Fill the pipe with concrete - it won't bend and rust out if you crown
the top. Use some rope guys at about 40% of the height and the vertical
will last a lot longer.
On 1/15/2011 5:09 AM, jimlux wrote:
> On 1/14/11 6:27 PM, K2VI wrote:
>> Ok so tomorrow is the day i start this project. I have a 5' piece of
>> 1 1/2" fence post pipe. It will be almost 4' cemented in gound and
>> 14" above ground. I will have a zero five 43' vertical on the pipe.
>> All i hear is schedule 40 plumbing pipe however i have on hand this
>> fence pipe. I see no reason not to use it since the vertical has very
>> little wind load.
> A vertical has a surprisingly large wind load. 2" tube has a drag of
> about 1.5 lb/ft in a 60 mi/hr wind
> A quick calculation shows that at 40 mi/hr the wind load is 0.7 lb/ft
> for a total wind load on your 43 feet of 29.5 lbs, and a bending moment
> at the base of 640 ft lb. Go up to 70 mi/hr and you're up to up to 2.1
> lb/ft, 90 lb total load, and almost 2000 ft lb bending moment.
> Just as a comparison, 2" schedule 40 iron pipe (2.375"od, 0.154" wall)
> will fail in bending at around 2400 ft lb.
> your 1.5" fence post pipe is pretty small, and probably pretty thin.. My
> catalogs actually don't show 1 1/2" for post, just top rail (1 3/8"
> seems to be the standard size)
> I did find some other fence post online
> There's a 1 5/8" Allied SS15 post, 0.072 wall thickness, which they
> claim is 570 ft lb bending moment as well as SS20 and SS40, which have
> thicker walls and are stronger.
> In any case, your 43 foot antenna, even in fairly light winds, might
> bend a thin wall steel pipe. On the other hand, it's probably fairly
> easy to fix. If it bends, you yank it out and put a new pipe in.
> I attach verticals to standard wire fence "T" posts, and when the wind
> blows too hard, the post bends, I get the jack out, yank the post and
> drive another.
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