Buckling - Compatibility Torsion

What is compatibility torsion?

Often torsion contributes little to the force distribution. It is then logical to set the torsional stiffness equal to zero. However, it may be that an element contributes little to the force distribution but must undergo the same deformations as the structure. Torsion that occurs in this process is called compatibility torsion. This can result in a considerable torsional moment in the element, which is only correctly calculated if a realistic torsional stiffness is used.

Reinforced concrete

Reinforced concrete beams without prestressing lose much of their torsional stiffness when the concrete cracks. In some cases, the cracked stiffness can be only 18% of the uncracked stiffness.

Concrete beam grids

If a beam grid is calculated with uncracked torsional stiffness, part of the load will be carried by torsional moments and part by bending moments. However, if the cracked torsional stiffness is used, only small torsional moments will occur and almost all load will be carried by bending moments. In the latter case, we must apply more longitudinal reinforcement but can suffice with much fewer stirrups.