CHARACTERISTIC STUDY ON CONCRETE STEEL DOUBLE SKINNED TUBES AND CONCRETE FILLED STEEL TUBES UNDER AXIAL COMPRESSIVE STRENGTH

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1 International Journal of Civil Engineering and Technology (IJCIET) Volume 8, Issue 10, October 2017, pp , Article ID: IJCIET_08_10_115 Available online at ISSN Print: and ISSN Online: IAEME Publication Scopus Indexed CHARACTERISTIC STUDY ON CONCRETE STEEL DOUBLE SKINNED TUBES AND CONCRETE FILLED STEEL TUBES UNDER AXIAL COMPRESSIVE STRENGTH Deepak K.P Post Graduate Student, Department of Civil Engineering, Amrita University, Coimbatore, India Ananth Kumar. M Assistant Professor, Department of Civil Engineering, Amrita University, Coimbatore, India ABSTRACT The present work aims at a comparative study on the axial compressive strength of concrete filled steel tubes and concrete filled double skinned steel tubes with both inner and outer tubes filled with concrete and the variation of compressive strength is studied keeping the area of steel and concrete used as constant for both the cases. The impact of the variation of inner and outer steel tube thickness towards compressive strength is also studied and the stress-strain relationships are recorded. The specimens are also modelled in finite element software ASNYS workbench 16.0 and the obtained results were in accordance with the experimental results. It was found that concrete steel double skinned steel tubes of outer tube thickness higher than inner tube thickness showed an increase in load carrying capacity whereas the concrete steel double skinned tubes of inner tube thickness higher than outer tube thickness showed a reduction in axial compressive strength when compared with concrete. The concrete steel double skinned steel tubes showed an enhancement in ductility thus it is more favorable to use double skinned steel tubes for construction as it prevents sudden failure and can give indications prior to failure. Keywords: ANSYS, Concrete Filled Steel Tubes, Concrete Steel Double Skinned Steel Tubular Columns, compressive strength. Cite this Article: Deepak K.P and Ananth Kumar. M, Characteristic Study On Concrete Steel Double Skinned Tubes and Concrete Filled Steel Tubes Under Axial Compressive Strength, International Journal of Civil Engineering and Technology, 8(10), 2017, pp editor@iaeme.com

2 Characteristic Study On Concrete Steel Double Skinned Tubes and Concrete Filled Steel Tubes Under Axial Compressive Strength 1. INTRODUCTION Over the years, the construction industry has witnessed various composite structures for high performance column design to fare better in terms of high strength, stiffness, ductility and seismic resistance. Concrete-filled steel tube (CFST) columns are one such composite structure that outperforms the conventional column system in various different aspects. The CFST column is a composite structure which combines the advantages of steel and concrete. Steel columns possess high tensile strength, ductility and better seismic performance by improved resistance to dynamic loads. The steel tube aids as a formwork during the casting the concrete, which aids in faster pace of construction and reduction in construction cost. The steel tube performs the function of both longitudinal and lateral reinforcement for the concrete encased by it, thereby eliminating the necessity of providing additional reinforcements and also aids in providing confinement for the concrete core and thus improving the compressive strength of concrete core. Concrete filled double skinned steel tubes are structures having 2 steel tubes and the concrete is sandwiched between the 2 tubes. These tend to have higher ductility when compared to their single tube counterparts and also has improved fire resistance [15]. The double skinned tubes can resist sudden failure when used along with high strength concrete. The adopted method is similar to that of determination of axial compressive strength of novel hybrid composites [16].The studies involving Concrete filled double skinned steel tubes with both the inner and outer tubes filled with concrete are very limited. The behavior of such a specimen when compared to that of concrete filled steel tube in axial load carrying capacity, effect of variation of tube thickness and its stress strain relationship is carried out with the criteria for comparision being the use of equal area of steel and concrete for both specimens. The specimens are also modelled in the finite element software ANSYS workbench 16.0 and the model is validated 2. EXPERIMENTAL PROGRAM A total of 12 specimens were tested for the compressive strength and stress strain behavior. The specimens were casted two at a time and tested for axial compressive strength, one of concrete filled steel tube and the other concrete steel double skinned steel tube, where the area of steel and concrete used in both cases were kept as constant. A total of three trials were done with the same configuration in order to arrive at a commendable result. The next set of tests were conducted by reducing the outer tube thickness and increasing the inner tube thickness for concrete steel double skinned steel tubes and comparing it with single skinned counterparts of equivalent area of steel and concrete Steel Tubes 12 mild steel tubes of dimensions 100mm diameter, 300mm height, 4mm thickness and 6 steel tubes of 50mm diameter, 4mm thickness were prepared. In order to incorporate the criteria of equal area of steel for both single skinned and double skinned tubes, some of the tubes had to be turned in a lathe in order to remove excess thickness to achieve this. Facing of all the steel tubes had to be performed in order to ensure a smooth level surface so a small amount of metal had to be removed on each face thus reducing the height from 300 to 285mm. The specifications of the steel tubes are given in Table: editor@iaeme.com

3 Deepak K.P and Ananth Kumar. M Table 1 Specifications of steel tubes Concrete filled Steel Tubes Steel Mild Steel Length 285mm Thickness of steel tube 4mm Concrete steel double skinned steel tubes Steel mild steel Length 285mm Thickness of outer tubes 3mm, 2mm Thickness of Inner tubes 2mm, 4mm 2.2. Concrete The specifications of concrete used are given in table2. Brand Type Specific Gravity Size Bulk density Water absorption Size Bulk density Water absorption Table 2 Specifications of concrete Grade Minimum Cement content Maximum w/c ratio Workability Degree of supervision Type of aggregate Maximum cement content Cement ACC PPC 2.9 Fine Aggregate 4.75mm passing 1.615kg/l 8.14% Coarse aggregate 20mm passing 1.58kg/l 8.14% Concrete M20 300kg/m mm good Crushed angular aggregate 450kg/m editor@iaeme.com

4 Characteristic Study On Concrete Steel Double Skinned Tubes and Concrete Filled Steel Tubes Under Axial Compressive Strength The mix proportioning done based on IS is given in Table 3. Table 3 Concrete Mix ratio Concrete mix ratio by weight Mix Ratio Cement Fine aggregate Coarse aggregate Water 1 : 1.89 : kg/m kg/m kg/m kg/m Casting The specimens were casted in such a way that one concrete filled Steel tube and one concrete filled double skinned steel tube was casted at a time. The base plate with the groove was cleaned and the surface was oiled. Then concrete was filled in the steel tube in 5 layers with 25 tamps for each layer. The top surface was levelled ad finished with a trowel and was kept for setting for hours Curing The specimens were water cured for a period of 28 days by completely immersing them in water Testing Axial compressive loading of specimen was done on a compression testing machine of capacity 2000kN. A deflection dial guage was used to measure the linear strain. Compression testing of concrete cube specimen was also done. Uniform loading was done at a pace of 5kN/sec and its corresponding deformations in the intervals of 10kN were recorded. The specimens were loaded till the load dial reaches the ultimate load, stays there for a while and then it suddenly deflects in the reverse direction where considerable amount of deformation of steel tube was observed Experimental result discussion It was observed that the concrete steel double skinned steel tube with 3mm thick outer diameter and 2mm thick inner steel tube diameter exhibited an increase of 6.6% in axial compressive strength when compared to concrete filled tubes of 4mm thickness of equivalent area of steel and concrete for all the three trials. Whereas the concrete steel double skinned steel tubes of 2mm thick outer tube and 4mm thick inner tube exhibited a reduction of 8.43% in axial compressive strength. All the specimens failed due to the crushing of concrete in the steel tube resulting in the linear buckling of steel tubes. Bulging of steel tubes were observed near the ends of the specimen either at the top or at the bottom editor@iaeme.com

5 Deepak K.P and Ananth Kumar. M Figure 1 CFT left and CSDST 1 right A hollow steel tube and concrete cubes of the mix design used was tested for finding out the yield strength of mild steel and compressive strength of concrete for the purpose of feeding the values to analysis equation Stress-Strain and load deflection plots The stress strain relationships and load deflection plots of three different configurations used are given below. Here, CFT denotes concrete filled steel tube 4mm thick. CSDST 1 denotes concrete steel double skinned tubes of 3mm outer tube thickness and 2mm inner tube thickness. CSDST 2 denotes concrete steel double skinned tubes with 2mm outer tube thickness and 4mm inner tube thickness. Figure 2 Stress strain relationships editor@iaeme.com

6 Characteristic Study On Concrete Steel Double Skinned Tubes and Concrete Filled Steel Tubes Under Axial Compressive Strength Figure 3 load deflection relationships From above graphs, it is well evident that the double skinned steel tubes undergo more deformation prior to failure and can thus be considered to be more ductile. 3. FINITE ELEMENT ANALYSIS The modelling of the specimens was done in ANSYS workbench Element Type Steel The element type SOLID45 is chosen for steel tube modelling. This element comprises of eight nodes and has 3 degrees of freedom at each node. The element is capable of exhibiting creep, plasticity, swelling, large deflection and large strains Concrete For concrete core, the element type SOLID65 is chosen. This element type can exhibit features such cracking in tension and crushing in compression, creep and plastic deformation. It is an eight noded element with 3 degrees of freedom at each node Model Details A total of 3 specimens were modelled. A concrete filled steel tube of 100mm diameter, 4mm thickness and 285mm in height, a concrete steel double skinned tube with outer tube of 100mm diameter and 3mm thickness, inner tube of 50mm diameter and 2mm thickness with a specimen height of 285mm, a concrete steel double skinned tube with outer tube of 100mm diameter and 2mm thickness, inner tube diameter of 50mm and 4mm thickness with a specimen height of 285mm Results The deformations for each specimen, the load deflection curves, the stress-strain relationships were obtained. The ultimate load attained by each specimen denoted as Pu FEA is tabulated in Table editor@iaeme.com

7 Deepak K.P and Ananth Kumar. M Figure 4 Total deformation of CFT Figure 5 Total deformation of CSDST 1 Figure 6 Total deformation of CSDST editor@iaeme.com

8 Characteristic Study On Concrete Steel Double Skinned Tubes and Concrete Filled Steel Tubes Under Axial Compressive Strength Figure 7 Load deflection relationships Figure 8 Stress strain relationships It was observed that CSDST 1 attained the highest strength, followed by CFT and CSDST 2. It can be established from the graph that CSDST 2 shows most ductile behavior followed by CSDST 1 and CFT. These results are conforming to the results obtained from the experiment and thus the model can be validated using ANSYS. 4. ANALYTICAL STUDY The equation proposed by Giakoumelis and Lam [7] who modified the equation from ACI code taking into account the confinement effect, was used to verify the experimental results. =1.3 + Where = Ultimate load = Area of concrete = Compressive strength of concrete = Yield strength of steel. = Area of steel The ultimate load carrying capacity predicted by this equation, represented as Pu Equation for each specimen is shown in table editor@iaeme.com

9 Deepak K.P and Ananth Kumar. M 5. CONCLUSIONS Concrete filled double skinned steel tubes of 3mm thick outer tube and 2mm thick inner tube showed an increase of 6.4% in compressive strength when compared to concrete filled steel tubes of 4mm thickness based on experimental results and an increase of 4.8% based on finite element analysis results. The concrete filled double skinned steel tubes of 2mm thick outer tube and 4mm thick inner tube showed a reduction of 5.825% in axial compressive strength when compared to concrete filled steel tubes of 4mm thickness based on experimental results and a reduction of 8.37% based on finite element analysis results. Based on the experimental results, it is evident from the stress strain graphs that concrete steel double skinned tubes show more ductile behavior than that of concrete filled tubes. The results obtained from finite element analysis also exhibited similar behavior of specimens. 6. DISCUSSIONS The concrete steel double skinned steel tubes with outer tube thickness greater than inner tube thickness has improved compressive strength when compared to concrete filled steel tubes having equivalent area of steel and concrete. The increase in concrete confinement in case of concrete steel double skinned tubes could be responsible for enhancing its compressive strength. The concrete steel double skinned tubes with thickness of inner tube greater than outer tube experienced a reduction in compressive strength compared to concrete filled steel tubes of equivalent area of steel and concrete. As the area of steel is more where the lever arm distance is maximum for concrete filled steel tubes, the moment of resistance of concrete filled steel tubes will be greater than that of concrete steel double skinned tubes It can be assumed that there exists a transition region, above which the effect of moment of resistance is predominant and below which effect of confinement is predominant, which is dependent on the thickness and diameters of both the inner and outer tubes. Pu Experiment Table 4 Results Trial 1 Trial 2 Trial 3 Pu Equation Pu FEA Pu Experiment CFT 1000 kn 940 kn kn CSDST kn 940 kn kn Pu Equation kn kn Pu FEA Pu Experiment Pu Equation Pu FEA kn kn kn kn 1115 CFT 1060 kn 931 kn kn 966 kn kn kn 1063 CSDST kn 931 kn kn 966 kn kn kn 974 It is more advantageous to use concrete steel double skinned tubes as they provide better warning prior to failure thus one will have the time to take precautionary safety measures. Due to their enhanced ductility and not much considerable variation in compressive strength, when compared to single skinned steel tubes they can perform better as earth quake resistant structures editor@iaeme.com

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11 Deepak K.P and Ananth Kumar. M [17] M.S. Vijaykumar and Dr. R. Saravanan. Analysis of Epoxy Nano Clay Composites Compressive Strength during Salt Spray Test. International Journal of Mechanical Engineering and Technology, 8(5), 2017, pp [18] M.S. Vijaykumar and Dr. R. Saravanan. Analysis of Epoxy Nano Clay Composites Compressive Strength during Tropical Exposure Test. International Journal of Mechanical Engineering and Technology, 8(5), 2017, pp [19] Z.Tao, L.H Han, X.L Zhao (2004) Behaviour of concrete-filled double skin (CHS inner and CHS outer) steel tubular stub columns and beam-columns Journal of Constructional Steel Research 60 pp [20] Mini K.M., Jayanarayanan K, Akila.P, (2016), Novel hybrid composites based on glass and sisal fiber for retrofitting of reinforced concrete structures Journal of Construction and Building materials; 133 (2017) editor@iaeme.com