Orientation of the Cross Section
For 3D beams, the orientation of the cross section in the plane perpendicular to the beam axis is an important property. The bending stiffness of most cross sections is highly anisotropic. Only a few cross sections, like circular pipes and square sections, have the stiffness which is the same in all directions. Even in this case, the section orientation will still usually be important for a correct evaluation and interpretation of stresses.
Any cross section has a local y-z coordinate system, which correspond to the two principal area moments of inertia,
I
yy
and
I
zz
.
I
yy
is defined as the moment of inertia for bending around the y-axis,
Whether the local y-axis or z-axis is used as the stiffer direction is not important as such. For the built-in cross sections (H, T, C, hat) the convention is that
I
zz
>
I
yy
, so that the local y-axis the stiffer direction.
There are two methods by which you can supply the cross-section orientation information. In either case, it is the orientation of the local y-axis that is described.
Reference Point
When you use the reference point input method, you give the coordinates of a point in space. The local y-axis is perpendicular to the beam axis and located in the plane described by the beam element and the given point.
Orientation vector
When using orientation vector input, you provide an approximate direction of the y-axis. This given vector is then adjusted in the plane given by the beam and vector, so that a y-axis perpendicular to the beam is obtained.
Once this direction has been established, it is possible to further rotate it around the beam axis. This is particularly useful for L-shaped sections, where the angle between a coordinate system aligned with the flanges and the principal axes often is provided in design tables.
See the documentation of the
Section Orientation
node for details about how the local directions are computed from the given input.
