Geotechnical Engineering - Fadum Chart 1

[HarshParmar]

Hi Everyone,

I have question about finding accurate values from fadums chart, in exams, because the visual judgement would differ values in decimals and answers will be deviating. This issue, I found while solving tutorials and its respective answers.
Moreover, in one of the example in tutorial of finding stress under vertical stress distribution of rectangular shape, length of the foundation is 6 m, width is 3 m and depth at which the stress to be found is 2 m. In this case we can assume a value from fadum chart as near to 0.227 as shown in solution, but after we calculate m and n, and then we try to pluck it in fadums expression of influence factor (attached snap) the tan inverse term goes negative and hence the influence factor turns out to be negative and so the stress.
I am not sure , how to interpret this or what's wrong with it.

Thanks in Advance

 

[CarlB]

According to Bowles, when the denominator of the arctan term is negative, need to add pi to it.

 

[HarshParmar]

Thank you CarlB. It helped a lot. Can you just let me know which book or article is this?

 

[CarlB]

From textbook by Joseph E. Bowles, Chapter 5.

http://civilcafe.weebly.com/uploads/2/8/9/8/28985467/foundation_analysis_and_design-_bowles_5ed_.pdf