seismic loads on water tank

huangyhg

seismic loads on water tank
i have two 2800 gallon plastic awwa certified potable water tanks.  they are each 8 ft diameter and 8 ft high.  the foundation is 6" of fine sand as recommended by the manufacturer.  total weight full is 8000 lb each.  i am in seismic zone 3 and have 3 ft of stable sandy loam soil below with gravel and clay mix below that.  the tanks are on fairly level ground and have no structures other than the pump house within 75 ft.
the local building officials are requiring me to provide tie downs for seismic loads.  the tank manufacuter doesn't provide any method of tieing the tanks down and states that he has never been required to tie down the tanks even in california.
i feel the tanks are stable but i need some suggestions on how to calculate the seismic loads for a water tank.  most of the articles i have found are for fully anchored tanks or steel tanks that are much higher than they are wide.  any suggestions or references and suggested methods for tieing down large plastic potable water tanks is appreciated.  since the manufacturer does not recommend strapping the tanks down and restraining the tanks improperly could rupture them it is a tricky situation.
thanks.

hi,
the best references i know specifically for water tanks are awwa d-100 and api-650.  these are probably the references that you have already looked at.  doing a quick run through the 8'x8' tanks using api-650 for seismic overturning would indicate that the tanks would need to be anchored per that spec.  awwa d-100 gives similar results to api (i just guessed at the weight of the plastic tanks, but it's pretty insignificant compared to the water). these specs are geared for tanks larger than what you have.
looking at api-650 appendix j which covers small shop fabricated tanks (in api terms, a small tank is up to 20 foot diameter), in the section titled "anchoring", they state:
"because of the proportions of shop-assembled storage tanks, overturning as a result of wind loading must be considered.  if necessary, adequate provisions for anchoring shall be provided"
they don't mention any anchorage due to seismic, and your tanks may be ok if you just have to consider wind (depending on their empty weight).
i don't have the most current version of api-650, but if you want me to fax you something i can (obviously it only covers welded tanks)
hope that helps rather than just "muddying" the waters!
tom wolfe
how did you come up with 8000 pounds? full.
pennpoint
6" fine sand on 3 feet of sandy loam and clay gravel
eight feet high water pond would require some foundation, not a sand pit, before it is anchored, even without the earth tremors
mikehydrophys
mdshydroplane
pennpoint
i listened to the vendor rather than going back to my original calculations.  the full weight including the tank is 24000 lb.  
mikehydrophys
the bearing load on the soil under the full tank is less than 500 psf static.  why do you say it is inadequate for a sand pit.