I am having trouble agreeing with our structural dept. about how the value of restraint forces from the pipe to vessel shall be calculated. We have a 100ft tall, 7'-8" dia. vessel with a 24" dia. line coming out of the top and dropping down the length of the vessel. We are designing the pipe to B31.3 which states that all seismic must comply with the Universal Building Code (UBC). In that code there is formulae under section 1632.2 DESIGN FOR TOTAL LATERAL FORCE, which states:

Fp=4.0*Ca*Ip*Wp or alternatively

These formulae seem to assume that the seismic event is acting on the bottom the attachment (pipe) and the forces get larger for each guide as you go up the vessel. Since the pipe is attached to the top of the vessel, it appears to my intuition that the exact opposite is the case. After modeling the Vessel and the pipe and running a dynamic seismic analysis my intuition was confirmed. The forces that Caesar II produces are similar in magnitude but opposite in the fact that they are smallest at the top of the vessel and get larger as the you look down the vessel.

My question is thus;
"Can I defend the values of Caesar II vs. those of the UBC in respect that the pipe is hanging from the top and not supported at the bottom?"

"Can anyone direct me to example seismic calculations of pipe hanging from pressure vessels?"

The codes that I have reviewed do not appear to have any specific discussion on this particular application. Any input would be greatly appreciated.