When you have multiple versions of ProSteel 3D installed on the same machine you MUST control which one opens when you start the program. This is done through a few different elements.
The AutoCAD version of ProSteel 3D is a demand load program be default. This means that you have to use one of the ProSteel 3D functions to get ProSteel 3D to start. When you have multiple versions of ProSteel 3D installed this must be disabled. To disable demand loading you need only rename 1 file. In all ProSteel 3D versions it exists in the PRG folder of the installation, in ProSteel 3D R18.0 it is located in (default install path)
C:\Program Files\Bentley\ProSteel 3D\R18.0\PLATFORM\Prg\Rename_to_ACAD.rx (where PLATFORM is the AutoCAD version that ProSteel 3D is running on)
Rename this file so that it is called ACAD.rx. Do this for every version of ProSteel 3D that is installed on the computer.
For the newer versions of ProSteel / ProStructures one needs only to set demandload to 2 for each of the ProSteel / ProStructures running.
This means explicitly use the desktop shortcut or start menu shortcut only!! do not double click a drawing, do not drag a drawing to the shortcut. Double-click the shortcut then once the program is running open the file.
ProSteel 3D models are not backwards compatible, so make sure of the version you are using before saving a model as once a model goes up a version it cannot go back again (ProSteel 3D versions, not AutoCAD versions).
Doing this means that ProSteel 3D will start as soon as you double-click the shortcut. If you want AutoCAD to start without ProSteel 3D running then you must add a profile switch to the AutoCAD shortcut. Refer to the section in the AutoCAD help files;
User's Guide > The User Interface > Customize the Drawing Environment > Customize Startup