x =B/2 =20/2 =10 m
y =L/2 =30/2 =15 m
m =x/z =10/11 =0.91
n =y/z =15/11 =1.4
Entering Fig. 4.8 with n =1.4, intersecting the m =0.9 curve, I =0.185
Finding the increase in stress below the corner of the loaded area: s z =Δs v =qo I =(50 kPa)(0.185) =9.25 kPa
For center of the loaded area, multiply the above result by 4: s z =Δs v =(4)(9.25 kPa) =37 kPa
Since OCR =1, use Eq. 8.12: