Mly is present only in 3D, and so is the torsional moment
Mlx described below. The torsional stiffness of the beam is defined using the torsional constant
J given by
A special feature of some unsymmetrical cross sections is that they twist under a transversal load that is applied to beam centerline. As an example, this would be the case for a U-profile under self-weight, loaded in the stiff direction. It is only a load applied at the shear center which causes a pure deflection without twist. This effect can be incorporated by supplying the coordinates of the shear center in the local coordinate system (
ey,
ez). A given transversal load (
flx,
fly,
flz), which is defined as acting along the centerline, is then augmented by a twisting moment given by