Fencing advice.

[paulmitchell1984]

Hi guys and gals,
Iv been asked to make some steel fence posts for a friend of a friend..easy peasy I hear you say, well yeah there not too bad, but I'm unsure what size steel to go for.

Iv attached an image/sketch (hopefully),

Basically they are wanting to put a 3ft wooden fence on top of a stone wall again 3ft tall which has a triangular top.

My plan is to have 2ft buried in the ground and cemented in and 1 fixture on the back of the wall, this will then follow the slope up and then go vertical from there with brackets for them to fit their fence to.

Now I was thinking 50x50x5 box but my god the price has gone up so wondering what do people think I could "get away" with. Apparently it does get windy where it is so I'm thinking it's going to act like a big sail.

The span of the wall is approximately 23ft, so was thinking 1 post every 8ft or should I go for more posts?

Sorry for the long post just need some advice from somebody who might have had a similar situation/setup.

 

[gaz1]

walk away ive had that one before

the only way you will get away with that build is to build it like a double pole fence post as our windy area will just bend them

they wont want that as it will intrude into there garden and they dont want that to happen

you will then get the blame for not doing it right

even if you use yorkshire blow through fencing i dont think there wanting it that way but there intention is to use pinelap board fencing

alternative is to use a 4x4 and add a piece to the top of the wall and the wind will still affect it

 

[Agroshield]

The way you have drawn it, 90% or more of the wind force on the fence will be seen as a bending force at the point the post meets the wall restraint. So the post at that point has to resist that force. The joint between post and wall tie has be strong enough to transfer that force into the tie. The tie will see both tension and compression depending on which way the wind blows. Moving the tie upwards into the sloping part of the wall will reduce the lever arm, the bending moment and the axial force.

Do some basic calculations of pressure on the fence using this:

Calculation of wind load on free standing walls & parapets - Eurocode 1

Calculation of wind pressure load on free-standing walls and parapets (net pressure coefficients) for zones A, B, C, D according to Eurocode 1 (EN1991-1-4)

eurocodeapplied.com

Then convert that pressure from kN/m2 to kN/m run of fence. Then convert to kN per post at your desired spacing. Make a mock up and push the post* with that many kN and see what happens.

* in both directions, with varying frequencies.

Edit: the only part of the calculator that is puzzling is the basic wind velocity, but if you put 'basic wind velocity map uk' into Google, you will get a map that tells you the appropriate value for the fence's location.

 

[Dev]

What a great website.

 

[gaz1]

i will give you a truss method

1.2mm thick truss

take note though it would need to be both ways for the push on both sides of a fence so 3mm angle iron can be used

but like ive said owners dont want to lose there garden they want the job done right for as little cost as posible