Straight and curved steel box girder bridges - Computational Analysis and Design of Bridge Structures - page 237

(a) Normal stress (b) Shear stress
Figure 8.5 (a) Normal and (b) shear stress components of longitudinal bending stress.
where:
V is the shear force
I is the moment of inertia of the section
Q is the first moment of area under consideration
t is the width of the section where shear stress is considered
Normal and shear components of longitudinal bending stress are illus-
trated in Figure 8.5a and b, respectively.
8.1.1.2 Bending distortion
When any vertical load is applied on a box girder, bending distortion in trans-
verse direction, or local transverse bending, occurs at the same time as longitu-
dinal bending. This local bending effect could be significant before a box girder
is closed on top. The AASHTO guide specifications (2003) state that if the box
girder does not have a full-width steel top flange, the girder must be treated as
an open section. In open box girders, this distortion causes outward bending
of the webs, upward bending of the bottom flange, and in-plane bending of the
top flange (Figure 8.4b). The transverse bending could cause the cross section
to change shape. Therefore, to prevent bending distortion, the top bracing (ties
and struts) as shown in Figure 8.2 is usually placed between top flanges.
8.1.2 torsional effects
In their studies (Hsu 1989; Hsu et al. 1990; Fu and Hsu 1995), Hsu and
Fu modified Vlasov's theory on curved thin-walled beams (Vlasov 1965;
originally developed for open sections such as I-girders shown in Equation
7.1) to represent the behavior of both open and closed sections for box
girder analysis. The torsional load (Figure 8.4c) causes the section to
1. Rotate rigidly (mixed torsion)
2. Deform (torsional distortion)