Page 322 - Civil Engineering Formulas

256 CHAPTER TEN

Thickness of stiffener, in (mm), should be at least b f b /71.2, where b is the stiff-
ener width, in (mm); and f is the flange compressive bending stress, ksi (MPa).
b
The bending stress in the stiffener should not exceed that allowable for the material.

HYBRID BRIDGE GIRDERS

These may have flanges with larger yield strength than the web and may be
composite or noncomposite with a concrete slab, or they may utilize an
orthotropic-plate deck as the top flange.
Computation of bending stresses and allowable stresses is generally the
same as that for girders with uniform yield strength. The bending stress in the
web, however, may exceed the allowable bending stress if the computed flange
bending stress does not exceed the allowable stress multiplied by
2
#(1 ) (3 # # )
R 1 (10.39)
6 #(3 #)
where ratio of web yield strength to flange yield strength
# distance from outer edge of tension flange or bottom flange of
orthotropic deck to neutral axis divided by depth of steel section
ratio of web area to area of tension flange or bottom flange of
orthotropic-plate bridge

LOAD-FACTOR DESIGN FOR BRIDGE BEAMS

For LFD of symmetrical beams, there are three general types of members to
consider: compact, braced noncompact, and unbraced sections. The maximum
strength of each (moment, in kip) (mm kN) depends on member dimensions
and unbraced length, as well as on applied shear and axial load (Table 10.3).
The maximum strengths given by the formulas in Table 10.3 apply only
when the maximum axial stress does not exceed 0.15F A, where A is the
y
area of the member. Symbols used in Table 10.3 are defined as follows:
F steel yield strength, ksi (MPa)
y
3
3
Z plastic section modulus, in (mm )
3
3
S section modulus, in (mm )
b width of projection of flange, in (mm)
d depth of section, in (mm)
h unsupported distance between flanges, in (mm)
M smaller moment, in kip (mm kN), at ends of unbraced length of member
1
M F Z
u y
M /M is positive for single-curvature bending.
1 u