A bar joist is a manufactured truss of unknown members and unknown configuration. Most programs including STAAD just use a bar joist like a dummy beam to transfer the reaction loads to a supporting member. So you couldn't use it in your model to do anything more than a simple beam. It can't be used to resist lateral loads. See excerpt from the manual below.
"In STAAD, selfweight and any other member load applied on the joist is transferred to its end nodes through simply supported action. Also, in STAAD, the joist makes no contribution to the stiffness of the overall structure."
I have used joists in frames before, not in a long time though.
I figured the size of the beam I needed and then specified a joist with similar properties (moment of inertia) and end moments (had to separate these into dead, snow, wind and seismic for the mfr).
Never specifically tried using the bar joist option in STAAD.
