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American Steel Design
Section 2
2-68
CASE 2: PNA IN STEEL BEAM
Define:
C
s
= Compressive force in slab = 0.85 . f
c
. b . t
C
b
= Compressive force in steel beam
T
b
= Tensile force in steel beam
C
s
+ C
b
= T
b
Since magnitude of C
b
+ magnitude of T
b
= A
s
. f
y
Substituting for T
b
as (A
s
. f
y
– C
b
), we get:
C
s
+ C
b
= A
s
. f
y
– C
b
C
b
= (A
s
. f
y
– C
s
) x 0.5
Determine whether the PNA is within the top flange of steel beam,
or inside its web.
C
f
= Maximum compressive force carried by flange
= A
f
. f
y
where, A
f
= Area of flange
If C
f
>= C
b
, PNA is in the flange.
If C
f
< C
b
, PNA is in the web.