Viewpoint A series of articles in which engineers give their personal views on aspects of structural engineering practice

Aims of structural design Peter Campbell, JP, DIC, CEng, FIStructE, ACIArb Campbell, Reith & Partners

A good design has certain typical features- simplicity, unity and necessity.

Simplicity is common to excellence in all the arts-that impression of ease which is usually the result of intense effort. The structure in which evident difficulties have been painfully overcome lacks excellence. Simple does not mean elemen- tary; a structure suited to its purpose and easily constructed may be of great analytical complexi- ty.

Unity means a single thing, not just a heap. The unity of a small-span bridge will be quite other than that of a power station; but both are formed of separate parts which are made to serve the whole, and if the whole and parts are well designed, the unity will be seen.

There, however, is a superficial unity which is to be avoided. How easy to hide a chaos of unrelated parts behind a screen of bold and eye- catching pattern! And how easily-and with what disastrous results-the parts can show through!

The last feature, necessity, is perhaps the key. How magnificent the result if there is nothing in a structure but that which is necessary! This is not a paradox; the mind is a t rest in the accep- tance of necessity, but is uneasy in the presence of the wasteful, the superfluous, the factitious.

Not that these characteristics are to be aimed for directly. They are that by which good design is recognised; they are attained by the single- minded pursuit of the specific purposes of a structure-its function, safety and economy.

A design that satisfies functional requirements practically and economically is the least that is expected of the designer. The very magnitude of his responsibilities, however, imposes on him the ceaseless pursuit of excellence.*

Design and analysis It is essential for the structural engineer to understand and acknowledge the difference bet- ween structural design and structural analysis, two important but entirely different procedures, necessary for the attainment of a satisfactory structural solution.

*Aims of srructural design, report of the Institution of Structural Engineers, London, 1969. (Available from the Institution price f 2 to members and E3 to others.)

The structural design should ensure that the structure fulfils its intended functions, which in- clude the support of load, watertightness, ther- mal and sound insulation, fire resistance, resistance to toxicity; and when constructed, an adequate resistance to weathering, ageing, fatigue, ill-use, accident and other causes of deterioration.

The structural design is implicitly concerned with structural safety, not simply for its own sake, but because in almost every case the securi- ty of human life depends on the adequacy of the structure. The adopted structural solution should always be achieved at minimum cost, employing the greatest economy in the use of materials and construction equipment. In considering the economics of the proposed solution, one should attempt to equate first cost with future maintenance costs, and ultimately it may be necessary to concern oneself with future demoli- tion costs, when structures do not lend themselves readily to demolition-a problem seldom considered at the design stage.

The process of design is an evolutionary pro- cess, worked through with the closest collabora- tion between client and the design team. This process involves appreciation of the re- quirements, the formulation and appraisal of alternative solutions, until the preferred solution evolves. The design process is concerned not only with the nature and form of the solution, but also with economy and an appreciation of the construction techniques that will be required, and the ease with which the structure will be built. All of this work is carried out with the minimum use of mathematical calculation. It is when the preferred solution is defined in detail, and only then, that a full structural analysis is made, the purpose of the analysis being to con- firm the structural integrity of the solution, and quantify the elements of construction in detail. The design process is rounded off when the details are fully interpreted by means of drawings and schedules, and the specifications are fully in- tegrated with the design requirements.

The construction process is, of course, an ex- tension of the design process, which must be carefully monitored by the designer. At this stage, other experts become involved in the con- version of an idea into reality, and they will have a contribution to make. Any deviation from the original intentions proposed on grounds of economy, or speed of construction can be assess-

ed properly, in terms of the implications, only by the designer, who will also be required to ad- judicate when the inevitable incidents arise on the construction site that require resolution.

Safety Absolute freedom from danger cannot be at- tained, no matter how much money be spent on the structure.* The philosophy of safety is therefore an important consideration in the mind of any responsible designer. A perhaps over- simplified concept of safety would require that:

-the structure is able to support its own weight, plus all the applied loads that are like- ly to occur;

-the substructure is capable of supporting those loads in relation to the nature of the subsoil in depth;

-the structure be stable in all directions, when subjected to the anticipated applied loads;

-the structure is, in every respect, able to sus- tain the applied loads, whether they be material or thermal, with acceptable levels of deformation and movement.

The fulfilment of these objects requires the prediction of three limiting states, the first being the serviceable state, the second when the struc- ture becomes unserviceable, and the third the failure state. Between each of these states, one must predict and confirm an adequate and accep- table load factor, if one is to understand fully the ultimate performance of the structure being con- sidered.

Responsibilities The designer has a dual responsibility, to both his profession and to society. T o the former he contributes his skill, integrity and innovative ability, and in return he receives the support, counsel and guidance of his colleagues through professional institutions and learned societies.

T o society he has the responsibility to reduce to the minimum the element of risk implicit in any construction procedure; to ensure that the built environment, in terms of its design and quality, is of the highest standard; and to spend societys resources with care, ensuring the minimum of waste.

The Structural EngineerlVolume 61A/No. llJanuary 1983 3