Structural Engineering & Statics
where the load goes
Concept

Three dimensional equilibrium — where it appears

The six equations a rigid body satisfies in space: three of force and three of moment. A structure drawn in elevation has three of them checked and three assumed, and the assumed three are where an out-of-plane load, a torsion or a plan eccentricity goes unnoticed.

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

56.8 kN13.6 kN49.6 kN120 kNthree legs · rank 3 · determinateΣV, ΣMx and ΣMy all close to 1e-14 kN

Six equations, and the drawing shows three

Ninety essays have taken place on a piece of paper, where equilibrium is three equations and a structure is a diagram. The real object has six, the extra three are the ones nobody writes down, and the difference between three legs and four is not a matter of degree.

equilibrium · three dimensional equilibrium 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.

Free bodyIndeterminacyBucklingCentre of gravityDeterminacyEquilibriumErectionForce polygonGraphic staticsLoad pathMoment about an axisRank

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