[TowerTalk] Windloading

[Kurt Andress]

Merry Christmas!
Thanks to all for making my subscription to the reflector enjoyable and
educational.

John Duncan wrote:

>Regarding stacked arrays when figuring windloading do you simply add the
windload ratings >of each antenna together to determine what size tower
>one would need or is there some other magic formula?

There are lots of formulae that allow for the analysis of towers. No simple
way to figure out how much more area one can put on the tower when
distributing them over several placements compard to a single lumped
placement at the tower top.

Without doing a complete analysis of the tower, the safest thing to do is
stick to the rated area regardless of placement. This is a very conservative
approach as tower will have lower stresses etc with the distributed loads.
But, it is better than just guessing and getting it wrong.

More below


alsopb wrote:

> This brings up an interesting questions.
>
> Say your tower is rated for a 15 sq foot antenna.  What if I put this
> antenna not at the top of a tower with 3 sets of guys but at the
> location of the second (or first) set of guys.  Would there be extra
> capacity in the tower left to then put something else on top?

Yes.

> In otherwords, does 15 sq foot rating assumes the antenna is at the top?

For the Rohn configurations, Yes!

> Distributing the loading differently could get you extra capability?

In most cases, Yes.

> What failure mode is limiting and how does distributing the load
> vertically impact it?

Ok, will try to address this in as compact a manner as possible.

The limiting condition of any tower can vary. There are so many variables in
geometry and materials that it is impossible to make statements that would
be accurate for all configurations.

Here are the prime areas of concern that I have seen with my guyed tower
models.

1) Guy line loads.
These are caused by the windload on the tower itself and the point loads
from the antennas, mast, coax, and wind on the guys

2) Combined stresses in tower at the base.
This is caused by the vertical compression in the tower caused by the guys
reacting the horizontal loads, tower bending caused by guy cable stretch,
tower torque, and horizontal shear due to wind load on the bottom span. The
primary contributors are compression and bending.

3) Buckling between the guy points.
The problem is not pure Euler (long column, pure compression) buckling but
what's called beam/column buckling where there is compression and bending.
The additional bending loads are caused by the windload on the tower between
the guys and guy cable stretch.
In this case the tower gets out of column between the guys and when that
happens the bending stresses go through the roof pretty quickly and the
thing falls down on itself.
The actual failure mode is often buckling of the tower legs after the entire
section got too far out of column.
This buckling problem usually occurs when the distance between sets of guys
gets too large.


Ok, now let's look at what happens if we alter the antenna placements.

For a tower with all the antenna load at the top, The top set of guys will
have the highest load.
This is often why we see towers spec'd with larger cable for the top set.
The compression in the tower will be also the highest. This is because the
top guy set has the worst angle to react the antenna load. It has to pull
harder than any other guy set to equal the horizontal wind load. Because it
has the highest angle to the ground it will result in the highest
compression in the tower.

If we move the entire antenna load to a lower set of guys the guy load and
tower compression will be lower. This is entirely due to the improvement in
the angle for the lower set.
This may require using a larger cable for that set.

There are lots of other possibilities, like we move 1/2 or 1/3 of the area
to other locations etc.

The only way I trust to figure out how much antenna can go where is to model
and analyse the situation.
Otherwise stick to the published information or just guess! Which, BTW has
been successful for many. Not my recommendation.

Oh yeah, there is a better way to guess if you are gonna do it. Find someone
else who either by design or guess, has a configuration like what you want
to do. If his tower has survived the winds you expect you probably have
enough empirical evidence to do the same. Just be careful to not wander very
far from the "proven" configuration and hope you didn't miss on your wind
estimates.

73, And Happy Holidays, Kurt


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YagiStress - The Ultimate Software for Yagi Mechanical Design
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