It's an interesting question. I have been told, by an engineer with
considerable tower modeling experience, that the BPC25G is a weakness in
the Rohn pier pin setup for 25/45G, because friction between the base
plate and the concrete base prevents it from achieving the full degree
of twisting freedom that a classic single point base affords. That
said, it makes sense to support all three legs on the concrete, and as
someone pointed out, that pretty well requires either a square base or a
larger-diameter round one.
73, Pete N4ZR
The World Contest Station Database, updated daily at www.conteststations.com
The Reverse Beacon Network at http://reversebeacon.net, blog at
reversebeacon.blogspot.com,
spots at telnet.reversebeacon.net, port 7000 AND now
at arcluster.reversebeacon.net port 7000
On 10/27/2011 1:48 PM, Joe Subich, W4TV wrote:
>
> > Sorry Joe, but Rohn 45G and "pier pin" adds up to a guyed tower to me.
>
> The discussion I had with the PE was about tower bases in general. In
> *that* discussion we did not differentiate. However, since the Rohn
> documentation states that the CB1G *must* be square, there must be a
> logical reason. I would not attempt to use undersize concrete -
> particularly when the concrete would not fully support the base plate
> (BPC25G) for the pier pin installation.
>
> 73,
>
> ... Joe, W4TV
>
>
> On 10/27/2011 1:36 PM, Pete Smith wrote:
>> Sorry Joe, but Rohn 45G and "pier pin" adds up to a guyed tower to me.
>>
>> 73, Pete N4ZR
>>
>> The World Contest Station Database, updated daily at
>> www.conteststations.com
>> The Reverse Beacon Network at http://reversebeacon.net, blog at
>> reversebeacon.blogspot.com,
>> spots at telnet.reversebeacon.net, port 7000 AND now
>> at arcluster.reversebeacon.net port 7000
>>
>>
>>
>> On 10/26/2011 7:47 PM, Joe Subich, W4TV wrote:
>>>
>>> > Does this apply to a guyed tower?
>>>
>>> Our discussion was about towers in general ... there was no mention
>>> of guyed vs. self-supporting or single pier vs. multiple pier (e.g.,
>>> Rohn SSV with its wide base).
>>>
>>> In any case, looking at the Rohn web catalog will show that CB1G is
>>> specified for all heights of Rohn 25 - except the tallest at 110 MPH.
>>> Two guy anchors are used in the three tallest 110 MPH designs and
>>> those towers are specified for GB2G. CB1G is a 2' 6" x 4' 0"
>>> square base ... CB2G is a 3'0" x 4' round base. The Rohn document
>>> is quite emphatic that CB1G *must* be square.
>>>
>>> The trends are consistent in the 45, and 55 documents as well -
>>> CB1G may be used for the shortest towers but must be square. Larger
>>> round piers are specified for taller towers and those with higher
>>> wind ratings. Even the smallest of the 65 towers start with CB2G.
>>>
>>> 73,
>>>
>>> ... Joe, W4TV
>>>
>>>
>>> On 10/26/2011 5:21 PM, Pete Smith wrote:
>>>> Does this apply to a guyed tower? I can understand big overturning
>>>> forces in a self-supporter, but most of the forces acting on a guyed
>>>> tower base are vertical, aren't they?
>>>>
>>>> 73, Pete N4ZR
>>>>
>>>> The World Contest Station Database, updated daily at
>>>> www.conteststations.com
>>>> The Reverse Beacon Network at http://reversebeacon.net, blog at
>>>> reversebeacon.blogspot.com,
>>>> spots at telnet.reversebeacon.net, port 7000 AND now
>>>> at arcluster.reversebeacon.net port 7000
>>>>
>>>>
>>>>
>>>> On 10/26/2011 5:10 PM, Joe Subich, W4TV wrote:
>>>>> I asked the same question of a well known Professional Engineer a
>>>>> couple of years ago. His answer was quite simple and made a lot
>>>>> of sense ... the round base can be used instead of a square one
>>>>> if the diameter of the round base is the same as the *diagonal*
>>>>> measurement of the square base.
>>>>>
>>>>> In your case, for a 2' 6" (30") square base the round equivalent
>>>>> would be 42" diameter *not* 36" as you propose. The reason has to
>>>>> due with projected surface area - the area that resists overturning
>>>>> - not volume (or weight) of the concrete.
>>>>>
>>>>> 73,
>>>>>
>>>>> ... Joe, W4TV
>>>>>
>>>>>
>>>>> On 10/26/2011 1:28 PM, Richard Thorne wrote:
>>>>>> I'm finally getting some dirt work done in the next day or two at
>>>>>> my new
>>>>>> place which will include holes/concrete for my 45g tower.
>>>>>>
>>>>>> I plan on using a pier pin install, so does it matter if the base is
>>>>>> square or round? The rohn book calls for a 2' 6" square hole 4' deep
>>>>>> (I'm designing the tower based on 90mhp winds with a height of
>>>>>> 120').
>>>>>> It would be easier to use a 36" auger to dig the hole. Since I'm
>>>>>> going
>>>>>> with a pier pin I don't see that there would be any twisting
>>>>>> moments on
>>>>>> the concrete base, it would only be there to hold the tower up. But
>>>>>> I'm
>>>>>> not an engineer, hence the question.
>>>>>>
>>>>>> Thanks
>>>>>>
>>>>>> Rich - N5ZC
>>>>>>
>>>>>>
>>>>>> _______________________________________________
>>>>>>
>>>>>>
>>>>>>
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>>>>>>
>>>>> _______________________________________________
>>>>>
>>>>>
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>>>>>
>>>> _______________________________________________
>>>>
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>>>
>>>
>>
>
>
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