Structural Engineering & Statics
where the load goes
Concept

Load reversal — where it appears

A change in the sign of a member's force between one load case and another, turning a tie into a strut or a sagging moment into a hogging one. It changes which variable decides the member — an area for tension, a radius of gyration for compression — so the previous design contains no information about the new requirement.

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

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 54.454.493.3116.7116.793.354.454.4-93.3-116.7-124.4-124.4-116.7-93.3-35.0-21.0-14.0-21.0-35.0-73.2-73.252.331.410.510.531.452.3tensioncompression2 carrying nothing

The tie that spends an afternoon as a strut

A tension member is chosen by its area and nothing else. A compression member is chosen by how that area is arranged. So a member whose force reverses under some load case is not merely being asked for the same number with the other sign — it is being designed against a different variable, and the same steel can carry seventy times more or less depending on a shape nobody chose for that purpose.

stability · load reversal

Named alongside it

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

BalanceBearingBracingBucklingCentre of gravityCompressionCounterweightCritical loadDead loadEquilibriumFree bodyLever arm

All concepts