50

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.

51

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

52

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

57

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

59

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

66

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.

67

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.