Hello, it is not clear from your question:

1. which of the ATENA Graphical User Environments you are using, i.e., ATENA Engineering 2D or ATENA Engineering 3D, or ATENA Science with GiD?

2. if you have already read ATENA Troubleshooting, "3.3.2 Problem creating an opening (More than two surfaces are connected to the edge n.XX!)"? The latest version of the document is available from our web (Products - ATENA - Documentation).

In ATENA 2D, you can define arc segments. In GiD, you can define curved lines and surfaces. In ATENA 3D, you need to replace each arc with a reasonable number of straight segments. Please note the smaller segments you use, the shorter element edges you need while meshing (see also ATENA Troubleshooting, 3.3.6 I have a bug; when I click on the button Generate in order to generate the mesh, I get the following error: CCFEModelGenerateExc: T3D generator error code: 10 Source line: 157, file: ..\..\Sources\T3d\atena_cpp.cpp, last modified at: Fri Nov 19 13:5139 2010).