In a message dated 9/10/01 9:52:16 AM Pacific Daylight Time,
blueis@sprintmail.com writes:
> My question is simple. I live in a "70 mph" wind zone according to what I
> can determine. Can I SAFELY use the 70 mph figure or should I build in
> additional capacity by purchasing a stronger tower?
I guess my question is, "Why wouldn't you use the value for your
application?".
How are you going to purchase a 'stronger' tower? All the towers are
basically the same in terms of construction and there is no other version for
higher windspeeds. What happens is that guy wire sizes are increased plus the
tower capacity is downrated for higher windspeeds. This is pretty obvious by
looking at the Rohn catalog. Crank-ups don't have guys but they are
definitely downrated as well.
Typically ultimate tower strength for guyed towers is limited by leg
capacity and for crank-ups it's the cable capacity.
The windspeed ratings that are used by the TIA-222 Tower Standard and the
UBC (Universal Building Code as well as other building standards) are
determined by standards of their own based on historical data and an accepted
formula and parameters.
> Using a 'lower spec'
> Trylon tower (eg, T400 vs. T600) means getting a taller tower at
> approximately the same price. (Remember: size does matter!) What would
> you recommend?
Self-supporting towers like the Trylon Titans are a slightly different
case. While the limiting factor is still leg strength, as you take off a top
section - the tower capacity increases because now you've got a bigger top
section; i.e. the T200 has a #2 top section, the T300 has a #3 top section,
T400 has a #4, etc.
You still need to consider your windspeed and your proposed load. A more
esoteric exercise would be to model each height with regard to desired paths
and see what the results look like.
> How reliable are the 'wind speed' tables? Do I stand a
> *reasonable* chance of long service and no tower failure using the 70 mph
> figure? Or does this all boil down to how lucky I feel?!?
Luck has NOTHING to do with it. All towers are designed and manufactured
with a significant amount of overhead built in but the manufacturer's
published specs are the ones to go with. While sometimes you may have gusts
that exceed your local windspeed rating, this is also taken into account in
the calculations.
If you follow the LXC Prime Directive of "DO what the manufacturer says",
you'll have a safe, reliable installation for years to come.
Yes, higher is better and so is stronger. I will never discourage anyone
from over-engineering something.
This all gets down to what I call the "Sleep Factor"; that is, how will
you sleep when that big storm rolls through and you're laying in bed
wondering if you did everything right? Part of it has to do with experience
but even a neophyte tower owner/installer will be fine by doing everything
right. If you'll be more comfortable knowing your tower is much stronger but
a little lower, then go that way.
Experienced professional engineers have designed all the structural stuff
to accepted standards. Personally I trust them.
Here's an option: get the shorter but stronger one and put it up. If you
get comfortable in the future and want to go higher, just get the next top
section and install it. Voila - higher tower.
>
> The more I read, the less likely it is that I can make a decision!
You just hafta read the 'right' stuff (and quit listening to your
buddies) and things get a little more obvious. If you have these kind of
trepidations, hire a PE and have him calc everything out for you. That should
increase your confidence level.
Cheers, Steve K7LXC
Tower Tech
Champion Radio Products
List Sponsored by AN Wireless: AN Wireless handles Rohn tower systems,
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