Dear Mr.Mandeen,
First of all I really thank you for your kind answer.
As I understood from your reply it seems that the method which is used in PV Ellite for calculating the horizontal shear force on one leg (in vertical vessels) is a little different with the method developed in MOSS book. because as it comes to the mind the total shear force which is calculated by PV Ellite (based on MOSS I think) should be devided by 4 to get the shear on each leg.I was wondering in which stage of this calculation PV Ellite is different from MOSS.(because I use Moss book as a refrence of designing pressure vessels and it help me to use PV Ellite better)
So appreciated with your help!
Regards,

Dear Kamali,

Both in D. Moss's book and in PV Elite, the horizontal shear is distributed proportional to the stiffness of the leg perpendicular to the applied force relative to stiffness of the other legs (quoted from the Moss book).

Some legs will bear more of the shear force than the other legs. If you divide the horizontal shear by the number of legs then you are not designing for the worst case.

I hope this helps.