Page 110 - Marine Structural Design

86 Part I Siructural Design Principles

The elastic buckling stress of lateral buckling may be derived from column buckling theory
and is given by:
mel = n . E I" (N /mm2) (4.34)
A1
where I, is the moment of inertia of the longitudinal, including attached plate flange, in cm4,
A is the cross-sectional area of the longitudinal, including the attached plate flange, in cm2, 1
is the span of the longitudinal and II is a buckling coefficient, which depends on the end
supports (for an ideal case, n=O.OOl).
It should be noted that the section properties of the longitudinals used in the buckling
evaluation should be the deducted net properties with a corrosion allowance.
Torsional buckling mode:
n2EI,
+
+
(m
a,, = - 3) 0.385E: ( N / mm ') (4.35)
io4 r,iZ
where
(4.36)

where I,, is the warping constant of the longitudinal about the connection of the stiffener to

the plate, in cm6, Ip is the polar moment of inertia of the longitudinal about the connection of
the stiffener to the plate, in cm4, I is the span of the longitudinal, in m, IT is the St. Venant's
moment of inertia of the longitudinal (without the attached plate), in cm4, m is the number of
half-waves (usually varying from 1 to 4), and C is the spring stiffness exerted by the
supporting plate panel.

Web and flange buckling:
For the web plate of longitudinal, the elastic buckling stress is given by:

(4.37)

where t, , is the web thickness, in mm, and h, is the web height, in mm.
For flanges on angels and T-beams, the following requirement should be satisfied:

br
-515 (4.38)
*f
where b, is the flange breadth and t, is the flange thickness.

Eq~(4.29) to (4.33) may also be applied to calculate the critical buckling stress for profiles
and hence to conduct buckling evaluation. Refer to PART II of this book for further details of
buckling evaluation and safety factors.