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Steel Designers' Manual - 6th Edition (2003)
For each section, there is a ‘change’ value of n. Formulae for reduced plastic modulus
and the ‘change’ value are given below.
(1) Universal beams, universal columns and bearing piles
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
than the change value, the plastic neutral axis lies in the tension flange and the
formula for higher values of n must be used. 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.
Major axis bending:
Reduced plastic modulus: Change value:
2
Srx= K1 - K2n for
( D-2T) t
n< A
Srx= K3( 1 - n) ( K4 + n) for
( D-2T) t
n≥ A
where
Minor axis bending:
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
K1 = Sx 2
A
K2
=
4t
2
A
K3
=
4B
2DB
K4
= -1
A
K1 = Sy 2
A
K2
=
4D
2
A
K3
=
8T
4BT
K4
= -1
A
Notes on section dimensions and properties 1155
(2) Joists
Major axis bending:
If the value of n calculated is less than the lower change value (n1), the plastic
neutral axis is in the web and the formula for lower values of n must be used.
If n is greater than the higher change value (n2), the plastic neutral axis lies in