Intersection of Line Elements with Shell Elements

Fine shell element have stiffness in the two out of plane bending rotational axis, the out plane direction axis and the two enplane directional axis. In a full 6 degree of freedom space this leave one missing axis, i.e. the rotation  about the axis normal to shell element, known as the "drilling" degree of freedom.

Nice reference here.

https://www.youtube.com/watch?v=9cS_X7P6D90

When line elements intersect with finite element surface it is often not understood about he missing shell element drilling DOF, hence the line element will not be connected to the finite element surface in this axis.

In the case below of the column connected to slab, the rational axis of the column (theta-Y) is not connected to the slab.

This is not generally a concern in building models. However if the column is not restrained or fixed at any other node point in the theta Y axis, then the column could produce an instability at analysis time as it is free to spin  torsional about it's own axis.

The common fix for this issue is to provide a theta - Y nodal restraint at the base of the column.