Structural Engineering & Statics
where the load goes
Concept

Compatibility torsion — where it appears

Torsion that exists because a member is stiff rather than because nothing else can carry it, and which therefore disappears if the member is allowed to rotate.

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

40 kNm26.7 kNm13.3 kNmthe step at the load is 40 kNm, and the diagram closes

The internal force with no diagram

A cut through a member reveals four things, and this collection has drawn diagrams for three of them. The fourth is a torque, it obeys exactly the same rules, and whether it exists at all can depend on a decision the designer is free to make.

internal-forces · 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.

TorsionCracked sectionDistribution factorEquilibrium torsionFixed end momentInternal forcesJoint stiffnessLoad pathRedistributionServiceabilityShear centreShear flow

All concepts