API 650 Chapter 5.12, UPLIFT Loads

[senthil83]

As per API 650 clause 5.12 tank anchorage, table 5-21a shows uplift load calculation formula including seismic case as (4xMrw/D)-W2(1-0.4Av).

In this W2 is given as dead weight of the shell and roof. since for seismic moment (Mrw) calculation we are including the content weight(liquid weight) inside tank.
but in the uplift load calculation, why we are not considering the W2 as Dead weight of tank + liquid weight?

thanks
senthilb

 

[IFRs]

The sloshing of the liquid in the tank causes uplift on one side of the tank and compression on the other side of the tank.
If the liquid is causing uplift, it can not resist uplift in the same location at the same time.

 

[Geoff13]

In order for the liquid weight to resist the uplift the tank bottom must lift off the foundation. This effective liquid weight is calculated in 5.11.2.3 for unanchored tanks.

However, since 5.12 is designing anchor bolts then by definition the bottom can't lift and no liquid weight resists the uplift. Thus no liquid weight is included in W2.

 

[IFRs]

Geoff's answer was better than mine. Use his...