Straight and curved steel I-girder bridges - Computational Analysis and Design of Bridge Structures - page 196

and the skew angle is equal to or less than 20°. Otherwise place cross frames
normal to the girders.
The behavior of the steel girder bridges may be grouped as either (1) straight
and nonskewed or (2) curved and/or skewed bridge. According to G13.1
by the AASHTO/NSBA Steel Bridge Collaboration (2011), the behavior
of curved and skewed steel girder bridges can be broadly divided into two
categories:
Basics —Curved or skewed steel girder bridges, or both, experience the
same effects of gravity loading (dead load and live load) as straight girder
bridges.
Curvature and skew effects —Torsional and warping stresses, flange lat-
eral bending, load shifting and warping, and twisting deformations.
In Section 7.1.2, different effects will be characterized as effects of
curvature.
7.1.2 various stress effects
Early steel bridges are primarily straight and simple-span bridges and can
be analyzed by hand. The advent of computers can easily handle indeter-
minate structures, such as continuous span bridges, but are still mainly
straight bridges subjected to major-axis shear and bending moment effects
of the main girders. A curved girder and/or skewed girder bridge, in addi-
tion to the basic vertical shear and bending effects, will be subjected to
torsional effects (Nakai and Yoo 1988). Torsion in steel girders causes
both normal stresses and shear stresses. Because I-shaped girders are in
opened sections and thus have low St. Venant torsional stiffness, they
carry torsion primarily by means of warping. The total normal stress in an
I-shaped girder is a combination of any axial stress, major-axis bending
stress, lateral bending stress, and warping normal stress (Figure 7.3). The
total shear stress is the sum of vertical shear stress, horizontal shear stress,
St. Venant torsional shear stress (generally relatively small), and warping
shear stress (Figure 7.4). For nonskewed straight steel bridge analysis, only
the major-axis bending stress (second term on Figure 7.3) and the vertical
shear stress (first term on Figure 7.4) are dominant, and the rest of the
terms can be ignored in the design phase, but have to be included in other
load combinations for code checking.
The relatively low St. Venant torsional stiffness of I-shaped girders is a
result of their open cross-sectional geometry. The St. Venant torsional shear
flow around the perimeter of the cross section can develop only relatively
small force couples. Without significant force couples, compared to the close
section (described in Chapter 8 for steel box girder bridges), the ability of
I-shaped girders to carry torque through St. Venant torsional response is low.
I-girders carry torsion through the combination of pure torsion and
restrained warping. Diaphragms and/or cross frames provide lateral