Question: 

Why is this Vertical Force Factor used in AutoPIPE Vessel's report for Earthquake load case per EN1998-1 :2004+A1:2013, when the factor is not mentioned in the actual code?

Answer: 

Correct, the Vertical Force Factor is not explicitly referenced in the code. This factor allows users the flexibility to consider all, none, or a portion of the vertical loading in their design process.

The program's help section lacks sufficient information regarding this setting. Additional details will be included in a forthcoming version of the program.