DOUBLE SKIN TUBULAR COLUMNS CONFINED … the two layers by providing proper compaction. ... Self-curing compound was added ... Figure 3 shows the concrete filled CFDST and CFST column

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International Journal of Civil Engineering and Technology (IJCIET) Volume 7, Issue 6, November-December 2016, pp. 536–543, Article ID: IJCIET_07_06_059

Available online at http://www.iaeme.com/IJCIET/issues.asp?JType=IJCIET&VType=7&IType=6

ISSN Print: 0976-6308 and ISSN Online: 0976-6316

© IAEME Publication

DOUBLE SKIN TUBULAR COLUMNS CONFINED

WITH GFRP

Parvati T S and Dr. P.S. Joanna

Department of Civil Engineering, Hindustan Institute of Technology & Science, Chennai, India

ABSTRACT

This paper presents a comparative experimental study on the performance of the Concrete

Filled Double Skin Tubular (CFDST) columns with and without Glass Fibre Reinforced Polymer

(GFRP) wrapping under axial compression. The CFDST columns consist of cold formed steel

square hollow section as the outer skin, a circular poly vinyl chloride (PVC) tube as its inner skin

with fly ash concrete filled in between the two layers. Tests were also conducted on Concrete Filled

Steel Tube (CFST) columns with and without GFRP wrapping for comparison. The CFDST

specimens with GFRP wrapping exhibited 25% more load carrying capacity and 14% more energy

dissipation capacity when compared with unwrapped CFDST specimens. By providing GFRP

wrapping, PVC inner tube and fly ash concrete, a sustainable structural member with greater

economy and increased strength could be achieved.

Key words: GFRP, Concrete Filled Double Skin Tube, PVC tube, Axial compression, Fly ash

concrete.

Cite this Article: Parvati T S and Dr. P.S. Joanna, Double Skin Tubular Columns Confined with

GFRP. International Journal of Civil Engineering and Technology, 7(6), 2016, pp.536–543.

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1. INTRODUCTION

Concrete filled double skin tubular (CFDST) members consist of two concentric steel tube with concrete

sandwiched between them. CFDST members have all the advantages of a concrete filled steel tubular

(CFST) member with lesser self-weight and greater section modulus. Due to lesser self-weight and higher

load carrying capacity CFDST members have found wide spread application in high rise bridge piers[1],

electrical poles[2] etc.

Studies on the CFDST members by many researchers reveal that these members have better capacity

under bending, compression and also they exhibit better response to cyclic loading[ 3,4]. Tao et al.

conducted tests on stub columns and beam- columns with circular hollow section for both inner and outer

tubes [5]. It was concluded that the specimens exhibit ductile behaviour because of the concrete infill.

Zhao and Grzebieta reported that increased strength and ductility were observed for the CFDST sections

when square hollow sections were used as inner and outer tubes [6]. Han et al. described the behaviour of

the CFDST beam-columns with square hollow section as outer tube and circular hollow section as inner

tubes[7]. A mechanics model was proposed for describing the confinement factor which represents the

extent of the composite action between steel and sandwiched concrete. The investigations carried out on

stub columns and beam-columns under cyclic loading , both point to the fact that the CFDST sections have

enhanced strength, ductility and energy dissipation [1,3]. A model was proposed by Liang and Fragomeni



to predict the strength and ductility of the CFDST sections by non-linear analysis [8]. Based on the

experimental studies on stub columns Uenaka et al., reported that the failure of the specimen was mainly

by local buckling due to the shear failure of the concrete[9]. Yuan and Yang carried out experimental

investigations on CFDST sections with outer octagonal section and the inner PVC–U pipe[10]. The study

demonstrated that the PVC-U pipe could be a better replacement for steel tubes as they are cheaper, have

lighter weight and provide good formwork for the concrete. To reduce the susceptibility of steel to

atmospheric conditions Wang et al., wrapped CFDST section having steel tubes as the inner skin with

GFRP sheets[11]. The study revealed that the GFRP sheet in addition to protecting the steel tubes also

provides confinement to the concrete.

This paper presents a study on the behaviour of the CFDST columns with outer steel tube of square

cross section and an inner PVC tube of circular cross section. Tests were conducted on CFDST columns

with and without GFRP wrapping. CFST specimens with and without GFRP wrapping were also tested for

comparison. Load carrying capacity and the failure pattern of the specimens under axial compression were

observed.

2. EXPERIMENTAL PROGRAM

2.1. Preparation of Specimens

Eight specimens were subjected to axial compression which included four CFDST specimens and four

CFST specimens. Two CFDST and two CFST specimens were wrapped with GFRP sheets of 3mm

thickness. Cold formed steel tubes of 3 mm thickness and 700 mm height were used for all the steel

columns. The CFDST specimens had square hollow section of 100 mm x 100 mm cross section as outer

tube and a PVC pipe of 50 mm diameter having 3mm thickness as the inner tube. The space between the

two tubes were filled with M30 grade fly ash concrete with 40% cement replaced with fly ash. The CFST

specimens considered as the control specimens had 100 mm x100 mm square steel tubes completely filled

with M30 grade fly ash concrete. Table 1 shows the details of the test specimens.

Table 1 Details of the test specimen

S No Specimens Description

1 CS1 CFST columns

2 CS2

3 PVC1 CFDST columns with

inner PVC tube 4 PVC2

5 WCS1 CFST columns with

GFRP wrapping 6 WCS2

7 WPVC1 CFDST columns with

inner PVC tube and

wrapped with GFRP 8 WPVC2

To provide GFRP wrapping, an initial layer of chopped strand mat of 0.1 mm thickness was placed on

the treated surface of the column followed by a layer of woven roving mat of 2.5 GSM (grams per square

metre). This was followed by two layers of shredded GFRP sheets and the finishing layer. Isothalic resin

was used to ensure proper bonding between the steel and the GFRP sheets. It was ensured that there was no

airlock while the resin was applied to the sheet. Once the resin was dry, the ends were finished by drilling

Double Skin Tubular Columns Confined with GFRP


of the excess length. The PVC tubes were placed at the centre of the square hollow sections and held in

position by small steel pieces attached to the inner side of the outer tube. Concrete was then filled in

between the two layers by providing proper compaction.

2.2. Material Properties

The material properties of the GFRP sheets were studied by carrying out coupon tests on three flat

specimens. The GFRP specimens exhibited a yield stress of 66.69 N/mm2. The material properties of the

cold formed steel tubes were also gathered by conducting coupon tests on the samples cut from the square

hollow steel tubes. The steel tubes exhibited yield strength of 479 N/mm2. The concrete filled in the

specimens were M30 grade fly ash concrete with 40% cement replacement by fly ash. The concrete used in

the specimens were mixed with a ratio of cement, sand, aggregate as 1: 1.86:2.9 and a water- cement ratio

of 0.45. Glenium super plasticizer was added to increase the workability. Self-curing compound was added

to aid curing of the concrete. Three cubes were cast and cured under similar conditions. The average cube

compressive strength at 56 days was 30.8 N/mm2. Figure 1 shows the fabricated specimen. Figure 2 and

Figure 3 shows the concrete filled CFDST and CFST column specimen respectively.

a) CFST b) CFDST

Figure 1 Fabricated Specimens

Figure 2 Concrete filled CFDST specimens Figure 3 Concrete filled CFST specimen with inner

PVC tube

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2.3. Test Set-up

Tests were performed in a 1000 kN capacity Universal Testing

data logger. The short column specimens were placed on the testing machine and the axial load was

applied on the specimen directly as the loading ram was a solid steel plate. The load was applied at the rate

of 0.8kN/sec and the test was stopped when the load was found to reduce with increase in axial

displacement. Figure 4 shows the test set up.

3. RESULT AND DISCUSSIO

3.1. Failure Modes

Figure 5 shows the load-deformation curves of the

exhibited an elastic response indicated by the linear portion of the load deflection graph. On further

loading, there was a gradual increase of load till the peak value. Beyond peak load, different spec

exhibited different load-deformation patterns. Control specimens CS1 and CS2 exhibited a round bend in

the load deflection graph indicating non

observed in the specimens CS1 and CS2 at th

an inclined fold at one-third height indicating shear failure in the specimen.

The CFDST specimens with inner PVC tubes also exhibited the non

round bend in the graph. ‘Elephant foot’ formation was observed at the top of the

and PVC2.

GFRP wrapped CFST specimens WCS1 and WCS2 also exhibited a gradually increasing load but with

a sharp yield point followed by a plastic zone. Bulge formation was o

de-bonding of the GFRP laminate from the steel tube in WCS1. WCS2 exhibited bulge formation at mid

height of the column with the rupture of GFRP laminate.

Figure 5 Load-Deformation Curve


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Tests were performed in a 1000 kN capacity Universal Testing Machine and the data were collected by a



N/sec and the test was stopped when the load was found to reduce with increase in axial

displacement. Figure 4 shows the test set up.

Figure 4 Test Set up

RESULT AND DISCUSSION

deformation curves of the eight specimens.On application of load, the specimens


loading, there was a gradual increase of load till the peak value. Beyond peak load, different spec

deformation patterns. Control specimens CS1 and CS2 exhibited a round bend in

the load deflection graph indicating non-linear response of both steel and concrete. Local buckling was

and CS2 at the lower end of the column. The specimen CS2 also developed

third height indicating shear failure in the specimen.

The CFDST specimens with inner PVC tubes also exhibited the non-linear response validated by the

lephant foot’ formation was observed at the top of the


a sharp yield point followed by a plastic zone. Bulge formation was observed at the top of the column with

bonding of the GFRP laminate from the steel tube in WCS1. WCS2 exhibited bulge formation at mid

th the rupture of GFRP laminate.

Deformation Curve Figure 6 Inward folding of inner PVC tube

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Machine and the data were collected by a



N/sec and the test was stopped when the load was found to reduce with increase in axial

On application of load, the specimens


loading, there was a gradual increase of load till the peak value. Beyond peak load, different specimens

deformation patterns. Control specimens CS1 and CS2 exhibited a round bend in

linear response of both steel and concrete. Local buckling was

e lower end of the column. The specimen CS2 also developed

linear response validated by the

lephant foot’ formation was observed at the top of the column specimens PVC1


bserved at the top of the column with

bonding of the GFRP laminate from the steel tube in WCS1. WCS2 exhibited bulge formation at mid

Inward folding of inner PVC tube


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Wrapped CFDST specimens WPVC1 and WPVC2 exhibited a slightly varied load

The load-deflection curve of the WPVC2 specimen was similar to WCS1 and WCS2 w

point and a plastic zone. Whereas, WPVC1 exhibited a rounded peak followed by plastic zone and when

loaded further the specimen exhibited an increased peak load. This increase in load capacity may be

attributed to the strain hardening eff

The wrapped CFDST specimens failed with multiple folds and subsequent rupture of the GFRP

lamination. The PVC inner tubes

inward folding at the end of testing

Figure 7 shows the load deformation curve of WPVC1 indicating the number of peaks developed in the

specimen. Figure 5 shows the load

deflection of 15mm.

Figure 7

3.2. Load Carrying Capacity

Test results of the column sections are given in Table 2.

average peak load of 895.02kN and the CFDST specimens (PVC1 and PVC2) was able

average peak load of 713.2kN.The GFRP wrapped CFST specimens failed at an average load of 910.9

and the wrapped CFDST specimens exhibited an average maximum load of 894.

S No Specimens

1

2

3

4

5

6

7

8


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Wrapped CFDST specimens WPVC1 and WPVC2 exhibited a slightly varied load

deflection curve of the WPVC2 specimen was similar to WCS1 and WCS2 w



attributed to the strain hardening effect.


PVC inner tubes of all CFDST specimens (both wrapped and unwrapped) exhibited

at the end of testing (Figure 6) indicating the loss of confinement


specimen. Figure 5 shows the load-deflection curves with single peak as the curves are plotted to a

Figure 7 Load-deformation curve for WPVC1

Test results of the column sections are given in Table 2. CFST specimens (CS1 and CS2

kN and the CFDST specimens (PVC1 and PVC2) was able

average peak load of 713.2kN.The GFRP wrapped CFST specimens failed at an average load of 910.9

and the wrapped CFDST specimens exhibited an average maximum load of 894.

Table 2 Test results of column sections

Specimens Peak Load (kN) Average Peak

Load (kN)

CS1 891.97 895.02

CS2 898.08

PVC1 671.22 713.2

PVC2 756.02

WCS1 910.89 910.91

WCS2 910.94

WPVC1 877.9 894.33

WPVC2 910.75


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Wrapped CFDST specimens WPVC1 and WPVC2 exhibited a slightly varied load-deflection pattern.

deflection curve of the WPVC2 specimen was similar to WCS1 and WCS2 with a sharp yield




(both wrapped and unwrapped) exhibited

s of confinement (Elchalakani et al., 2002).


deflection curves with single peak as the curves are plotted to a

CFST specimens (CS1 and CS2) exhibited

kN and the CFDST specimens (PVC1 and PVC2) was able to sustain an

average peak load of 713.2kN.The GFRP wrapped CFST specimens failed at an average load of 910.91 kN

and the wrapped CFDST specimens exhibited an average maximum load of 894.33 kN .

Average Peak

Load (kN)

895.02

713.2

910.91

894.33



0

100

200

300

400

500

600

700

800

900

1000

CS PVC WCS WPVC

Pe

ak

Lo

ad

(kN

)

Specimen

The CFST columns (both wrapped and without wrapping) exhibit higher load carrying capacity than

the CFDST specimens. But when wrapped, the CFDST specimens failed at a load equivalent to the CFST

specimens. Also, the wrapped CFDST specimens exhibited 25% more load carrying capacity than the

unwrapped specimens. This increase in the load capacity compared to unwrapped specimens may be

attributed to the confinement effect offered by GFRP laminates to the CFDST section. Figure 7 shows the

strength variation in the specimens. The failure pattern of the wrapped CFDST specimen is shown in

Figure 8. Figure 9 and Figure 10 shows the failure pattern in all the CFST and CFDST specimens

respectively.

Figure 7 Strength variation of the specimens Figure 8 Failure pattern of WPVC specimen

Figure 9 Failure pattern of CFDST specimens Figure10 Failure pattern of the CFST specimens

3.3. Energy Absorption or Energy Dissipation

The energy absorption is measured by the area under the load-deformation diagram. The energy absorbed

up to 20mm deformation was calculated. The energy absorption capacity of the various column specimens

are given in Table 3.

WPVC specimens have nearly 14% more absorption capacity than the PVC specimens. Thus by

providing GFRP wrapping the energy absorption capacity is greatly increased indicating adequate

confinement provided by the wrapping.

Bulge

fo

r

m

ati

on



Table 3 Energy Absorption capacity

SNo Specimens Energy

Absorption (kN-

mm)

Average Energy

Absorption (kN-

mm)

1 CS1 9682.76 9611.24

2 CS2 9539.72

3 PVC1 6847.27 7542.21

4 PVC2 8237.16

5 WCS1 9578.94 9887.16

6 WCS2 10195.39

7 WPVC1 7839.44

8580.29 8 WPVC2 9321.15

4. CONCLUSION

Axial compression tests were conducted on four CFDST and four CFST specimens and the following

conclusions were drawn

• The wrapped CFDST specimens with inner PVC pipe exhibited 25% more load carrying capacity than the

unwrapped CFDST specimen.

• The energy absorption capacity of the wrapped CFDST specimen with inner PVC pipe is 14% more than the

CFDST specimen without wrapping.

• The wrapped CFDST columns exhibit nearly the same load carrying capacity as the wrapped CFST

specimens.

• The GFRP wrapping protects the steel tube from atmospheric effects and adoption of fly ash concrete

ensures the column to be a sustainable member with lesser self weight and greater economy.

Due to the high load carrying capacity, lesser weight and better energy absorption capacity, GFRP

wrapped CFDST columns could be used in regions of high seismic activity.

REFERENCE

[1] Elchalakani, M., Zhao, X.-L. & Grzebieta, R. Tests on concrete filled double-skin (CHS outer and SHS

inner) composite short columns under axial compression. Thin-Walled Structures 40, 2002,pp.415–441.

[2] Li, W., Han, L.-H. & Chan, T.-M. Tensile behaviour of concrete-filled double-skin steel tubular

members. Journal of Constructional Steel Research 99, 2014,pp.35–46.

[3] Han, L.-H., Huang, H., Tao, Z. & Zhao, X.-L. Concrete-filled double skin steel tubular (CFDST) beam–

columns subjected to cyclic bending. Engineering Structures 28, 2006,pp.1698–1714 .

[4] Tao, Z. & Han, L.-H. Behaviour of concrete-filled double skin rectangular steel tubular beam–columns.

Journal of Constructional Steel Research 62, 2006, pp. 631–646 .

[5] Tao, Z., Han, L.-H. & Zhao, X.-L. Behaviour of concrete-filled double skin (CHS inner and CHS outer)

steel tubular stub columns and beam-columns. Journal of Constructional Steel Research 60, 2004,

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[6] Zhao, X.-L. & Grzebieta, R. Strength and ductility of concrete filled double skin (SHS inner and SHS

outer) tubes. Thin-Walled Structures 40, 2002 , pp.199–213 .

[7] Han, L.-H., Tao, Z., Huang, H. & Zhao, X.-L. Concrete-filled double skin (SHS outer and CHS inner)

steel tubular beam-columns. Thin-Walled Structures 42, 2004 , pp.1329–1355 .



[8] Liang, Q. Q. & Fragomeni, S. Nonlinear analysis of circular concrete-filled steel tubular short columns

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[9] Uenaka, K., Kitoh, H. & Sonoda, K. Concrete filled double skin circular stub columns under

compression. Thin-Walled Structures 48, 2010, pp.19–24 .

[10] Yuan, W. & Yang, J. Experimental and numerical studies of short concrete-filled double skin composite

tube columns under axially compressive loads. Journal of Constructional Steel Research 80, 2013,

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[11] Wang, J., Liu, W., Zhou, D., Zhu, L. & Fang, H. Mechanical behaviour of concrete filled double skin

steel tubular stub columns confined by FRP under axial compression. Steel and Composite Structures

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[12] Ali S. Shanour, Ahmed A. Mahmoud, Maher A. Adam And Mohamed Said, Experimental Investigation

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Documents

DOUBLE SKIN TUBULAR COLUMNS CONFINED … the two layers by providing proper compaction. ... Self-curing compound was added ... Figure 3 shows the concrete filled CFDST and CFST column