Frost Protected Shallow Foundation Design for Heated Tank

[DWags33]

I am looking into using a frost protected foundation for an 80' dia. digester tank foundation. The digester contents will be at approx. 90 deg F. I cant quite follow the detail for heated structures because I don't think insulation placed on the exposed edge of the concrete will hold up (the tank is offset 8" back from the edge of the concrete the pad is 6" above grade). See the attached detail for clarity. Can anyone suggest how I can modify the FPSF design to account for this exposed concrete? ATI-100 = 2500, MAT = 43 deg F. Simply designing the FPSF as if it were an unheated building (70" horizontal extension of insulation) is not economical.

 

[CarlB]

I don't believe a tank must have a foundation that meets IBC or IRC. Does AWWA or applicable code require this? I've seen many bulk fuel tank designs in Alaska with similar shallow ringwalls, or none, with no perimeter insulation. It appears to me, by inspection only, that with 90 degree tank contents, there is no chance for frost penetration below ringwall, even without the subgrade insulation. You might analyze it with a constant outside temperature of -20F (or other design temperature") and show it works.

 

[DWags33]

Thank you for your response. We have always frost protected our tanks to prevent issues with rigid piping in case of frost heave. Could you tell me where to find the procedure for analyzing the chance of frost penetration as you described?

 

[CarlB]

I don't know of any simplified way to analyze temperatures for this 2-d problem, this would best be handled with a finite-element program. You could take some measures to reduce the cold-bridge, such as extend insulation to top of concrete and protect with flashing. Extending the wing insulation would help; 2-feet is typical minimum, could go further. And, insulation could be placed directly under the footing.
Another measure would be to thicken the NFS layer under the footing.

 

[BigH]

Why not use flexible connections to the tanks instead of fixed rigid connections? That was always are recommendations for large storage tanks in Ontario when I worked there. Also, in Ontario at the various Lake Ontario or Lake Erie refineries or in Sarnia, I don't ever remember (1970-1995) seeing concrete ring walls - tanks always put on 1.5 m of a granular sand and gravel pad which extended out beyond the tank. Some of the tanks went under a fair amount of settlement - which was the big reason for flexible connections

 

[geotechguy1]

Could you not just attach insulation to the small vertical and horizontal areas of exposed concrete you've shown and provide a layer of soil to protect it? Or alternatively some other form of protective coating or layer - the supplier should be able to help with this. The vertical portion is just an extension of the buried vertical portion usually. The horizontal part would need to be physically attached to the concrete on the bottom or otherwise held down somehow - once again the best source of info will be to get on the phone with a supplier / manufacturer of the product you're using.

There's some useful info about protecting the exposed insulation in this manual:

https://www.homeinnovation.com/~/me...de-to-Frost-Protected-Shallow-Foundations.pdf

 

[MJCronin]

Agree with BigH ....

Flexible piping connections could solve several issues at the same time, including seismic protection, settlement protection and frost heaves

Take a look at this thread and tell us what you think ....

https://www.eng-tips.com/viewthread.cfm?qid=483315

MJCronin
Sr. Process Engineer