Analysis of Composite Steel (CFST) Columns -FRC Infilled at Elevated Temperature Using MATLAB

Analysis of Composite Steel (CFST) Columns -FRC Infilled at Elevated Temperature Using MATLAB

Arpitha P

PG Student, M Tech Dept of Civil Engineering

Ghousia College of Engineering Ramanagaram

E Ramesh Babu

Associate Professor Dept of Civil Engineering

Ghousia College of Engineering Ramanagaram

Abstract: – Combining different materials in a single structural member to take advantage of the good qualities that they both have separately has always been a recognized strategy in building industry. Concrete filled steel tubular columns (CFST) are a type of composite columns in which the combined action of steel and concrete leads to an exceptional structural behavior. In this case, the compressive strength of the element increases due to the passive confinement that the steel tube generates on the concrete core. Simultaneously, the local buckling of the steel tube is improved due to the support of the concrete core which prevents it from suffering this phenomenon inwards. In concrete filled tubular columns (CFST) the combined action of steel and concrete results in many positive attributes at ambient temperature: high load-bearing capacity with smaller cross- section size, aesthetics, high stiffness and ductility and reduced construction cost.

Keywords: Genetic Algorithm, MATLAB, VBA, Composite steel Column.

allows not only retaining all the advantageous features of the empty section but also developing new excellent properties. This fact reveals the synergy existing when these two elements work in conjunction. Figure 1.1 shows the behavior of a steel tubular column, a reinforced concrete (RC) stub column and a concrete filled steel tubular stub column without reinforcing bars. As it can be observed, the summation of the steel tube and the RC columns ultimate strength is less than that achieved by the CFST column Han et al. 2014.

1. INTRODUCTION

   This chapter introduces the structural technology of concrete filled steel tubular columns and their fire resistance characteristics. The benefits of using these composite columns over the other alternatives are also described. The three problems which constitute the whole fire analysis of a CFST column are explained: the fire dynamics, the thermal and the structural analysis

   1.1. Background

   Combining different materials in a single structural member to take advantage of the good qualities that they both have separately has always been a recognized strategy in building industry. Concrete filled steel tubular columns (CFST) are a type of composite columns in which the combined action of steel and concrete leads to an exceptional structural behavior. In this case, the compressive strength of the element increases due to the passive confinement that the steel tube generates on the concrete core. Simultaneously, the local buckling of the steel tube is improved due to the support of the concrete core which prevents it from suffering this phenomenon inwards.

   Structural hollow sections (SHS) are the most efficient steel sections in resisting compression loads. Generating a new composite section by filling the hollow tube with concrete

   Concrete filled steel tubular (CFST) columns have many positive attributes at ambient temperature for building industry: high load-bearing capacity with smaller cross- section size, attractive appearance, high stiffness and ductility, high seismic resistance and reduced construction cost and time since no formwork is necessary.

   Among the most commonly used shapes of CFST columns, it can be found circular and rectangular cross-sections Figure 1.2, although new shapes, such as elliptical profiles, are appearing. A wide variety of concrete infills can be used such as plain concrete, bar-reinforced concrete or fiber reinforced concrete. As mentioned above, the combination of these elements permits the design of columns with reduced cross-section. However, when free spaces and higher net usable surfaces are desirable, the cross-section of the CFST column can be even smaller if high strength materials are employed, solution which is hugely applied by high-rise buildings designers.

2. Fire behavior of CFST columns

The use of concrete filling offers a practical alternative for providing the required fire resistance in steel hollow structural section columns without the need of additional protection Kodur and Lie 1995 and Twilt et al.1996.This is due to the heat sink effect produced in the composite section because of the low thermal conductivity of concrete and the mechanical contribution of the concrete core, which helps to support the applied load and also prevents the inward local buckling of the steel.

The increment in the fire resistance rate (FRR) can be magnified with the use of internal reinforcement. The fire resistance reached by bar-reinforced concrete (RC) filled tubular columns is higher than that achieved by steel tube columns filled with fiber reinforced concrete (FRC), which, at the same time, is higher than the FRR shown by plain concrete filled hollow steel section columns.

1. Properties of fiber reinforced concrete column at elevated temperature

   In recent years, the construction industry has shown significant interest in the use of fiberreinforced concrete due to the advantages it offers over traditional plain concrete. The use of steelfibers as reinforcement in plain concrete not only enhances the tensile strength of the composite system but also reduces cracking under serviceability conditions. Further, steel-fibers improve resistance to material deterioration as a result of fatigue, impact, shrinkage and thermal stresses. The improvements in material properties, which improve structural performance, have extended the use of fiber-reinforced concrete to applications in the area of fire.

   A number of experimental investigations have been conducted up to date with the aim to observe the fire response of concrete composites. Particularly, the studies have focused on the effect of a type, shape and content of fibers on the mechanical properties of concrete composites, mostly compressive and tensile strength including elastic modulus. Namely, it concerns steel fibers, synthetic fibers and a mix of steel and polypropylene fibers which are widely used in the concrete industry. There are also a few investigations which deals with carbon fibers and glass fibers.

   The mechanical properties that determine the fire resistance of structural members are the strength and deformation properties at elevated temperatures of the materials of which the members are composed. For concrete, the important mechanical properties are the compressive and tensile strength of the concrete and the deformations caused by load, creep and thermal expansion. These properties are usually expressed in stress-strain relations, which are used as input data in mathematical models for the calculation of the fire resistance of concrete members.

2. Material properties

The properties of the fiber reinforced concrete which were used in the model were determined by an experimental test

within the framework of Models of Fiber reinforced concrete columns in case of fire. The amount of steel fibers used in the fiber concrete mix was 1.7% by mass. Experimental tests were performed on conventional bodies at normal temperatures and elevated temperatures of 200 ° C, 400 ° C and 600 ° C. On the basis of the acquired knowledge a material model was made, which is to serve for numerical simulations of composite steel columns on the spatial model.

WORK FLOW

• Defining Structural problm.

• Determination of Objective Function, Design Variables and Constraints.

• Development of VBA (visual basic application) code for design.

• Development of MATLAB programme.

• Solving problem Using MATLAB.

  MATLAB Programming

  1. Predicted and experimental results:

     CONCLUSIONS AND FUTURE WORKS 5

     CONCLUSIONS AND FUTURE WORKS:

     • The ultimate shear strength of FRC is less than the ultimate tensile capacity of FRC. It is less than by about 33%, 39% and 53% for temperatures of 200, 400 and 600 respectively.

     • The steel fibers improves the post crack behavior of FRC significantly.

     • Thecompressive strengthof the cube is about 20% higher than the compressive strength of the cuboid made from the same material. Unlike the compressive strength, the tensile strength of the cube and cuboid are almost the same.

• FRC column fails in compression by crushing when the axial load is applied without any eccentricity at any temperature. FRC column experiences a combined failure under the application of the load with some eccentricities.

• The orientation of cracks in the FRC column models with eccentric loading are aligned in a diagonal manner which indicates that the presence of a shear stress along with the flexural and compressive stresses.

• The failure modes of a column is greatly affected by the boundary condition and loading environment

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