Material index — where it appears
Named by 3 essays across 3 fields — each of them below, with the objects they name alongside it.
The ranking belongs to the load case
Every table of material properties ever printed ranks by strength, and every one of them answers a question nobody asked. What a structure wants is the least mass for a stated performance, and the combination of properties that gives it changes with the shape of the member — so timber beats steel four to one as a beam and loses to it as a tie, without either material changing.
The weight that has to be known before it can be found
Every other load arrives from outside and can be looked up. A structure's own weight depends on how big it is, and how big it is depends on the load — so the first calculation on any project is a fixed point, and the fraction of a member spent carrying itself turns out to be the square of its span as a fraction of a span it can never reach.
The same span, four ways
A beam, a truss, an arch and a cable can all cross the same gap under the same load, and the choice between them is usually described as a matter of judgement or of taste. It is neither. Each carries the load by a different mechanism, each mechanism has a different exponent, and an exponent decides the ordering at every span rather than at some spans.
Named alongside it
The objects these essays reach for when they reach for this one.
FunicularLoad pathMembrane actionSelf weightSpan scalingStiffnessArchCableCarbon fibreCostDead loadDeflection