ASCE 7-10 Chapter 15 Seismic Loading on Tanks

[CordialEngineer]

I've been tasked with designing a foundation system for a 60' tall, 50,000 gallon ground-support tank used for liquid storage. My firm has had a older "rule of thumb" reference on tank seismic design from IBC 2000.

However, looking through ASCE 7-10's seismic requirements, it would seem section 15.7.6 would be the appropriate method to use in finding the seismic base shear for the tank. I've yet to find a good design example for ASCE 7-10 Ch. 15, if anyone could point me to one? But I have specific questions as well:

1. How is W_c (portion of liquid weight sloshing) calculated? Is this provided from the tank manufacturer typically? Further, are there any approximate methods out there?
2. Is T_L the same as it would be for typical buildings? (i.e. using Figure 22-12)
3. At what elevations are the impulsive and convective base shears applied? I imagine the convective effects would occur at a higher elevation than the typical impulsive effects?

I would appreciate input on any of these questions. Thanks!

 

[SlideRuleEra]

1. Sloshing liquid is usually addressed through API 650. See this article, "Steel Tanks, Seismic Design of Ground Supported Liquid Storage Steel Welded Tanks", from the March 2007, issue of Structure Magazine. The article is out of date, but still helpful.

http://www.structuremag.org/wp-content/uploads/2014/09/C-CodesStandards-SteelTanks-Mar071.pdf

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[JStephen]

The impulsive and convective portions of the contents, and the heights at which they are applied, will generally come from the tank design codes- API-650, AWWA D100, etc. That should also be part of the calculations done by the tank manufacturer, although it would quite commonly get figured by both parties. The T-L is the same as for buildings (IE, it's not a building property, it's a ground-movement property).
Generally, the approach will be to do all the calculations per API or AWWA, then check for any adjustments required by ASCE 7.

 

[mrsutton]

There are two tank design methodologies that ASCE7 Chapter 15 refers back to:

API-650 (mostly used for welded steel tanks found in tank farms)

AWWA D100 (water tanks of any material and occasionally elevated up on stands)

I've only anchored tanks per AWWA D100 but if you can get your hands on it it has all the equations to determine the sloshing and impulsive force as well as the height of the force application. The terms include a lot of hyperbolic sine and cosine terms if I recall. But it's a prescriptive process.

The overturning values per AWWA are a little better (as in less conservative) than those found by simply treating the tank as a rigid mass per the Chapter 15 simplified approach.

 

[JedClampett]

Get a copy of ACI350.3-06. That has all the variables you're asking about defined (Wc, etc.).
But I'll warn you, you'll be using buttons on your calculator you didn't even know you had.

 

[CordialEngineer]

All, thanks for the references. I will look into getting a copy of API-650 and AWWA D100, as suggested.

We actually have a copy of ACI350.3-06, which seems to present the same dynamic model that ASCE 7-10 is using (Housner 1963). If I'm analyzing a steel tank, are ACI350.3 equations for Tc, Wc, etc. still applicable considering it was written for concrete structures? In other words, do those equations apply to "flexible" structures as they do "rigid" concrete structures?

 

[EnriqueO]

Hi Cordial Engineer and all,

I have the same task at my firm, the design of a 30,000 gallon water storage flat-bottom concrete tank, in Southern California, and haven't been able to find an example on how to do this, any help will be greatly appreciated.