Foundation bearing stress calculation

[stephenbell50]

I am seeing Compress run two different types of calculations for foundation bearing stress (f_c). It is the same vessel and I'm running two separate operating loads (a separate calc for each load).

The heavier operating load (2,240,000 kg) causes Compress to run the equation for f_c as follows:

Foundation bearing stress (governing)

Ac = pi*(Do2 - Di2) / 4 - N*pi*db2 / 4
= p*(9182 - 8312) / 4 - 32*p*5.32 / 4
= 118,802.6 cm2

Ic = p*(Do4 - Di4) / 64
= p*(9184 - 8314) / 64
= 1.1453E+10 cm4

fc = N*Ab*Preload / Ac + (1 + 0.14*SDS)*W / Ac + M / 2*Do / Ic
= 32*14.2438*0 / 118,802.6 + (1 + 0.14*0.284)*3,184,834.87 / 118,802.6 + 10*5,778,801.9 / 2*9,180 / 1.1453E+10
= 51.034 kgf/cm2

As fc <= 100 kgf/cm2 the base plate width is satisfactory.

The lighter operating load (1,308,000 kg) is giving

Foundation bearing stress

fc = Cc / (((tc - ts) + n*ts)*(d / 2)*K2)
= 100*2,058,549.02 / (((435 - 1.66) + 7.8686*1.66)*(8,745 / 2)*1.8814)
= 56.057 kgf/cm2

As fc <= 100 kgf/cm2 the base plate width is satisfactory.

k = 1 / (1 + fs / (n*fc))
= 1 / (1 + 541.583 / (7.8686*56.057))
= 0.448868

Why does Compress choose which equation to use?