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Type of Assembly

For many simple structures, particularly those carrying significant levels of overall bending, the highest levels of stress occur at the extreme edges of the structure, while the centre parts carry much lower levels. As a result, more efficient structures, in terms of the amount of material used, can often be achieved by concentrating it at the extremes, leaving only enough elsewhere to provide the necessary links to act as a single structure.

A simple example of this is the I-beam, where the material concentrated in the top and bottom flanges carries the direct stresses imposed by a bending load, while the vertical web provides the necessary shear connection between them. Trusses, monocoque structures, and sandwich panels all use similar principles of using space and configuration to increase the structural efficiency of the material used by concentrating it at the extremes of the shape available. Two factors limit this strategy; either the material becomes so thin that local buckling effects dominate, or the amount of material needed to make the connections exceeds that saved by using a larger shape.

Effects of Space and Shape Constraints

The relative positions of load and reaction points are the primary geometrical constraints on the design of the structure. There are often further spatial restrictions, which are a necessary requirement for the artefact to perform its function, but which constrain the shape to a less than structurally ideal form. The most frequent of these is are definitions of inviolable spaces. This may be simple, such as the minimum clearance under a bridge over a roadway, or more complex, like the space necessary to clear the movement envelope of a mechanism. It may be more flexible in requiring an access route of a certain size for services or people, or some continuous route for air flow, but not specifying where or what shape it must be.

There will also usually be a mutually agreed specification of the interface geometry with other parts at a defined position. The most significant space constraints are those which force the structure to adopt a configuration not well suited to the load patterns imposed. One of the earliest design considerations must be to seek the best compromise between any extreme shape constraints and the resulting cost in structural efficiency and thus increased weight, cost and other effects.