On 4/1/2012 3:30 PM, Jon wrote:
> The 2x2x4 chunk of concrete is called ballast.
> Similar to the keel of a sailboat...helps to keep it upright.
As far as I can see the base does not act like a keel which is dynamic.
That base is a dead weight. That chunk of concrete normally only does
two things. It keeps the tower from sinking into the ground and it keeps
the base from slipping sideways. In many instances a dirt base will
work just as well. ROHN used to sell them, but I'm assuming they
stopped due to misuse of those bases.
First is an over simplification: Given a 40' tower with a "single"
bracket at 30 feet. The ratio above the bracket to below the bracket is
3:1. On a really windy day if you have a 400# wind load at the top of
the tower the side force at the base is 400/3 or 133.3# in the opposite
direction.
When the tower has a "single" bracket the base picks up a relatively
"small" side force in a direction opposite to the direction of the wind,
the magnitude of which depends on the distance ratio between the top of
the tower and bracket and between the bracket and base.
But In this case it's a 35' tower bracketed in "two" places with the top
one being near the top of the tower. I'm going to assume 10' from the
top of the tower at 25' (It's likely closer) and say again at 15' there
is very little of this fulcrum effect on the base.
Using a pinned base in this case allows for some play while the concrete
or dirt base does not.
It's unlikely that the end of the house is capable of supporting 815#.
Some will and some won't.
The wind load is figured from the exposed tower and antennas with no
guys. Figure the wind speed at what ever it is for your area be it 70,
80, 90, or what ever. The entire wind load is figured against that
bracket which is why they do not use a pinned base. That base may
greatly reduce the load on the bracket. There are too many unknowns for
me to even try to figure the loads. Distance from the base to each
bracket, how high the tower extends above the top bracket, the area of
the antenna(s), and the top wind speeds for your area are needed. Once
the wind load is figured don't forget a safety factor.
ROHN gives a wind load for the bracketed tower at a given height.
73 and good luck.
Roger (K8RI)
>
> On Sun, Apr 1, 2012 at 2:24 PM, Dave Cook<wa0ttn@netdave.com> wrote:
>
>> Hi guys, I'm in the process of relocating my 35' freestanding Rohn 25G
>> tower
>> after 8.5 years on the air. I figured this time I'll bracket it to the
>> house, which is what I should have done in the first place. But I've run
>> across a few problems with the Rohn specifications:
>>
>>
>>
>> 1. The tower will be bracketed to the house mid-way up and also near
>> the top of the tower with an eave bracket. The specifications for the
>> brackets "must support a minimum horizontal force of 815 pounds". I can't
>> find any calculations for how they came up with this number. And it seems
>> very excessive for a modest 35 foot tower bracketed in two places.
>>
>> 2. The specs also state that the base must be a "fixed base", rather
>> than a pinned base. Again, in this case I don't understand why a pinned
>> base
>> wouldn't be sufficient for a well-bracketed tower. Why would the base
>> experience any lateral loading?
>>
>> 3. The specs also state that the base has to be the same 2x2x4 feet
>> of
>> concrete that was spec'ed for self-supporting and guyed towers. In this
>> case
>> I can understand somewhat having a solid chunk of concrete for the thing to
>> rest on, but the entire weight of the tower and antenna will be, by my
>> calculations just a little over 200 lbs. It doesn't seem to me that it
>> needs
>> a 2000 lb chunk of concrete to keep that from sinking into the soil.
>>
>>
>>
>> So in short, I'm just wondering if Rohn is simply CYA'ing themselves, or
>> are
>> these specs really to be taken literally?
>>
>>
>>
>> Thanks and 73,
>>
>> -- Dave WA0TTN
>>
>> _______________________________________________
>>
>>
>>
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