The nature of design is uniquely personal. For this reason the following description of how the author understands design may not be recognised by all readers.
Design is an interactive process, with signals shuttling between three notional centres in the brain, those responsible for appreciating beauty, for accumulating experience, and the brain's calculator. It most definitely is not just a sketching exercise, nor is it a logical, linear process.
When a client proposes a commission for the design of a new bridge, across say a river, with an outline description of the purpose of the bridge and the characteristics of the river, the first engineering response is to imagine a solution that appears to fit the facts as they have been described (often incorrectly), and is usually based on some idea that has been tried before, has been imagined or read about, or is the extrapolation of a previous idea, pushing it further towards some logical conclusion. As more information on the project becomes available, the suitability of this first idea is tested and then reinforced, modified or dropped in favour of another 'guide' idea. This process of imagining a solution and then subsequently confronting it with the facts is in the author's view the essence of creative design.
At the earliest stages of this design process, calculations are carried out. They are in general very simple, to compare the cost of alternatives, and to check on the sizes of members. For most bridges and other civil engineering projects, the structure can be notionally simplified to the point where the bending moments and shearing forces may be estimated by simple manual means, generally to within 15-20 per cent of the correct value.
Similarly, the loading on the bridge can be reduced from the pages of the code of practice definition to its simplest basics. From these simple beginnings, the size of members, the density of reinforcement, the intensity of prestress and the basic deflections of the structure can be calculated. Of course one makes use of books with charts of bending moments and deflections for beams and portals and safe load tables for columns and of codes of practice, not to check for detailed compliance, but to remind oneself of limiting stresses, load combinations, load factors etc. If one is very computer literate, simple computer models may be invaluable, as long as one can produce them almost automatically, without struggling to understand manuals, sign conventions etc. One must at all costs not engage one's brain into an 'analysis mode', or one's creativity will be swamped by one's intellect.
It is very important that, at this early stage, all calculations are kept conservative, so that one is not deluding oneself about the feasibility of a favoured option. These initial calculations allow the designer to develop his understanding of how the structure works, of how the forces flow. They also enable him to put sizes to members, and so to gain a first insight into the appearance of a structure. The aesthetics of structures are critically dependent on member sizes, and how size varies along a member.
The diagrams of normal forces, bending moments, shear forces and torques may be drawn along the members, and stresses calculated. As the structure is better understood, the logic of how it works becomes apparent. Member sizes may be refined to improve economy, to provide reserves of strength and to affect the appearance. However, one must not defy the basic logic of the structure; one cannot make a member excessively slender in defiance of the structural logic, just because it looks better; there must be a concordance between function and appearance. This does not mean that the size of members is dictated by their stress levels, but that one must not act contrary to the structural logic. This usually leaves a considerable margin for discretion in sizing members. For instance, two members that are equally stressed may be given different sizes for the sake of appearance.
At this stage, it may become clear that the structure is evolving in a way that is not satisfactory, either technically or aesthetically. When one embarks on this design process, it is frequently not clear what the nature of the destination will be. An essential part of design is the readiness to tear up what one has done and start again. An engineer who does not have the courage, or the time, to recognise that he is engaged in a dead end and to start again cannot pretend to be a creative designer.
Was this article helpful?
The use of dumbbells gives you a much more comprehensive strengthening effect because the workout engages your stabilizer muscles, in addition to the muscle you may be pin-pointing. Without all of the belts and artificial stabilizers of a machine, you also engage your core muscles, which are your body's natural stabilizers.