Structural Engineering & Statics
where the load goes
Concept

Reaction — where it appears

The force or moment a support applies to a structure in return for holding it. It is not a load and it cannot be chosen: it is whatever equilibrium requires, up to whatever the support can supply.

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

05101520-600-400-200200400distance along the beam (m)bending moment (kNm)primarysecondarythe loadeverythingpushed down by 51.0 kN with nothing applied at all

The prestress that pushes back

On a simply supported beam a tendon is an internal matter and changes no reaction. Put the same beam on three supports and the tendon lifts it off the middle one, the support refuses, and the force it takes to hold the beam down is a reaction produced with no load applied at all.

internal-forces · secondary prestress 2700 kNthe leaf, 900 kN3600 kN on the trunnionno moment, at any angle9 mW·a = 8100 kNm and C·r = 8100 kNm — a product, not a weightinertia 9908 t·m² against 7431 for the leaf alone

Balanced, and four times as heavy

A counterweight cancels a moment about a pivot, and that is the only thing it cancels. The bearing beneath carries both weights, the inertia rises as the square of the radius, and a load that moves cannot be balanced at more than one position at all.

equilibrium · counterweight

Named alongside it

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

BalanceBearingCentre of gravityConcordant profileContinuityCounterweightDead loadEccentricityEquilibriumEquivalent loadFree bodyIndeterminacy

All concepts