Dear mtma, please see ATENA 3D User's Manual, 3.7.2.2.7 Shell Macroelements (if you work in ATENA Engineering 3D) or  ATENA-GiD User's Manual, 5.3.2 Shell Material (if you use the ATENA-GiD interface of ATENA Science) for basic information about defining shell elements.

A quaote from the ATENA-GiD User's Manual:
Unlike volume elements, a single shell per volume thickness works well in bending. In other words, placing 2 or more shell elements above each other ("above" refers to the shell thickness direction) is not a good idea. Instead, use a single shell per thickness with more internal layers to improve precision.

I would further recommend to read ATENA GiD User's Manual, 5.7 Finite Element Mesh or ATENA 3D User's Manual, 3.7.4 FE Mesh for general information about meshing in ATENA.

Dear mtma,
please see ATENA Troubleshooting, 2.2.6 Floating point problems - Division by zero, Loss of >7 digits on the matrix diagonal, Warning: Sign of diagonal changed when reducing equation, Floating point exception: Multiple floating points traps, Zero or negative jacobian, InvertA: zero determinant, CCMaterials Extended AExc: CC5ParamYield F:project_stress_on_f_division by zero, and similar