Would like to add one thing. Once you declare the structure as STAAD SPACE all the members would be considered as frame members with ability to carry axial+bending+shear. I think you would like the top and bottom chords to behave that way. However if you would like to declare the bracings connecting the top and bottom chord as trusses, you can declare these accordingly by going to General > Spec page and selecting the options Beam > TRUSS.
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