint region in mm

Dl – Initial diagonal length in mm

hj – Depth of the joint region in mm (Db =Total depth of beam)

bj – Breadth of the joint region in mm (DC =Total depth of

column)

u y

139

Table 8.17 Joint Distortion

Sl.No SpecimenJoint distortion (mm)

M20 M25 M60

1 O Series 0.040 0.0375 0.025

2 S Series 0.035 0.03 0.020

3 F1 Series 0.0225 0.02 0.0150

4 F 2 Series 0.025 0.022 0.0185

5 F 3 Series 0.0275 0.025 0.0175

6 F 4 Series 0.03 0.0275 -

7 F 5 Series 0.035 0.030 -

8.9.2 Summary

Table 8.17 shows the joint distortion of all the specimens. From the

table it is observed that the specimen in F1 series had less joint distortion

compared to all the specimens. Thus the addition of steel fibre increases

effectively the confinement of concrete there by reduce the joint distortion.

8.10 GENARAL BEHAVIOUR AND FAILURE MECHANISMS

8.10.1 General

The development of cracks in each specimen during testing were

carefully observed and recorded by marking the cracks at the peaks of each

loading cycle.

8.10.2 Crack Pattern and Failure Mode

The crack pattern of all the specimens are shown in Figures 8.48 to

8.54 shows the crack pattern at ultimate load of failure. From the

experimental observations it is revealed that in ordinary specimens at

approximately 5mm displacement, all joints showed evidence of hair cracks

and in seismic and fibrous specimens the hair cracks formed only in the

140

second cycles. The cracks of conventional joints increased in width and

resulted in spalling of concrete (especially the concrete cover zone) and

eventually exposing the steel reinforcing bars. In almost all the specimens

tensile cracks were developed at the interface between the column and beam.

The specimens failed due to the advancement of crack width at the interface

between beam and column. A clear vertical cleavage was formed at the

junction of all the specimens. From the crack pattern it is observed that the

width of crack went on decreasing from O1, S1, F1, F2, F3, F4 and F5.The

width of crack was found to decrease when the polypropylene fibre content

was increased. The first crack load was also high when polypropylene fibre

gets increased.

Figure 8.48 Crack Pattern of Figure 8.49 Crack Pattern of

Specimen II O1 Specimen II S1

Figure 8.50 Crack Pattern of Figure 8. 51 Crack Pattern of

Specimen II F 11 Specimen II F 21

141

Figure 8.52 Crack Pattern of Figure 8. 53 Crack Pattern of

Specimen II F 31 Specimen II F 41

Figure 8.54 Crack Pattern of Specimen II F51

8.11 COMPARISON OF EXPERIMENTAL RESULTS WITH

ANALYTICAL RESULTS

Table 8.18 shows the load deflection values obtained using Abaqus

Finite Element analysis and experimental results. Figure 8.55 shows the load

deflection curves obtained using ABAQUS Finite Element analysis and

experimental results for the specimen II F 22 cast using M20 concrete. From

the figure it is observed that the load values obtained in the Finite element

analysis are much higher than the experimental load values of about 10%.

142

This is due to the stiffer model designed in the Finite Element analysis. It is

observed that the Finite Element results are very nearly coincide with the

experimental results and are in good correlation with the experimental results.

Table 8.18 Ultimate Load and Deflection of Finite Element Analysis

and Experimental

Sl.

No.

Specimen

Id

ABAQUS Result Experimental Results

Ultimate

load (Pu)

kN

Ultimate

Deflection

(mm) ( u)

Ultimate load

(Pu)

kN

Ultimate

Deflection

(mm) ( u)

Up-

ward

Down-

wardUp-ward

Down-

ward

Up-

ward

Down-

ward

Up-

ward

Down-

ward

1 II O2 12 12 26.57 29.48 10.6 13.4 30 15

2 II S2 16 16 29.73 32.24 12.2 15.4 30 30

3 II F12 20 20 36.25 40.08 16.2 17.2 40 30

4 II F 22 20 20 46.53 48.97 16.6 17.8 45 45

5 II F32 16 16 42.87 44.72 16 15.2 45 30

6 II F 42 16 16 35.65 38 14.5 16.2 45 30

7 II F 52 12 12 31.68 32.24 11.2 12.0 30 30

Figure 8.55 Comparision of Load Deflection Curves of II F22 (M25)

Specimen Obtained in ABAQUS and Experimentaly

143

8.12 SUMMARY

From the experimental results it is found that the specimens

cast by using constant 1.5% of steel fibre and optimum 0.2% of

polypropylene fibre have maximum ultimate load carrying capacity, energy

absorption capacity and shear strength. The polypropylene fibre in addition to

the steel fibre increases the ductility factor but at this % the ultimate load and

energy dissipation capacity was found to be reduced. The specimens cast by

using steel fibre had more stiffness compared to the specimens cast by using

hybrid fibre. The HFRC (Hybrid Fibre Reinforced Concrete) joints undergo

large displacements without developing wider cracks when compared to the

ordinary joints The Finite Element results were compared with the

experimental results. It is observed that the Finite Element results very nearly

coincide with the experimental results.

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