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CHAPTER-5
TEST PROCEDURE AND TEST RESULTS OF CSCC BEAMS
5.0 GENERAL
This chapter deals with the experimental set up for pure bending, pure torque, combined bending
and torsion. The experimental procedures are also included for the confined steel concrete
composite beams (CSCC). The test results for composite beam subjected to pure bending, pure
torsion and combined bending and torsion are tabulated. This section also reports the behaviour
of these beams such as separation of sheet, local buckling, formation and development of cracks,
crushing of compression concrete and yielding of tension steel under pure bending, pure torsion
and combined bending and torsion.
5.1 EXPERIMENTAL SETUP AND TESTING PROCEDURE FOR PURE BENDING
5.1.1 Testing Procedure
For pure bending a total number of eight composite beams with an effective span of 2.1m
(classified as Group A – A1T1, A1T2, A2T3 and A2T4) were tested. The position of the supports,
inclinometer and dial gauge points were marked on the beams. The beams were tested for two
point loading.
Two point loading was done in order to apply pure bending on the beams. All the beams were
designed to fail by flexure only. In order to determine the curvature of the beams, the
inclinometer readings were taken. For the measurement of deflections, dial gauges were located
at seven places, one at mid-span, two under the load points, two at 1/6 of the span at the bottom
of the beams and two on the top of beams at supports. The test setup is shown in Fig.5.1.
The beams were tested at a rate of loading of 30kN/min. The test was carried out until the
formation of waves due to buckling of sheets of the beams. The beams began to yield and the
behaviour of the beams was keenly observed from the beginning till collapse. A careful
observation was made from the initial separation of sheet, propagations of cracks and failure of
bracings connecting the sheet and concrete. After that the beams were tested for finding the
ultimate load carrying capacity by removing all the dial gauges and the inclinometer setup till
failure.
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Fig. 5.1 Test Setup for Pure Bending
From the readings obtained from the dial gauges, the load-deflection curves, moment-curvature
curves and moment-rotation relationships were determined.
Table 5.1 Test Results for Pure Bending
S.
No
Beam
ID
Dimensions
(mm)
Thickness of
the Sheet
(mm)/spacing
of bracing
(mm)
Ultimate
Load
(kN)
Average
Ultimate
Load
(kN)
Ultimate
Bending
Moment
(kNm)
1 A1T1 150 x 230 x 2300 1.2 / 100 132.8
2 A1 T1 150 x 230 x 2300 1.2 / 150 132.6 132.7 46.5
3 A1T2 150 x 230 x 2300 1.5 / 100 138.5
4 A1T2 150 x 230 x 2300 1.5 / 150 139.2 138.9 48.5
5 A2T3 150 x 300 x 2300 1.2 / 100 194.0
6 A2 T3 150 x 300 x 2300 1.2 / 150 193.8 193.9 68.0
7 A2 T4 150 x 300 x 2300 1.5 / 100 182.8
8 A2 T4 150 x 300 x 2300 1.5 / 150 181.4 182.1 64.0
5.1.2 Observation about the Behaviour of Beams
The failure started with the initial separation of sheet in the form of waves due to local buckling
followed by yielding of the beams. The first crack was observed on the specimen followed by the
appearance of several cracks which propagated in the inclined manner upon further increase of
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load. At failure, crushing of compression concrete and failure of bracings and yielding of tension
steel were observed.
Fig. 5.2 Failure Pattern of Beam in Pure Bending
Fig. 5.3 Failure Pattern by Crushing of Concrete in Pure Bending
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5.1.3 Variation of Load with deflection
Fig.5.4 compares the load-deflection responses of CSCC beams under pure bending. The load
deflection curves of CSCC beams are linear up to yield load and have a long plateau beyond
yield load.
Fig. 5.4 Load vs. Deflection for Pure Bending
It was also found from the above graph that beams with 100mm spacing of bracings have shown
higher values of toughness which is an indication of the energy absorption capability of the
beams. The variation in dimension of the beams increases the ultimate load to a great extent.
5.1.4. Moment vs. Curvature (M/)
Using the inclinometer readings, curvature at mid-section was calculated and moment curvature
diagram was drawn. The curvature at any load was obtained by dividing the arithmetic sum of
the average compressive strain and average tensile strain in the constant bending moment zone
by the distance between the lines of measurement of strains as shown in Fig.5.5.
Fig.5.6 shows the moment-curvature relationships of CSCC beams under pure bending. It was
inferred that, the curvature increases with the increase in load in all the beams and it was nearly
linear up to yield load and it was non linear in inelastic region. The value of curvature was
calculated as shown below:
0
50
100
150
200
250
0 2 4 6 8 10 12
Load
inkN
Deflection in mm
A1T2
A1T1
A2T3
A2T4
54
D8
79mm
230mm
388mm
79mm D9
200mm
Fig. 5.5 Inclinometer Arrangement on Beam
Curvature = axisneutralofDepth
topatStrain
Depth of Neutral Axis from the level of top dial gauge
Tc
cxAxisNeutralofDepth
388
Depth of Neutral axis from the top of beam
= Depth of NA from the level of dial gauge – 79
Curvature = axisneutralofDepth
topatStrain
Curvature = 388
000525.00003.0
= 2.13 x 10-6
Rad/mm
55
Fig. 5.6 Moment vs. Curvature in Pure Bending
It was also found from the above graph that beams with 100mm spacing of bracings have shown
lower values of curvature for higher values of moment than beams with 150mm spacing of
bracings. The reason is attributed to the enhancement in stiffness due to closely spaced bracings
which contributed additional confinement to the beams. The variation in dimension of the beams
increases the ultimate bending moment value to a great extent.
5.1.5. Moment vs. Rotation
The rotation at the maximum bending moment zone at any load stage was obtained by using the
inclinometer as shown earlier. Moment rotation diagrams were drawn using the readings
obtained from the inclinometer and shown in Fig. 5.7.
The value of rotation was calculated as shown below:
38822
89
x
DD
radiansx
00013.03882
03.002.0
2
0
10
20
30
40
50
60
70
80
0 0.00001 0.00002 0.00003
Mo
me
nt
in k
Nm
Curvature in radians/mm
A1T2
A1T1
A2T3
A2T4
56
79mm D8
230mm
388mm
79mm D9
200mm
Rotation
Fig. 5.7 Layout of Inclinometer Arrangement
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Fig. 5.8 Moment vs. Rotation in Pure Bending
Fig. 5.8 shows the moment-rotation characteristics of the CSCC beams under pure bending. The
rotation was initially more in the case of beams with 150 mm spacing of bracings than the beams
with 100mm spacing. But with higher moments the rotation decreases in case of beams with 150
spacing of bracings. The reason is due to the enhancement in stiffness of closely spaced bracings
confined at top of the beams in pure bending region.
5.1.6 Moment vs. Flexural Rigidity
The flexural rigidity was computed as the ratio of applied bending moment to the curvature in
the constant bending moment zone.
Flexural rigidity (EI) value was calculated as shown below:
EI = Curvature
Moment
Moment = 6
WL
Curvature = axisneutralofDepth
topatStrain
0
10
20
30
40
50
60
70
80
0 0.001 0.002 0.003 0.004
Mo
me
nt
in k
Nm
Rotation in radians
A1T2
A1T1
A2T3
A2T4
58
1.14E+12
1.64E+12
2.14E+12
2.64E+12
3.14E+12
3.64E+12
4.14E+12
4.64E+12
0.00 20.00 40.00 60.00
Fle
xura
l Rig
idit
y in
kN
mm
2
Moment in kNm
A1T1
A1T2
A2T3
A2T4
Fig 5.9 Moment vs. Flexural Rigidity in Pure Bending
Fig. 5.9 shows the moment vs. flexural rigidity relationships for the CSCC beams under pure
bending. The value of flexural rigidity decreases with the increase in moment. For beams with
100 mm spacing of bracings the moment carrying capacity was found to be higher than the
beams with 150mm spacing. This shows that the closely spaced bracings enhances the flexural
stiffness of the beams.
5.2. EXPERIMENTAL SETUP AND TESTING PROCEDURE FOR PURE TORSION
5.2.1 Testing Procedure
For pure torsion a total number of eight composite beams with an effective span of 2.1m
(classified as Group D – D1T1, D1T2, D2T3, D2T 4 each two) were tested. For the application of
torsion, torsion brackets and supports, which permit rotation and twisting, were fabricated in the
laboratory. The experimental setup is shown in Fig.5.10 and 5.11. The test specimen was erected
on the supports. After the erection of beam, loading points were fixed at the middle third points
of span. The load distributing beam was placed over these supports by proper centering and
checked for its verticality.
Then the proving ring was mounted on the centre point of the load distributing beam. Then the
main central hydraulic jack attached at the bottom of loading frame was lowered to the top flat
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surface of the proving ring. The ram of the jack was made to butt against the top of the proving
ring.
After erection of the specimen, proving ring, jack etc. the torsion brackets were inserted at the
over hangs of the beam on either side. The ends of the beams were connected to the torsion
brackets with a lever arm made of steel truss for applying loads. The cantilever truss was held in
position. A rigid steel beam was kept over the top of the trusses diagonally for applying the load.
Glass plates were fixed on the top of the beam at the ends projecting perpendicular to the axis of
the beam and deflectometers were kept under projected ends for measuring the deflection under
different loads. These deflections were used for calculating the twist.
The beam with the torsion brackets were supported on flexible supports in such a way that the
beam could be subjected to torsion. The weight of the beam was taken into consideration for the
calculation of torque. The deflection readings were also measured at 1/3 and 1/2 of the span of
the beam.
Fig. 5.10 Test setup for Pure Torsion
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Fig. 5.11 Test setup for Pure Torsion
Table 5.2 Test Results for Pure Torsion
S.No Beam
ID
Dimensions
(mm)
Thickness of
the Sheet
(mm)/spacing
of bracing
(mm)
Ultimate
Load
(kN)
Ultimate
Torque
(average)
(kNm)
Angle of
Twist(ө)
(radians)
x 10-5
1 D1 T1 150 x 230 x 2300 1.2 / 100 31.92
2 D1 T1 150 x 230 x 2300 1.2 / 150 31.90 14.84 0.18593
3 D1 T2 150 x 230 x 2300 1.5 / 100 34.09
4 D1 T2 150 x 230 x 2300 1.5 / 150 34.02 15.85 0.15558
5 D2 T3 150 x 300 x 2300 1.2 / 100 40.40
6 D2 T3 150 x 300 x 2300 1.2 / 150 40.30 18.79 0.10181
7 D2 T4 150 x 300 x 2300 1.5 / 100 44.70
8 D2 T4 150 x 300 x 2300 1.5 / 150 44.50 20.80 0.05465
From the dial gauge readings, twisting moment was found by multiplying the lever arm distance
and the total load obtained in the dial gauge divided by 2.
5.2.2 Observation about the Behaviour of Beams
Each specimen was tested to failure by applying the load in increments. At failure diagonal
cracks were formed at the top face and rotation occurred about an axis near the top face. The
angles of cracks were found to be close to 45°. The widest crack on the top face was inclined and
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extended across the vertical faces. Thus a hinge was formed on the top face. The failure was well
defined and followed by failure of shear connectors in the sides and spalling of concrete resulting
in separation of sheet. The failure at bottom face was prevented by the confinement of sheet.
Fig. 5.12 Failure Pattern of Beam D1T1 under Pure Torsion
Fig. 5.13 Failure Pattern of Beam D2T1 under Pure Torsion
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Fig. 5.14 Failure Pattern of Beam D2T3 under Pure Torsion
Fig. 5.15 Torque vs. Twist of Beams under Pure Torsion
The torque twist relations are shown in Fig. 5.15. From the graph it was inferred that the spacing
of the bracings have influence on the torque- twist relationship.The closely spaced bracings
contributed more resistance to twist. This is owing to the reason that the bracings act as ties
carrying tension which restrict the torsional deformation thus enhancing the torsion carrying
capacity of the beam.
0
1
2
3
4
5
6
7
8
9
10
0 0.05 0.1 0.15 0.2 0.25 0.3
Torq
ue
in k
Nm
Twist in radians
D1T2
D1T1
D2T4
D2T3
63
The ultimate torque also increases to a small extent due to the variations in dimension of the
beams.
5.3. EXPERIMENTAL SETUP AND TESTING PROCEDURE FOR COMBINED
BENDING AND TORSION
5.3.1 Testing Procedure
For combined bending and torsion a total number of sixteen composite beams with an effective
span of 2.1m (classified as Group B and C – B1T1, B1T2, B2T3 & B2T4 & C1T1,C1T2, C2T3 &
C2T4 each two) were tested. Group B specimens were subjected to 30% of ultimate
experimental torque followed by the flexure till failure. Group C specimens were subjected to
60% of ultimate experimental torque followed by the flexure till failure. Both groups of
specimens were subjected to flexural failure only.
The supports were fabricated in such a way that combined bending and torsion could be applied
on the compositebeams. The torsion brackets with a loading truss held the ends of the beams.
For applying torsion uniformly, a pair of hydraulic jacks was used. For applying flexural load,
two point loading arrangement was made on the beam and the load was applied by means of a
hydraulic jack fitted to the loading frame as shown in Fig.5.16. A proving ring of capacity
500kN (50T) was used to measure the load.
Fig. 5.16 Test Setup for Combined Bending and Torsion
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The torque was found by multiplying the load on the truss by the distance between the point of
application of the load and center of the beam. Weight of the truss with bracket was also
considered in the calculation of torque. For measuring the angle of rotation glass plates were
fixed on the top of the beam at both ends and the deflections were measured with the help of
four deflectometers for different loadings.
Table 5.3 Test Results for 30% Torque and Bending till Failure
S. No Beam
ID
Dimensions
(mm)
Thickness of
the Sheet
(mm)/spacing
of bracing
(mm)
30% of
Ultimate
Torque
(kNm)
average
Ultimate
Bending
Moment
(kNm)
average
Angle of
Twist (ө)
(radians) x
10-5
average
1 B1 T1 150 x 230 x 2300 1.2 / 100
2 B1 T1 150 x 230 x 2300 1.2 / 150 3.43 43.02 0.21593
3 B1 T2 150 x 230 x 2300 1.5 / 100
4 B1 T2 150 x 230 x 2300 1.5 / 150 4.05 47.15 0.18558
5 B2 T3 150 x 300 x 2300 1.2 / 100
6 B2 T3 150 x 300 x 2300 1.2 / 150 4.74 67.87 0.12181
7 B2 T4 150 x 300 x 2300 1.5 / 100
8 B2 T4 150 x 300 x 2300 1.5 / 150 4.07 68.12 0.07465
Table 5.4 Test Results for 60% Torque and Bending till Failure
S.No. Beam
ID
Dimensions
(mm)
Thickness of
the Sheet
(mm)/spacing
of bracing
(mm)
60% of
Ultimate
Torque
(kNm)
average
Ultimate
Bending
Moment
(kNm)
average
Angle of Twist
(ө)
(radians)
x 10-5
average
1 C1 T1 150 x 230 x 2300 1.2 / 100
2 C1 T1 150 x 230 x 2300 1.2 / 150 7.2 23.9 0.28593
3 C1 T2 150 x 230 x 2300 1.5 / 100
4 C1 T2 150 x 230 x 2300 1.5 / 150 8.69 26.78 0.25558
5 C2 T3 150 x 300 x 2300 1.2 / 100
6 C2 T3 150 x 300 x 2300 1.2 / 150 10.92 34.8 0.18181
7 C2 T4 150 x 300 x 2300 1.5 / 100
8 C2 T4 150 x 300 x 2300 1.5 / 150 11.16 39.40 0.14645
65
Fig. 5.17 Failure Pattern of B Group beams
Fig. 5.18 Failure Pattern of C Group beams
5.3.2. Observation about the Behaviour of Beams
For those beams tested under combined bending and torsion, in the initial stages the angle of
twist and twisting moment increases linearly. However after the formation of cracks the
behaviour was nonlinear. The length of the linear portion of the torque-twist relationship
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decreased when the twisting moment value increases. When failure occurred the sheets were
separated from concrete and the bracing at the top delays the failure and cracks widen
appreciably at failure. Rotation of beam at failure occurred about an axis near the top face.
However, in Group C beams upward deflection was well prominent where the widening of
cracks started from bottom which was invisible and was noticed from the separated sheet in the
sides predominantly.
5.4 General Behaviour Prior to Failure under Combined Bending and Torsion
For the beams tested for low value of twisting moment, i.e. in Group B Beams the first cracks
appeared at mid depth of the vertical faces and were inclined approximately at 45° to the axis of
the beam which is in agreement with St. Venant’s theory of torsion. For the lower value of
twisting moment the appearance of the cracks were delayed on the top face. Nevertheless, cracks
generally appeared on the top face within one or more increment of first cracking and twisting
moments considerably less than the ultimate torque. Neither the quantity nor the distribution of
reinforcement affected the appearance of the initial development of cracks.
For the beams tested for high value of twisting moment, i.e. in Group C Beams the first crack
was inclined and observed on the face in which diagonal tension stresses due to transverse shear
and torsional shear are additive. On further loading, the diagonal cracks extended towards the top
and bottom faces and a plastic hinge was developed in the vertical side of the compression zone
which lead to the failure of the beam.
The first cracks would have occurred either in the bottom face or lower portion of the vertical
faces after the separation of sheet (which could not be observed due to confinement of concrete).
On the bottom face these cracks occurred nearly perpendicular to the axis of the beam and nearly
vertical on the vertical faces. On further loading, the cracks progressed vertically on the vertical
face and inclined subsequently. The beams that were tested for high value of twisting moment
exhibited negative deflection also in the later stages of test usually had zero deflection or a small
positive deflection up to the level of twisting moment at which cracking occurred.
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5.5 General Behaviour at Failure under Combined Bending and Torsion
When the beam could no longer sustain the loads to which it was subjected or could not sustain
any further increase in loads, it was considered to have failed and the corresponding forces were
considered to be the failure forces. In all tests, application of load was carried well beyond
observance of peak values of twisting load and transverse load. Most of the beams failed while
the load increment for flexure was being applied. All the beams tested exhibited appreciable
ductility.
At failure, the angle of twist was lower and the vertical deflection was found to increase due to
high transverse loads. At this time the traverse load and twisting load decreased distinctly. Due
to load maintaining feature of the transverse loading equipment the previous level of transverse
load was quickly regained. However further application of twist to reach the previous level of
twisting load resulted only in further rotation of the beam. The magnitude of the twisting load
slowly decreased with time.
The beams that were subjected to low value of twisting moment at failure, rotated about an axis
in the vicinity of the top face. The beams that were subjected to higher value of twisting moment
would have rotated about an axis located in the vicinity of bottom face after the separation of the
sheet which was invisible due to confinement.
Failure of a beam was accompanied by a large rotation and widening of crack on the face of the
beam other than the one adjacent to the axis of rotation. In most cases, the cracks which widened
at failure and defined the failure plane were the cracks which appeared at the moments
considerably lower than the ultimate moment. These cracks were essentially a continuous crack
which spiraled around the three faces of the beam.
Many of the beams that were tested with low or high value of twisting moment had an axis of
rotation adjacent to the top or bottom surface of the beam and developed a Z-shaped crack at
failure which connected the failure crack appearing on the vertical face.
68
With these details of the experimental study and with the details of theoretical investigation in
Chapter 6, the interaction between the bending moment vs. twisting moment of CSCC beams
under combined bending and torsion is well explained in the Chapter 7 and Chapter 9.