If you analyze the model with load that has both the lateral and gravity load involved, then after performing the analysis, if Staad reports instability warning message then it indicates that the system has overturned and the gravity load is not sufficient enough to stabilize the overturning effect due to the lateral loading.
Please following the following link
http://communities.bentley.com/products/structural/structural_analysis___design/f/5932/t/95633
