When attempting to use the Cut Solids By Curve tool with some of the beams in my 3D model I am getting an "element not valid for tool" message, while with others it works as expected. I've confirmed that they are all valid Structural members and were placed in CONNECT Edition, but regardless of how the cutting shape is oriented the process just won't work for these particular beams.
Defect 1004460 has been filed to address this issue which is specific to Structural sections composed of multiple members such as Double Angles, Double Channels, Plate Girders, etc.. In the interim, there are two workarounds available:
1. Using the Modify Structural Attributes command, temporarily change the existing Section Name to a single member section such as Angles, I Shapes, Rectangular Shapes, etc..
Generate the cut using the Cut Solids by Curve command, then change the Section Name back to the original section assembly.
2. Use the Construct Openings tool instead of Cut Solids by Curve. Note that the cutting shape needs to be oriented so that it cuts in a downward direction and the tool will need to be used multiple times since each instance creates an opening for only one "face" of the section assembly as shown in this video: