The third limit state for beams is Web Local Buckling, or
WLB for short. This type of buckling occurs when the
width-thickness ratio is not large enough to withstand the
moment on the beam. The way to prevent this type of
buckling is to limit the with-thickness ratio.
The limits can be computed for web local buckling. The
width-thickness ratio is compared to
λ
p
λ
p
and
λ
r
λ
r
. Then the maximum moment can be calculated.
When,
bt<
λ
p
b
t
λ
p
(1)
there is no WLB and the cross section is compact because,
M
n
=
M
p
M
n
M
p
(2)
M
p
=
F
y
Z≤1.5
M
y
M
p
F
y
Z
1.5
M
y
(3)
from the yielding state.
When,
λ
p
<bt<
λ
r
λ
p
b
t
λ
r
(4)
the graph is linear, and therefore a linear interpolation between
M
p
M
p
and
M
y
M
y
is used for the maximum moment.
And finally, when
bt>
λ
r
b
t
λ
r
the graph is non-linear, the web is non-slender, and there
is an equation to find the maximum moment in Appendix F of
the Specification section of the Manual
(page 16.1-96).