[TowerTalk] Pad and Pier for ROHN SSV. Pad and Pier Foundations

[Jim Lux]

At 09:00 AM 7/12/2005, Michael Keane K1MK wrote:
>On Mon, 11 Jul 2005 at 00:37:06 <isp at bnjcomp.com> wrote:
>
> > So I can compare where do I find what "normal" or
> > "average" soil conditons means?
>
>One would expect to be able to find something about the assumed soil 
>conditions somewhere in the drawing notes, typically in the notes to the 
>foundation drawing(s).
>
>Normal soil usually has a specific, technical meaning. One will commonly 
>see a drawing note such as: "Foundations designed in accordance with 
>ANSI/TIA/EIA normal soil" or other words to that effect.
>
>EIA-222-F defines normal soil as:
>
>"A cohesive soil with an allowable net vertical bearing capacity of 4000 
>pounds per square foot and an allowable net horizontal pressure of 400 
>pounds per square foot per lineal foot of depth to a maximum of 4000 
>pounds per square foot."
>
>Be warned though, the "normal" soil defined in EIA-222 is just a set of 
>parameters intended for bidding purposes so quotes can be prepared and 
>compared on a common basis when the customer does not provide a 
>geotechnical report as part of an RFQ.
>
>EIA-222 calls it normal soil but the mechanical properties are not the 
>properties of an average soil; some properties of "normal" soil are quite 
>atypical.
>
>
>And don't assume that the standard foundation design has been 
>substantially over-engineered in order to cover all or most cases. The 
>foundation design for normal soil is used as a quoting standard. A 
>manufacturer is not going to deliberately place itself at a competitive 
>disadvantage by designing a foundation that requires more cubic yards of 
>concrete than what's needed to achieve the safety factors specified in the 
>EIA-222 standard.
>
>73,
>Mike K1MK


Mike hits the nail exactly on the head.  The mfr drawing is designed to 
provide a common basis for comparison.  The going in assumption is that it 
will be reviewed by a competent person before using it blindly, just like 
any other engineered component.  Kind of like bolts. The fact that the bolt 
is graded and certified at a particular level doesn't mean it works for all 
bolt applications.  The user is expected to use judgement on whether they 
need a grade X bolt for their particular need.

That competent review doesn't necessarily have to be a licensed engineer. 
It's all about risk acceptance posture:  If you're putting up a tower down 
on the lower 40 and the only thing around for 2 miles is sheep or cows, the 
standard of review is substantially different than if you're putting up 
that tower on a 5000 square foot lot in a crowded subdivision.