Materials and foundations for a large gate project?

[WillEndure]

This is the gate to my yard. As you can see the posts are collapsing. Whoever built this gate did not use galvanized posts and they are rusted out at ground level :-(

 

The height of the gate is 2.4 meters.

The larger RHS leaf is 3.0 meters wide. The smaller LHS leaf is 2.2 meters wide.

 

Just considering the larger RHS leaf, I estimate its weight to be under 300kg. Maybe 250kg, but lets say 300kg to be on the safe side.

 

I am thinking of putting in new posts, in galvanized steel box section.

 

What size of steel box section do you think is required to support the 300kg leaf, such that the deflection of the post is small (under 5mm)?

What size of concrete foundation will be needed in terms of depth and area? I will build a rebar cage to strengthen the concrete.

How deep should the posts be embedded into the concrete foundation?

 

Any other thoughts or tips? Thinking of doing most of the work myself, but I might get someone to do the foundations and set the posts.

 

Thanks for your help!

[Big Jimbo]

The first thing I would say is that if you intend using the same gates, you are going to be replacing the existing posts with something the same size ? It does not look like you have the width for meatier posts, if you want to use the same gates. If you are not taking high things through the gate, you might be better off having the posts extended to above the gates. Fitting a crossmember to keep the two posts apart, and stop them bending in towards each other. (Think goal post)

[WillEndure]

Considered that, but sometimes I do take things through that are taller than the gate so would be better without a cross member.

 

Current posts are round, going to replace with square as it will make it easier to bolt on the hinge mechanism. Going to completely rebuild the gates. Might also even them up so RHS and LHS are more equal in size, as that will help with the weight.

 

Any thoughts on my actual questions? Size of box section, size of foundations?

[Mr Punter]

I have seen this sort of setup with a welded steel u shape, like a goal post but inverted so the crossbar is in the ground.

[WillEndure]

Crossbar in the ground is defnitely a good idea. Unfortunately there are services coming through the gate - telephone, and maybe some or all of water, drainage, electrics, town gas! I know the BT telephone lines are for sure as there is a sealed trench for it comes through my yard. The others I will need to investigate.

 

That also complicates the foundations, as you can just see in the picture the right post is close to a manhole cover. I am guessing its most likely to be the water connection to the building under there. But its a bit close to the right post to dig out a decent sized foundation.

[WillEndure]

I put the gate measurements into ChatGPT - o1 reasoning mode. Asked it to calculate what strength of posts needed, and size of foundation. It told me:

 

* 150x150x10mm mild steel galvanized box section would deflect about 5mm under that gate weight.

* Concrete foundation with rebar cage should be 600x600mm to 800x800mm and only needs to be 1m deep for a post height of 2.4m.
* Bottom of post should stop 0.2m above the bottom of the foundation to allow complete encasement in concrete.

 

This gives a post depth of 0.8m embedded into the concrete.

 

I am wondering if that is right? The rule of thumb I have been given is bury a post to 1/2 the height of the post that is above the ground. So for a 2.4m post, I should embedd 1.2m into the concrete? Requiring a foundation depth of 1.4m.

I am pretty sure ChatGPT was right about the steel bending calculation. No idea if its recomendations on foundation size are right.
 

[ToughButterCup]

6 minutes ago, WillEndure said:

I put the gate measurements into ChatGPT - o1 reasoning mode. Asked it to calculate what strength of posts needed, and size of foundation. It told me:

....

This gives a post depth of 0.8m embedded into the concrete.

...

I am wondering if that is right?

...

No idea if its recomendations on foundation size are right.
 

 

Ask a few AIs.

I am finding that DeepSeek gives a more detailed answer to a question that I also asked of ChatGPT. They both gave the same answers, one answer was more detailed than the other. DeepSeek gave a more reasoned  answer.

Have a quick check to see if there's a public AI for Structural Engineering......

[Mr Punter]

23 minutes ago, WillEndure said:

No idea if its recomendations on foundation size are right

It will be down to ground conditions.  You may need to mess around with a bit of hand digging and some shuttering around the services.  I would go 800 x 800.  It is no fun digging out a post that has failed.

[WillEndure]

21 minutes ago, Mr Punter said:

It is no fun digging out a post that has failed.

 

That is exactly what I will be doing for the current posts! But yes, even less fun to have to do it all a second time.

 

If I have to remove concrete from around the current set of posts, what is the best tool to do that with? Will a hand-held pneumatic drill do it? Or you would need something beefier like the kind of drill fitting on the back of a JCB?

This could be the part of the job that I pay someone else to do! 😃

[WillEndure]

BTW, 800 x 800 sounds good size wise for foundations. Do you think posts need to be 1.2 meters deep for a gate height of 2.4 meters? Or will shallower be sufficient?

[Mr Punter]

For timber fence posts my mate digs a 30" deep hole, so 750 deep.  For your gates, the extra 450mm sounds good.  Make sure the ground is nice and stable.  If it is crappy made ground you could go deeper.  You will already have a hole where the failed posts were, so you know that you will need to make the new ones bigger and better.  Can you fix to the building either side, or is that a no-no?  If you are allowed, get some angle drilled and welded on before you get this galved.  Resin fix to the buildings.

[WillEndure]

I think I won't need to fix it to the building - the current one has ties into the brickwork but they pulled out when the post collapsed, so were not adding much strength. The real problem with the current gate is that the posts rusted out. It was actually strong enough to hold the gate.

 

Was going to buy the box section already galved and cut to the correct length. Then I can bolt on the bits of angle for the hinges. The main thing is that its galved near and below ground level, where it is most likely to be damp.

[Cliffpope]

Why do you need posts? Why not just bolt hinges to the concrete walls?

[WillEndure]

Wall on the right is brick. Wall on the left is my neighbours house which is covered in external insulation a.k.a cottage cheese.

 

I'm not sure the brick will take the weight of the gate pulling sideways on it - I think more likely to pull the fixings or the bricks out.