Elementary Behaviour of Composite Steel and Concrete Structural Members

Deric J. Oehlers, Mark A. Bradford

• Publisher: CRC Press
• Year: 1999
• Print ISBN: 9780367091828
• eBook ISBN: 9781136407031
• Topic: Technik & Maschinenbau
• Subtopic: Bauingenieurwesen

This book is aimed at developing the elementary analysis skills, familiarity and intuitive feel for composite construction that is required by undergraduate and graduate students, and by structural engineers. It does not require a prior knowledge of advanced analysis and design techniques, but builds on simple concepts such as statics and the mechanics of materials. A topic is first introduced by a brief description, with numerous carefully-chosen examples forming an integral part of the main text. Working through the examples allows the reader to gain a full understanding of the subject, as a technique is illustrated by its application to the design of new structures, or the important area of assessing and upgrading existing structures.The techniques described for the analysis of standard structures form a basis for understanding the way composite structures work, and these techniques are applied to many non-standard forms of composite construction that are rarely covered in national standards, if at all. The book is an essential purchase for all undergraduate and postgraduate students of structural and civil engineering, as well as all practitioners.

1 Introduction

1.1 Composite Structures

Composite steel-concrete structures are used widely in modern bridge and building construction. A composite member is formed when a steel component, such as an I-section beam, is attached to a concrete component, such as a floor slab or bridge deck. In such a composite T-beam, as shown in Figure 1.1, the comparatively high strength of the concrete in compression complements the high strength of the steel in tension. Throughout this book, we will refer to the steel and concrete as the components of the member, which are further made up of elements, such as the flanges or web of the steel I-section component, or the reinforcement in the slab.

Figure 1.1 Composite T-beam

The fact that each material (steel or concrete in Figure 1.1) is used to take advantage of its best attributes makes composite steel-concrete construction very efficient and economical. However, the real attraction of composite construction is based on having an efficient connection of the steel to the concrete, and it is this connection that allows a transfer of forces and gives composite members their unique behaviour. In this book, we will make considerable reference to the behaviour of this connection at the interface between the steel and concrete components, and will attempt to demonstrate that the connection between the steel and concrete characterizes the composite member.

There are a number of structural arrangements in which the steel and concrete act in this symbiotic composite fashion. In simply supported bridge construction, the concrete slab is subjected to compressive forces, and this slab is supported typically by steel I-section components, as depicted in Figure 1.1. The connection between the steel and concrete is in the form of mechanical shear connectors, which allow the shear transfer of the forces in the concrete to the steel and vice versa, and which also prevent vertical separation of the concrete and steel components. There are many forms of mechanical shear connectors as shown in Figure 1.2. The most common, however, is the stud shear connector shown in (a), which consists of a head and a plain shank connected to the steel component by a weld collar. These stud shear connectors are considered in Chapter 5, and in bridges particularly, their efficiency may be reduced by fatigue loading, as discussed in Chapters 14 and 15.

Figure 1.2 Mechanical shear connectors

It is worth noting that in composite T-beams the way in which the beam is constructed affects greatly its response to load. Buildings generally have the floors supported by closely spaced props, as shown in Figure 1.3(a), which carry all of the wet concrete loading applied to the steel component, so that the latter component does not contain any significant bending moments. This is called propped construction. On the other hand, environmental constraints in the construction of bridges usually prevent props from being used, as in (b), so that the flexural stiffness and strength of the steel component must carry the weight of the wet concrete. This method of construction, which is also experienced in pretensioned prestressed concrete bridge construction, is called unpropped construction. The ramifications of propped and unpropped construction on the flexural behaviour of beams are considered in Chapter 3.

Figure 1.3 Propped and unpropped beams

Composite columns are also used widely in practice to resist predominantly compressive loading, and they may take the form of an encased I-section, as shown in Figure 1.4(a), a concrete-filled rectangular steel section, as in (b), or a concrete-filled steel circular tube, as in (c). The use of high strength-high performance concrete is now finding its way into composite column construction, where concrete strengths may be more than twice the strength of ‘normal’ concrete. Short or stocky composite columns tend to fail by squashing, and their strength is governed by the strength of the cross-section. Stocky columns are considered in Chapter 7. Long or slender columns, on the other hand, tend to fail by a combination of material and geometric nonlinearities, and their strength is governed primarily by the phenomenon of buckling. Slender columns are considered in Chapter 8.

Figure 1.4 Composite column sections

Most modern flooring systems in buildings use a concrete slab with a cold formed profiled steel sheeting element about 1 mm thick as its soffit, as shown in Figure 1.5(a). This is a special form of composite member where the steel forms permanent and integral formwork for the concrete component, and the composite action is achieved by embossments in the sheeting as in (b) to (d), and by some chemical bonding between the concrete and steel sheeting. Commonly, the ribs of the profiled sheeting are orthogonal to the centreline of the I-section component which supports it, and the stud shear connectors are welded through the thin steel sheeting into the top flange element of the steel component. There is thus shear connection in the longitudinal beam direction by way of the mechanical shear connectors, as well as in the direction transverse to the steel I-beam component by the embossments in the profiled sheeting. The resulting behaviour of this system is thus referred to as double composite action.

Figure 1.5 Composite profiled slabs

Another way of forming composite beams is by filling trough girders that are fabricated from profiled sheeting with concrete as shown in Figure 1.6. The resulting profiled beam bears a resemblance to the composite profiled slab in that the steel sheeting is used as permanent formwork and acts compositely with the concrete. An extension of this concept in flooring construction would be to produce a flooring formwork consisting of profiled troughs and profiled sheeting, and then to pour concrete so as to produce a fully composite profiled beam-slab system.

Figure 1.6 Profiled trough girders

It is now commonplace in high-rise buildings to use a combination of these composite forms of construction. For example, the columns may be concrete-filled tubes or encased I-sections, and these are connected to the core of the building by steel I-beam components. These I-beams are then made composite by laying steel profiled sheeting onto their top flange elements, welding stud shear connectors through the profiled sheeting into the flanges and pouring the concrete slab to form the flooring of the building storey. The I-sections must be connected to the columns by some form of mechanical connection, which is not to be confused with the mechanical shear connectors considered so far. Such composite connections are still the subject of vigorous on-going research, and are not treated in this book.

The composite members and composite forms of construction described in the previous discussion represent only the common applications, and the use of steel and concrete to form these types of composite member is only limited by the imagination of the designer. For example, in retrofitting deteriorating concrete beams or slabs to improve their performance, or to increase their resistance to earthquake loading, steel plates may be glued or bolted to the concrete component, and these plated members must be analysed by the theories based on composite analysis. We shall see that although this text deals with the elementary behaviour of composite members, there are a number of concepts peculiar to this form of construction, and the basic principles that are established must be borne in mind if the designer is to take advantage of composite action in his or her final design solution.

1.2 Design Criteria

1...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright
• Contents
• Preface
• Notation
• 1. Introduction
• 2. Sizing of Members
• 3. Elastic Analysis of Composite Beams
• 4. Rigid Plastic Analysis of Simply Supported Beams
• 5. Mechanical Shear Connectors
• 6. Transfer of Longitudinal Shear Forces
• 7. Stocky Columns
• 8. Slender Columns
• 9. Composite Beams with Service Ducts
• 10. Local Splitting
• 11. Post Cracking Dowel Strength
• 12. Rigid Plastic Analysis of Continuous Composite Beams
• 13. Lateral-Distortional Buckling
• 14. General Fatigue Analysis Procedures
• 15. Fatigue Analysis of Stud Shear Connections
• Index