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Re: Pole Foundation Embedment

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At 02:18 PM 11/9/99 EST, you wrote:
>I have difficulty in interpreting pole foundation embedment formula given in 
>UBC.
>
>Required depth, d, of embedment is a function of S1, which is defined as 
>"Allowable lateral soil bearing pressure as set forth in Table 29-B, based on 
>a depth of one-third the depth of embedment."
>
>Is it, therefore, correct to say that, S1=Allowable Lateral Bearing X (Depth 
>of embedment)/3?  Also, would it not be prudent to discount the frost depth 
>in this calculation?

        "YES" to both of these questions.


>One of the footnotes to table 29-B, states that the lateral bearing value 
>"may be increased the amount of the designated value for each additional 
>depth to a maximum of 15 times the designated value." So, if d=10 feet in 
>clayey soil, is the lateral bearing equal to 1000 psf/ft depth of embedment 
>or should it be an average value of (100+1000)/2 = 550 psf/ft depth of 
>embedment? Taking advantage of this footnote will make the solution 
>iterative, because while 'd' is dependent on S1, S1 itself becomes a function 
>of 'd'.

        The last sentence above is true. It is an iterative process as you
note. However, the "increase" of lateral bearing with depth has always been
the initial, simple, linear multiple of the 100 psf/foot of depth (or other
listed value) times the depth that is of interest. If depth is to be 10ft on
a trial basis, lateral bearing for S1 is 100 times one-third of ten, or 333
psf. The "may be increased..." footnote provision refers only to that, not
to some additional step that is optional. [Once again, inarticulate code
wording acts to confuse code users.]

        There is a maximum value of depth for purposes of calculating S1 and
S3 for their use in the two formulas, but this does not act to prohibit use
of depths deeper than 15 ft. Note that in the definition of "d" the maximum
value of depth "for purpose of calculating lateral [soil bearing] pressure"
is 12 ft, not 15 ft.  

        I have tracked and followed this pole fdn. topic in Code Committee
for a number of years. Chap 29 became Chap 18 for the 94 edition. I bought
my 97 edition in loose leaf, and all of its Chap 18 has somehow "gotten
loose" from me since I am still struggling with Chap 16 and 23. So I'm
unsure, in spite of committee work, if the 97 version remains unchanged from
94. 

Charles O. Greenlaw  SE   Sacramento CA