Steel Water Tank Overturning Toe Pressure

[PEStructural]

What is the approach/equation to calculate the overturning toe pressure on a concrete ringwall in a seismic event? I have a 89'x32'H welded steel tank that is in seismic zone 4 and is resting on a concrete ringwall (not bolted down).

Any help is greatly appreciated.

Thanks,

Mike

 

[prex]

If you go to the site below, under Vessels -> API Std.650 -> E.5.1 you'll find a calculation form that should solve your problem.

prex

http://www.xcalcs.com

Online tools for structural design

 

[JStephen]

Normally, you'd calculate the bearing under the ringwall using P/A +/- MC/I. This is assuming that the ringwall itself doesn't uplift on the light side. The load P would include water directly over the ringwall. The ringwall can be widened inward to increase P if required.

For an unanchored tank with partial uplift, there is actually a higher calculated compressive load in the tank shell than what the P/A+MC/I would indicate. However, AWWA D100 has some wording to the effect that you calculate bearing under the ringwall as though the tank were anchored, and accept that there will be some local soil yielding in the event of a strong earthquake.

 

[PEStructural]

Thanks for the help! One question for JStephen, is the C/I (or section modulus) for the ringwall or for the entire foot-print of the outside diameter of the ringwall?

Thanks again.