Page 237 - Marine Structural Design

Part I1

Ultimate Strength

Chapter 11 Ultimate Strength of Cylindrical Shells

11.1 Introduction
11.1.1 General
Cylindrical shells are important structural elements in offshore structures, submarines and
airspace crafts. They are very often subjected to combined compressive stress and external
pressure, and must be designed to meet the strength requirements. A theoretical load end-
shortening curve representing unstiffened cylindrical shells under axial compression is shown
in Figure 1 1.1. For a perfect shell, the stress-strain relation is linear until the bifkcation point,
B, where buckling occurs and load-carrying capacity decreases sharply. For an imperfect shell,
the stress-strain relation starts is non-linear from an early stage of loading, buckling occurs at
point L without showing obvious bifurcation phenomenon.

Figure 11.1 Stress Strain Relations for Perfect and Imperfect Shells

Strength of imperfect cylindrical shell may be significantly lower than the bifurcation load.
The design of cylindrical shells is based on the modification of the theoretical predictions
using a knockdown factor for imperfection effect.