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Types of Seismic Loads Acting on a Tank That Need To Be Designed For

Types of Seismic Loads Acting on a Tank That Need To Be Designed For

Types of Seismic Loads Acting on a Tank That Need To Be Designed For

(OP)
I have got a very basic question about the types of seismic loads acting on a horizontal round tank that need to be considered and designed for. And where and in which direction those loads will be acting?

Say, the tank is seated on a saddle, which is in turn supported at the top of a pedestal about 4 feet from the finished floor elevation.  The tank is supported on two wide pedestals.  For simplicity's sake, weights or dimensions of the tanks have not been included.

In an earthquake event, an instantaneous lateral earthquake load, F will be applied on the tank.  Say, F is applied normal to the longitudinal dimension of the tank. I.e. front end of the tank sticking out of the page.

The magnitude of this load will depend on the mass of the tank, the stiffiness of the structural system, etc.

Ok, we can calculate V, a seismic base shear that is determined by;

V = CsW,

where Cs is the seismic response coefficient, and W is the effective weight of of the structure.

Question 1: Do we need to include only V in our load calculations, and not F?  Since Summation of F will equal V, will they not cancel each other if we included both?  V will act at the base of the tank, which is also at the top of the pedestal? Is that correct?

Question 2: Overturning Momen (OTM).  Here is where I am having a bit of difficulty, and would appreciate some help. How do we calculate OTM, and in which direction is it acting?  Is OTM calculated as F * h, where h = distance of CGM of Tank to the Base? Or is it calculated as OTM = V * h.  I am thinking that it is F*h. And where is OTM acting?  Is it acting at the base of the tank, i.e. top of the pedestal?  Which direction is OTM acting?  If OTM = F*h, would it be acting in a manner that tend to overturn the tank?

Appreciate any feedback and comments.


 

RE: Types of Seismic Loads Acting on a Tank That Need To Be Designed For

The OTM can act in any direction, so the connections and footing must be designed accordingly.

As whether to use V or W, for any story, to get the appropriate story shear, you take the weight of the story times the "H" above the seismic base, and compute the % for that particular story of the total for all the stories summed - Ws X Hs / Sum W X H.  The V base shear then can thus be broken down to individual story shears, or element shears to be applied to the structure for your connections and analysis.

Thus you can separate the overturning for the tank from the pedistal.  However, the overturning of the tank at the top of the pedistal will be less than at the base of the pedistal.  It's just accounting.   

Mike McCann
MMC Engineering

 

RE: Types of Seismic Loads Acting on a Tank That Need To Be Designed For

Sounds like a fuel tank. If you are in the US you should get API 650 which details seismic tank forces including those applied by stored liquids along with ASCE 7's nonbuilding structures chapter. The API is more detailed.

RE: Types of Seismic Loads Acting on a Tank That Need To Be Designed For

API 650 does not address horizontal tanks.  I am not aware of any publication that really goes into much detail about the seismic loading of horizontal tanks.  ASCE 7 will give the seismic load as noted in the original post.

On the question of F versus V, the actual loading consists of the ground moving while inertia of the tank itself is resisting motion.  But it is usually convenient to treat this as just an applied force F proportional to the mass.

If you treat the seismic loading as a force F applied at the centroid of the tank mass, then calculation of moment at any point uses the distance from the centroid to that point.  For a typical tank with welded saddles, you'd be concerned about moment at the base of the saddle.

RE: Types of Seismic Loads Acting on a Tank That Need To Be Designed For

Dont forget to include the "Sloshing" forces from any liquid inside the tank.

RE: Types of Seismic Loads Acting on a Tank That Need To Be Designed For

Sloshing is usually not considered on a horizontal tank.

On vertical tanks, consideration of sloshing gives a reduction in the loading, due to the longer sloshing period.

RE: Types of Seismic Loads Acting on a Tank That Need To Be Designed For

I have also considered that sloshing would result in a reduction of loading for vert tanks and used the total liquid mass for seismic, believing that this would be consrvative.
I have not seen any definitive confirmation of this in literature
and would appreciate any references addressing this.  

RE: Types of Seismic Loads Acting on a Tank That Need To Be Designed For

SAIL3- In API-650, Eq. E.4.6.1-4 and 5 require that Ai<=Ac.

In looking at Section 15.7.6.1 in ASCE 7-05, it looks like that is not necessarily the case with the procedure given (it should be with tanks of any size, but not inherently).  However, that section also allows the rigid-mass option for the smaller tanks where that would likely be the case.

RE: Types of Seismic Loads Acting on a Tank That Need To Be Designed For

(OP)
Thank you for all your responses.  I would like to get back to the topic of loads acting on the tank (F, V and OTM).

msquared48, you said that, the OTM can act in any direction.  That is true, of course, although it can only act in one direction at any one time.  The four directions are, of course, +X, -X, +Z, -Z.  Let's say the earthquake force, F is acting in the +X direction.

Question 1:  What is the magnitude of OTM, how is it calculated, and what is its direction?  Is it F x height from the center of mass of the tank to the base of the saddle?

JStephen stated that, "If you treat the seismic loading as a force F applied at the centroid of the tank mass, then calculation of moment at any point uses the distance from the centroid to that point.  For a typical tank with welded saddles, you'd be concerned about moment at the base of the saddle."  This tallies with my own observation.

Question 2:  What is the magnitude of F?  I am able to calculate the seismic base shear, V, from the seismic response coefficient, Cs and the effective weight, W.  Is F equal to V, but acting in the opposite direction?

Question 3:  Which loads do I input into my load combinations for the purpose of design?  I am thinking that it is only the seismic base shear, V and the overturning moment, OTM.  Comments, please.

RE: Types of Seismic Loads Acting on a Tank That Need To Be Designed For

seismic acceleration is not just horizontal, it can act vertically or any other direction. This may have some impact on how you anchor the structure down (both the pedestal and the tank).

RE: Types of Seismic Loads Acting on a Tank That Need To Be Designed For

Your force F acting on the center of mass of the tank is equal to your base shear V in this case. In a multi-story building you would distribute V vertically amongst the stories and get an F for each level. In this case you have only one level so F = V.

You need to use appropriate load combinations. The seismic chapter in ASCE 7 details how you calculate E for putting into the load combinations. This includes a vertical seismic effect that effectively lowers the dead load, which is what is resisting the overturning moment.

Without a sketch it is hard to say what you are looking at, but you should be designing for overturning about the base of the structure also, which would be the finished floor level and not just the base of the tank.

Your moment arm for your seismic load will be the centroid of the mass to the floor, and the moment arm for the resisting dead load will be half the width.

Since the seismic load acts with the same force in all directions, the shorter dimension direction is going to control the overturning.

RE: Types of Seismic Loads Acting on a Tank That Need To Be Designed For

Check out ACI 350.3-06 for rectangular tanks (not covered by API).

RE: Types of Seismic Loads Acting on a Tank That Need To Be Designed For

Quoting Saudi Aramco specs for Horizontal tanks, refers to non-building structures in its 2005 code

SAES-M-001
Structural Design Criteria
for Non-Building Structures


"5.5 Earthquake Loads (E)
5.5.1 Except for API STD 650 ground supported storage tanks, earthquake loads shall be computed and applied in accordance with SEI/ASCE 7,unless otherwise specified.

Commentary Note:
The earthquake loads in SEI/ASCE 7 are limit state earthquake loads and this should be taken into account if using allowable stress design methods or applying load factors from other codes. Earthquake loads for API STD 650 storage tanks are allowable stress design loads. ASCE's Guidelines for Seismic Evaluation and Design of Petrochemical Facilities may also be used as a general reference for seismic design.

5.5.2 Seismic zones, effective peak acceleration, effective peak velocity and site soil coefficient shall be determined in accordance with SAES-A-112, "Meteorological and Seismic Design Data". All plant area structures shall be considered essential facilities.

5.5.3 Earthquake loading shall be determined using SEI/ASCE 7, Section 9.14 for non-building structures (as defined in SEI/ASCE 7, Section 9.14.1.1 and Table 9.14.5.1.1). Non-building structures include but are not limited to elevated tanks or vessels, stacks, pipe racks, and cooling towers.

5.5.4 The importance factor "I" for non-building structures shall be determined from SEI/ASCE 7, Table 9.14.5.1.2. The Importance Factor "I" for nonbuilding structures in petrochemical process units shall be seismic group II, giving an importance factor "I" of 1.25 except for minor or insignificant structures which may be designated seismic group I with "I" of 1.0 by the Supervisor, Civil Engineering Unit, Mechanical & Civil Engineering Division, Consulting Services Department, on a case-bycase basis.

5.5.5 For the load combinations in Section 4.2 the following designations are used:

Eo = Earthquake load considering the operating load case
Ee = Earthquake load considering the empty load case"

It must be referring to ASCE 7-02, by quote at other sections.

RE: Types of Seismic Loads Acting on a Tank That Need To Be Designed For

Dears
I am thanking all of your participations; I am specializing in my company to design such these structures.
Load types affect the structure:
1.    Hydrostatic pressure (conventional liquid pressure)
2.    Walls (or structural) inertia pressure initiated from the acceleration of the structure.
3.    Impulsive water pressure initiated from the acceleration of the structure
4.    Convective Water Pressure initiated from the waves generated by the overall system acceleration.
5.    Seasonal temperature variation
6.    Immediate intermediate and long term shrinkage strain stresses.

All of these loads have to be taken in ultimate and service load combinations where some of these combinations are SRSS combination. I believe that ACI 350.3 and its commentary is the best reference in case that ACI is applicable in your project.
 Any more detailed information needed please contact me at allamcivil@hotmail.com

Regards

Allam Abdelmajid
 

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