DESIGN of BEAMS for FLEXURE or
BENDING STRESS
3) Partially-Compact sections
The stresses caused by the
bending moment are known as bending
or flexure stresses, and the relation
between these stresses and the bending
moment is expressed by the flexure
formula.
A) LATERALLY
BEAMS
Fb = Fy [0.79 – 0.00076 bf /tf √ Fy ]
Flange width – thickness ratio
bf > 170 < 250
2tf √Fy √Fy
SUPPORTED
When Lb < Lc use Fb = 0.66Fy
fb = M/S
Where
Lc = 200bf
√Fy
where S = section modulus
1) Compact sections
Lb = unsupported length of
compression flanged
Fb = 0.66 Fy
When Lb > Lu use Fb = 0.60Fy
Flange width – thickness ratio
Where
bf < 170
2tf √Fy
Lu = 138000
Fy d
bf tf
Web Depth – thickness ratio
When Lb > Lc and Lb < Lu
dw < 1680
tw
√Fy
2) Non-Compact sections
Fb = 0.60 Fy
Flange width – thickness ratio
bf > 170
2tf √Fy
use Fb = Fy [0.79 – 0.00076 bf /tf √ Fy ]