The Design of Modern Steel Bridges - TEDI

Chapter 2
Types and Properties of Steel
2.1 Introduction
Steel used for building bridges and structures contains:
(1) iron
(2) a small percentage of carbon and manganese
(3) impurities that cannot be fully removed from the ore, namely sulphur and
phosphorus
(4) some alloying elements that are added in very small quantities to improve
the properties of the finished product, namely copper, silicon, nickel,
chromium, molybdenum, vanadium, columbium and zirconium.
The strength of the steel increases as the carbon content increases, but some other
properties like ductility and weldability decreases. Sulphur and phosphorus have
undesirable effects and hence their maximum amount is controlled. Steel used
for building bridges may be grouped into the following three categories:
(1) Carbon steels – only manganese, and sometimes a trace of copper and
silicon, are used as alloying elements. This is the cheapest steel available for
structural uses where rigidity rather than strength is important. It comes with
yield stress up to 275 N/mm 2 and can be easily welded. The Euronorm EN
10025 steels to grades S235 and S275, and the North American steels to
grade 36 of AASHTO M270 and ASTM A709 and earlier ASTM A36 belong
to this category.
(2) High-strength steels – these cover steels of a wide variety with yield
stress in the range of 300 to 390 N/mm 2 . They derive their higher strength and
other required properties from the addition of alloying elements mentioned
earlier. The Euronorm EN 10025 steels to grade S355, and the North American
steels to grade 50 of AASHTO M270 and ASTM A709, and earlier ASTM
A572 belong to this category.
(3) Heat-treated carbon steels – these are the steels with the highest
strength, and still retain all the other properties that are essential for building
bridges. They derive their enhanced strength from some form of heat treatment
after rolling, namely normalisation or quenching-and-tempering. The European
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