The Design of Modern Steel Bridges - TEDI

174 The Design of Modern Steel Bridges
(2) For the support region:
or
or
sa þ sa y
r 2
sa þ sa y
r 2
sE
sE sa
sE
sE sa
þ sb >j s 0 y
>j ðs 0 y
sa >j Rs 00
y
where Rs00 y is the strut strength of a strut made of material yield stress s00 y ,
and s 00
y ¼ s0y sb
sbÞ
ð6:14aÞ
and R is the ratio of the strut strength to material yield stress of a strut made of
material yield stress s00 y .
Alternatively
sa
Rs0 y
þ sb
s0 y
>j 1 ð6:14bÞ
Comparing equation (6.13) with this equation, it should be noted that ssu is
less than Rs0 y ,asRis calculated for a strut of a lower effective material yield
; this reflects the benefit of the applied bending moment not being
magnified by the axial load over the support section.
stress s 00
y
6.8 Effect of variation in the bending moment of a girder
The applied axial stress in a longitudinal stiffener in a stiffened flange varies
according to the shape of the bending moment diagram on the box or plate
girder. Such a stiffener may be checked as a uniformly compressed strut with
an equivalent applied axial load equal to that occurring at a distance 0.4L from
the heavily stressed end. However, for a relatively slender strut, i.e. one with
high ‘l/r’ ratio, subjected to a substantial variation in the axial loading at its
two ends, such an approximate method may be too conservative. A more accurate
method, in which the magnitude and location of the maximum bending
moment is calculated for such a strut with initial out-of-straightness imperfection,
is given in Reference [2].
6.9 Transverse stiffeners in stiffened compression flanges
In the design of longitudinal stiffeners, it has been assumed that the transverse
stiffeners shall provide adequate support to the longitudinal stiffeners, so that