PANDA2 analysis that doesn't employ those approximation used in optimization might

be slightly higher than zero due to approximation errors. The result is that the PANDA2

optimum design maybe corresponds to a slight higher safety factor than the one used in

optimization. Another correction formula based on the actual safety margin from

PANDA2 analysis is proposed as the following

 (k+1) 1+m (k)
 s > (8-4)
 Pf (k)

The correction is also only done for those safety factors that correspond to active failure

modes.

 The optimization problem is formulated as

 minimize W =W(b,h,tz,0,,t2> 2)
 such that
 a (8-5)
 1- <0,i =1, N
 S,(k+1) g


The process is repeated until the optimum converges and the reliability constraint is

sati sfied.

 Two design iterations are carried out using Equation. (8-3) and (8-4) using

PANDA2 optimization, respectively. The results are shown in Table 8-19 and Table 8-

20. It is seen that the second design (S2=1.437514) in Table 8-19 has much lower

probability of failure than the desired probability of failure predicted by using the safety

factor calibrated by PSF. It must be noted that the critical failure mode switches from in

plane shear failure of the isogrids as of the first design (S =1.2) to transverse tensile

failure of the tank skin as of the second design, which causes the oscillation in design

convergence.