SEISMIC FORCE ON TANKS.

[Jandra11]

Hi I have some clarification on asce/ubc/ibc in computing the lateral force of the whole tank including the liquid inside. Do this codes consider the correcting force of the liquid while experiencing earthquake? or we just consider the liquid to be part of the total weight of the tank and multiply it by certain factor to get the seismic lateral force?

 

[jittles]

Typical seismic design of tanks (petroleum, water, ASCE) splits the seismic loading into two pieces. The impulsive component is due to the weight of some portion of the liquid that is assumed to move with the tank. Then you combine that with the convective (sloshing) component for the part of the liquid that will slosh about.

I don't recall off the top of my head if simply assuming 100% of the liquid goes along with the tank is an appropriate conservative assumption.

 

[Agent666]

The two components noted by jittles are both destabilising, i.e. increasing the seismic force at their maximum effect. You are not interested in the overturning when the two forces are perhaps working in opposite directions cancelling each other to some degree, as this doesn't correlate to the maximum overturning force and base shear experienced by the tank.

The only 'correcting force' of the liquid as you termed it (reducing the overturning) is the self weight of the liquid and tank itself, this would be considered based on your code load factor for self weight or contents, I would guess in combination with the lateral force component.

 

[TLHS]

ASCE 7, in the seismic design of structures not similar to buildings section, has requirements for tanks. This includes formulas to account for sloshing. The ASCE-7 seismic provisions are appropriate to use unless the governing specification for the tank has modern seismic requirements.

 

[SocklessJ]

I've not actually done the seismic design for a tank, but I do remember reading a case study where a tank failed. The analysis only considered the empty and full cases, but the worst case was when the tank was partially filled.

 

[IFRs]

API 650 Annex E is all about seismic stability and stresses on aboveground storage tanks. If these are your interest, post in the Storage Tank Engineering forum...

 

[bones206]

There is at least one scenario I know of where you can consider the tank and its liquid contents as a rigid mass. I recently used this simplified approach to design the foundation of a 14 ft diameter, Seismic Category B, Risk Category III tank.

From ASCE 7-10:

15.7.6 Ground-Supported Storage Tanks for Liquids

15.7.6.1 General

Ground-supported, flat bottom tanks storing liquids shall be designed to resist the seismic forces calculated using one of the following procedures:

a. [highlight #FCE94F]The base shear and overturning moment calculated as if the tank and the entire contents are a rigid mass system per Section 15.4.2 of this standard.[/highlight]

b. Tanks or vessels storing liquids in Risk Category IV, or with a diameter greater than 20 ft (6.1 m), shall be designed to consider the hydrodynamic pressures of the liquid in determining the equivalent lateral forces and lateral force distribution
per the applicable reference documents listed in Chapter 23 and the requirements of Section 15.7 of this standard.

c. The force and displacement requirements of Section 15.4 of this standard.

 

[Jandra11]

thank you for spending time on my post
@jittles yeah I agree that is conservative to consider that the liquid movement will be the same as the tank during seismic occurrence. We are trying to make the foundation as small as we can because of the limited area due to the existing structures along the area.Hence, we are looking for some design guides on considering the affect of the liquid movement to the total lateral seismic force, and what will be the effect on the foundation. whether it will lessen the required area of the foundation or not. thank you

@agent666 base on your suggestion we need to design the foundation considering the movement of the tank and the content will be rigid? if thats the case i think we should proposed pilings. thank you.

@TLHS thank you ill check that one. btw considering this sloshing effect will it make any difference on designing the foundation or it only concern about designing the tank itself?

@sockless thank you for that information but right now we are focusing on how are going to design the foundation considering the minimum load required by the code.

@IFR thank you ill check that one too

@bones yeah thank you for all the notes you mention. just for information the tank is around 3 meters in diameter 9m height and the estimated total weight is 75 tons.