Structural Engineering & Statics
where the load goes
Concept

Material selection — where it appears

Choosing a material for a member by comparing a combination of properties rather than any single one. The combination depends on what is held constant and what the design is free to change, so the same eight materials rank differently for a tie, a beam and a plate.

Named by 2 essays across one field — each of them below, with the objects they name alongside it.

00.1%0.2%0.3%0.4%0.5%0.6%050100150200250300350strainstress, N/mm²mild steelaluminiumconcretetimber

The one number a stronger steel does not change

Geometry beats material almost everywhere on this site. Stiffness is the exception in the other direction — it cannot be bought at all, because every steel ever made has the same elastic modulus.

materials · elastic modulus 2.62.833.23.43.63.811.522.5density log₁₀(kg/m³)modulus log₁₀(GPa)mild steelhigh-strength steelaluminiumconcretetimbercast ironcarbon fibreglassE/ρ — a tieE^½/ρ — a beamE^⅓/ρ — a plate

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.

materials · material index

Named alongside it

The objects these essays reach for when they reach for this one.

Elastic modulusSpecific stiffnessStiffnessBucklingCarbon fibreCostDensityDesignDimensional analysisMaterial indexProof stressScaling

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