CHAPTER 1 INTRODUCTION

Transcription

1 CHAPTER 1 INTRODUCTION 1.0. GENERAL Modern civilization relies upon the continuing performance of civil engineering infrastructure ranging from industrial building to power station and bridges. For the satisfactory performance of the existing structural system, the need for strengthening is inevitable. Commonly encountered engineering challenges such as increase in service loads, changes in use of the structure, design and/or construction errors, degradation problems, changes in design code regulation and seismic retrofits are some of the causes that lead to the need for new techniques to upgrade the performance of the structures. Though concrete a versatile construction material has several advantages due to its compressive strength and mouldable shape, it has its own tensional limitation and poor ductility. Ductility is an important characteristic of a structure to resist earthquake, impact and blast loading. Steel has excellent ductile property. Hence a judicious combination of structural steel and concrete utilizing the strength possessed by them and suppressing their weakness resulted in the composite construction. The present day demands in construction on parameters such as strength, safety, serviceability, satisfactory and reliable performance expected of a structure apart from economical solutions has also made it imperative to use steel concrete composite construction techniques COMPOSITE CONSTRUCTION A structural member composed of two or more dissimilar materials joined together to act as a unit is referred as composite structure. Joining two dissimilar materials to form a composite member does not only combine the collective strengths of the two materials, forming a union between relevant materials actually enhances their physical characteristics and makes the composite stronger than the sum of their strengths. An example in civil engineering structures is the steel-concrete composite beam in which a steel wide-flange shape (I or W shape) is attached to a concrete floor slab. The many other kinds of composite beams include steel-wood, woodconcrete and plastic-concrete or advanced composite materials concrete. 1

2 1.2. STEEL CONCRETE COMPOSITE CONSTRUCTION In order to design the structural member with maximum efficiency and minimum cost, steelconcrete composite construction is adopted. It is a powerful construction concept in which compressive strength of concrete and the tensile strength of steel are almost effectively used. Steel and the concrete have almost the same thermal expansion apart from an ideal combination of strength. Hence, these essential different materials are completely compatible and complementary to each other. Steel-concrete composite beams are today widely used for bridges and industrial buildings. In large scale construction, steel and concrete are most frequently used combinations for composite beams. The concrete lends the composite mass, stiffness and compressive strength and reduces deflection and vibration in the slab. The steel members give the beam its tensile strength with excellent strength to weight ratios and rapid construction times. Steel-concrete composite beams have long been recognized as one the most economical structural systems for both multistory steel buildings and steel bridges. Buildings and bridges require a floor slab to provide a surface for occupants and vehicles respectively. Concrete is the material of choice for the slab because its mass and stiffness can be used to reduce deflections and vibrations of the floor system and to provide the required fire protection. The supporting system underneath the slab, however, is often steel because it offers superior strength-weight and stiffness-weight ratio, ease of handling and rapid construction cycles. Since both the steel and concrete are already present in the structures, it is logical to connect them together to better utilize their strength and stiffness. Almost all the advantages of steel and concrete structures are combined together in steel-concrete composite construction. The main advantages in utilizing steel-concrete composite construction are saving in the weight of steel. The other advantages in using steel-concrete composite section are: Most effective utilization of materials like concrete in compression and steel in tension. High ductility of steel leads to better seismic resistance of the composite section. Steel component has the ability to absorb the energy released due to seismic forces. Ability to cover large column free area. 2

3 Faster construction by utilizing rolled and/or prefabricated components. Keeping span and loading unaltered, smaller sections are required compared to noncomposite construction. Minimum disturbance to traffic in bridge construction CONFINED STEEL CONCRETE COMPOSITE BEAM (CSCC BEAM) A modern composite construction concept was initially developed in North America and is now used extensively in the UK where it has been further developed and redefined. Early application involved concrete encased steel beams for which the concrete served primarily as fire protection. However it was recognized that both the strength and the stiffness of the encased member were increased compared to the bare steel. Many improvements to composite systems have occurred during the past 40 years. Among these is the introduction and wide spread use of composite steel decks that serve initially as work platform and concrete formwork and as slab reinforcement in resisting loads after the concrete hardens. It is initially supported by secondary beams, metal decks. The secondary and the primary beams work efficiently leading to economy. It should be that the combination of concrete cores, steel frames and composite floor construction has become the standard construction method for multistoried commercial buildings in several countries. Much progress has been made example in Japan, where the structural steel/reinforced concrete frame is the standard system for the tall buildings. The main reason for this preference is the suitability of the sections and members to resist repeated earthquake loadings, which require a high amount of resistance and ductility. Composite construction is popular for building and the bridges as well because of the following aspect: Fast track construction Reduction in overall weight of the structure Higher stiffness Better seismic resistance Economy Service and building flexibility Assembly 3

4 The greatest impetus for the composite construction was the development of the welded headed stud. In the 1950 s headed studs provide mechanical shear connection between the top flange of the steel beam and cast in place of concrete floor slab. As a result of experimental investigation carried out in the past by many researchers with different ways of bonding between steel and concrete, an innovative study has been done in this project to understand the behaviour of concrete beam shuttered with cold formed steel sheet which acts as a composite beam by means of shear connectors and bracings viz. Confined Steel Concrete Composite Beam (CSCC Beam). This is the new idea to the composite structure world. And this can be a replacement for hot rolled steel beams or RCC beams in small to medium sized building. Fig. 1.1 CSCC Beam The cold formed steel sheet is bonded to surface of concrete by means of shear connectors. Stud shear connectors are used to take up the bond between sheet and concrete. The passive confinement by the cold formed sheet in the sides and bottom influences the strength and ductility of the system. These beams are simple to fabricate and provides very good confinement of concrete SHEAR CONNECTORS One of the most important parts of a composite beam is the fixing points of shear connectors between the two materials. The correct connection of the two parts of the composite allows the materials to act as a unit and gives the composite beam its inherent strength. A composite beam is constructed generally by casting a reinforced concrete slab on the top of the steel beam. 4

5 Composite action between the steel and the concrete is achieved by means of mechanical connectors by the effective transfer of shear at the interface between concrete and sheet elements. These shear connectors are generally dubbed as shear connectors. They are typically connected by welding to the top of the flange of a steel beam and cast within the concrete slab. The transfer of longitudinal shear forces at the interface between both components is mostly realized by headed shear studs. It is only through this connection that composite action is achieved. Without these connectors, the slab acts independently and analysis is relatively simple. Shear connection significantly increases the strength and stiffness performance of composite beams. A composite beam can be made to be considered to have full shear connection or partial shear connection, proportional to the amount of shear connections. Shear connectors can take the form of either headed studs, channels or high strength structural bolts. These shear connectors are typically studs welded to the steel beams and set into the concrete slab. The number and size of these shear connectors are carefully calculated as they represent a critical part of the composites mechanical performance. Design of the shear connection is important because it affects: 1. Choice of a suitable ultimate strength design method for the composite beam 2. Vertical deflection of the composite beam. 3. Economics and speed of construction. 4. Stability of the steel beam and safety against collapse. Fig. 1.2 Stud Shear Connector 5

6 Fig. 1.3 Alternate Beam Types used in Composite Constructions 1.5 COLD FORMED STEEL SHEET Although the cold rolled products were developed during the First World War, their extensive use worldwide has grown only during the last 29 years because of their versatility and suitability for a range of lighter load bearing application. The wide range of available products has extended their use to primary beams, floor units, roof trusses and building frames, ranging from purlins to roof sheeting and floor decking. Generally these are available for use of basic building element for assembly at site or as prefabricated frames or panels. These thin steel sections are cold formed. The thickness of steel sheet used in cold formed construction is usually 1 to 3 mm. Much thicker material up to 8mm can be formed if galvanized material is not required for the 6

7 particular application. With this least thickness the sheet is used as a form work and reinforcement. The beams using the profiled sheeting are referred as profiled composite beams. There are other strength related aspects to profiled composite construction some of which are the subject of ongoing research. However they increase the compressive strength and decrease the deformation of confined concrete by offering resistance to the lateral bulging of concrete. 1.6 RESEARCH GAP In engineering applications, physical response of any structure to a system of external force is very much important. Experimental programmes are usually carried out to predict the physical response of structure. The literature for such experimental programme in composite beam is scanty and no codal provision is available in Bureau of Indian Standards (BIS). Previous research by Oehelrs 13 showed that the side profile sheet increased the flexural strength and makes the concrete more ductile than RCC. Richard. P and Nguyen N. 38 studied the cold formed steel stiffened channels and concrete subjected to shear, bending and combined effect of both. Khandeker M. Anwar Husain 27 studied the strength and failure modes of concrete composite beam under flexure. Although many experiments have been done to investigate the behaviour of composite beams under flexure, very less investigations has been carried out on the behaviour of composite beams under torsion and under combined bending and torsion as per author s knowledge. Before a design procedure can be formulated, it is essential to obtain a better understanding for the behaviour of composite beam under pure bending, pure torsion, and combined bending and torsion. Hence an extensive experimental program on the behaviour under combined loading is considered as research gap and it is hoped that this investigation will make a contribution to that end. 1.7 RESEARCH METHODOLOGY In view of the complex nature of the problem of studying the interaction between torsional and flexural strength of the composite beam, an approach is made to conduct number of experiments 7

8 to observe the behaviour of the beams under pure bending, pure torsion and combined bending and torsion. Based on the observations from the experiments, equations are developed for the interaction and the validity is finally established by the comparison of experimental and theoretical results. 1.8 RESEARCH PLAN WORK PLAN EXPERIMENTAL STUDY THEORETICAL INVESTIGATION NUMERICAL VALIDATION MATERIAL STUDY ELEMENT STUDY GENERATION EQUATION FOR BM AND TM USING FINITE ELEMENT ANALYSIS 1)TEST FOR AGREGRATES, 2) TENSION TEST FOR COLD FORMED SHEET, 3)CUBE COMPRESSION TEST TESTING OF BEAMS UNDER 1)PURE BENDING 2)PURE TORSION 3)COMBINED BENDING AND TORSION 1.9 OBJECTIVES OF THE PRESENT STUDY The following aspects are aimed in this thesis. 1. To observe the behaviour of CSCC beam subjected to pure bending, pure torsion and combined loading (bending and torsion) with particular regard to their behaviour at failure. 2. To develop the interaction equation between bending and torsion under the combined loading (bending and torsion) and to predict the mode of failure. 3. To study the effect of bending moment on the torsional strength of beam. 4. To study the effect of shear connectors on the ultimate strength of beam. 5. To study the effect of bond force on the ultimate strength of beam. 8

9 6. To study the effect of dimension of beam on the ultimate strength of beam. 7. To study the effect of spacing of bracing on the ultimate strength of beam. 8. To compare the deflection of CSCC beams with a numeric tool finite element analysis using ANSYS software version SCOPE OF THE PRESENT STUDY This programme includes testing of 32 simply supported rectangular CSCC beams. The beams are divided into four groups as A, B, C, D. The beams under group A, B, C, D were designed to be tested in pure bending,, 30% of torsion and bending till failure and 60% of torsion and bending till failure and pure torque respectively. Beams of nominal cross-section 150 X 230 mm and 150 X 300 mm with the thickness of the sheet of 1.2 mm and 1.5 mm were tested. Their overall length was kept as 2300 mm. M25 Grade of concrete and F e 415 Grade of steel are used for casting the beams THESIS ORGANISATION Areas of the literature that were particularly important to the conduct of the research are reviewed in Chapter 2 of the thesis. Chapter 3 describes the preliminary tests on materials related to test specimen of CSCC beams to understand the material properties under composite action. Chapter 4 describes the test specimen, equipment, procedure and experimental programs on composite beam under bending, torsion and combined torsion and bending. Chapter 5 describes the test results and observations about the failure pattern of CSCC beam under combined bending and torsion. Theoretical investigation to determine ultimate strength to predict the behaviour of the confined steel concrete composite beam (CSCC Beam) under combined loading of bending and torsion is explained in Chapter 6. These models are based on the fundamental stress strain laws, equilibrium equation and strain compatibility behaviour in combined bending and torsion. Chapter 7 describes the evaluation of experimental results with theoretical analysis. In this a comparison is made with theoretical investigation for the torsional strength of the CSCC beam and the experimental results tested for the same beams. Simulation of the beams under combined loading are performed in Chapter 8 using commercially available finite element package ANSYS version 11.0.Results from the simulation were compared with theoretical and experimental study. Chapter 9 gives the discussion of results. Conclusion from this work together with the suggestion for the future work is presented in Chapter 10. 9