Design philosophies

1. INTRODUCTION

The precise objectives of structural design vary from one project to another. In all cases, the avoidance of collapse is an important - if not the most important - requirement and an adequate factor of safety must be provided. In this context, the structure must be designed in order to fulfil both strength and stability requirements. These concepts are illustrated in Figure 1 in which a long thin rod is subject to tension (Figure 1a) and compression (Figure 1b). In the case of tension, the load resistance of the rod is governed by strength, which is the ability of the material to carry load without rupturing. The rod can only carry this load in compression if it remains stable, i.e. it does not deform significantly in a direction perpendicular to the line of action of the applied load. The stiffness of the structure is yet another important characteristic, concerned with resistance to deformation rather than collapse. This is particulary important in the case of beams whose deflection under a particular load is related to their stiffness (Figure 1c). Large deformations are not necessarily associated with collapse, and some brittle materials, such as glass, may rupture with little prior deformation. Other considerations may also need to be included in the design process. They include: quantifiable behaviour such as deformation, fatigue, fire resistance, and dynamic behaviour; considerations such as durability and sustainability and service accommodation which may influence both detail and overall concept, but in a more qualitative way; and appearance, which is largely a subjective judgement. In addition considerations of economy are likely to be a significant influence on the great majority of structural designs. In this context, questions of speed and ease of construction, maintenance and running costs, as well as basic building costs, are all relevant. The relative importance of each of these aspects will vary depending on circumstances.

The approach to structural design is dealt with in Process of design, which describes how the designer might begin to accommodate different requirements, many of which will exert conflicting pressures. In this lecture, the focus is on how a satisfactory structural design can be achieved through a rational analysis of various aspects of the structure's performance. It is worth emphasising that the process of structural design can be considered as two groups of highly interrelated stages. The first group is concerned with defining the overall structural form - the type of structure, e.g. rigid frame or load bearing walls, the arrangement of structural elements (typically in terms of a structural grid), and the type of structural elements and material to be used, e.g. steel beams, columns, and composite floor slabs. A high degree of creativity is required. The synthesis of a solution is developed on the basis of a broad understanding of a wide range of topics. The topics 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. A satisfactory solution depends more on the creative ability of the designer.

The later stages are concerned with the more detailed sizing of structural components and the connections between them. By now the problem has become clearly defined and the process can become more formalised. In the case of steelwork, the process generally involves selecting an appropriate section size although in some circumstances the designer may wish to use a non-standard cross-section which, for execution, would then need to be made up, typically by welding plates or sheets or standard sections together into plate girders or trusses.

Design regulations are largely concerned with this stage of detailed element design. Their intention is to help ensure that buildings are designed and constructed to be safe and fit for purpose. Such design legislation can vary considerably in approach. It may be based simply on performance specification, giving the designer great flexibility as to how a satisfactory solution is achieved. An early example of this is the building laws published by King Hummarabi of Babylon in about 2200 BC. They are preserved as a cuneiform inscription on a clay tablet and include such provisions as 'If a builder builds a house for a man and does not make its construction firm and if the house which he has built collapses and causes the death of the owner of the house, then that builder shall be put to death. If it causes the death of the son of the owner of the house, then a son of the builder shall be put to death.' The danger, and at the same time the attraction, of such an approach is that it depends heavily on the ability of the designer. Formal constraints, based on current wisdom, are not included and the engineer has the freedom to justify the design in any way.

The other extreme is a highly prescriptive set of design rules providing 'recipes' for satisfactory solutions. As these can incorporate the results of previous experience gained over many years supplemented by more recent research work, they might appear to be more secure. However, such an approach cannot be applied to the conceptual stages of design and there are many cases where actual circumstances faced by the designer differ somewhat from those envisaged in the rules. There is also a psychological danger that such design rules assume an 'absolute' validity, and a blind faith in the results of using the rules may be adopted.

Clearly, there is a role for both the above approaches. Perhaps the best approach would be achieved by specifying satisfactory performance criteria to minimise the possibility of collapse or any other type of 'failure.' Engineers should then be given the freedom to achieve the criteria in a variety of ways but also be provided with the benefit of available data to be used if appropriate. Perhaps the most important aspect is the attitude of the engineer, which should be based on simple 'common sense' and include a healthy element of scepticism of the design rules themselves.