Civil Engineering Reference
In-Depth Information
of the more complex methods available by fracture mechanics testing and analysis
(Anderson, 2005).
Failurebyaccumulatedfatiguedamage(initiationandpropagationofsmallcracks)
caused by repeated cycles of tensile stress is of primary concern in the design of
steel railway superstructure members and connections. The fatigue life, or number
of cycles to failure (generally taken as through-thickness fracture of a component),
depends on the frequency and number of load cycles, load magnitude (in particular,
stress range), member size, and member details. A fracture mechanics approach to
fatigue design is not generally used for ordinary steel bridge design (Fisher, 1984;
Kulak and Smith, 1995; Dexter, 2005). Therefore, the stress-life approach, recom-
mended for the design of steel bridges by AREMA (2008), is outlined further in this
chapter.
The serviceability criteria (or limit states) of deflection and fatigue often govern
important aspects of the structural design of steel railway bridges.
5.3.2
S
TEEL
R
AILWAY
S
UPERSTRUCTURE
D
ESIGN
5.3.2.1
Strength Design
ThestrengthdesignofmembersandconnectionsasrecommendedbyAREMA(2008)
is performed through elastic structural analyses and the ASD method. The ASD
methodology divides the ultimate and yield stress of the steel by an FS to determine
allowable stresses. Yield stress is associated with plastic deformation and ultimate
stress with fracture. Internal stresses in members and connections must not be greater
than the allowable yield or fracture criteria. As indicated in Chapter 2, the yield and
ultimate stresses for tension, compression, and shear are all expressed in terms of the
yield and ultimate tensile stresses.
The FS for tensile stresses recommended by AREMA (2008)
(
9
/
5
=
1.80
∼
1
/
0.55
)
is greater than the typical allowable tensile stress FS
(
5
/
3
1
/
0.60
)
used in building or highway bridgeASD because of the high magnitude dynamic and
cyclical live load regime of steel railway bridges. Further considerations relating to
the larger FS for steel railway superstructures are the fracture (cold weather service),
corrosion (industrial and wet environments), and damage susceptibility (railway and
highway vehicle contact) of the superstructure due to location.
The FS for ASD design of compression members is generally taken as between
1.9 and 2.0 because of stability issues relating to unintended eccentricities and initial
curvature of compression members. However, for short axial compression members
thatwillyieldpriortobuckling,theFScorrespondingtocompressiveyielding(related
to tensile stresses, see Chapter 2) of 9
/
5
=
1.8 could be used. A cubic polynomial
equation (representing a quarter sine wave) is an appropriate transition function for
a compression member FS (Salmon and Johnson, 1980) and can be applied to the
AREMA (2008) recommended FS for axial compression stresses as shown in
Fig-
end condition) (see Chapter 6),
L
is the length of the member,
r
is the radius of
gyration of the member, and
C
cr
is the limiting or critical value of (
KL/r
) at pro-
portional limit (0.50
F
y
) to preclude instability in the elastic range (Euler buckling).
=
1.67
=
