Civil Engineering Reference
In-Depth Information
2
Steel for Modern
Railway Bridges
2.1 INTRODUCTION
Modern steel is composed of iron with small amounts of carbon, manganese, and
traces of other alloy elements added to enhance physical properties. Carbon is the
principal element controlling the mechanical properties of steel. The strength of steel
may be increased by increasing the carbon content, but at the expense of ductility and
weldability. Steel also contains deleterious elements, such as sulfur and phosphorous,
that are present in the iron ore.
Steel material development in the latter part of the twentieth century has been
remarkable. Chemical and physical metallurgical treatments have enabled improve-
ments to many steel properties. Mild carbon and high-strength low-alloy (HSLA)
steels have been used for many years in railway bridge design and fabrication.
Recent research and development related to high-performance steel (HPS) metallurgy
has provided modern structural steels with even further enhancements to physical
properties.
The important physical properties of modern structural bridge steels are
• Strength
• Ductility
• Fracture toughness
• Corrosion resistance
• Weldability
2.2 ENGINEERING PROPERTIES OF STEEL
2.2.1 S
TRENGTH
Strength may be defined in terms of tensile yield stress,
F
y
, which is the point where
plastic behavior commences at almost constant stress (unrestricted plastic flow).
Strength or resistance may also be characterized in terms of the ultimate tensile stress,
F
U
, which is attained after yielding and significant plastic behavior. An increase in
strength is associated with plastic behavior (due to strain hardening) until the ultimate
tensile stress is attained
(Figure 2.1)
.
The most significant properties of steel that are
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