The Behaviour and Design of Steel Structures to EC3

N.S. Trahair, M.A. Bradford, David Nethercot, Leroy Gardner

• Publisher: CRC Press
• Year: 2017
• ISBN:
  9781351990011
• Edition: 4
• Topic: Technology & Engineering
• Subtopic: Engineering General
• Index: Technology & Engineering

The fully revised fourth edition of this successful textbook fills a void which will arise when British designers start using the European steel code EC3 instead of the current steel code BS5950. The principal feature of the forth edition is the discussion of the behaviour of steel structures and the criteria used in design according to the British version of EC3. Thus it serves to bridge the gap which too often occurs when attention is concentrated on methods of analysis and the sizing of structural components.

Because emphasis is placed on the development of an understanding of behaviour, many analytical details are either omitted in favour of more descriptive explanations, or are relegated to appendices. The many worked examples both illustrate the behaviour of steel structures and exemplify details of the design process.

The Behaviour and Design of Steel Structures to EC3 is a key text for senior undergraduate and graduate students, and an essential reference tool for practising structural engineers in the UK and other countries.

Chapter 1

Introduction

1.1 Steel structures

Engineering structures are required to support loads and resist forces, and to transfer these loads and forces to the foundations of the structures. The loads and forces may arise from the masses of the structure, or from man’s use of the structures, or from the forces of nature. The uses of structures include the enclosure of space (buildings), the provision of access (bridges), the storage of materials (tanks and silos), transportation (vehicles), or the processing of materials (machines). Structures may be made from a number of different materials, including steel, concrete, wood, aluminium, stone, plastic, etc., or from combinations of these.

Structures are usually three-dimensional in their extent, but sometimes they are essentially two-dimensional (plates and shells), or even one-dimensional (lines and cables). Solid steel structures invariably include comparatively high volumes of high-cost structural steel which are understressed and uneconomic, except in very small-scale components. Because of this, steel structures are usually formed from one-dimensional members (as in rectangular and triangulated frames), or from two-dimensional members (as in box girders), or from both (as in stressed skin industrial buildings). Three-dimensional steel structures are often arranged so that they act as if composed of a number of independent two-dimensional frames or one-dimensional members (Figure 1.1).

Structural steel members may be one-dimensional as for beams and columns (whose lengths are much greater than their transverse dimensions), or two-dimensional as for plates (whose lengths and widths are much greater than their thicknesses), as shown in Figure 1.2c. While one-dimensional steel members may be solid, they are usually thin-walled, in that their thicknesses are much less than their other transverse dimensions. Thin-walled steel members are rolled in a number of cross-sectional shapes [1] or are built up by connecting together a number of rolled sections or plates, as shown in Figure 1.2b. Structural members can be classified as tension or compression members, beams, beam-columns, torsion members, or plates (Figure 1.3), according to the method by which they transmit the forces in the structure. The behaviour and design of these structural members are discussed in this book.

Figure 1.1 Reduction of a [3-D] structure to simpler forms.

Figure 1.2 Types of structural steel members.

Structural steel members may be connected together at joints in a number of ways, and by using a variety of connectors. These include pins, rivets, bolts, and welds of various types. Steel plate gussets, or angle cleats, or other elements may also be used in the connections. The behaviour and design of these connectors and joints are also discussed in this book.

Figure 1.3 Load transmission by structural members.

This book deals chiefly with steel frame structures composed of one-dimensional members, but much of the information given is also relevant to plate structures. The members are generally assumed to be hot-rolled or fabricated from hot-rolled elements, while the frames considered are those used in buildings. However, much of the material presented is also relevant to bridge structures [2, 3], and to structural members cold-formed from light-gauge steel plates [4–7].

The purposes of this chapter are first, to consider the complete design process and the relationships between the behaviour and analysis of steel structures and their structural design, and second, to present information of a general nature (including information on material properties and structural loads) which is required for use in the later chapters. The nature of the design process is discussed first, and then brief summaries are made of the relevant material properties of structural steel and of the structural behaviour of members and frames. The loads acting on the structures are considered, and the choice of appropriate methods of analysing the steel structures is discussed. Finally, the considerations governing the synthesis of an understanding of the structural behaviour with the results of analysis to form the design processes of EC3 [8] are treated.

1.2 Design

1.2.1 Design requirements

The principal design requirement of a structure is that it should be effective; that is, it should fulfil the objectives and satisfy the needs for which it was created. The structure may provide shelter and protection against the environment by enclosing space, as in buildings; or it may provide access for people and materials, as in bridges; or it may store materials, as in tanks and silos; or it may form part of a machine for transporting people or materials, as in vehicles, or for operating on materials. The design requirement of effectiveness is paramount, as there is little point in considering a structure which will not fulfil its purpose.

The satisfaction of the effectiveness requirement depends on whether the structure satisfies the structural and other requirements. The structural requirements relate to the way in which the structure resists and transfers the forces and loads acting on it. The primary structural requirement is that of safety, and the first consideration of the structural engineer is to produce a structure which will not fail in its design lifetime, or which has an acceptably low risk of failure. The other important structural requirement is usually concerned with the stiffness of the structure, which must be sufficient to ensure that the serviceability of the structure is not impaired by excessive deflections, vibrations, and the like.

The other design requirements include those of economy and of harmony. The cost of the structure, which includes both the initial cost and the cost of maintenance, is usually of great importance to the owner, and the requirement of economy usually has a significant influence on the design of the structure. The cost of the structure is affected not only by the type and quantity of the materials used, but also by the methods of fabricating and erecting it. The designer must therefore give careful consideration to the methods of construction as well as to the sizes of the members of the structure.

The requirements of harmony within the structure are affected by the relationships between the different systems of the structure, including the load resistance and transfer system (the structural system), the architectural system, the mechanical and electrical systems, and the functional systems required by the use of the structure. The serviceability of the structure is usually directly affected by the harmony, or lack of it, between the systems. The structure should also be in harmony with its environment, and should not react unfavourably with either the community or its physical surroundings.

1.2.2 The design process

The overall purpose of design is to invent a structure which will satisfy the design requirements outlined in Section 1.2.1. Thus the structural engineer seeks to invent a structural system which will resist and transfer the forces and loads acting on it with adequate safety, while making due allowance for the requirements of serviceability, economy, and harmony. The process by which this may be achieved is summarised in Figure 1.4.

The first step is to define the overall problem by determining the effectiveness requirements and the constraints imposed by the social and physical environments and by the owner’s time and money. The structural engineer will need to consult the owner; the architect, the site, construction, mechanical, and electrical engineers; and any authorities from whom permissions and approvals must be obtained. A set of objectives can then be specified, which if met, will ensure the successful solution of the overall design problem.

Figure 1.4 The overall design process.

The second step is to invent a number of alternative overall systems and their associated structural systems which appear to meet the objectives. In doing so, the designer may use personal knowledge and experience or that which can be gathered from others [9–12]; or the designer may use his or her own imagination, intuition, and creativity [13], or a combination of all of these.

Figure 1.5 The structural design process.

Following these first two steps of definition and invention come a series of steps which include the structural design, evaluation, selection, and modification of the structural system. These may be repeated a number of times before the structural requi...