Structural Engineering & Statics
where the load goes
Concept

Moving load — where it appears

A load whose position is a variable rather than a given, so that the design quantity is a maximum over positions. Every effect has its own worst position, and the position that maximises one is generally not the position that maximises another.

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

the station being watched, x = 3unit load, at its worst position2.100shaded: where a spread load must stand to make this quantity worstthe horizontal axis is where the load is, not where the beam is cut

The worst place to stand

A bridge is not designed for a load. It is designed for a load that moves, and for every station along it there is a different position of that load that does the most damage.

internal-forces · influence line 12012080resultantmidspan05101520020040060080010001200station along the spangreatest moment ever seen there1160.2 at 9.88Barré: 1160.3 at 9.88

The train that is worse than its heaviest axle

An influence line says where to stand one load. A vehicle is several loads at fixed spacings, and the worst arrangement never puts the heaviest one at the peak.

internal-forces · influence line 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

Named alongside it

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

Axle trainInfluence lineMoment envelopePattern loadingBalanceBarre ruleBearingBending momentCentre of gravityCounterweightDead loadEquilibrium

All concepts