[TowerTalk] ROHN tower anchor question

[Tower2sell@aol.com]

Your question is very simple. You are missing some information.

First ROHN references "NORMAL SOIL" Normal soil is referenced in the 
ANSI/TIA/EIA-222-F, 1996, and is mainly used for bidding purposes when 
someone wants to compare prices to of bids from several tower suppliers. The 
build officials are under no obligation to permit the use of NORMAL SOIL 
foundations without a geotechnical report and engineering design.

What is NORMAL SOIL? As far as I've found it located in Normal, IL. and the 
transportation costs are very high. (You can add your own puns)

OK really. 

For an anchor the  forces from the anchor is broken down into a horizontal 
and vertical component. (you do remember trig. sin(X) and cos(X)) 

The vertical force is resisted by the weight of the concrete  and soil above  
the anchor. The angle of soil uplift cone is assumed to be 30 degrees and the 
compacted back fill is specified as a minimum of 100 pounds per square foot.  
Normal soil is assumed to have no ground water. The effects of ground water 
is to reduce the effective weight of the soil (100-62.4) Code specified 
safety factors are used.

For the horizontal load, the lateral bearing area times the allowable bearing 
stress. You are done. How do you calculate the allowable NORMAL SOIL lateral 
bearing capacity you ask?  The formula. Take the depth of the center of the 
block times 400 and limit it to 4000 psf. (note 10 ft = 4000 psf for those 
mathematically impaired)

Someone said that undisturbed soil is better than disturbed soil. This may or 
may not be the case. I've seen a lot of bad undisturbed soil. A properly 
compacted backfill can be better than the existing soil. In some cases you 
can dig a hole and place the earth back in the hole at a higher density that 
you will need more dirt to fill the hole than came out of it.  Soil 
compaction is a complex art and requires education and experience; not 
necessarily an engineering degree, but contracting experience.

OK I hope this email helps.

Let the "NORMAL SOIL" puns begin.

Tower2sell@aol.com





In a message dated 7/29/00 6:18:43 PM Central Daylight Time, 
km5up@airmail.net writes:

<< ROHN drawing C620643 titled "Standard Concrete Anchors" shows uplift and
 lateral capacity in lbs for each of the anchors. How does ROHN derived at
 these numbers?
 
 For exampe a 3 foot by 3 foot by 1.5 foot of concrete that is buried 4 foot
 down into the ground has a up lift capacity of 3,490 lbs. and a lateral
 capacity of 5,850 lbs.  The concrete weighs 1,890 lbs.  NOTE is provided by
 ROHN that states "The soil is cohesive with an allowable net vertical
 bearing capacity of 4000 pounds per sq. ft.(psf) and allowable net
 horizontal pressure of 400 psf per lineal foot of depth to a maximum of 4000
 psf."
 
 How do they crunch the numbers to determine the the uplift and lateral
 capacity of the anchor?
 
  >>

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