When an older strutural file from AECOsim SS3 is opened in AECOsim SS5, the custom rules would not load or are not present.
In order to add your own custom rules file in AECOsim Building Designer SS5 you need to add them to the SS5 workspace. By default the Structural Rules file is located in the ..\Dataset_US\rules\Structural.rul file. Just rename the existing rule file located in the SS5 folder (e.j. Structural.xyz) and then add your custom Structural.rul.