Structural Engineering & Statics
where the load goes
Concept

Accidental eccentricity — where it appears

A mandatory offset of a storey force from the centre of mass, covering mass and stiffness that are not where they were assumed to be.

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

Also named here as centre of rigidity, diaphragm, plan torsion — the same set of essays touches all of them, so they are one junction rather than several.

centre of masscentre of rigidity1000 kNwest wall500 kN direct+ 39 torsional= 539 kNeast wall500 kN direct+ 39 torsional= 539 kNsouth wall0 kN direct− 19 torsional= 19 kNnorth wall0 kN direct+ 19 torsional= 19 kNcentre of rigidity at x = 15.00 m, y = 9.00 mtorsional radius r = 17.02 m against a plan radius of 10.10 mtorsionally stiff by the usual criterion

The corner that moves most

A lateral force is shared out in proportion to stiffness only if it passes through the centre of rigidity, which is not the centre of the plan and not the centre of mass. The distance between the two is a torque, and the wall that pays for it is the one furthest away and carrying least.

structures · plan torsion 0.020.1011025000.20.40.60.8floor-plate stiffness ÷ wall stiffnessshare of the storey forcethe middle wallan end walltributary arearigid plate← soft plate

The floor is a beam lying down

A floor plate spans horizontally between the walls that resist a lateral load, carries a distributed inertia load, and has chords, a web and a span-to-depth ratio like any other beam. Its stiffness decides whether the walls share the load by their stiffness or by the area of floor nearest them — and the familiar tributary answer turns out to be neither limit.

structures · diaphragm

Named alongside it

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

Centre of rigidityDiaphragmLateral systemLoad sharingPlan torsionShear wallChord forceCollectorDeep beamEccentricityErection stabilityIn plane stiffness

All concepts