Page 247 - Marine Structural Design

Chapter I I Ultimate Strength of Cylindrical Shells 223

reduction of the load carrying capacity, as effect of initial deflection on column buckling.
Therefore fabrication tolerance is applied to the stiffeners.
General Instability
General instability involves buckling of both the stringer and ring stiffeners together with the
shell plating. Due to the catastrophic consequences this failure mode may result in, restrictions
are applied in the design codes on the second moment of inertia for the ring stiffeners. Such
restrictions are to assure that the buckling strength for general instability mode is 1 to 4 times
of that for stringer-stiffened cylinder buckling.
11.4.2 Radial Pressure
External pressure may be applied either purely radially, known as “external lateral pressure
loading”, or all around the shell (both radially and axially), known as “external hydrostatic
pressure loading”. Potential failure modes include:
Local buckling of the panels between stringer stiffeners,
Stringer buckling,
General instability,
Local stiffener tripping,
Interaction of the above failure modes.
The formulation for collapse pressure pkc may be found from API Bulletin 2U (1987) and
Das et a1 (1992,2001).
Baht et a1 (2002) modified the formulae in API Bulletin 2U (1987) and suggested the
following elastic buckling equation:

(11.38)
Capanoglu and Baht (2002) proposed to use the following equation for the geometric
parameter k, :
1 (1 1.39)
1 + (L, L,)* 0.01 IM,’

where the imperfection parametera, may be taken as 0.8. The plasticity correction factor 4 in
Section 10.1.6 may then be used to derive inelastic buckling strength.
11.4.3 Axial Compression and Radial Pressure

A simple interaction equation for the strength of stringer and ringer stiffened cylinders under
combined axial compression and external pressure may be expressed as:

(1 1.40)