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Re: To Carl Sramek: Re: A new angle on "Sliding"

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<<I am always interested in anchors so when you said "but the anchors that were there were mostly outdated design and inadequate."  you got my attention.  Could you elaborate on this point a little. Thanks again, Kenneth S. Peoples, P. E.>>

Ken.

Sure.

The outdated anchors that I refer to are "strap" anchors that appeared to be commonly used back in the 70's and into the 80's.  I don't think AWWA D100 actually forbids these type today, but the more modern tanks observed typically had chair-type anchors with substantial anchor bolts.  Of course, my observations are for a specific region, and may not apply to areas that are not earthquake country.  

These strap anchors typically consisted of a plate about 4 inches wide and 1/4 inch thick.  They were embedded in a concrete ring footing about 2 to 3 feet deep and the bottom end of the plate was bent at 90 degrees to provide the anchorage.  The upper end of the plate extended from the top of the footing about 1 foot and was bent over about 3 inches to make contact with the tank and welded to the tank.

These straps exhibited really cruddy, non-ductile behavior.  They were broken, bent, buckled, pulled out of the concrete, and the concrete was broken at the straps (not all of these at once, of course).  What made it worse, was that once the tank edge lifted up and the straps pulled out or buckled, they might restrain the tank shell from coming back down on the foundation.  One case, the tank was lifted up 3/4 inches and the buckled straps would not let the shell come back down.  You could put your tape measure under the tank 6 to 8 inches in.

Most of the tanks we looked at dated back to the 70's and before and did not appear to be designed for overturning, since the anchors appeared to be "courtesy" anchors spaced at 6 to 8 feet.  That's what I meant by "inadequate."  Anchors consisting of chairs with anchor bolts also failed, but they failed in a more predictable manner, such as stretching of the bolts or failure of the bearing plate.    

So, the bad news is that the anchors were not adequate designed relative to current practice.  The good news is that the earthquake was only a moderate quake, less than .10g ground acceleration, and therefore the minimal anchors allowed us to observe the performance of the tanks, since if the tanks had been more substantially anchored, there would likely have been little or no damage observable.  Considering that uplift definitely occurred in the tanks, and that there was no loss of contents (that we know of), has to be a positive observation.  

The steel tanks that were observed are what are referred to as "standpipes" (no, I don't know why).  I guess their use is a regional thing, since I don't see these type around southern California.  These are basic flat bottomed steel tanks on ground that are relatively tall.  The ones we looked at varied from 100,000 gallons to almost 5 million gallons.  They varied from 24 feet to 144 feet tall with diameters from 15 feet to 90 feet, so there was quite a range.  Typical aspect ratios were about 2.5 to 3 to 1.  Although most of these tanks had high aspect ratios, I believe that the uplift behavior occurs in low profile tanks also.  One tank with damaged anchors was only 40 feet high with a 70 foot diameter.

BTW, these observations were made while I was working for FEMA under DMJM's contract to provide post-earthquake support following the Nisqually earthquake in Washington.  I had really only been superficially exposed to tanks previously, so this was quite enlightening to be reviewing tank performance instead of counting cracks in plaster.  I was partnered with Jan Niclas, an engineer from the Seattle region, and we collected data from over 40 tanks that had observed damage (standpipes, concrete tanks, elevated tanks, and buried tanks included).  Please feel free to e-mail me off-list if you'd like additional info.

Carl Sramek
Los Alamitos, CA



 

   

  

   

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