Wind Load on Chain Link Fence With Privacy Screen? 4

[psychedomination]

Hi there,

I'm working on a project, where I need to reinforce a chainlink perimeter fence. The fence recently failed in high winds (not surprising as the posts are undersized and they were not attached to a footing).

I'm trying to determine how best to analyse this. The fence has a privacy screen that is the fake grass type of stuff. This is necessary from a Building Control perspective.

So this leads to my main question :

1. Due to the privacy screen, do I consider the fence as a solid panel? (doing so is leading to some very large overturning moments, which is causing the foundation size to be kind of ridiculous). If not, what guidance can I follow to conservatively estimate what the wind load should be?

Typically for a chainlink fence, I've seen the footings as mass concrete circular pours (~1' diameter up to 4' deep and the post just cast into the pour).

However, the load on the fence from the wind is approximately 13 kN, which leads to an overturning moment of around 20kNm (depending on footing depth).

The current fence posts are spaced at 20'. I plan on putting an intermediate post in, cutting the tributary width in half, however, this still leads to the 13kN load and 20kNm overturning moment.

Is there another practical way that I can reduce the overturning moment/force, other than adding another mid-post?

See fence condition below :

 

[3DDave]

That situation begs for the fence posts to be welded to helical vehicle suspension springs. Let the fence lie down and then come back when the wind dies down. Add some high-friction slip joints on the angled struts to keep the fence from slapping vehicles parked nearby.

 

[bugbus]

Specify a certain number of flaps cut into the screen?

Then again, this would only work in one direction in your case.

 

[ALK2415]

Not only the vertical members resist the bending moment (due to wind pressure) - the cross bars and tension cables (Top & Bot) will work as unit in redistribution of applied forces
see attached drawings [CAD & PDF] from

http://www.wbdg.org

[open chain-links fence]
Link
all your sections should be water-proof galvanized type [Not the cheap epoxy coated type]
Another Remark; that wind don't act in large wave front, meaning "their intensity" act in small front of the fence line perimeter and not the whole line of fence.
Also try to increase the pole-footing depth to 6', since top soil lateral Bearing Capacity is questionable..

 

[ALK2415]

@ 3DDave
Dave you have some pretty mind imagination [but not applied due to privacy condition ]

 

[XR250]

Why does your demand stay the same when you cut the trib. in half?

 

[LittleInch]

The posts in the photo are a lot less than 20 feet apart. Looks more like 6 to 8 feet max.

That privacy grass is adding a huge amount of both weight and wind load.

Can't you use something which has a much lower wind resistance? I guess it depends on exactly how much privacy you need / want.

You could build the fence out of bricks....

Attaching that covering to that fence is the height of ridiculousness without someone realising that the force will increase by probably a factor of 10.

A solid wall is probably what you have in terms of analysis.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[ALK2415]

@ LittleInch
here we say someone problems, is other-one benefits
therefore, should carefully model my structure, when it come to wind/seismic loadings
even with simple cases as chain-links fence

 

[cliff234]

My suggestion is to consider the fence solid. (It already looks like it is close to being solid.) And since it's a privacy screen, even if it's partially solid today, tomorrow it might be 100% solid.

 

[psychedomination]

@3DDave thanks but may be a bit tricky; there's a main road on the other side of the fence.

@gussmr thanks that may work, although I think I will first try to find a privacy screen that allows wind to pass through (if something like that exists).

@Adn26 thanks, I'll take a look at the documents you attached. I agree about the galvanizing. The current fence posts all failed as a result of the posts corroding and shearing off at the base.

@XR250 the demand doesn't stay the same. Those loads are a result of the reduced tributary width. The original loads would be double.

@LittleInch you are correct. I posted this question late at night; that's a typo. The existing fence post spacing is 10'. I plan to include a mid-post, which would reduce the tributary width to 5'. I'm going to do a bit of research for privacy screens that have lower wind resistance.

 

[dik]

Is this of any use?

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/raw/upload/v1657633458/tips/Chainlink_Fence_g1qxvq.sm[/url]

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1657633458/tips/Chainlink_Fence_yog5xo.pdf[/url]

and...

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1657633089/tips/CLFMI-Product-Manual-revised-March-2017-1_xrw7ku.pdf[/url]

and, another...

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1657652779/tips/WLG-Updated-1.1.18_v2ygdd.pdf[/url]

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[EZBuilding]

Have you seen the Embedded pole approach in IBC Chapter 18? This is how I typically address similar foundation situations.

To your question of what can be done to reduce the moment - possibly introducing diagonal kickers down to a separate foundation element. This would reduce your demands on the foundation at the chain link post - but will likely be a costlier option than a deeper foundation element.

 

[3DDave]

If the posts are rotted off at the ground, then no amount of in-ground resistance will help. Is this a location where it snows and salt are used? Very popular to scrape that mixture and pile it against the fence.

In any case, if it was overload due to wind and not otherwise, then it would fall over and remain in the street. Since that's not the case then more extreme measures aren't required to avoid high-wind damage.

 

[MotorCity]

The prudent thing to do is consider it 100% solid. I do not see daylight coming through any point in this fence. I don't think you could determine the actual porosity of this material with any reasonable degree of accuracy unless it is provided by the manufacturer.
You don't want your fix to fail and have the finger pointed at you because you assumed this material was 78.93% solid for example. I would not expect the typical 1'x4' fence post footings to work for a fence like this. Follow the numbers, big solid fences = big solid footings