Value for money and building a strong fence

[functor]

Hello,

On one side of my house I have a concrete-like fence which seems really solid so far. On the other side is a cheap wooden fence which recently with the strong winds broke in a few places. I am looking to replace it with a more solid fence that wont break with a bit of wind.

I have heard you can have a wooden fence with cement foundations, is this good enough or should I go for a concrete one like the other side? What would be the cost differences?

The fence is aprox 18-20 meters longs.

Untitled

• functor
• 18 May 2013
• 1

Untitled

• functor
• 18 May 2013
• 1

 

[r896neo]

A well constructed timber fence will be perfectly strong. The main reason concrete posts are used is for longevity.

The timber one you have looks like very flimsy budget panels and 3'' posts which are too small.

The concrete fence you have actually looks like concrete slotted posts with concrete gravel boards slotted between. This is not a particularly common way to make a fence but certainly a very strong one that will last a long time. Its effectively a thin wall rather than a fence.

Posts whether concrete or timber (If timber use 4'') need to be concreted at least 18'' into the ground. Concrete posts will be about 2 or maybe even 3 times the cost but will obviously last longer.

The face of the fence can be finished in either close boards pallisade boards solid panels etc etc, thats mainly a personal choice.

The cheapest and easiest option will be boarding rather than panels as less precision is required when positioning posts.

 

[Porque223]

If you have an idea of the wind speeds that broke the fence you can figure out the minimum strength for the new fence. Try Googling

"wind force" PSI

for the formula.
As I recall the force on the fence increases with the square of the wind speed.

 

[functor]

So basically, for the highest speed recorded in my area in the last year (15m/s)

I do:
0.5 * C * D * V^2
0.5 * 1.05 * 1.25 * 15^2 = 147.65 N/m^2

How do I calculate what kind of posts I need with this?

 

[r896neo]

Don't make this more complicated than it needs to be. A fence with 4'' posts at adequate depth will be more than strong enough.

Your fence broke probably because it was terrible quality or poorly installed not because it was under engineered.

I put one up right on the shore line with nothing in front of it to shield it from the wind and nothing has broken it down in the 3 years its been there.

 

[functor]

Anyone know how much it would cost to get someone + the materials to install a regular 2mx20m timber fence with 4'' posts and dug 18'' under (as you pointed out )?

 

[Porque223]

So basically, for the highest speed recorded in my area in the last year (15m/s)

I do:
0.5 * C * D * V^2
0.5 * 1.05 * 1.25 * 15^2 = 147.65 N/m^2

How do I calculate what kind of posts I need with this?

The strength of materials calculations can get messy so I'd look at existing fences and ask the makers what wind speed they are designed to withstand. They may not want to tell you but I have seen graphs of fence construction and design max wind speed.

For your ~30 MPH I don't think the fence needs to be so strong that it is ugly or uses steel uprights.