CEMP-E
TI 809-07
NOVEMBER 1998
fully effective element, otherwise they are less than fully effective. While such an effect may
appear to be detrimental there can still be a significant amount of strength remaining. The elastic
critical buckling factor, k of a stiffened element can increased by a factor of 9.3 over an
unstiffened element.
kp 2E
fcr =
(Eq 2-1)
2
(
)
w
12 1- m 2
t
1.052 w f
(Eq 2-2)
l=
k t E
when
l 0.673... b = w
(Eq 2-3)
when
l > 0.673... b = rw
(Eq 2-4)
0.22
1-
l
r=
(Eq 2-5)
l
Where:
= an element slenderness factor
k = a plate buckling coefficient = 4 for a stiffened element, and 0.43 for an unstiffened
element
w = the flat width of the element
t = the thickness of the element
f = the design stress in the element determined at the Nominal Moment (Mn) based on the
effective section properties
fcr = the critical elastic buckling stress for the plate
E = the modulus of elasticity of the element
= an effective width factor
= Poisson's ratio (0.3 for steel) in the elastic range
d. Element Slenderness. A review of the slenderness factor shows the relationship of the
width to thickness ratio to design stress. Very thin members are less effective at higher stresses.
The AISI specification limits element w/t ratios to be less then 60 for flanges that are stiffened
with simple lips (C and Z sections), and flanges that are stiffened with elements that are stiffened
(hat sections) to less than 500. When elements have a w/t ratio greater than 30 and 250 they will
display a noticeable waviness before reaching their design capacity, this does not effect the
members final design capacity.
e. Simplified Section Properties. A simplified approach to calculating section properties
uses the centerline of each section element. Section properties are calculated by using the
corners and flat element dimensions. When calculating the moment of inertia the corners are so
small they can be ignored. When a lip is used to stiffened an element the moment of inertia of
2-6
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