The Design of Modern Steel Bridges - TEDI

Table 4.2 Failure probabilities of old designs
Structural components Range of failure
probability of old designs
Compression members 0.2 10 6 – 0.8 10 17
Tension members 1.0 10 8 – 0.5 10 10
Beam compression flanges 0.15 10 11 – 0.8 10 19
Beam tension flanges 0.1 10 14 – 0.8 10 27
Beam webs 0.5 10 4 – 0.3 10 8
Plates in compression 0.25 10 5 – 0.25 10 9
Weighted average 0.632 10 6
Table 4.3 gm factors derived from calibration
Optimised g m2
values, for use with
gf3 ¼ 1.1 and g m1 ¼ 1.08
Range of failure
probabilities
Aims of Design 89
Final g mð¼ g m1g m2Þ
values, for use with
gf3 ¼ 1.1
Struts 0.88 0.3 10 5 – 0.1 10 6
1.05
Yielding of
beam flanges
0.97 1.0 10 6 – 0.6 10 7
1.05
Buckling of
beam webs
1.12 0.25 10 5 – 0.2 10 6
1.05–1.25
Buckling of
stiffened flanges
1.15 1.0 10 6 – 0.3 10 6
1.20
Buckling of
plates in
compression
0.97 0.15 10 5 – 0.15 10 6
1.05
Ties 0.98 0.15 10 5 – 0.25 10 6
1.05
Lateral buckling
of beams
– – 1.20
these with the corresponding figures in Table 4.2, it may be noted that these
ranges are considerably narrower than the ranges of failure probabilities
achieved in old designs.
From these results it was concluded that a rationalised single g mð¼ g m1g m2Þ
value of 1.05 was appropriate for all components that fail by yielding in
tension or compression. Slender struts exhibit a sudden drop in the load carried
after reaching maximum strength, and hence a g m value higher than that
derived as optimum was advisable; hence the same value of 1.05 was adopted
for struts. The design rules for stiffened compression flanges underwent further
rationalisation in the treatment of the strength and stiffness of the flange plates;
to reflect the consequent changes in the mean failure probability, a g m value of
1.20 was finally adopted.