[TowerTalk] Rohn questions
k6ll@juno.com
k6ll@juno.com
Thu, 9 Apr 1998 14:45:13 +0100
I missed the original posting on this, but here's my two cents.
>> Since Rohn's wind loads are substantially higher for a house
>bracketed 70'
>> tower (at 56' and 28') (32 sq. ft. at 70 mph) than a tower with
>their two
>> recommended guys at 35' and 65' (9.5 sq. ft. at 70 mph assuming
>tower has 8
>> sq.
>> ft. for sidearms and 6 transmission lines coming up the sides), how
>much
>> extra
>> wind load, if any, would be gained by using 3 guys at 23'
>increments?
In a guyed tower, lateral wind forces are converted to downward
compressive forces in the tower legs, because the guys come
down to the ground at an angle. At some wind speed, the tower
bottom section will fail in compression, if nothing else fails
first. If you add more sets of guys then the mfr recommends, this
failure mode will occur at a lower windspeed, because the
preloads on the additional guy sets will cause additional
compression, even when the wind is not blowing.
In a bracketed tower, lateral wind forces are transferred as
lateral forces to the guy bracket, since it is at a 90 degree
angle to the tower. There is no compressive force on the
bottom tower section due to wind. The primary failure
mode then becomes exceeding the Moment Capacity, parameter "M"
on the Rohn drawings. This failure would probably occur just
above the top bracket, assuming that the bracket or structure
did not fail first.
Dave Hachadorian, K6LL
Yuma, AZ
K6LL@juno.com
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