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RE: Seismic Base Shear - Tank With Supported Bottom

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In my opinion the weight of the pile caps should be included in the seismic
lateral force at the top of the piles. Presumably the piles are intended to
carry the vertical loads down to a stiffer stratum, perhaps to bedrock. I
view the seismic force as primarily being transmitted to the structure thru
this stiffer stratum, since there is likely little vertical pressure or
friction between the bottom of the tank and the soil below. Since the pile
caps have mass and inertia, they will also be affected by lateral
acceleration. However, the effects of soil resistance to lateral movement at
the pile cap level should also be considered.  

Also, as I read 1997 UBC 1634.4, it appears to allow tanks to be designed
according to 1634.3 without confirming that the period is less than 0.06
sec. (Note: I believe that the reference in Section 1634.4 to Section 1632
should be to 1634.3.) However, I question the basis of using the 0.7 factor
included in Equation (34-1); in my opinion the full ground acceleration
should be used in this equation for ground supported tanks, i.e. the tank
accelerates at the same rate as ground acceleration assuming a rigid

> -----Original Message-----
> From: Sprague, Harold O. [mailto:SpragueHO(--nospam--at)]
> Sent: Wednesday, July 24, 2002 3:41 PM
> To: 'seaint(--nospam--at)'
> Subject: RE: Seismic Base Shear - Tank With Supported Bottom
> Kevin,
> Apparently this is a very short, stiff tank to have a period 
> of 0.06 second
> or less to invoke 1634.3.  Since this is a tank I would 
> suggest that you use
> 1634.4.  Even better would be the seismic provisions 
> contained in ACI 350.3
> to develop the impulsive and convective fluid forces.
> In regard to the pile cap, normally the pile cap is not 
> added.  Pile caps
> are normally embedded in the soil.  It is the soil that is 
> shaking which
> should accelerate the pile cap which causes an inertial 
> response of the
> tank.  
> You need to calculate the impulsive and convective response 
> in the tank.
> Regards,
> Harold O. Sprague
> > -----Original Message-----
> > From:	Klarkowski, Kevin [SMTP:Kevin.Klarkowski(--nospam--at)]
> > Sent:	Wednesday, July 24, 2002 4:26 PM
> > To:	'seaint(--nospam--at)'
> > Subject:	Seismic Base Shear - Tank With Supported Bottom
> > 
> > When calculating the seismic base shear for a tank with 
> supported bottom
> > per the Rigid Structures Criteria of UBC section 1634.3, 
> should the weight
> > of the foundation(pile cap) be included in W when the foundation is
> > pile-supported?
> > 
> > Kevin J. Klarkowski 
> > Jacobs - Green Bay 
> > kevin.klarkowski(--nospam--at) 

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