Compress foundation bearing stress calculation

[arbor]

I noticed that the equation for foundation bearing stress calculation for the skirt base ring design is different from the referenced book by Jawad & Farr. Is this an error or there is some reason behind the change? Thank you,

 

[arbor]

The equation in compress is
fc = Cc / (((tc - ts) + n*ts)*(d / 2)*K2)

However, in the referred book is
fc = Cc / ((ts + n*tc)*(d / 2)*K2)

Thank you,

Arbor