Page 106 - Marine Structural Design

82 Part I Structural Design Principles

4.5.2 Local Bending Strength of Plates
In the design rules, the minimum required plate thickness is defined as a function of stiffener
spacing, design pressure, and allowable stress. This criterion may be derived fbm plate
theory: A plate panel between two stiffeners and two girders can be simplified and considered
as a rectangular plate under uniform lateral pressure p, with all edges fixed.

S I

Figure 4.1 1 Plate
Based on the plate theory, the maximum stresses are given as follows:

Max(0) = -pip*s2 (at the center of the long edge) (4.20)
t2

0 = (at the center) (4.21)
t
If the aspect ratio of the plate (Us) is greater than 2, p1 =0.5,p2 =0.25 are to be used as
correction factors for the aspect ratio. For plates with an aspect ratio greater than 2, which are
designed against maximum stress at the center, the required minimum thickness is:
t=- 0.5Sfi 4 (4.22)
& =z

where, 0 is the allowable local bending stress, p is the design pressure, and s is the spacing. In
actual design, a corrosion allowance should be added to the calculated thickness.
Allowable bending stresses should be determined by taking into account the plate location, the
stiffening system, and the material strength. Each classification society has its own definition
of allowable stresses.

In the classification rules, formulae are available for design of plating under lateral pressure,
and for the determination of plate thickness. Between classification rules, there is certain
difference in the way corrosion allowance is handled.