Structural Engineering & Statics
where the load goes
Concept

Centre of gravity — where it appears

The point through which a body's whole weight can be taken to act. A hanging load rotates until it lies directly beneath the hook, because that is the only attitude in which the moments balance, so a lift whose pick points do not straddle it arrives out of level.

Named by 2 essays across one field — 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 hook100 kN57.7 kN57.7 kN60°57.7 kN of compression, in the load2040608000.511.52sling angle (degrees)leg force ÷ load30°

The angle that doubles the force

A crane hook takes the weight and nothing else. What holds the load is a set of legs running down to it at an angle, and each leg is a two-force member — so at thirty degrees from the horizontal each of two legs carries the entire load, and the difference goes into the thing being lifted as compression.

equilibrium · rigging

Named alongside it

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

EquilibriumFree bodyBalanceBearingBucklingCounterweightDead loadErectionForce polygonGraphic staticsIndeterminacyLever arm

All concepts