I do not believe an Rp of 3 for B31 piping is too small. While ASCE 7 lists Rp=12 for such piping, B31E says Rp shall not exceed 3.5 when used with the allowable stresses found in that document.
But 4.6 g is huge. What does the rest of your horizontal acceleration equation look like? (Note that the equation uses Sds, not Ss.)