The origin of the word design is the Latin 'designare', to draw. In classical times, the stability of a structure depended on its shape, which could be drawn by those with the special skills. Design now has a much-widened meaning embracing the concept of anything from bridges to floats for a carnival.
In the context of bridge engineering, design means the conceptual phase, where harmony is created out of the tumult of data which includes:
• the physical characteristics of the site;
• the technical aspects concerned with the strength of materials and the theory of structures;
• the specified design life of the bridge and the maintenance regime;
• the various regulations that must be complied with;
• the economic and time constraints that have to be met;
• the form of contract under which the bridge is to be built;
• the effect the new bridge will have on the community, either by its scale, its appearance, or by the changes it will make to the local environment;
• the wishes of the bridge owner.
An inspired designer may attain a state of grace, where original ideas combine with technical expertise and past experience to create the perfect solution that best fits all the data, and which in hindsight appears obvious.
Design must be followed by the detailed justification of a project, the analysis, to demonstrate that it is safe and complies with the relevant regulations. This analysis is followed by the preparation of drawings which are needed to communicate to the contractor the information required to build the structure, and the preparation of the contractual documentation. Although requiring skill and care, these latter phases of the process are different in nature to the initial conceptual design; they are more mechanical, and do not require the combination of technical expertise, aesthetic sense and imagination that are characteristic of conceptual design.
However, in many cases, design is the name given to the mechanical analysis of the structure, and even to the whole process. This is more than a semantic quibble. Analysing structures is principally a mathematical, mechanical procedure, whereas design is largely a matter of judgement in weighing up the importance of the many relevant criteria. If the two processes are given the same name, one tends not to notice the relative weight given to each. The mediocrity of many projects that are built is the result of the shortening, or virtual absence of the conceptual, the design phase.
Design and analysis are not strictly sequential. Although clearly design must start first, design development continues in parallel with analysis as ideas evolve or as the analysis gives rise to further insights into the behaviour of the structure, and opens new design possibilities. The analysis is an important part of understanding the structure. It needs to start with simple models which are easy to check, and only gradually build up to the final verification of the structure as a whole. As the analysis is carried out on a model of the structure, not the structure itself, the designer must always question whether the results represent reality or whether they have been distorted by the assumptions made in preparing the model.
The designer cannot delegate the analysis, he must remain in charge and needs the required knowledge and skills, in addition to his abilities to imagine, innovate and communicate.
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