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RE: A new angle on "Sliding" - unanchored tanks

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AWWA D100 does permit "unanchored" tanks in seismic regions, although it
states that "properly designed anchored tanks retain greater reserve
strength to seismic overload than unanchored tanks". A study of seismic tank
failures by the American Lifelines Alliance (associated with FEMA and ASCE)
indicated that "anchored tanks should perform better than unanchored tanks."
Consequently, I specify that tanks in seismic regions "shall be anchored
tanks". Tank designers often request an option to provide an unanchored tank
but I stick with my specified requirements based on the improved seismic

William C. Sherman, PE
CDM, Denver, CO
Phone: 303-298-1311
Fax: 303-293-8236
email: shermanwc(--nospam--at)

> -----Original Message-----
> From: Yousefi, Ben [mailto:Ben.Yousefi(--nospam--at)]
> Sent: Thursday, November 14, 2002 4:37 PM
> To: seaint(--nospam--at)
> Subject: A new angle on "Sliding"
> 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. His point is that, there is 
> sufficient friction
> between the tank wall and the foundation to resist the 
> seismic forces from
> the tank itself and any sloshing forces from the water!
> I have not in, my entire professional life, seen a case where 
> a structure of
> this magnitude would not use any positive anchorage from the 
> superstructure
> to the foundation. There is some code language in the UBC, 
> such as sections
> 1605.2.3 and 1809.3, that deal with anchorage of walls and 
> buildings. And to
> me it is clear that what is being referred to in those 
> sections is positive
> anchorage. However, the designer is insisting that using 
> friction is an
> acceptable method of resisting seismic forces in tank design. 
> Any input from
> those experienced with this type structure is appreciated.
> Ben Yousefi, SE
> San Jose, CA

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