variations in the material and geometric parameters. Fuzzy set analysis was employed to

model the uncertainties in the response. Buckling loads of the thin walled stiffened panels

are highly sensitive to geometric imperfections. Elseifl et al. (1999) applied a convex

model to represent the worst-case geometric imperfection. The results obtained with this

convex model were compared to the traditional probabilistic models used to account for

uncertainties in imperfections.

 This chapter presents reliability-based designs of isogrid stiffened panels. The

problem is to minimize the weight of the stiffened panel subj ect to a reliability constraint.

The reliability-constraint is evaluated by Monte Carlo simulation, which requires a large

number panel analyses. The PANDA2 software (Bushnell, 1987) is employed to analyze

the stiffened panels. PANDA2 uses a combination of approximate physical models, exact

closed form (finite-strip analysis) models and 1-D discrete branched shell analysis

models to calculate pre-buckling, buckling and post-buckling responses with highly

efficient analysis. PANDA2 also provides limited deterministic global optimization based

on multiple starting points strategy. Under compressive loads, the load carrying capacity

of stiffened panels is greatly affected by geometric imperfections due to fabrication. The

effects of geometric imperfections are taken into account in PANDA2 software directly

by modifying the effective radius of a cylindrical panel and indirectly by redistributing

the pre-buckling stress resultants over the various segments of the panel. Various

geometric imperfections, such as global, local, inter-ring, and general ovalization of

cylindrical panels are considered, with a sophisticated methodology to identify the most

detrimental imperfections. (Bushnell, 1996).