Future plans here call for a free-standing screen porch, raised about 8-10' off of the ground. I guess you would call it a gazebo? Anyhow, the question is what kind of footings to build it on. It will be 16 X 16, with 8 6X6 posts. I know I need adequate diagonal bracing. My concern is about uplift in high winds. If the floor is 8' off of the ground, the top of the roof will be over 20' high. I'm concerned about the whole thing toppling in a wind storm. We're in SC, so I don't need to go deep with the footings for frost, but how deep should I go to counter uplift? I figured on some kind of reverse flare, either hand dug, or using Bigfoot forms. Am I being paranoid after living in hurricane country for 30 years? Our local wind design load is 90 MPH.
Depending on where you are along that line of hills, there can be some healthy funneling gusts. If you are well fastened to a spread footing, uplift or overturning has to pull up an inverted cone of soil. I've been to a link that had fire tower plans of many heights and sizes.
heres the plans site
http://www.fs.fed.us/database/acad/lt/
A similar height, is is this design but it includes 4 off guy lines to secure it, as well as the footings
28ft high design, 4 footings 3'6" deep x 3' square
http://www.fs.fed.us/database/acad/lt/LT_Standard_Lookout_Tower_28ft_Type_CT_2.pdf
Thanks for the link. I can't get any of their pdf's to load, even after leaving it for 1/2 an hour ???
Wind load shouldn't be much of an issue around here, I hope. Design requirement is for 90 MPH. In front of the porch is an ICF house. Everywhere else are 75-100' trees in all directions. Even on the roof of my house, it never gets very windy; most of it roars overhead in the trees. Once the trees start coming down, all bets are off anyway. Our soil is sandy clay, and it gets a firm grip on anything embedded in it. I had to use a 6' pry bar to pull a 4X4 that had been in the ground 2' deep for a couple years. I would think any kind of reverse taper would be very hard to pull up. Since I will have 8 footings, I would think 3' deep with a reverse taper should do the trick. As much as I don't want to, I guess I could call the building department and see if they have any suggestions or requirements.
Wow, that's a goldmine of engineering information. Thanks UK4x4.
I was able to view most of the documents. The server or network that hosts those tower plans seems a bit dodgy. Responding in fits and starts. Maybe try again later.
I was able to load the large PDF files but they are hard to read and should probably be printed on a large format plotter/printer.
Here is a section on the deadman anchor for the guy wire cable
(https://i1009.photobucket.com/albums/af219/countryplans/guy-cable.jpg?t=1400074262)
Your thought about a visit to the permit desk is probably the most direct.