....re-read and though probably obvious, felt the need to clarify, what I said only takes into account forces from turbine thrust - thought that was what you were asking for some reason. Of course you will want to take into account guy wire tension, some tower weight, force from wind hitting tower, severe weather events. 80 foot, 5 inch pipe towers for that size turbine that I have seen usually use about a yard of concrete at each anchor. Hope that helps some.