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

[Michael Keane K1MK]

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. 

As one example, normal soil is defined as having an allowable bearing pressure of 4000 psf. There are soils that do have an allowable bearing pressure that meets or exceeds 4000 psf, but they are extreme cases. Under the UBC only a  Class 1 material, described as massive crystalline bedrock, is presumed to have a allowable bearing pressure of 4000 psf without a test. 

And another example, the lateral pressure per unit depth for normal soil is defined as of 400 psf/ft. Again, this value lies towards the extreme high end for typical soils. And it is a parameter that's very relevant for a self-supporting tower as it directly effects the ability of a foundation design to resist overturning moments.

Whether the standard foundation design for "normal soil" shown on the manufacturer's drawings meets performance or code requirements for a specific set of soil properties is something that needs to be verified by a civil/structural engineer. 
 
Those who proceed to build a foundation to the drawing without checking the validity of the design assupmtions don't often encounter immediate, catastrohpic failure because of the safety factors built into the standard foundation design.

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
--
Michael Keane K1MK
k1mk at alum.mit.edu