http://www.ctresources.info/ccp/paper.html?id=4953

In multi-storey buildings made of reinforced concrete, lateral loads are often resisted by specially arranged shear walls. Shear wall components may be planar, are usually located at the sides of the building or in the form of a core which houses staircases or elevator shafts. When the coupling action between the piers separated by openings becomes important, some of the external moment is resisted by the couple formed by the axial forces in the walls due to the increase in the stiffness of the coupled system by the connecting beams. Actually, the deformation of a coupled shear wall subjected to lateral loading is not confined to its plane. Studies considering in-plane, out-of-plane and torsional deformations in the investigation of coupled shear walls are called non-planar coupled shear wall analyses. In non-planar coupled shear walls, both the flexural and torsional behaviours under external loading have to be taken into account in the analysis. When thin-walled structures are twisted, there is a so-called warping of the cross-section and the Bernoulli-Navier hypothesis is violated.