Concept

Section — where it appears

The cut face of a member, and the set of geometric properties read from it. Almost every internal force on this site is a statement about a section, which is why the members that have no natural section — a gusset, a deep region, a plate — need one to be invented.

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

Deliberately the wrong shape

Concrete in compression follows a curve, and no design office has ever integrated it. Every code in the world replaces it with a rectangle of reduced depth and reduced intensity, and the answer is right to a fraction of a per cent — not because the shapes are similar, which they visibly are not, but because a bending calculation only ever asks a stress distribution two questions.

sections · stress block

The section calculation with no formula in it

Every section result on this site is a closed form, and each was derived once for one arrangement of material. Slice the section instead, give each strip the strain a curvature puts it at, and move the neutral axis until the axial force balances — and the same twenty lines answer for a cracked section, a confined one, a prestressed one and a composite one, having been told nothing about any of them.

sections · fibre model

Named alongside it

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

Equilibrium Moment curvature Neutral axis Plane sections Bending stress Centroid Composite action Concrete Confinement Cracked section Crushing strain Curvature

All concepts