Seems no one quoted the K7LXC Directive:
DO WHAT THE TOWER MANUFACTURER SAYS.
Don
N8DE
Quoting Bert Almemo <balmemo@sympatico.ca>:
>
>
> Hi guys,
>
> Thanks for all responses, comments and suggestions regarding the dimensions
> of a concrete base for a tower or other support structure.
>
> It seems almost everyone wants to elaborate on the issue and also wants to
> put my question into different scenarios. Again, not being a structural
> engineer, isn't there a general rule as to what design provides the best
> support for a free standing tower or similar structure regardless of other
> circumstances?
>
> I was hoping to get some indication from the experts on TT which way to go
> as I'm not spending thousands of dollars to use the services of a structural
> engineer. I just want an opinion from you experienced guys. It's only a 55ft
> free standing tower with a medium sized tri-bander on top. So far I'm more
> confused than when I started this thread. Thanks.
>
> 73 Bert, VE3OBU
>
>
>
>
>
> -----Original Message-----
> From: towertalk-bounces@contesting.com
> [mailto:towertalk-bounces@contesting.com] On Behalf Of kr7x1@verizon.net
> Sent: Tuesday, September 02, 2008 12:24 PM
> To: Al Williams
> Cc: towertalk@contesting.com
> Subject: Re: [TowerTalk] Dimensions of a tower concrete base
>
> Hi Al:
>
> In the case of a cantilever (free standing) structure say like a ham tower,
> the wind or seismic force developed is horizontal (lateral). This force
> creates a cantilever moment at the base equal to w*L^2 / 2 where w is the
> lateral force per unit length and L is the height of the structure. This is
> the cantilever moment at the base. The lateral load also has to be resisted
> by the base along with the moment. The base horizontal (lateral) reaction is
> w*L. Then the weight of the structure also creates a force to be resisted by
> the base which is the weight of the appurtances (antennas, rotators,
> feedlines, the foundation weight, north facing fake owl, etc).
>
> Thus the base (foundation) needs to be designed to resist the vertical load
> of the structure, the lateral load and the cantilever moment due to the
> horizontal wind or seismic enviromental loading.
>
> In a nut shell, the foundation has to resist being pushed into the ground,
> moved through the ground and rotated in the ground all at the same time.
>
> Foundation strength or resistance to the applied loading is totally a
> function of the soil characteristics at the location being considered.
> This is the big variable. The IBC has a table that gives conservative
> generalized values to use for these soil characteristics if a geotechnical
> analysis and report is not available for the specific location.
>
> Regards
> Lonberg Design Group, Ltd.
> H.S. Lonberg, P.E.,S.E.
> President
>
>
>
> On Tue, Sep 2, 2008 at 8:18 AM, Al Williams wrote:
>
>> Please explain the difference between "lateral force" and "the moment
>> inherent in this type of structure"
>> and how they effect on tahe strength of the foundation.
>>
>> k7puc
>>
>>
>> ----- Original Message ----- From: "Hank Lonberg" <kr7x1@verizon.net>
>> To: "'Bert Almemo'" <balmemo@sympatico.ca>; <towertalk@contesting.com>
>> Sent: Monday, September 01, 2008 9:14 PM
>> Subject: Re: [TowerTalk] Dimensions of a tower concrete base
>>
>>
>>> Bert:
>>>
>>> The best answer to your generalized questions is; it depends.
>>>
>>> Depends if the foundation is supporting only vertical load as in a
>>> guyed structure.
>>> Depends if the foundation is to support a cantilevered or free
>>> standing structure with both lateral force, vertical force and the
>>> moment inherent in this type of structure.
>>> Depends if the designer is trying to utilize a spread footing type of
>>> foundation.
>>> Depends if the designer is trying to utilize a post type of
>>> foundation.
>>> Depends heavily on the soil's characteristics and strength that you
>>> are trying to utilize.
>>>
>>> So you see, your question does not have a closed form of solution,
>>> but depends on what you are trying to accomplish and what type of
>>> loading the structure you are trying to support will impart to the
>>> foundation.
>>>
>>> A case can be made for either the shallow spread footing type of
>>> foundation or the smaller footprint deeper foundation. The trick is
>>> to size them adequately for the loading condition versus the soil
>>> conditions expected.
>>>
>>> Gut level, the amount of concrete for either design is most likely
>>> similar.
>>>
>>> Regards
>>> Lonberg Design Group, Ltd.
>>> H.S. Lonberg, P.E.,S.E.
>>> President
>>>
>>> -----Original Message-----
>>> From: towertalk-bounces@contesting.com
>>> [mailto:towertalk-bounces@contesting.com] On Behalf Of Bert Almemo
>>> Sent: Monday, September 01, 2008 8:51 PM
>>> To: 'Bob Maser'; towertalk@contesting.com
>>> Subject: Re: [TowerTalk] Dimensions of a tower concrete base
>>>
>>> Hi Bob,
>>>
>>> Thanks for your reply. Yes, I think you missed the point.
>>> Sorry! I'm talking
>>> about the concept in principle not if the total weight is exactly the
>>> same.
>>> According to the experts, the weight of the base has very little to
>>> do with the support of the structure. The weight only makes up for
>>> about 10% of the forces needed to support a given structure. 90% or
>>> so comes from the soil/dirt/sand enclosing the base. I'm not an
>>> expert in structural engineering and can't probably explain this
>>> properly.
>>>
>>> Anyway, back to my question - in principle is there a difference in
>>> force/strength to support a structure if the base is deeeper or
>>> shallower having basically the same weight and volume?
>>>
>>> 73 Bert, VE3OBU
>>>
>>>
>>>
>>>
>>>
>>> -----Original Message-----
>>> From: Bob Maser [mailto:bmaser@tampabay.rr.com]
>>> Sent: Monday, September 01, 2008 8:49 PM
>>> To: Bert Almemo
>>> Subject: Re: [TowerTalk] Dimensions of a tower concrete base
>>>
>>> The last time I looked, 3x3x6=54 and 4x4x4=64 so 64 will weigh more
>>> than 54 will. Am I missing something?
>>>
>>> Bob W6TR
>>> ----- Original Message -----
>>> From: "Bert Almemo" <balmemo@sympatico.ca>
>>> To: <towertalk@contesting.com>
>>> Sent: Monday, September 01, 2008 2:35 PM
>>> Subject: [TowerTalk] Dimensions of a tower concrete base
>>>
>>>
>>>> In general, is it better to go deep than wide when digging
>>> for a tower
>>>> base,
>>>> i.e. does a deeper base give better support than a shallower
>>> one with
>>>> basically the same volume? Is 3x3x6 better than 4x4x4 with
>>> everything else
>>>> being equal. Thanks.
>>>>
>>>> 73 Bert, VE3OBU
>>>>
>>>>
>>>> _______________________________________________
>>>>
>>>>
>>>>
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>>
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