Determining depth of zone of influence 2

[NearBeer]

Thank you for taking time to answer what I hope is a basic question. I am trying to figure out how far down the zone of influence for a footing extends. I borrowed a CE Reference Manual (Lindeburg) and came across a formula that gives me area of the zone of influnce: A=((B+2(h cot 60))+((L+2(h cot 60)). On the diagram in the book associated with this formula, "B" is the width of the footing. Also, there is a brief explanation that talks about the 60 deg zone versus a 63.4 deg zone (for a 2:1 slope) but nothing about how "h" is defined/determined. Are we to assume that the depth, h, of the zone is a twice that of the width of the footing?

I appreciate your input(s).

 

[msucog]

i believe the reference you mention suggests 2B for a column footing. others use 2 1/2 B for column footings. notice "column" and not "strip". deeper for strip.

 

[BigH]

Typically, it is about 2B (full width) for spread footings extending to 4B or so for strip footings. Rectangular footings would be in between. This is for uniform soils. However, if you have, say, a stronger material at top (say Et=10Eb), the induced stresses are attracted more to the upper layer and the depth of influence (10 % of applied) would therefore reduce. You can see approximation of the effects of different "strength/stiffness" layers in Poulos and Davis' Elastic Solutions to Soil and Rock Mechanics (which is now downloadable on the net).

http://www.ce.ncsu.edu/usucger/PandD/PandD.htm

 

[NearBeer]

Got it...thanks for the info!