Gate posts depth

[Chris]

I am going to be building a couple gates for the entrance to my property. They will be 6-8 feet tall and 8' wide each. I am putting in steel posts to attach them to. Is three feet underground good enough? They will be set in concrete and also concrete filled. Steel will be either 3/16" or 1/4" wall thickness.

 

[oldognewtrick]

What diameter post?

 

[Chris]

Guess I forgot that part. 4" square tube

 

[oldognewtrick]

http://www.quikrete.com/AtHome/Video-Setting-Posts.asp

 

[nealtw]

Will the swing end of the gate have a wheel to carry the weight?

 

[bud16415]

Go 4 or 5 foot deep. You have the equipment to dig with and why take a chance 2’ of steel is cheap compared to a cocked gate.

 

[Chris]

Ok I can do 4'. My posts are ten footers and I already have them and don't want to spend another two hundred bucks on steel.

 

[Chris]

No wheel. Extra heavy duty readable hinges and .090 wall thickness on the gate material so it should only weigh a couple hundred pounds max

 

[nealtw]

Leverage is not always your friend. So you have the weight of the gate plus a few children hanging on it and then the lever action, the shape of the concrete and the soil type to be considered.
Other than that I have no idea on how to work it out. If the center of the gate is 4 ft out and 4 ft high, I think then you have 8ft of leverage so the weigth becomes something like your 200lbs times 8 so 1600 ft pounds of torque at the ground

I would install a slightly larger galvinized tube in the concrete to just above ground level and bolt your post to that.

 

[bud16415]

We had a 90 day wonder design a post for a satellite dish here in the plant. You know the dish like the one you nail up to the corner of your “Aluminuminium” / mobile home. He set a 6 inch pipe into a hole with a form 3x3x3 with 6 pieces of rebar going all 3 directions and attached to the pipe. Then poured in a yard of concrete. The pipe is about 6’ high. That sucker is designed.

Your poles don’t have to go to the top of the gate only to the top of the top hinge.

 

[slownsteady]

If your posts go high enough above the gate, you can use a guy wire to take some strain off the end of the gate.

 

[bud16415]

The guy wire will take strain off the hinges of the gate but won’t change the reaction moment in the ground or the bending moment at the ground. That’s a function of the weight of the gate and the distribution of the weight across the gate.

 

[slownsteady]

The guy wire will take strain off the hinges of the gate but won’t change the reaction moment in the ground or the bending moment at the ground. That’s a function of the weight of the gate and the distribution of the weight across the gate.

Not familiar with the term "reaction moment" or "bending moment". Engineering terms?

 

[nealtw]

Not familiar with the term "reaction moment" or "bending moment". Engineering terms?

Think about where lever contacts a fulcrum

 

[slownsteady]

moment refers to time, i would guess, so what would cause a bending moment? Is there an equation there?

 

[nealtw]

moments and minutes refer to degrees of a circle in this case or something like that, some smarty will show up and correct me.
http://www.learneasy.info/MDME/MEMmods/MEM30006A/Area_Moment/Area_Moment.html

 

[havasu]

OMG, my head is hurting! Stop and speak engrish for us mentally challenged saps!

 

[nealtw]

OMG, my head is hurting! Stop and speak engrish for us mentally challenged saps!

Most of this stuff is over my head too. I just try to put it into language that I can understand so I can nod my head when the geeks talk.

Anything supporting weight at an angle has a bend factor.

Most of the weight of will be carried with to top hinge if the posts is overloaded it will bend, but not the part above the hinge or below the ground. I think it will be under the most stress right where the bend starts at the ground if it is fixed in an unmoable object so that is where it would fail and that is what I think of as the bending moment, as that is the place you would measure the degrees of bend before it failed. This the reson flag poles are biggest at the ground and get small as they go up.

All that aside my concern was more the weight of the concrete and the condition of the soil that would be rquired to make the base an unmovable object.

 

[havasu]

I've seen what you are talking about, but can't figure how it would be secured to the post. It seems with anything holding tension at that point, it would become real fragile and snap apart. I suppose a turn buckle would be used, but the gate has hinges which are utilized when swinging open. Do you hinge the fulcrum point as well?

 

[nealtw]

No you are going off in the wrong direction. The post can be over sized so it won't bend. But the first question was ( how deep) If the soil is good it would be a certain size of concrete with a certain weight. If the soil is loose sand , easly shoved I would think the concrete itself has to be heavy enough all by itself to counter all the loads that might be applied to it.