Civil Engineering Reference
In-Depth Information
hydrogen-induced cracking at welds eliminated (provided correct measures are taken
to eliminate hydrogen from moisture, contaminants, and electrodes). Welding of HPS
steels using low-hydrogen electrodes is done by submerged arc welding (SAW) or
shielded metal arc welding (SMAW) processes (see Chapter 9).
Toughness is significantly increased through reductions in sulfur content (0.006%
max.)andcontrolofinclusions(bycalciumtreatmentofsteel).Thefracturetoughness
of HPS is, therefore, much improved with the ductile to brittle transition occurring at
lower temperatures (the curve shifts to the left in
Figure 2.2)
.
Higher toughness also
translates into greater crack tolerance for fatigue crack detection and repair procedure
development. HPS steels meet or exceed the CVN toughness requirements specified
for the coldest climates (Zone 3 in AREMA, 2008).
The weathering properties of HPS are based on quench and temperedASTMA709
Grade70Wand100Wsteels.Chromium,copper,nickel,andmolybdenumarealloyed
for improved weathering resistance. Improved weathering resistant steels are under
developmentthatmightprovidegoodserviceinevenmoderatechlorideenvironments.
Hybrid
∗
applications of HPS steels with HSLA steels have proven technically and
economically successful on a number of highway bridges (Lwin, 2002) and may be
appropriate for some railway bridge projects.
2.4 STRUCTURAL STEEL FOR RAILWAY BRIDGES
There is no increase in stiffness associated with higher-strength steels (deflections,
vibrations, and elastic stability are proportional to the modulus of elasticity and not
strength). Also, because fatigue strength depends on applied stress range and detail
(see Chapter 5), there is no increase in fatigue resistance for higher-strength steels.
Therefore, the material savings associated with the use of higher-strength steels (with
greater than 50 ksi yield stress) may not be available because deflection and fatigue
criteria often govern critical aspects of ordinary steel railway superstructure design.
The steel bridge designer must carefully consider all design limit states (strength,
serviceability, fatigue, and fracture), fabrication, and material cost aspects when
selecting the materials for railway bridge projects.
2.4.1 M
ATERIAL
P
ROPERTIES
The following material properties are valid for steel used in railway bridge
construction:
490 lb/ft
3
• Modulus of elasticity (Young's modulus),
E
=
29
×
10
6
psi
=
29,000 ksi
• Coefficient of thermal expansion,
• Density,
γ =
10
−
6
per
◦
F
α =
6.5
×
• Poisson's ratio,
0.3 (lateral to longitudinal strain ratio under load)
• Inaccordancewiththetheoryofelasticity,shearmodulus,
G
υ =
=
E/
[
2
(
1
+ υ
)
]
10
6
psi)
(11.2
×
∗
An example is the use of HPS steels for tension flanges in simple and continuous girders.
