# [TowerTalk] Dimensions of a tower concrete base

kr7x1 at verizon.net kr7x1 at verizon.net
Tue Sep 2 12:24:25 EDT 2008

```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

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.

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:
>>
>>
>> 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
>> 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
>> 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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