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RE: Tank Deadman

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Harold Sprague
Krawinkler, Luth & Assoc.
4412 W. Eisenhower Blvd.
Loveland, CO 80537
Voice: 970 667-2426
Fax: 970 667-2493
Email: hsprague(--nospam--at)klaalov.com


-----Original Message-----
From:	ErnieNSE [SMTP:ErnieNSE(--nospam--at)aol.com]
Sent:	Wednesday, March 11, 1998 2:22 PM
To:	seaoc(--nospam--at)seaoc.org
Subject:	Re: Tank Deadman

I'm designing a concrete deadman for an underground storage tank subjected to a bouyant force due to water level rising above an empty tank. I'm using two flat straps restraining the tank and connected to the 12 inch thick concrete slab below the tank. In calculating the downward forces resisting the bouyant force, how much of the soil above the tank and the deadman can I use.?  Reference NAVFAC DM 7.2; p 7.2-171.  You can develop a wedge with an inclined failure plane.  If the condition occurs regularly the inclined failure plane is not as reliable.
How much is the net weight of the soil (soil weight minus bouyant force due to the volume of water displaced by the soil, in PCF)? It depends on the soil, but a reasonable value for sandy soils is 110 pcf - 62.4 pcf which will give you a net downward resistance to buoyancy of 47.6 pcf.  Don't forget to subtract the density of water also from the concrete.

Can I use the horizontal projected area of the soil directly above the tank and the concrete slab deadman up to the soil surface above? If the water table is lower than the surface you can generally use the wedge to the soil surface. (See DM 7.2.)  Using the wedge is a common practice in dead man design for anchoring guyed towers.  The wedge may not be appropriate in all cases.  A consistently high water table in sandy soils in a high seismic area would be prone to liquefaction and you could not develop the wedge.  In this case use a theoretical vertical failure plane at the perimeter of the foundation.

Can anybody who have experience in this design give me comments, suggestions or reference material.  Safety factors vary.  I have seen SF of 1.1, 1.5, and 2.0.  
I would tend to use 1.5 with a vertical failure plane for a consistently submerged condition in a high seismic area.  You will get some wedge action, but it is difficult to quantify.  
For a condition with a water table about half way from the base of excavation to the surface I would use the wedge and a safety factor of 1.5.  If you don't have a geotech, it is up to your judgement.

Thanks in advance.
Ernie Natividad

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