Water Storage Tanks

[Zulak]

I'm checking anchor bolts on a flat-bottom welded steel water storage tank that has a 42' diameter, and is 106' tall. It has (8)-1.375" anchor bolts, some slightly corroded.

With the tank built in the 1950's, I'm assuming that it was only designed for wind (not seismic). The tank appears in good condition (needs painting) and has performed well over the past half century, so I don't feel that a seismic analysis is required, or warranted. Is this an accurate assumption? Is there some portion of the AWWA Standards that requires a seismic upgrade for older water tanks?

Also, anyone have a guess as to the probable wall thickness for such a tank?

Thanks in advance for any insights.

 

[JStephen]

I'm not aware of any requirement to upgrade tanks to current seismic designs, although many owners in high seismic zones have done so. If so, it would be a regional requirement. Adequate performance when there haven't been any earthquakes is certainly no guarantee that it will perform well if there is an earthquake. If checking for seismic loads per current standards, it would help tremendously to have a set of drawings show foundation details.

Shell thickness is usually 2.6HD/12750, where H is depth to the bottom of the shell course, D is diameter, both in feet. This may be rounded up to fractional thicknesses, or may have had a corrosion allowance added to it as well. Shell plate would be thickest at the bottom and thinner farther up.

It is common, but not universal, practice to use the same thickness for repads as used for the shell plate, so you can get an idea there as well.