When the load combinations are not “enveloping”, the program declares the load combinations as “seismic” or “not seismic” appropriately. However, when the load combination is “enveloping”, the program does not declare the load combinations as “seismic” as enveloping forces at different stations can come from any of the included combos. In addition, the load combination forces loose associations. Therefore, it is not recommended to design using this type of combinations when seismic loading are present.
Please note that the program internally design for each load combination specified in the "Design Load Combination" list and report the result for the worst one. In view of the above, it is advised to design independently for all load combinations contained in the enveloping combo.