D1.J.6 Chord Selection and Qf Parameter
Qf is a factor to account for the presence of nominal longitudinal stress in the chord. When calculating Qf for the joints, the moments used in the chord stress calculation will be from the computer node results and not the representative moments underneath the brace. If the moment varies significantly along the chord, it is more accurate to use the actual chord moment in the middle of the brace foot print. The tests reported in Reference I were performed with a constant moment along the chord. Thus for a local joint check, the local chord moment (under the brace) should be used.
STAAD calculates Qf based on the moment at the chord member. The chord member can be selected automatically by initial screening by the program (based on geometry and independent of loading) or specified in the External file.
In the automatic selection of the chord two collinear members (5 degree tolerance) are used to identify the chord. The chord is then selected from one of the two members based on the larger diameter then thickness or then by the minimum framing angle; for T joints the first member modeled will be selected as the chord.
You should confirm that the chord either be assigned by the program or the user is representative of the local chord moment for the brace in question.