selfweight in staad
You can create a load case and have the self-weight load assigned to the structure and subsequently you can look at the static check results for that load case which will give you the total weight of the structure.
The following Wiki explains how to check the selfweight of plates only, but the same can be used for a whole structure: How to find total weight of plates - RAM | STAAD | ADINA Wiki - RAM | STAAD | ADINA - Bentley Communities