A shell-like metal structure is one that supports loads in a manner similar to that of a thin shell. That is, the majority of its load-bearing behavior is provided by membrane actions in which forces are carried from point to point by biaxial tension or compression and by shear in the plane of the shell. In addition to the membrane action, shell-like structures can include some degree of bending resistance. Shell-like structures are often divided into two classes, although the distinction between them is often not obvious, continuous, and discrete. Continuous shell structures are basically curved plates, which may be stiffened in orthogonal or skew directions. Discrete shells are reticulated structures that are curved in space. Continuous shell structures are considered first.
A typical stiffened shell is shown in Fig. 18.1. Stiffening elements in the meridional direction are called stringers. They are usually small and spaced close together. Stiffening elements in the circumferential direction are called ring stiffeners. They are often large rings spaced fairly far apart. There are four modes in which buckling can occur for this structure. First, there is local buckling; the plate spanning between the stringers and stiffeners can buckle. Second, the shell plate and stringer combination spanning between the stiffeners can buckle; this is called bay instability. In the third form of instability, general instability, a large portion of the plate, stringers, and stiffeners buckles. In addition, the stringers or stiffeners may undergo torsional buckling as curved columns. This form of instability is not unique to shell structures and is considered in other chapters of this guide.
Shell design codes usually address the first three buckling cases. There are two ways in which stiffened shells can be described. The first approach is to "smear" stiffeners and stringers across the shell plate and analyze the system as a continuous orthotropic plate. The second approach is to use the split-rigidity concept. This approach is described here in conjunction with discrete shell structures.