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Metal bridges may be constructed of cast iron, iron, steel and aluminum alloys. Cast iron is an alloy of iron that contains carbon, along with varying amounts of silicon and manganese. Cast iron is hard and brittle owing to its high carbon content and therefore is inferior for most purposes to wrought iron. Wrought iron is soft and ductile. Cast iron is strong in compression but weak in tension. Wrought iron, on the other hand, is as strong in compression as cast iron, but it also has much greater tensile strength. The Iron Bridge, spanning the River Severn near Coalbrookdale in Great Britain, is generally considered the first cast iron structure.

In the 19^th century, bridges were erected using wrought iron. Then cast iron and wrought iron were succeeded by high-strength steel. It is stronger and superior to iron in both tension and compression. Due to its rigidity and durability steel is simply the best building material. It is easily treated compared to other building materials. Low alloy steel offers superior quality owing to the fact that it contains such rare elements as nickel, chromium, manganese, etc. Currently stainless steel grades are widely used in both space and building industries. Such steel does not need any painting because of the oxide film on its surface.

Metal superstructures which are suited to medium and long spans, and made a good showing for the length between 50 and 1, 500 m, varying the bridge appearance with continuous and discontinuous beams (fig. 8.1a), frames, arches, trusses (fig. 8.1b and d) cantilever structures, combined systems, etc. The length of the structures ranges within 150 m for discontinuous beams and within 500 m for cantilever-beam spans.

Frame span length is within 100 m limits. Trusses are employed for spans of 200 m in length, and in the case of 300 m, arch spans are built.

Metal bridges offer the following advantages: high-strength building material; great permissible span length; durability; large-scale production of bridge construction elements; convenience at erection, maintenance and overhaul.

The disadvantages of metal bridges include considerable steel consumption, corrodibility, high dynamic sensitivity, and great maintenance costs (in case of painting).

Metal elements used in construction can be jointed by welding (fig. 8.2a and b) rivets (fig. 8.2b and c) or bolts (fig. 8.2c). Welding is the greatest cost saving method but is not as reliable as riveting or bolting. Rivets are not widely used because driving them is rather complicated, and bolts are commonly used (fig. 8.2).

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