Structural Engineering & Statics
where the load goes
Concept

Torsional stiffness — where it appears

A member's resistance to twist per radian, GJ over its length, which for an open section is two orders of magnitude below a closed one's.

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

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 20%40%60%80%100%020406080100120torsional stiffness left, as a fraction of the uncracked valuetorque in the spandrel (kNm)compatibilityit goes awayequilibriumit does not100 kNm40 kNmcracking sheds a factor of 2.47 of torque, and the floor beam pays 18% more sagging moment

The torsion that goes away if you let it

A spandrel beam attracts a torque in proportion to its own torsional stiffness. Crack it and the stiffness falls by a factor of four, the torque falls with it, and nothing has failed — because the floor beam it was competing with picks up exactly what was shed. A canopy hung off the same spandrel is a different animal entirely.

internal-forces · compatibility torsion

Named alongside it

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

Accidental eccentricityCentre of rigidityCompatibility torsionCracked sectionDiaphragmDistribution factorEccentricityEquilibrium torsionFixed end momentInstantaneous centreJoint stiffnessLateral system

All concepts