Introduction

Structural design involves both conceptual design and detailed element sizing.

The process of structural design can be considered as two highly interrelated stages. The first is concerned with defining the overall structural form - the type of structure (eg. rigid frame or loadbearing walls), the arrangement of structural elements (typically in terms of a structural grid), and the type of structural elements and material to be used (for example steel beams, columns and composite floor slabs). This stage involves a high degree of creativity. The synthesis of a solution is developed on the basis of a broad understanding of a wide range of topics. These include structural and material behaviour, as well as a feel for the detailed implications of design decisions made at this stage - for instance recognising how deep a beam may need to be for a particular purpose. Formalised procedures are of little use at this stage and a satisfactory solution depends more on the creative ability of the designer.

The structure is first analysed to determine the effects within individual components.

In order to study these conditions the behaviour of the building, as determined during the initial stage of design, must be modelled to establish the distribution of forces throughout individual elements of the structure. This might typically involve the determination of bending moments and axial forces in the beams and columns which together form the structural frame of a building. This process involves some idealisation and simplification of the real behaviour, but well established methods of analysis are available for different structural forms.

Codified design rules provide little guidance on such methods but are generally more concerned with the sizing of individual elements on the basis of the forces so determined.

The detailed design process is often iterative and takes account of structural loading, material strength and required safety factors in relation to individual member sizes.

The design of structural steelwork, in common with other materials, is typically based upon trial and error procedures. BS 5950 provides a basis for checking the performance of individual members of known proportions. In doing so due consideration must clearly be given to the following points:

• anticipated level of loading on the structure
• material strength
• size of the structural member
• required safety factor

Loads, material strength and safety factors are generally defined and the principle is to select a suitable section size for the particular conditions.

Of these it is the cross-sectional dimensions of the beam or column which are varied in the trial and error approach until a satisfactory solution has been achieved. The designer may also have some choice in terms of the specification of material strength, but loadings and safety factors are largely prescribed. In limit state design there is a strict definition of terms associated with loading, material strengths and safety factors, and these are discussed below.

Structural Eurocodes, which are similar in principle to BS5950, are due to be introduced and replace national codes. The structural Eurocode for steelwork is EC3, and for composite construction is EC4, and these are based on similar principles to BS5950.