Structural Engineering & Statics
where the load goes
Concept

Specific stiffness — where it appears

Stiffness per unit weight, the measure by which a folded or hollow shape beats a solid one without any change of material.

Named by 2 essays across 2 fields — 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 flat: I = 6750 mm⁴folded: I = 67.50×10⁶ mm⁴300gain 10000× = (100)²limited by buckling of the leg: the leg goes at 27 N/mm²

Folded until it spans

A flat sheet has a second moment of area of B·t³/12 and will not span anything. Folded, the same material has B·t·h²/12, and the gain is exactly the fold depth over the thickness, squared — a ratio with no material in it and no width in it.

structures · folded plate

Named alongside it

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

StiffnessBucklingEfficiencyElastic modulusForm findingLocal bucklingMaterial selectionOne way spanningOptimisationPlate bucklingPlate slendernessProof stress

All concepts