SEED Guides Selecting Structural Configuration < >

It is also important to understand the issues by which the design of the structure will be evaluated. These may simply be strength, cost or mass or there be may complex relationships between a number of issues which must be considered. For example, the extra cost of higher strength materials or more complex manufacture may be set against a resulting saving in weight or volume. Balance or stability may be more important than overall mass. Other issues may include organisational constraints on the materials and manufacturing capability available. The efficiency of the structure is the measure of how well it copes with these issues and hence the success of the design. The overriding requirement is for safety and reliability in sustaining any foreseeable imposed load cases without failure. Once that is assured then the other criteria may be applied. The level of analysis needed to ensure these criteria are met, varies with the circumstances, particularly the consequences of failure. A list of those usually considered is given below.

Mass (total, distribution, static balance)   >
Strength (yield, ultimate, fatigue)            > ratios of any
Stiffness (static and dynamic)                 > combination
Cost                                                     >
Ease of manufacture/assembly
Durability
Aesthetics
Portability
Corrosion Resistance
Other (weatherproof, aero-/hydrodynamic resistance, etc.)

Some criteria have been subject to considerable analytical research to discover the ideal structure. For example, the most efficient strength to weight ratios of trussed forms are obtained from "Mitchell" structures. These attempt to produce a three dimensional truss of small members meeting at near right angles which all carry loads of the same sense, either tension or compression. They were used by Mitchell and others to produce the "geodetic" structures of several aircraft, before and during World War 11, where their light weight and ability to sustain considerable damage without failure proved their usefulness. After the war, new types of construction, making use of the skin itself as a load carrying member, led to more efficient structures in a situation where damage resistance was less important. Considerable work on the costing of manufacture has made it possible to assess the financial effects of design proposals. (All structures may carry non-load-bearing cladding, fairings, weather protection or other fittings, which play no part in the overall structural behaviour but may distribute local loads and must be strong enough to support themselves.)