Civil Engineering Reference
In-Depth Information
the resistance to atmospheric corrosion as carbon steels (Kulak and Grondin, 2002),
whichmakestheiruseinbridgeseconomicalfromalifecycleperspective.Weathering
resistance can be estimated by alloy content equations given in ASTM G101.
∗
An
index of 6.0 or higher is required for typical bridge weathering steels.
Nonweathering steels can be protected with paint or sacrificial coatings (hot-dip or
spray applied zinc or aluminum). Shop applied three-coat paint systems are currently
used by many NorthAmerican railroads. Two, and even single, coat painting systems
are being assessed by the steel coatings industry and bridge owners. An effective
modern three-coat paint system consists of a zinc-rich primer, epoxy intermediate
coat, and polyurethane top coat. For aesthetic purposes, steel with zinc or aluminum
sacrificial coatings can be top coated with epoxy or acrylic paints.
2.3 TYPES OF STRUCTURAL STEEL
2.3.1 C
ARBON
S
TEELS
Moderncarbonsteelcontainsonlymanganese,copper,andsiliconalloys.Mildcarbon
steel has a carbon content of 0.15-0.29%, and a maximum of 1.65% manganese (Mn),
0.60% copper (Cu), and 0.60% silicon (Si). Mild carbon steel is not of high strength
but is very weldable and exhibits well-defined upper and lower yield stresses (Steel 1
in
Figure 2.1
).
Shapes and plates of ASTM A36 and A709 Grade 36 are mild carbon
steels used in railway bridge fabrication.
2.3.2 H
IGH
-S
TRENGTH
L
OW
-A
LLOY
S
TEELS
Carbon content must be limited to preclude negative effects on ductility, toughness,
and weldability.Therefore, it is not desirable to increase strength by increasing carbon
content and manipulation of the steel chemistry needs to be considered. HSLA steels
have increased strength attained through the addition of many alloys.
Alloy elements can significantly change steel phase transformations and properties
(Jastrewski, 1977). The addition of small amounts of chromium, columbium, copper,
manganese, molybdenum, nickel, silicon, phosphorous, vanadium, and zirconium in
specified quantities results in improved mechanical properties. The total amount of
these alloys is
<
5% in HSLA steels. These steels typically have a well-defined yield
stress in the 44-60 ksi range (Steel 2 in
Figure 2.1)
.
Shapes and plates of ASTM
A572, A588, and A992 (rolled shapes only) and A709 Grade 50, 50S, and 50W are
HSLA steels used in railway bridges.
A572 Grade 42, 50, and 55 steels are used for bolted or welded construction.
Higher-strength A572 steel (Grades 60 and 65) is used for bolted construction only,
due to reduced weldability. A572, A588, and A992 steels are not material toughness
graded at the mills and often require supplemental CVN testing to ensure adequate
toughness, particularly for service in cold climates.A588 andA709 Grade 50W steels
are atmospheric corrosion-resistant (weathering) steels. ASTM A709 Grade 50, 50S,
and 50W steel is mill certified with a specific toughness in terms of the minimum
∗
Other equations, such as theTownsend equation, have also been proposed and may be of greater accuracy.
