ANALYTICAL STUDIES ON CONCRETE FILLED STEEL TUBES

Transcription

1 INTERNATIONAL JOURNAL OF CIVIL ENGINEERING AND TECHNOLOGY (IJCIET) Proceedings of the International Conference on Emerging Trends in Engineering and Management (ICETEM14) ISSN (Print) ISSN (Online) Volume 5, Issue 12, December (2014), pp IAEME: Journal Impact Factor (2014): (Calculated by GISI) IJCIET IAEME ANALYTICAL STUDIES ON CONCRETE FILLED STEEL TUBES Vima Velayudhan Ithikkat 1, Dipu V S 2 1, 2 Civil Department, SNGCE, Kolenchery, India ABSTRACT In recent years, as a type of hybrid system, the concrete-filled steel tubular (CFST) columns are increasingly used in buildings and bridges. In concrete-filled steel tube (CFST) columns, the steel tube provides formwork for the concrete, the concrete prolongs local buckling of the steel tube wall, the tube prohibits excessive concrete spalling, and composite columns add significant stiffness to a frame compared to more traditional steel frame construction. The design of concrete filled steel tubes is considered to be difficult since a proper formula is not present to find out the axial load carrying capacity of such CFST short columns. The main aim is to find an approximate formula for finding the ultimate axial load carrying of rectangular concrete filled steel tube short column by obtaining relations between various material properties of CFST using a finite element model developed using ANSYS software and validating it against the experimental data obtained during literature survey. Keywords: Concrete Filled Steel Tubes, Contact Elements, Contact Status, Finite Element Model, Short Column, Ultimate Axial Load. 1. INTRODUCTION A concrete-filled tube (CFT) member consisting of a steel tube filled with concrete material realizes the importance of steel reinforcement to provide confinement for the concrete and to increase the load-carrying capacity of the composite member. From the structural point of view, the concrete filling in the steel hollow section not only prevents the occurrence of the steel s local buckling but it also enhances the ductility of the CFT member up to the ultimate load. In a composite column both the steel and concrete would resist the external loading by interacting together by bond and friction. The concrete and steel are combined in such a fashion that the advantages of both the materials are utilized effectively in composite column. Fig 1: Typical cross sections of Concrete filled Steel Tubes 99

2 2. FINITE ELEMENT MODELING The finite element model of the Concrete filled Steel Tube (CFST) was modeled using the software ANSYS ANSYS is a commercial FEM package having the capabilities ranging from a simple, linear, static analysis to a complex, nonlinear, transient dynamic analysis. The finite element model was modeled using direct modeling by both Bottom Up approach and Top Down approach. 2.1 Description of the model Short Rectangular Plain Cement Concrete filled Steel tube was modeled. For the present study, the cross section of 190 mm x 100 mm and length 390 mm was modeled with the thickness of steel tube as 4 mm. The grade of concrete was varied between 20 to 50 MPa and yield strength of steel varied between 475 to 495 MPa. The Poisson s ratio for steel is taken as 0.3. The correct simulation of composite action between concrete and steel tube is the single most important factor guiding the behavior of the CFT column. To model this interaction, the normal contact between the two materials is provided using friction, with the inner surface of the stiffer steel tube serving as the rigid surface and the outer surface of the concrete core as the slave surface. The coefficient of friction between the two surfaces is chosen as The study was continued even with thickness of steel tube as 5 mm. The boundary condition is that fixed at the bottom of the specimen and axially loaded at the top of the column specimen 2.2. Elements used to Model Concrete Filled Steel Tubes in ANSYS Software SOLID65 This is used for the three-dimensional modeling of solids with or without reinforcing bars (rebars). The element is defined by eight nodes having three degrees of freedom at each node: translations in the nodal x, y, and z directions. It is used model plain concrete infill Fig 2: SOLID65 Geometry SHELL63 Shell 63 element has both bending and membrane capabilities. Both in-plane and normal loads are permitted. The element has six degrees of freedom at each node translations in the nodal x, y, and z directions and rotations about the nodal x, y, and z axes. It is used to model steel tube Contact Elements These elements are used for modeling interface between Concrete and Steel. When two separate surfaces touch each other such that they become mutually tangent, they are said to be in contact. In the common physical sense, surfaces that are in contact have these characteristics: They do not interpenetrate. They can transmit compressive normal forces and tangential friction forces. They often do not transmit tensile normal forces. They are therefore free to separate and move away from each other. Contacts have changing-status nonlinearity. The contact elements used are CONTA 173 and TARGE 170 elements. Here the target surface is steel tube and contact surface is taken as concrete infill. The contact surface moves in to target surface. 2.3 Modeling of the Specimen The steel tube was modeled using Bottom Up approach and concrete infill is modeled using Top Down approach. 100

3 Fig 3: Completed Steel model The model is completed only after meshing them properly. Both steel tube and concrete infill are meshed of equal sizes to provide contact between them very easily. The contact pairs are provided using contact pair option. The inner side of the steel tube is selected as target surface and contact surface is contact infill. The contact between concrete infill and steel tube should be such that it will always have bonded contact with friction and gap between steel tube and concrete infill is always closed and not allowed to penetrate in to each other. There are five types of contact status available to provide between steel tube and concrete infill that is bonded, no separation, rough, frictional and frictionless contact. Fig 4: Model after applying Contact pairs The model is then fixed at bottom side and an axial load is applied on the top of the short column specimen. Fig 5: Completed model of column specimen 101

4 3. BUCKLING ANALYSIS 3.1 Buckling Buckling is a critical phenomenon in structural failure. It is the failure of structures under compression load. Also buckling strength of structures depends on many parameters like supports, linear materials, composite or nonlinear material etc. Also buckling behavior is influenced by thermal loads and imperfections. Buckling proceeds either in stable or unstable or equilibrium state. Stable: In which case displacements increase in a controlled fashion as loads are increased, that is the structure's ability to sustain loads is maintained. Unstable: In this case deformations increase instantaneously, the load carrying capacity nose- dives and the structure collapses catastrophically. Neutral equilibrium: This is also a theoretical possibility during buckling - this is characterized by deformation increase without change in load. Buckling and bending are similar in that they both involve bending moments. In bending these moments are substantially independent of the resulting deflections, whereas in buckling the moments and deflections are mutually inter-dependent - so moments, deflections and stresses are not proportional to loads. 3.2 Types of Buckling Analysis ANSYS supports two types buckling analysis. They are as listed below. For the present work, both types of buckling analysis is done for getting the accurate result Eigen Value Buckling Analysis Eigen value buckling analysis predicts the theoretical buckling strength (the bifurcation point) of an ideal linear elastic structure. This method corresponds to the textbook approach to elastic buckling analysis: for instance, an Eigen value buckling analysis of a column will match the classical Euler solution. However, imperfections and nonlinearities prevent most real-world structures from achieving their theoretical elastic buckling strength. Thus, Eigen value buckling analysis often yields unconservative results, and should generally not be used in actual day-to-day engineering analyses. Fig 6: Table of Eigen Buckling Analysis 3.2.2Non-Linear Buckling Analysis Nonlinear buckling analysis is usually the more accurate approach and is recommended for design or evaluation of actual structures. This technique employs a nonlinear static analysis with gradually increasing loads to seek the load level at which the structure becomes unstable. Using the nonlinear technique, model can include features such as initial imperfections, plastic behavior, gaps, and large-deflection response. In addition, using deflection-controlled loading, we can even track the post-buckled performance of the structure (which can be useful in cases where the structure buckles into a stable configuration). 102

5 4. RESULT OBTAINED FROM ANSYS Fig 7: Table of solution controls for Non Linear Analysis The aim of carrying out both the types of buckling analysis was to have a more accurate result for obtaining the relation between material and geometric properties so that new equation going to be developed becomes reasonable with the obtained experimental results values. The obtained relations are plotted in graphs as shown in figure (9) and (10). Fig 8: Detailed Summary of Eigen buckling analysis for A-14-1 It is observed that both Eigen Value Buckling Analysis and Non linear buckling analysis (time history output) output for a sample model specimen A-14-1 (table 1) Fig 9: Relation between ultimate load and f c for t = 4 mm 103

6 Fig 10: Relation between ultimate load and Yield strength of steel for thickness of steel tube, t = 4 mm 5. FORMULATION OF NEW EQUATION FOR ULTIMATE LOAD CARRYING CAPACITY For developing a better equation for finding the ultimate load carrying capacity of Concrete filled Steel Tube Column, we consider the basic equation for axial load: Axial load, P = Stress (σ) x Area of cross-section (A) (1) So from this, we assume that the axial load carrying capacity of Concrete filled Steel Tube Column can be written as: Axial Load, P = σ cc x A c + σ sc x A sc (2) where σ cc = Permissible stress in Concrete under direct compression in N/mm 2 A c = cross sectional area of concrete in mm 2 σ sc = Permissible stress in Steel in N/mm 2 A sc = cross sectional area of Steel in N/mm 2 Now find the original load carrying capacity of short rectangular Concrete filled Steel Tube by considering the relation between steel thickness and material properties of materials used in specimen to consider the bond developed at the interface of the Concrete infill and Steel Tube. The bond is assumed to be bonded always without any gap and coefficient of friction assumed between the steel and concrete interface was 0.25: Thus, a factor of strength developed between the concrete infill and steel tube when bonded with friction, let it be n b = 1.20 (3) Now to find the factor that contribute to the load carrying capacity of Concrete filled Steel Tube due to the mix of concrete can be found out by using the figure 9, it can be seen that for the same thickness of steel tube, changing the mix proportions of concrete is 20%: Hence we can introduce a factor of Strength developed due to change in contact of status of steel tube with mix proportions of concrete, n c = 1.2 (4) Now to find the factor that contribute to the load carrying capacity of Concrete filled Steel Tube due to the grade of steel can be found out by using the figure 10, it can be seen that for the same thickness of steel tube, changing the grade of steel is 40%: Hence we can introduce a factor of Strength developed due to change in contact of status of steel tube with grade of steel, n s = 1.4 (5) Hence, from (2), (3), (4) and (5),the outputs obtained from the behaviour of Concrete filled steel tube by finite element modeling in ANSYS 10.0, through thesis work, suggest an equation or formula to find the ultimate load carrying capacity of Short Rectangular Plain Concrete filled Steel Tube as: 104

7 Pu = n b x n c x n s x (σ cc x A c + σ sc x A sc ) (6) Where n c =factor of Strength developed due to change in contact of status of steel tube with mix proportions of concrete = 1.2 n s =factor of Strength developed due to change in contact of status of steel tube with grade of steel= 1.4 n b =factor of strength developed between the concrete infill and steel tube when bonded with friction = 1.20 σ cc = Permissible stress in Concrete under direct compression in N/mm 2 A c and A sc = cross sectional area of concrete and steel in mm 2 respectively. 6. COMPARISON OF RESULTS To provide the needed experimental data for validation of my thesis work, the experimental data was collected from the experiment conducted by Schneider in 1998, Han on 2002, Liu et al in 2003 and again in 2004 which was given in the journal namely Numerical Modeling of Rectangular Concrete-Filled Steel Tubular Short Columns by Heaven Singh and P.K Gupta in the International Journal of Scientific & Engineering Research, Volume 4, Issue 5, May Table 1: Details of Experimental Data Specime n label B H T B/H B/t L fc fy Es Exp. Peak Axial load mm mm mm mm MPa MPa GPa kn R R A Table 2: Comparison of Ultimate Load obtained in ANSYS 10.0 with Experimental Data Sl.NO: Experimental Data (kn) ANSYS Output (kn) A Table 3: Comparison of Ultimate Load obtained in New Formula with Experimental Data Sl.NO: Experimental Data (kn) Result obtained using new formula(kn) A R R It is observed from Table 2 and 3, the new formula derived for finding the ultimate load carrying capacity of CFST tubes shows good agreement with the Experimental data s obtained for the same. Hence this formula can be used for short CSFT columns with rectangular cross sections to a limit. 7. CONCLUSION Today, use of Concrete filled Steel Tubes in Construction Industry is rising day by day in foreign countries. But due to lack of proper codal provisions in our Indian Standard Code, the uses of such Concrete filled Steel Tubes are not very much encouraged in our country. The Code provisions available in Indians Standards can be used for Reinforced Concrete Columns. So an attempt has been done for obtaining a new formula for finding out the ultimate load carrying capacity of Concrete filled Steel Tube Short columns using the behaviour pattern studied by finite element modeling of the structure in ANSYS 10.0 It is found that a new formula derived by adding some constants to the basic load-area relations proves to have good agreement with the available experimental data for the same. Though the obtained formula is for Shorts Columns, probably it can be said as theoretical columns, the attempt have been successful that makes to believe that the scope of future for this work will be a motivation for others to find new formulas that can be applied to the real structures 105

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