Design Examples of Steel and Steel-Concrete Composite Bridges - Finite Element Analysis and Design of Steel and Steel-Concrete Composite Bridges - page 236

plate of the upper and lower flanges of the cross section is taken as 0.2
the web height, which is equal to 600 mm, while the top plate width is
taken as 560 mm, to allow for welding with the bottom flange plate. The
flange plates have the same plate thickness of 30 mm. The choice of two
flange plates for the upper and lower flanges is intended to curtail the
top flange plate approximately at quarter-span as detailed in the coming sec-
tions. It should be noted that the web height value ( L /10) is an acceptable
recommended [1.9] value for railway steel bridges constructed in Great Brit-
ain and Europe. This value is an initial value for preliminary cross-sectional
estimation. The cross section has to be checked, classified, designed, and
assessed against deflection limits set by serviceability limit states. To classify
the cross section chosen,

s

235
f y

r

235
275

¼

e ¼ ¼ 0 : 924

C 1 ¼ 284mm, t fl ¼ 60, C 1 = t fl ¼ 284 = 60 ¼ 4 : 73 9 0 : 924

¼ 8 : 316 Main plate girder flange is class 1 ð Þ:

C 2 ¼ 2984mm, t w ¼ 16, C 1 = t fl ¼ 2984 = 16 ¼ 186 : 5 > 124 0 : 924

¼ 114 : 58 Main plate girder web is class 4 ð Þ:

To calculate the bending moment resistance, the effective area should be
used. Considering web plate buckling, the effective area of the web part in
compression (see Figure 4.16 ) can be calculated as follows:

k s ¼ 23 : 9

= :

300 1 6

l p ¼

p

23

¼ 1 : 462 > 0 : 673

28 : 4 0 : 924 : 9

1 : 462 0 : 055 3 1 ð Þ

r¼ ¼ 0 :

633

462 2

1 :

b eff ¼ 0 : 633 300 = 2 ¼ 95 cm,

Then, b eff1 ¼ 0.6 95 ¼ 57 cm and b eff2 ¼ 0.4 95 ¼ 38 cm as shown in
Figure 4.17 .
To calculate the elastic section modulus, the elastic centroid of the sec-
tion has to be located by taking the first area moment, as an example, around
axis y 0 - y 0 shown in Figure 4.17 , as follows:

6 ¼ 1088 cm 2 A¼ 60 3 2+56 3 2 + 207 1 : 6+38 1 :