Allowable Fence Post Deflection

[kxa]

I am designing an 8 foot high fence for a roof top and was wondering if anyone knows what the maximum recommended deflection for the top of the steel post should be. Of course, I have checked the stresses and designed the posts but can't find anywhere recommending L/120 or L/80, etc. as the limit.

 

[dik]

The one purpose of checking deflections is that the deflection doesn't interfere with the rest of the structure. Another purpose is the confort of the people relying on it. If I lean on a fence and it noticeably moves, I may not feel comfortable. As lomg as it's safe, the L/70 should be adequate. I'd likely opt for the L/120, just for comfort.

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

-Dik

 

[kxa]

Thanks for the reply. these are privacy fences and 6' to 8' high. I get about 1" deflection at the top which won't be interfering with anything. I was interested to see if anyone has come across any documents such as codes that I can reference in my calc's. You mentioned L/70. Was that from any document that can be noted in design calc's? Thanks

 

[dvd]

If you get 1" deflection at the post, what is the deflection at top, mid-span on the fence panel?

What is the post anchored to, and how likely is the long moment arm of the post to pull out fasteners, and/or stress the post mount?

I would picture some sort of knee brace on a free-standing post that would seriously limit deflection.

A sketch of your concepts might make for better comments.

 

[kxa]

The posts are 3"x3" tube steel and are welded to the base plates which anchored to the concrete slab. The 1" deflection is at the top due to 100 mph wind. I am not concerned with the attachments or anchorage. I just wanted to see if anyone has seen recommended limits for free standing cantilever posts.

 

[Tomfh]

Previous discussion here:

https://www.eng-tips.com/viewthread.cfm?qid=378515

Some more discussion:

https://www.lerchbates.com/news/deflections-in-deck-and-balcony-railings/

We often use H/75 for fences under design wind. That's a fairly stiff fence.

For context in Australia the industry standard for residential fence posts is 50x50x1.6SHS, for fences up to 1800 high. These will deflect 100mm+ under a service wind, i.e. H/18.

In summary, no-one will ever complain if your 8 foot fence moves 1 inch in a 100mph wind.

 

[gte447f]

wind deflection for a fence?... I would argue, who cares. As long as your deflection is elastic and there is no yielding (permanent damage), it's a non-issue in my book. If you want a calc to file away, check it for h/70 or h/75 as others have said and move on. As far as building codes go, they barely even address wind deflection of buildings, so I wouldn't expect to find any limits for a fence.

 

[LittleInch]

"In summary, no-one will ever complain if your 8 foot fence moves 1 inch in a 100mph wind."

And no one will be there to measure it either.....



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

 

[Enhineyero]

Sorry to hijack this post but cant help asking if its a flag pole, billboard, or a street sign, will there be particular deflection limits? Or don't bother-checking-deflection-as-no-one-would-care a reasonable arguement?

 

[EdStainless]

The only issue with a fence, especially on the top of a building is that when some big dude leans back against it with his beer in hand and feels the fence move 1" how sure will he and his friends be of the fence?

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P.E. Metallurgy, consulting work welcomed

 

[MotorCity]

EdStainless - the element you describe is a guardrail, not a fence. Guardrails have code prescribed loads that are intended to resist dudes leaning on it.

 

[gte447f]

Lateral wind deflection of flagpoles and street signs? Yes, who cares. I can't imagine that serviceability limit states would apply to flagpoles and street signs. Why would they, since they can't (or shouldn't) be occupied and nothing else is attached to them that can be damaged? Billboards, I know nothing about, but I think there may be some industry specific design guides or something that are utilized. I have no idea if they address lateral wind deflection.