"Member nn ref. node/axis - angle between AB and AC too small"
This error message may be displayed in the output window during the checking phase prior to an analysis. It occurs if the specified reference axis for a column is the vertical axis and in any other situation where the vector to the reference node is almost parallel to the longitudinal member axis (x).
GLOBAL AND MEMBER AXES
To understand what the error message means, look at the above diagram. The member x axis is AB and the y axis is computed using the vector cross-product of AB and AC. When the angle between AB and AC is zero the computation is impossible. If the angle is very small the computation is uncertain.
Fix the problem by double-clicking on the affected member and changing the reference node/axis. In many cases, the default reference axis is suitable.