[TowerTalk] Pier-pad foundation, under a house?

[Gene Smar]

Bryan:
 
     I concur with Joe.  Move the pier-and-pad (P-A-P) away from the house.  The way this design works is the vertical pier transfers the overturning forces from the tower/wind to the pad.  The pad then pushes up against the dirt above it and at an angle away from it.  If you were to dig underneath your house's foundation to install the P-A-P, these wind forces would be applied to the footer and walls of the foundation.  I'm positive these house systems weren't designed for that kind of stress.  
 
     A civil engineer friend of mine once suggested to me that my tower's similar P-A-P ought to be constructed away from my home's footer foundation by a distance equal to the depth of the bottom of the hole.  In my case the hole was 5 feet deep so that meant I needed to place the tower foundation hole at least 5 feet away from my home.  As it turned out, I built the tower much farther away than that.
 
     One more thing:  If you do go the P-A-P route, make sure you compact the dirt when you backfill the excavation.  Rent a powered tamper and use it on every foot of backfill depth around the P-A-P as you are backfilling the dirt.  The denser the backfill, the more weight there will be to resist overturning from the wind.  If the P-A-P were to be made heavy enough, you could just pour it on top of level ground.  But its been engineered to use the weight of (cheap) dirt instead of (less cheap) concrete to resist the wind forces.  
 
     Good luck with the project.
 
 
73 de
Gene Smar  AD3F
 
 



On 08/26/14, Joe Subich, W4TV 
wrote:


Don't even think of undermining the house. If you need to stick with
the 15' pad.move the appropriate distance away from your foundation.
The typical Florida home is slab on grade. Your area has enough issues
with sink holes, etc. I would not risk creating one artificially.

73,

... Joe, W4TV


On 2014-08-26 8:48 AM, Brian Amos wrote:
> You are going to need to get a structural engineer and a geotechnical
> engineer involved. Its very possible to do this, it won't be cheap, but
> done incorrectly you could cause some severe damage to your home. You will
> want to find good local engineers who work for smaller companies, a large
> company probably won't even take on a small job such as this. It also might
> be possible that they can redesign your foundation so you don't have to
> mess with the pad. Its very likely that they will save you money in the
> long run.
>
> Brian
> KF7OVD
> On Aug 25, 2014 8:49 PM, "Bryan Fields" Bryan at bryanfields.net> wrote:
>
>> I'm considering building a AN wireless tower at my property in St Pete
>> Beach,
>> and it must be rated for 120 MPH wind which means it's large foundation.
>> Good
>> news is the city says I'm good up to a 70' tower without a zoning
>> variance. I
>> only need stamped drawings for it.
>>
>> II want to keep it centered on my lot behind my house which is an L shape.
>> Looking at the foundation "K" which is required, I need a 7' pier and a 15'
>> pad under it. I'd like to have the 7' pier flush in the corner of the L
>> of my
>> house with the pad extending under the house.
>>
>> Has anyone ever done something like this before? I'm assuming I'll need to
>> support the foundation of the house with some steel beams and then dig out
>> under it.
>>
>> Thoughts?
>>
>> --
>> Bryan Fields
>>
>> 727-409-1194 - Voice
>> 727-214-2508 - Fax
>> http://bryanfields.net
>>
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