Straight and curved steel box girder bridges - Computational Analysis and Design of Bridge Structures - page 236

e

P /2 P /2 P /2 P /2

P

= +

(a) Loading components

P /2 P /2

= +

Longitudinal bending Bending distortion

(b) Bending load actions

P /2 P /2

= + Mixed torsion Torsional distortion

(c) Torsional load actions

Figure 8.4 General behavior of an open box section under gravity load showing separate
effect.

8.1.1.1 Longitudinal bending
A survey conducted by ASCE Task Committee on horizontally curved
steel box girder bridges revealed that box girders in the United States
typically have an average span-to-depth ratio of 23 for single spans and
25 for continuous girder spans (Heins 1978). For girders with such a large
span-to-depth ratio, any vertical load may cause significant longitudinal
bending and thus longitudinal bending stresses in the girder.
Assuming elastic behavior, normal stresses due to longitudinal bending,
f is given according to the beam theory as

M
S

f = (8.1)

where:
M is the bending moment
S is the section modulus

Shear stresses associated with the moment gradient also occur and are
calculated by

VQ
It

f

v =

(8.2)