Vertical Seismic Force in Piping as per UBC'97

Dear All,

In one of my project, I have to calculate Seismic force:

Where I Have considerd following values from UBC'97 & Contract Doc.

Importance factor = 1.0 [assumed Ip = 1.0 for TPP, Is it OK? Or I have to considerd 1.5, Please give your valueable comments]

Seismic Coefficient = 0.44 Na [For Zone 4 (Z=0.4), Soil Profile SD]

aP = 1.0

RP = 3.0

So, I calculate the total design lateral seismic force =.42*Wp as per code.

Now My Question is that as Seismic Zone-4 & as per contract Seismic events will occur frequently, So Should I also considerd the vertical Seismic force as two third of the total design lateral seismic force, If yes means I needed extra snubbers (Dynamic restraints) and If No means my system will fail, if such events will occur. Please give me light of your valueable experience.

Thanks in advance

Vikram Kapse