Sureshprsharma
One again I thank you for your response.
However I have to disagree. If you just consider the selfweight of the structure you will note that ther are reaction in Z direction, ie. there is participation of selfweight mass in the vibration, in my structure case resulting to reaction of 0.019 kN. If I add some load in Z direction like what I have in my structure, this should increase the Rz & not suddenly result in 0 reaction in Z direction . There is surely a bug in Staad with in this respect. If structure is giving resonses due to self weight with reaction in z direction, this can not suddenly disappear if I add mass along the beam and result in Rz as 0.000.