[TowerTalk] Dimensions of a tower concrete base

Tue Sep 2 18:47:32 EDT 2008

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 at contesting.com
[mailto:towertalk-bounces at contesting.com] On Behalf Of kr7x1 at verizon.net
Sent: Tuesday, September 02, 2008 12:24 PM
To: Al Williams
Cc: towertalk at 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 at verizon.net>
> To: "'Bert Almemo'" <balmemo at sympatico.ca>; <towertalk at 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 at contesting.com 
>> [mailto:towertalk-bounces at contesting.com] On Behalf Of Bert Almemo
>> Sent: Monday, September 01, 2008 8:51 PM
>> To: 'Bob Maser'; towertalk at 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 at 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 at sympatico.ca>
>> To: <towertalk at 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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>>> TowerTalk at contesting.com
>>> http://lists.contesting.com/mailman/listinfo/towertalk
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