On 12/7/2011 12:41 AM, Edwin Karl wrote:
> When computing windload, tubular surfaces are 66% of flat also a 30#
> tower is talking about 86 mph
> (as I recall, been a while since I did this) a 40# is 105 mph. Again, I
> am rusty on this stuff, but I know you take
> the cross section of a leg or element compute the impact pressure and
> multiply by .66 if it's round.
>
> I also recall towers are calculated at 150 (0r 175%) for a safety factor.
>
> Not sure if this gibberish means anything, but to do this right you need
> an engineer and data on the
> steel the tower was fabricated from. The rivets, not bolts or welding
> that connect the pieces need
> to factor in.
>
> Remember wind load goes all the way up the tower, not just on the
> antenna. The mast and rotor are a factor.
> Even the transmission line should be tucked into a corner so as not to
> increase the surface area.
I hope this makes sense:
Let's say for simplicity sake that a 70 MPH wind is 20# per sq fit and
the antenna is 16 sq ft. Not a super large monobander, but a good size
tribander or the Hy-Gain log periodic. That is 16 X 20 or 320#. At 70
feet that is an over turning moment at the base of 320 X 70 or 22,400
ft lb and we haven't even figured in the cross sectional area of the
tower plus accessories.
Its been a log time but I thought the wind pressure was not linear, but
went up with the square of the velocity where wind pressure is 1/2 the
density of air multiplied by the velocity squared, multiplied by the
shape factor of the object which is 0.66 for round objects and 1 for
flat objects. For a tower constructed of flat angle it's not quite that
simple as there is a bit more drag added due to the actual shape of the
tower even though most of it is open. I do not know what this would be
for an open lattis of a tower constructed of flat angle like the ROHN BX
and HBX series. The thing is that if it all remains constant except the
wind velocity I think we are talking about the square of the velocity
change.
The over turning force at the base due to the wind resistance of the
tower would be a summation from the top to the bottom, plus the wind
resistance of the antenna multiplied by the height above ground. I'm
more than a bit rusty and not a PE so I don't think it's hardly worth
the effort for me to try and figure out the formula but the cross
sectional area for wind resistance should be available for each section
used. . OTOH I'd also think we could simplify this and just figure the
wind resistance for each 8' or 10 foot tower section. True, it's crude
and only results in an approximation, but should be good enough. For
one the wind velocity normally is much less at ground level or even at
10 feet compared to 70 feet which just gives you a big safety figure.
73
Roger (K8RI0
>
> Overturn moment gets to be real big on taller towers and sections.
> (Think foot pounds). Failure of the bolts
> connecting to the stubs coming out of the ground could lead to a bad day.
>
> Of note, I have a pretty substantial tower and antenna (90' AN Wireless
> and 22 El Optibeam for HF)
> A recent windstorm bend the steel mast for a DB-224 used by a local
> repeater, my stuff stayed up and in
> one piece. If you are that concerned about the strength of the tower,
> maybe the installation should be re evaluated.
>
> 73!
>
> ed K0KL
>
>
> On 12/6/2011 10:13 PM, HansLG@aol.com wrote:
>> In a message dated 12/6/2011 4:43:51 P.M. Eastern Standard Time,
>> w2lu@rochester.rr.com writes:
>>
>> As I recall wind pressure "on a flat surface" at 70 mph is 20.6 lbs/sq
>> ft.,
>> so if the tower is rated for
>> 18 sq ft at 70 mph, would it be too much of a stretch to say to the zoning
>> board that since wind pressure at 90 mph is 34 lbs/sq ft, by simple math,
>> the tower should be good for 10.9 sq ft at 90 mph ?
>> Gene / W2LU
>>
>>
>>
>>
>>
>>
>> Nop! It is not that easy. You have to take the wind surface of the tower
>> into the calculation as well which makes it "a little more complicated".
>>
>> Hans - N2JFS
>>
>>
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