The Design of Modern Steel Bridges - TEDI

178 The Design of Modern Steel Bridges
where
l ¼ buckling half-wavelength ¼ BðDx=DyÞ 0:25
f ¼ l/B
te ¼ effective thickness ¼ t þð AsxÞ=B
B ¼ width of flange between webs
Ix ¼ moment of inertia of one flange stiffener
b0 ¼ spacing between stiffeners.
The above expression shows that an individual flange stiffener can be deemed
to be an isolated Euler strut with an effective length L e given by
We know that
and
where
Le ¼ l pffiffiffi ¼
2
B ffiffiffi
2
p Dx
Dy
0:25
Dx ¼ EIx
b0 ¼ EIxðN þ 1Þ
B
Dy ¼
Et 3
12ð1 v 2 Þ
N ¼ number of flange stiffeners in flange width B
v ¼ Poission’s ratio ¼ 0.3.
Hence
Le ¼ B pffiffi 2
12IxðN þ 1Þð1 v2Þ Bt3 ¼ 1:285 B
t
0:75
0:25
½IxðNþ1ÞŠ 0:25
To be on the conservative side, it is advisable to take
L ¼ 1:5 B
t
0:25
½IxðNþ1ÞŠ 0:25
6.11 A design example of stiffened compression flange
ð6:17Þ
Applied longitudinal stresses due to factored loads are shown below; further
partial safety factors of g f3 ¼ 1.1 and g m ¼ 1.20 are to be allowed for.
A flange stiffener can be taken as a strut composed of a flange plate 375 25
and an angle 200 100 12 (Fig. 6.8).