Structural Engineering & Statics
where the load goes
Concept

Compression — where it appears

A normal force that shortens the material it acts on, the sign convention of which is a matter of discipline rather than mechanics. What separates it from tension structurally is that a compression member can buckle, so its capacity depends on its length and its supports as well as on its area.

Named by 3 essays across 2 fields — each of them below, with the objects they name alongside it.

A Pratt truss of 8 panels. A Pratt truss under equal panel-point loads. The joint equilibrium equations were assembled and solved; 14 members came out in tension, 13 in compression and 2 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 A closed force polygon. The forces on a joint, laid tip to tail. Equilibrium is the statement that the polygon closes, and the gap when it does not is the out-of-balance force, to scale.

One drawing solves the whole truss

The method of joints solves a truss one joint at a time, and each solution is thrown away as soon as the next begins. Drawn instead of computed, the joints share their edges — every member's force appears once in a single figure, and the figure's own closure is the check.

structures · Truss A Pratt truss of 8 panels. A Pratt truss under equal panel-point loads. The joint equilibrium equations were assembled and solved; 14 members came out in tension, 13 in compression and 2 carrying nothing.

Halving the panel buys a shorter strut

Subdividing a truss into more panels of the same span and depth barely changes the chord forces, because the couple that carries the moment has not moved. What it changes is the length of every compression member, and a buckling capacity goes as the inverse square of a length.

structures · Truss

Named alongside it

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

BucklingSlendernessTensionTrussZero force memberBracingChord forceConnectionCritical loadDeterminacyEffective lengthEquilibrium

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