Structural Engineering & Statics
where the load goes
Concept

Two force member — where it appears

A member loaded at two points only, whose force must therefore act along the line joining them. Every sling leg, cable, hanger and pin-jointed bar is one, and the property is what makes a truss solvable and a sling force a division by a sine.

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

200 kN200 kNthe chord — and the line of action1.20 mbending moment = P × offsetpeak 240 kNmbending stress is 24.0 times the axial stress at the worst section

The member with only one direction

If a body is in equilibrium under forces applied at exactly two points, those forces are equal, opposite and along the line joining the points. It is three conclusions from two equations, it is the shortest real theorem in statics, and nearly everything on this site depends on it without saying so.

equilibrium · two force member 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 bodyBucklingCentre of gravityEccentricityErectionForce polygonFunicularGraphic staticsImperfectionIndeterminacyLine of action

All concepts