Steel used in bridges

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Steel used for bridges may be grouped into the following three categories:
(i)     Carbon steel: This is the cheapest steel available for structural users
        where stiffness is more important than the strength. Indian steels have
        yield stress values up to 250 N/mm2 and can be easily welded. The
        steel conforming to IS: 2062 - 1969, the American ASTM A36, the
        British grades 40 and Euronorm 25 grades 235 and 275 steels belong
        to this category.
        High strength steels: They derive their higher strength and other
(ii)
        required properties from the addition of alloying elements. The steel
        conforming to IS: 961 - 1975, British grade 50, American ASTM A572
        and Euronorm 155 grade 360 steels belong to this category. Another
        variety of steel in this category is produced with enhanced resistance
        to atmospheric corrosion. These are called 'weathering' steels in
        Europe, in America they conform to ASTM A588 and have various
        trade names like ' cor-ten'.
(iii)   Heat-treated carbon steels: These are steels with the highest
        strength. They derive their enhanced strength from some form of heat-
        treatment after rolling namely normalisation or quenching and
        tempering.
   
The physical properties of structural steel such as strength, ductility, brittle
fracture, weldability, weather resistance etc., are important factors for its use in
bridge construction. These properties depend on the alloying elements, the
amount of carbon, cooling rate of the steel and the mechanical deformation of the
steel.