STUDIES ON BEHAVIOUR OF CONFINED COLUMN UNDER AXIAL LOAD

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JOHN FRANCIS, K. And TENSING, D ijesird , Vol. II Issue VIII February 2016/498

STUDIES ON BEHAVIOUR OF CONFINED COLUMN UNDER AXIAL LOAD

1JOHN FRANCIS, K., ²TENSING, D

1ME Scholler, ²Pofessor, School of Civil Engineering, Karunya University, Coimbatore, India

1johnfkattoor@gmail.com, ²dtensing@rediffmail.com

Abstract- An experimental program was conducted to investigate the effect of shear connectors distribution and behaviour of thin- walled short concrete-filled steel tube (CFT) columns when subjected to axial load. Load deformation graphs were plotted for all 6 specimens and maximum load carrying capacity and deformation were studied and compared for specimens with different spacing of shear connectors. The study showed that the use of connectors enhanced the axial capacity load of CFT columns, the closer the shear connectors the higher the CFT capacity .

Keywords: Composite column ,Local buckling, Shear connector, Confinement

1. INTRODUCTION

Steel members have the advantages of high tensile strength and ductility, while concrete members have the advantages of high compressive strength and stiffness. Composite members combine steel and concrete, resulting in a member that has the beneficial qualities of both materials.

The two main types of composite column are the steel-reinforced concrete column (SRC), which consists of a steel section encased in reinforced or unreinforced concrete, and the concrete-filled steel tube (CFT) column, which consists of a steel tube filled with concrete. The confinement introduced by steel tube in the concrete core is an important aspect of the structural behaviour of CFT columns.

Concrete filled tubular (CFT) sections have advantages such as Higher axial load capacity, better ductility performance, larger energy absorption capacity , lower strength degradation .CFT columns also permits formwork economy,

since the steel tube can resist loads during the construction phase when the concrete filling is unable to contribute. The enhancement of structural properties of CFSHS columns is mainly due to the composite action of steel hollow section and core concrete. The confining effect by the steel hollow section causes the core concrete to behave in a tri axial stress state while the core concrete prevents the wall of the steel hollow section from buckling inward.

The steel tube supports axial load, confines concrete core, and eliminates the need for permanent formwork. The concrete core sustains the axial load and prevents or delays local buckling of the steel tube. Because of the importance of CFT, they have been under extensive investigation for many years. In CFT columns, it is of great practical and economic interest to have mechanical shear connectors at the interface between the concrete core and the steel tube to achieve the composite action with the help of natural bond. It is believed that the bond strength has a significant effect on the behaviour of the CFT column. Use of CFT columns improves mechanical properties under static and cyclic loading including strength, ductility, stiffness and energy-absorption capacity. A survey of the available literature showed that very little research has been performed to investigate experimentally the behaviour of small-size.

2. RESEARCH SIGNIFICANCE A lot of research has been focused on the

behaviour and stability of compact CFT sections under axial loading and on the use of high-strength steel and concrete.A few studies have examined the

stability of CFT columns with shear connectors under axial loading.

Study was carried out by P.K. Gupta^[6] on the behaviour of circular concentrically loaded concrete filled steel tube columns till failure.

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JOHN FRANCIS, K. And TENSING, D ijesird , Vol. II Issue VIII February 2016/499 Parameters for the study included the diameter and

D/t ratio of steel tube, grade of concrete. A nonlinear finite element model was developed to study the load carrying mechanism of CFTs using ANSYS. The model was validated by comparison of the experimental and computational results of load–deformation curves and their corresponding modes of collapse .Comparison of the experimental and computational results of load–deformation curves and mode of collapse is obtained and It is found that the actual and numerically simulated modes of collapse of different CFTs are quite similar to each other.

Dalin Liua^[2] presented an experimental investigation into the axial load behaviour of rectangular concrete-filled steel tubular (CFT) stub columns. A total of 26 specimens were tested under concentric compression. The material properties of steel and concrete were obtained from tension and crushing tests. The test parameters were material strengths, volumetric steel-to-concrete ratio, cross- sectional aspect ratio. A fibre model has been constructed to evaluate the nonlinear axial load behaviour of the specimens. The comparison of failure loads between the tests, design codes and the proposed model has been presented and it indicates that EC4 is unsafe to predict the ultimate capacity of CFT columns fabricated from mild steel and high- strength concrete.ACI, AISC and the proposed model conservatively estimate the failure loads of the specimens by 7, 8 and 2%,respectively.

Walter Luiz^[8] presented an experimental analysis of the confinement effects in steel concrete composite columns regarding two parameters:

concrete compressive strength and column slenderness. The confinement introduced by the steel tube in the concrete core is an important aspect of the structural behaviour of CFT columns. The failure mode of the specimens was a function of the L/D ratio and the concrete strength. The short columns (L/D =3) failed due to the crushing of the concrete core aggravated by local buckling of the steel tube after having reached the yielding stress of the steel . For columns with concrete filling of high compressive strength, the maximum load was reached with lower strain in comparison with the columns filled with ordinary strength concrete. The comparative analysis of the standard codes presented satisfactory results, mainly for the specimens with higher values of L/D ratio. AISC code showed result 10.4% below the obtained results. For short columns (L/D=3) the codes overestimated the increase of load capacity of CFT columns due to the confinement effect.

After the review of the previous research projects it is concluded that little information is available about the stability of CFT columns with shear connectors under axial loading. The results of an investigation carried out to study the behaviour of thin-walled short concrete-filled steel tubes under concentric compression with the presence of shear connectors are presented in this paper

3. EXPERIMENTAL PROGRAMME 3.1. MATERIAL PROPERTIES

3.1.1. Steel properties

Hot rolled steel sheet of thickness 2 mm was used for the fabrication. Yield strength of steel was 250 N/mm².Steel tube was fabricated by bending three sides in to suitable dimension and welding the flat plate in to the other side.

3.1.2. Concrete properties

Concrete of M25 grade was used to fill the column. The mix design is done for M₂₅concrete as per IS: 456 - 2000 and IS 10262-2009.

3.1.3. Shear connectors

In CFT columns, it is of great practical and economic interest to have mechanical shear connectors at the interface between the concrete core and the steel tube to achieve the composite action with the help of natural bond. The bond strength has a significant effect on the behaviour of the CFT column.6mm diameter bars with a length

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JOHN FRANCIS, K. And TENSING, D ijesird , Vol. II Issue VIII February 2016/500 of 96 mm is used as shear connectors. These shear

connectors are welded to the specimen.

3.2. TEST SPECIMEN

A series of six square hollow steel short columns sections filled with concrete were loaded to failure. Steel hollow sections were made from hot rolled steel sheet of 2mm diameter. The cross section of the specimen is 400x100x100 mm. The chosen dimension gave a h/t ratio of 50 to avoid local buckling. Concrete of M25 grade was use to fill inside the column. The studied parameters were the number and arrangement of the shear connectors. All other parameters such as column

size, column height, shell thickness, connectors section, steel and concrete qualities were not changed. Columns were named as C1, C2, C3, C4, C5, C6. Columns C1 and C2 were casted without providing shear connectors. Specimens C3 and C4 were welded inside with shear connectors of 6 mm diameter bars having a length of 96 mm. Spacing of the shear connectors provided was 6D_S. Columns C5 and C6 were also welded inside with shear connectors with a spacing of 9D_S.

.

Fig.1.Shear connectors Fig 2. CFT without shear connector

Table. 1. Description and details of the tested specimens studied.

Specimen Column details Column size (mm) Number of

specimens Stud diameter Stud distribution GROUP 1

C1, C2

CFT column without

shear connectors 400x100x100

2 NA

NA

GROUP 2 C3, C4

CFT column with shear connector of

spacing 6D_s

400x100x100

2 6mm 6D_s = 40mm

GROUP 3 C5, C6

CFT column with shear connector of

spacing 9D_s

400x100x100

2 6mm 9Ds = 60mm

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Fig 3.CFT with shear connector spacing 6D_S

Fig 4. CFT with shear connector spacing 9DS

3.3. TEST PROCEDURE

Specimens were marked with grid lines at a spacing of 5cm before testing. Specimen C1 was tested under fixed support conditions using removable caps made of mild steel. Other specimens were tested under hinged support conditions by placing base plates at top and bottom.

All the specimens were tested to failure using the 1000kN capacity UTM. The load was applied concentrically. Specimens were tested to failure under axial compression.

Fig 5.Test Setup

Fig 6. Failure mode of specimens a) C1, b) C2, c) C3, d) C4, e) C5, f) C6

4. EXPERIMENTAL RESULTS AND DISCUSSION 4.1. LOAD DEFORMATION GRAPHS FOR THE SPECIMENS

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Fig 7.Load vs. Deformation for CFT columns c1 vs. c2

424 374

0 50 100 150 200 250 300 350 400 450

0 5 10 15 20

Load in (kN)

Deformation in mm

c1 load vs deformation C2 load vs deformation

627.7 572

0 100 200 300 400 500 600 700

0 2 4 6 8 10 12

Load in (kN)

C3 LOAD VS DEFORMATION c4 LOAD VS DEFORMATION

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4.2. COMPARATIVE STUDY OF LOAD VS. DEFORMATION GRAPH

\

Fig 10 .C1 VS C3 VS C5 load defomation graph 564

623.2

0 100 200 300 400 500 600 700

0 2 4 6 8 10

Load in (kN)

C5 LOAD VS DEFORMATION C6 LOAD VS DEFORMATION

5

374 564 572

0 100 200 300 400 500 600 700

0 5 10 15

Load in (kN)

C1 LOAD VS DEFORMATION C3 LOAD VS DEFORMATION C5 LOAD VS DEFORMATION

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Fig 11. C2 VS C4 VS C6 load deformation graph

Table 2.Specimens Failure Loads and Deformation

Group Specimen Stud Spacing Failure Load (kN)

Deformation (mm)

Theoretical load

1

C1 N.A 374 12.10 327 kN

C2 N.A 424 12.30 327 kN

2

C3 6DS 572 5.80 -

C4 6DS 627.7 6.00 -

3

C5 9D_S 564 7.30 -

C6 9DS 623.2 6.10 -

4.3. DISCUSSION

Close observation of the results will lead to the following

[1] In all three specimens it was observed that local buckling of steel occur. It happened in the edges where stress concentration is more which is in accordance with the results obtained by Dalin Liu a et al

[2] It was observed that in axially loaded thin walled CFT without shear connectors Local buckling occurred suddenly due to lack of sufficient bond

between steel and concrete and this behaviour is in conformity with conclusion arrived by Sherif M.

Younes et al.

[3] After local buckling, concrete started taking the load. Upon reaching the maximum load concrete failed by crushing. There after post buckling strength was observed due to steel which confined the concrete.

[4] It was found that C3 and C4 columns with shear connectors having 6D_s spacing have marginally higher load carrying capacity compared to that of C5 and C6 column with 9Ds spacing.

424 627.7

623.2

0 100 200 300 400 500 600 700

0 2 4 6 8 10 12 14

Load in (kN)

C2 LOAD VS DEFORMATION C4 LOAD VS DEFORMATION C6 LOAD VS DEFORMATION

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JOHN FRANCIS, K. And TENSING, D ijesird , Vol. II Issue VIII February 2016/505 5. CONCLUSIONS

From the tested CFT thin-walled short columns with shear connectors subjected to axial loading, the following conclusions may be deducted:

1. Confinement provided by steel to concrete enhances the core concrete strength.

2. For axially loaded thin walled CFT's local buckling does not occur suddenly if sufficient bond is there between steel and column.

3. The use of connectors enhanced the axial capacity load of CFT columns, the closer the shear connectors higher will be the CFT capacity. This is attributed to the fact that the load transferred to the steel tube is increased by increase in the number of shear connectors.

REFERENCES

1. Ahmed Elremaily and Atorod Azizinamini, Behaviour and strength of circular concrete filled tube columns, Journal of Constructional Steel Research 58 (2002) 1567–1591 2. Dalin Liu a, Wie-Min Gho a,, Jie Yuan ,Ultimate capacity

of high-strength rectangular concrete-filled steel hollow section stub columns Journal of Constructional Steel Research 59 (2003) pp 1499–1515.

3. Dalin Liua,, Wie-Min Ghob, Axial load behaviour of high- strength rectangular concrete-filled steel tubular stub columns, Thin-Walled Structures 43 (2005) pp1131–1142.

4. Dalin Liu, Tests on high-strength rectangular concrete- filled steel hollow section stub columns, Journal of Constructional Steel Research 61 (2005) 902–911.

5. Georgios Giakoumelis, Dennis Lam, Axial capacity of circular concrete-filled tube columns, Journal of Constructional Steel Research 60, (2004), pp- 1049–1068.

6. P.K. Gupta, S.M. Sarda, M.S. Kumar, Experimental and computational study of concrete filled steel tubular columns under axial loads, Journal of Constructional Steel Research 63 (2007)pp 182-193

7. Sherif M. Younes, Hazem M. Ramadan, Sherif A. Mourad ,Stiffening of short small-size circular composite steel–

concrete columns with shear connectors, Journal of Advanced Research (2015).

8. Walter Luiz Andrade de Oliveira, Silvana De Nardin.Influence of concrete strength and length/diameter on the axial capacity of CFT columns, Journal of Constructional Steel Research 65 (2009) 21032110.

9. Young Bong Kwonan and InKyuJeong, Resistance of rectangular concrete-filled tubular (CFT) sections to the axial load and combined axial compression and bending, Thin-Walled Structures 79 (2014) pp 178-186.