[TowerTalk] windload rating of triex LM470e tower to be indep endently ver...

[Lonberg, Hank]

I understand the allowable increases and use the 1/3 for wind short term
loading. The increase in allowable due to width or depth is ok to a point
and would not have a problem with it if it were rock or a clean granular
type soil. I would have some problems if it were a clay or silt due to
potential shear rupture of the soil underneath the footing especially an
isolated spread or pier type foundation. For a large raft or mat it should
be ok.

Most tower foundations in the realm of ham radio are not very deep. The real
issue is that sometimes you do not have any experience with the local site
and have not seen it in person. I would rather be conservative when dealing
with the foundations as they are extremely variable and concrete is
relatively cheap to install the first time. Expensive to modify after the
fact.

Hank
 

	-----Original Message-----
	From:	Tower2sell@aol.com [SMTP:Tower2sell@aol.com]
	Sent:	Wednesday, September 27, 2000 11:50 AM
	To:	Hank.Lonberg@harrisgrp.com; towertalk@contesting.com
	Subject:	RE: [TowerTalk] windload rating of triex LM470e
tower to be indep endently ver...

	Hank,

	In reference to UBC Table 18-I-A, look at the foot notes.

	#2 allows for a 20% increase for each foot of depth or width up to a
maximum of 3 times. Plus it references section 1612.3.2 that allows a 1/3
increase in allowable stress using a "wind" load. 

	This means a Crystalline bedrock could go as high as
	4000 x 3 x 4/3 = 16000 psf - quite a bit.

	And for clay 16 feet deep:

	1000 x 3 x 4/3 = 5333 psf still much greater than most geotechs
would give.

	Take full advantage of the code allowables, they are not that bad.

	Something else to consider is that most (not all) catalog
foundations are overdesigned by their nature of trying to work with multiple
catalog towers. You can see this when the same foundation is specified for
various towers with increasing foundation reactions. A simple suggestion is
to specify the soil required for the specific catalog tower-foundation
combination. Then one might be able to use the UBC tables.


	Tower2sell@aol.com

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