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Re: A new angle on "Sliding"

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<<We are currently reviewing a steel water tank that is approximately 300 feet in diameter and approximately 40 feet high. The designer is proposing to not have any anchorage from the tank to the perimeter ring foundation that the tank wall is resting on.>> 

Ben.

I'm sure you can research this forever and get lots of differing opinions.  I looked at quite a few tanks after the Nisqually earthquake last year, and here's my opinion:

The issue isn't sliding, but rather uplift at the perimeter of the tank.  I assume you have a tank on ground with a thin bottom plate, say 1/4 to 1/2 inch.  During an earthquake the perimeter shell of the tank wants to lift up, and very little of the water is actually engaged to resist the uplift due to the thin bottom plate.  I recall that AWWA 100 has a method for calculating the net uplift on an unanchored tank and it uses something like 6 to 9 inches of water adjacent to the exterior shell to resist the uplift (hope this makes sense).  So on that basis, you should anchor the perimeter for uplift to avoid rupture of the tank at the connection of the shell to the base plate.  

Having said that, there is the argument that if you don't anchor the tank, the exterior edges have the capability to lift up and come back down without rupturing the tank and therefore you don't need to anchor it.  For low profile tanks, such as yours, that argument has a lot of merit, also.  There is the possibility of plate buckling (elephants foot) also, and I've heard the argument that anchoring the tank will actually contribute to that type of failure.  Weird, huh.   

However, I assure you that the edges of the tank will indeed lift up in a major earthquake if the tank is not anchored.  Most of the tanks I observed after the Nisqually earthquake had a higher aspect ratio than yours, and there was clear evidence that many of them lifted at the edges, as much as three inches.  Most were anchored, but the anchors that were there were mostly outdated design and inadequate.  Once the anchors failed they did more harm then good (that's a long story), but none of the tanks lost contents.  Of course I suspect that wouldn't have been the case if the earthquake magnitude had been much greater.  

In conclusion, I would suggest the following, though it sounds like I'm equivocating (did I spell that correctly):

1.  Get AWWA or API codes and anchor the tank.  Don't do a half-way job, since poorly designed anchors are worse than no anchors at all.
2.  If you really don't want to anchor the tank, then use AWWA or API to justify it.  There are several experts who will back you up, and you're probably going to be okay.  Make sure that any pipes coming into the tank have the flexibility to withstand some ups-and-downs, though.

Did that help?  

Carl Sramek
Los Alamitos, CA





   

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