double hollow sections

Question

Hi all, 
 I have an steel built model with double hollow sections (welded back to back), both in columns and beams and I wonder how I could model this in Staad. I also need to do a seismic model, so, the solution should allow to use the seismic model. 
 I know Staad has double angle sections (back to back), but not hollow sections, right? 
 If I put an offset between 2 points and I put 2 beams between these 2 points (one beam with the offset positive, and other with the offset negative) in a way, that they are really back to back, the doubt, is if Staad, considers the whole section (with the total area, inertia of both hollow sections, with the gravity centre of both sections, etc.). I think the answer is not, and so, this could not be a way to solve this issue. 
 Other option, is if I define sections in Staad with the area, inertia, etc. of both hollow sections. The problem here, is that Staad doesn't check those sections with the codes, because, it only checks typical sections, right? So, this could not be the answer either. Also, I am not very sure, if with these "manual sections", the seismic analysis could be done? 
 Other option, is if I could define 2 nodes very near (at the same distance where I put the offset), and linked them with the command master-slave. But I have never seen using this command in this way (I have only seen using when you are connecting a frame with slab defined with FEM or similar, etc.). So, I am not very sure if this could be a solution for this issue, and if yes, if someone could show & send an example of how is done. 
 Any other suggestion of how to model the double hollow sections will be welcome. 
 Thanks for advice. 
 Regards

Sye · Answer

Double profiles are available for some section types like wide flanges, angles, channels but not for HSS. The first approach you suggested with positive and negative offsets would work. You may need to break the beam into a few segments as opposed to defining the beam as a single member as that would ensure that the double profiles are connected at intermediate locations and not just at the ends. The analysis would correctly consider the properties.You will get duplicate member warnings during analysis but you can ignore those. As far as design goes, the members would be designed individually based on the forces developed in each. 
 The other approach would be to define these as User Table General type sections in which case the section would be considered as one section with combined properties. General sections defined through user tables can be designed as per most design codes following the rules applicable for wide flanges. If you know the properties for the combined profile, you can define these sections directly in STAAD.Pro. If not, you may define these sections in Section Wizard and get those into STAAD.Pro.

Sye · Answer

Using offsets is good enough as that establishes a rigid link automatically and you do not need to define master slave. 
 I was suggesting breaking up the member to simulate the fact that the two sections are connected along the length and should not deflect independent of each other. This is irrespective of whether you have seismic loading or not. I would split the member at quarter points may be. It's your judgment call. 
 I was referring to design by STAAD.Pro. You can define these user defined General Sections as outlined next 
 In the CONNECT Edition, from the ribbon menu choose Specification > User Table > User Table Manager > Select Section type as General and define the properties. Once the sections are defined, you can assign these sections using the User Table button within the Properties Whole Structure dialog box. Assign design commands to these members as you would normally do and these should then get designed by STAAD.Pro.