In a message dated 96-12-19 15:21:23 EST, you write:
>This raises an interesting question. If the lowest wind in the TIA
>standards is 70 mph, why do tower manufacturers specify tower capacity in
>terms of 50 mph wind?
>
>Since wind pressure increases with the square of the velocity, and 50 mph
>is roughly 1/sqrt(2) * 70 mph, this has the effect of presenting DOUBLE
>the capacity at 70 mph. (Other things being equal -- which they rarely
>are, since the tower face itself contributes considerable drag)
>
>Also, for crank-up units, shouldn't there be different ratings for the
>extended and nested positions?
>
>Sign me, Confused.
>
Dear Confused --
In the pre-TIA/EIA-222, UBC, et al. days, there wasn't much in the way
of standardization for structures and wind forces. Since then, many
standards, including the aforementioned ones, have been adopted. Many of the
crankups were designed before the current standards. And like antenna
manufacturers, crankup tower manufacturers sometimes play games with figures.
In order to upgrade the towers to a 70 MPH minimum would require: 1)
recalculating all of these towers (it would be expensive and no manufacturer
is really interested in doing it again since they've already spent the
engineering expense once); 2) down-rating ALL of the above towers to comply
with the (as you pointed out) MUCH larger wind pressures; 3) investing in
design and tooling for towers that really ARE capable of complying with the
these forces and loads.
Do you think that amateur radio operators (a bunch of widely recognized
cheap bastards) would be willing to pay for these more expensive towers? I
don't think so; they complain enough about the cost of anything as it is.
Everything gets back to manufacturing cost so the manufacturers are
unwilling to increase the cost of their products unless absolutely forced
into it.
Want to have a thoroughly frustrating project? Try getting a building
permit for one of these towers in a fairly brisk wind zone. In fact, try
getting one in a 70 MPH zone!
This just my opinion but based on my experience.
73, Steve K7LXC
TOWER TECH -- professional tower supplies and services for amateurs
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