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Steel Designers' Manual - 6th Edition (2003)
1156 Notes on section dimensions and properties
the part of the tension flange that is not tapered and the formula for higher
values of n must be used. If the value of n calculated lies between the lower
change value (n1) and the higher change value (n2), the plastic neutral axis lies
in the tapered part of the flange and then a linear interpolation between the
two formulae is used to calculate the reduced plastic modulus.
Reduced plastic modulus Change value
2
Srx = Srx1 = K1 - K2n ÏD
2 Ê B-t ˆ¸
for n£ n1
= Ì - T + tan(
q)
t
A AË ¯
˝
Ó
4 ˛
Srx = Srx2 = K3( 1-n) ( K4 + n) 2B
Ê B-t ˆ
for n≥ n2
= 1-
T - tan(
q)
A Ë 4 ¯
Ê n - n1
ˆ
Srx = Srx1 + ( Srx2 -Srx1)
Ë n - n ¯
for n1 < n< n2
where
Minor axis bending:
The same principles apply when the sections are loaded axially and bent about
the minor axis, lower and higher values of n indicating that the plastic neutral
axis lies inside or outside the web respectively.
Reduced plastic modulus Change value
S K K n n tD
A
S K n K n n tD
2
ry
= 1 - 2
for <
ry
= 3( 1 - ) ( 4 + ) for ≥
A
where
2 1
K1 = Sx 2
A
K2
=
4t
2
A
K3
=
4B
q = 8∞(
flange taper)
2DB
K4
= -1
A
K1 = Sy 2
A
K2
=
4D
2
A
K3
= 087 .
8T
4BT
K4
= -1
A
(3) Parallel flange channels
Major axis bending:
If the value of n calculated is less than the change value, the plastic neutral axis
is in the web and the formula for lower values of n must be used. If n is greater