DOUBLE SKIN TUBULAR COLUMNS CONFINED WITH GFRP

Transcription

1 International Journal of Civil Engineering and Technology (IJCIET) Volume 7, Issue 6, November-December 2016, pp , Article ID: IJCIET_07_06_059 Available online at ISSN Print: and ISSN Online: 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 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 editor@iaeme.com

2 Parvati T S and Dr. P.S. Joanna 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 2 CS2 3 PVC1 4 PVC2 5 WCS1 6 WCS2 CFST columns CFDST columns with inner PVC tube CFST columns with GFRP wrapping 7 WPVC1 CFDST columns with inner PVC tube and 8 WPVC2 wrapped with GFRP 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 editor@iaeme.com

3 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 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 N/mm 2. 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/mm 2. 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 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/mm 2. 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 editor@iaeme.com

4 Parvati T S and Dr. P.S. Joanna 2.3. Test Set-up Tests were performed in a 1000 kn capacity Universal Testing Machine and the data were collected by a 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 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 3. RESULT AND DISCUSSION DISCUSSIO 3.1. Failure Modes Figure 5 shows the load-deformation deformation curves of the eight specimens.on On application of load, the specimens 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 specimens spec exhibited different load-deformation deformation patterns. Control specimens CS1 and CS2 exhibited a round bend in the load deflection graph indicating non-linear non linear response of both steel and concrete. Local buckling was observed in the specimens CS1 and CS2 at the the lower end of the column. The specimen CS2 also developed an inclined fold at one-third third height indicating shear failure in the specimen. The CFDST specimens with inner PVC tubes also exhibited the non-linear non linear response validated by the round bend in the graph. Elephant lephant foot formation was observed at the top of the column specimens PVC1 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 observed observed at the top of the column with de-bonding bonding of the GFRP laminate from the steel tube in WCS1. WCS2 exhibited bulge formation at mid height of the column with th the rupture of GFRP laminate. Figure 5 Load-Deformation Deformation Curve Figure 6 Inward folding of inner PVC tube 539 editor@iaeme.com

5 Double Skin Tubular Columns Confined with GFRP Wrapped CFDST specimens WPVC1 and WPVC2 exhibited a slightly varied load-deflection pattern. The load-deflection curve of the WPVC2 specimen was similar to WCS1 and WCS2 with a sharp yield 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 effect. The wrapped CFDST specimens failed with multiple folds and subsequent rupture of the GFRP lamination. The PVC inner tubes of all CFDST specimens (both wrapped and unwrapped) exhibited inward folding at the end of testing (Figure 6) indicating the loss of confinement (Elchalakani et al., 2002). Figure 7 shows the load deformation curve of WPVC1 indicating the number of peaks developed in the specimen. Figure 5 shows the load-deflection curves with single peak as the curves are plotted to a deflection of 15mm. Figure 7 Load-deformation curve for WPVC Load Carrying Capacity Test results of the column sections are given in Table 2. CFST specimenss (CS1 and CS2) exhibited average peak load of 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 kn and the wrapped CFDST specimens exhibited an average maximum load of kn. Table 2 Test results of column sections S No Specimens Peak Load (kn) Average Peak Load (kn) CS CS PVC PVC WCS WCS WPVC WPVC editor@iaeme.com

6 Parvati T S and Dr. P.S. Joanna 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 Peak Load(kN) CS PVC WCS WPVC Specimen Bulge fo r m ati 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 editor@iaeme.com

7 Double Skin Tubular Columns Confined with GFRP Table 3 Energy Absorption capacity SNo Specimens Energy Absorption (knmm) 1 CS CS PVC PVC WCS WCS WPVC WPVC Average Energy Absorption (knmm) 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 [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 [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 [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 [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, pp [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 [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 editor@iaeme.com

8 Parvati T S and Dr. P.S. Joanna [8] Liang, Q. Q. & Fragomeni, S. Nonlinear analysis of circular concrete-filled steel tubular short columns under axial loading. Journal of Constructional Steel Research 65, 2009, pp [9] Uenaka, K., Kitoh, H. & Sonoda, K. Concrete filled double skin circular stub columns under compression. Thin-Walled Structures 48, 2010, pp [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, pp [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 17, 2014, pp [12] Ali S. Shanour, Ahmed A. Mahmoud, Maher A. Adam And Mohamed Said, Experimental Investigation of Concrete Beams Reinforced With GFRP Bars. International Journal of Civil Engineering and Technology 9IJCIET), 5(11), 2014, pp [13] Pradeepa. S, Gokul Raj. D and Divya M.R, A Review on Cost Assessment of Conventional Steel Structure and Square Tubular Sections Using Force Co-Efficient Method. International Journal of Civil Engineering and Technology (IJCIET), 7(4), 2016, pp editor@iaeme.com