The local coordinate system of a structure bar is a dextrorotary Cartesian system with the following orientation of axes:
The local x axis is always the longitudinal element axis, whose direction is defined from the beginning point to the end point.
The origin of the system is positioned at the beginning node of a bar. (See the following image.)
A - Beginning node, B - End node of a bar
The local axes y and z lie in the plane of the bar section and are arranged according to dextrorotary rotation. By standard,
the axes mentioned represent: y axis - the axis of the greater moment of inertia of a bar, while the z axis - the axis of
the lesser moment of inertia of a bar (depending on the bar section). When defining bars the following rules also apply:
When defining a vertical bar, the local x axis of the bar is vertical, while the local direction of the y axis is parallel
to the global Y axis
When defining a bar that is not vertical, the plane formed by the local axes x and z of the bar is vertical, while the local
z axis is directed upwards (towards the global Z).
Defining a GAMMA angle lets you rotate the bar cross-section with a local system, that is, the y and z axes, about the x axis of a bar element.