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RE: Flat Bottom Tank Anchorage

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On many of our new steel tanks we have installed both Hold-down anchors and
sliding restraint anchors. The holdowns typically have "chairs" consisting
of thick horizontal plates over vertical gusset plates welded to the tank
shell. The Anchor bolt hole is slotted to allow for shell movement. The
sliding restraints, which are typically spaced between the holdown anchors,
consist of steel tabs welded to the reservoir floor plate with a hole (not
oversized) for an anchor rod which is welded to the plate. The sliding
requirement depends on the demand (and the factor of safety) which may
exceed the resistance provided by friction. Note that many of our tanks are
near major faults and are single tanks in a pressure zone (I=1.5). 

To answer your specific question, I've seen & used a distribution based on
cosine of the angle from perpendicular to load for distribution of the shear
to the sliding restraints. Equal distribution would assume the floor plate
is rigid which doesn't seem reasonable. The floor plate is what transfers
the shear from the shell wall to the sliding anchors (or the ground if
friction is adequate). Don't forget to check the shell/floor plate weld.

Eric Fieberling SE
East Bay Municipal Utility District
Oakland CA

-----Original Message-----
From: YI [mailto:YI(--nospam--at)summit-sr.com]
Sent: Thursday, October 18, 2001 9:50 AM
To: 'seaint(--nospam--at)seaint.org'
Subject: RE: Flat Bottom Tank Anchorage


I don't think all those conditions are really relevant to my questions,
except that the tank is located in Zone 4, UBC does not allow you to resist
seismic load with any kind of friction, that's why all the anchor bolts need
to resist all the lateral load in shear, plus uplift of course, but that's
another subject. 
As far as shear is concerned, the very bottom ring of the tank wall should
have a unit shear stress tangent to the ring along the entire circumference
of the ring (picture a horizontal shear flow through a ring). My real
question is to figure out the maximum shear on any particular anchor bolt,
since the lateral force could be from any direction.  That's why I assumed 4
bolts for simplicity.  The unit shear stress in the bottom tank ring is not
uniform along the entire ring, it's at maximum where it's 90 degrees from
the the direction of the load, and minimum at where it's in line with the
direction of the lateral load.  Therefore the distribution of the lateral
load into each anchor is not uniform.  If it is assumed that there is no
shear stress at all for the anchor bolts located in line with the direction
of the lateral load, then the combined tensile and shear stress is not an
issue, since that's where the maximum tension would be.  However if that
assumption is incorrect, that might become a problem because of the conbined
stress ratio, since the bolt would be designed for tension only based on
that assumption.

If there are 50 anchor bolts for the tank, and you divide the lateral force
by 50 to get the shear force on each anchor, I think that would be
insufficient because some anchor bolts will have more than 1/50 of the
lateral force and some will have less than that, the maximum shear on each
anchor must be more than 1/50 of the total.

Yi Yang


-----Original Message-----
From: bob.ross(--nospam--at)wgint.com [mailto:bob.ross(--nospam--at)wgint.com]
Sent: Wednesday, October 17, 2001 6:43 PM
To: seaint(--nospam--at)seaint.org
Subject: Re: Flat Bottom Tank Anchorage


Is your design an API or AWWA tank? 
   Is it a tank supplying a supply during an emergency?
   In what Seismic Zone is the tank? 
   Where is the tank location and distance in relation to the nearest 
known active fault(s)?
   When anchor shear bolts are provided, (which do damage during an 
event), each should be assigned an equal share in resisting the seismic 
lateral and vertical loads. 

Pax e Gratia
Bob Ross
Robert P. Ross, P.E.
Principal Project Manager
Washington Group International,Inc.
Industrial Processes
17320 Red Hill, Suite 300
Irvine, Ca. 92614 
Mobile 562-254-4604
Office 949-222-3978
FAX 949-222-3985
E-mail: Bob.Ross(--nospam--at)WGINT.com

----- Original Message -----
From: YI <YI(--nospam--at)summit-sr.com>
Date: Wednesday, October 17, 2001 6:05 pm
Subject: Flat Bottom Tank Anchorage

> I have question regarding the design the anchor bolts for a 
> circular tank
> with bottom fully supported on concrete pad.
> When a lateral load is applied to the tank, assume no fiction 
> between the
> tank and the pad, the lateral load is resist by the shear capacity 
> of the
> anchor bolts.  The maximum shear force per anchor bolt is occured 
> on the
> bolt located 90 degres from the direction of applied load, and 
> minium  shear
> on anchor bolts occure on the bolts that are in line with the applied
> lateral load.  Consider the situation where there are only 4 
> anchor bolts on
> the ring of the tank base, will there be any shear force at all on the
> anchor bolts located in line with the direction of the applied lateral
> force?  Are all the lateral force resisted by the 2 anchor bolts 
> located 90
> degrees each side of the load direction?
> 
> Thanks in advance.
> 
> Yi Yang
> 
> 
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