At 07:02 PM 1/10/2006, Bill Coleman wrote:
>On Jan 9, 2006, at 5:32 PM, Thin Air Communications wrote:
> > I am considering using some thick wall (rigid) EMT for supports for
> > an 800'
> > wire loop skywire. I currently am using ornamental steel that
> > extends to
> > 50'. The steel has not yet collapsed but the masts have become
> > bowed due to
> > high winds and I don't have the real estate to guy it properly. So
> > I am
> > looking at thick wall EMT also known as rigid EMT.
>While EMT may LOOK sturdy, it is NOT a structural material. That
>said, I have used a 10 foot piece of 1.5" rigid EMT with an
>additional 12" steel nipple to support a Cushcraft R7000 at 8 feet.
>(eg 3 feet embedded in concrete)
>Despite it's thickness, EMT is a pretty wimpy steel.
The real problem is not the material, per se, but that it's made with a
fairly crummy weld along the length of it, and that's where it usually
fails. I built a 2 m diameter geodesic ball using EMT as the "structural"
material, and in the course of smashing over 100 ends to drill bolt holes,
I found out all about the structural properties.
> Certainly not
>nearly as strong as chrome-moly steel. However, I could see using
>even a stout chrome-moly tubing (not PIPE!) wouldn't go more than
>20-30 feet without requiring guying.
Whether it requires guying is more a matter of what deflection you're
willing to accept with wind/gravity loads. I've seen fairly tall masts
built of aluminum tubing. They flex (a lot) but don't fail (although with
aluminum, fatigue failure is an issue).
There's also lots of tall flagpoles around that are self supporting, and
are basically thin wall (but large diameter) tubing.
The real problem with EMT is the extreme variability. Every piece is
different, and some are outright crummy. However, it IS really, really
cheap. You could do a strategy of putting it up and then proof testing it
(hook a suitable rope/cable to the top and load it with say, 2 or 3 times
the load it will ever see in real life.) On the other hand, proof testing
might take more time and money than just buying a purpose designed mast.
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