COEngineer,
My post is possibly too late but I thought I would give some comments from a Colorado residential engineer in lieu of the comments from design expierience of dams and large retainging walls as posted earlier.
For all of my builders I use counterforts at a spacing of no more than 18ft (closer as the basement wall gets taller). The counterfort is typically 3ft (sometimes 4ft). I use them for ftg or piers. Becareful with piers to prevent uplift. You may need to extend the piers another 3 to 4 ft.
This is typical of most residential builders in the Denver area.
I'm curious if the PCA Rectangular Concrete Tanks gave you similar results?
jhoulette, I usually use it to break a wall that is 20ft+ long, sometime they dont want to deal with counterfort so I just beef up the horizontal. So @18ft sounds pretty good. How do you do the calc? Excel? How do you determine the over turning moment?
I have used RamConcept and yes I have created a spreadsheet. Overturning is based simply on the soil pressure along the given trib area for the counterfort. I have found that the overturning is usually the controlling factor in my spacing of counterforts/buttresses.
