I have recently installed a new basketball goal for my kids. I now need to install a net behind it as the balls tend to end up in the neighbor's yard over and over again and it drives their dogs crazy.
I need to go up 17' with the poles (top of the basketball goal backboard with rim at 10' is 13' so I want to go up 4' over that).
I purchased two 21' schedule 40 galvanized steel pipes (3") along with #4 rebar and some 5/8" stainless steel all thread that I plan on putting through the base of the poles in a criss cross fashion to prevent the poles from turning in the concrete.
My question is how deep do I need to bury the poles? There will be no "load" on the poles as the net weighs less than 40lbs total. I have read everything from two feet to 8 feet.. My plan was to dig 4' down and 16" in diameter flared at the bottom of the hole and then plumb the poles and fill with concrete. About have way through the fill I was going to throw in 4 – 3' #4 rebar vertically then continue to fill.
My other question was the use of a solotube as opposed to just a hole in the ground with a flare at the bottom. Should I use the solotube sans the flared bottom or just dig the hole and flare it?
We live in central Phoenix so frost/freezing is almost never a problem.
Thanks
Best Answer
overkill if you ask me. fundamentally this is just to hold up a net, correct? the only load on the net is going to be wind and the odd basketball.
what you need to worry about are the following:
1) overturning moment of the net/post assembly. this is how much force it will take you to pull or push the net/post assembly over or enough that it moves. just ball parking here, but a 2lb basketball lobbed over the backboard and missing may hit the net with a variety of forces (angle, speed of ball impact, tautness of the net, etc), but lets say for the sake of argument its equivalent to 20lbs of force. that means the torque on the base is going to be 14' x 20lbs = 280 ftlbs of force. this is also assuming the wind load on the net will be pretty negligible (large openings, thin web stranding)
2) assuming a safety factor of 4, lets work with 1200 ft lbs of force as the torque to be resisted. if you anchor a post in the ground, regardless of how you do it, the thing resisting the torque is the soil itself (and the mass of the base/post/net assembly). even in the loosest sand, 1200 ftlbs of force isnt very much. you could probably just drive the post into the ground 4 ft and it would be sufficient to resist the load described above.
3) if you do insist on using concrete (certainly easier than driving a 2" post 4ft into the ground with a post driver), you want the concrete to be installed so it is in direct contact with undisturbed ground. the undisturbed part is the key. as soon as you disturb the soil, compaction and liquefaction issues arise that make the post more susceptible to movement under torque or side loading conditions. if you put in a 16" post, it will help to resist the overturning moment better (large surface area on the post to push against and thus be pushed back on by the ground). do not use a sonotube. sonotubes are just there to make a post smooth and slippery so frost heaving minimizes vertical displacement of posts.
4) depth is governed by frost penetration. always put your foundation (whatever it is) at least 18" below max frost penetration. 4 ft is pretty standard everywhere in canada. just beware of going deeper than that as there are many services buried below ground. call before you dig.
5) you dont need rebar or threaded rod. once the concrete sets up around the pipe, it will never rotate in a million years under your scenario. the pipe itself is your rebar. if you are worried about, just get macrofibre reinforcement added to your mix when you pour it. you dont need it though.
6) flared sonotube bases are just to increase overall vertical load of a concrete post in soft soils. totally not applicable to what you are doing and will actually make your situation worse due to the loose compaction of the backfill over the base when you are done.
you may want to dial it back a few thousand notches and just use fiberglass pole whips and guy lines to hold up your net. much faster, more flexible installation, and far less permanent. will probably do a better job too.
hope that helps.
fyi - you never specified what size of pipe you are using. schedule only refers to the wall thickness and burst strength of a pipe. use 2". its readily available, and about the biggest you want or need to be manageable for the typical home diy type.