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Engineers design metal bridges using 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 to wrought iron for most purposes. 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 has much higher tensile strength.

a - The earliest cast-iron bridge across the River Severn in Great Britain	b - Steel Bridge with Long Middle Superstructure Spans

Figure 8.1 Metal Bridges

The Iron Bridge, spanning the River Severn near Coalbrookdale in Great Britain, is the first cast iron structure (fig. 8.1a), built in 1779 with semicircular arch spans. Each span was 30.6 m long, and has five haunch ribs.Abraham Darby designed the Iron Bridge after French engineers had cast a similar bridge a few years earlier, but they never completed their structure. Though the pedestrian Iron Bridge isa British national monument, it is still used.

Figure 8.2 Diagram of Metal Bridge (схема металлического моста)

a – Metal Girder (металлическая балка);

b – Riveted Superstructure (клёпаное пролётное строение);

c – Welded Superstructure (сварное пролётное строение);

d - Trussed Girder (сквозная ферма).

1 – Girder (балка); 2 – Web (стенка, ребро балки); 3 – Lower Chord (нижний пояс);

4 – Top Chord (верхний пояс); 5 – Angle Ties (уголки);

6 – Rivets (заклёпки); 7 – Cross Beam (поперечная балка);

8 – Longitudinal Ribs (продольные ребра); 9 – Weld Seam (сварной шов);

10 – Panel Length (длина панели моста); 11 – Bottom Chord of Truss (нижний пояс фермы);

12 – Top Chord of Truss (верхний пояс фермы); 13 – Diagonal Strut (раскос);

14 – Hanger (подвеска); 15 – Tie (стойка); 16 – Rib (ребро);

17 – Cross Bracing (поперечные связи);

18 – Longitudinal Braces (продольные связи); 19 - Cross Beam (поперечная балка)

In the 19^th century, bridge builders used wrought iron and later 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 allows easy metalworking compared to other building materials.

Low alloy steel offers superior quality owing to the fact that it contains small amount of elements like nickel, chromium, manganese, etc. Currently stainless steel grades are widely used in both space and building industries. Such steel does not need any surface protection because of the chromium oxide film on its surface.

Metal superstructures, which are suited to intermedium and long spans, have made good appearance lengths between 50 and 1,500 m with continuous and discontinuous beams (fig. 8.2a), frames, arches, trusses (fig. 8.2d) cantilever structures, combined systems, etc. The length of the structures is 150 m for discontinuous beams, and 500 m for cantilever-beam spans.

Frame span length is up to 100 m. Bridge engineers employ trusses for spans of 200 m, and for arch spans of 300 m in length.

Figure 8.3. Joints of Metal Bridge Components

(соединения элементов металлических мостов)

1 – Component (элемент); 2 – Weld Seam (сварной шов); 3 – Rivet (заклёпка);

4 – High Strength Bolt (высокопрочный болт)

Metal bridges offer the following advantages: high-strength building material; large possible span length; durability; large-scale production of bridge construction elements; constructability, access to maintenance and easy overhaul.

The disadvantages of metal bridges include considerable steel consumption, corrodibility, high dynamic sensitivity, and high maintenance costs (due to the need of painting).

Welding (fig. 8.3a), rivets (fig. 8.3b) or bolts (fig. 8.4c) are effective for joining metal elements used in construction. Welding is the largest costs saving method but it is not as reliable as riveting or bolting. Rivets are not widely used because use of them is rather complicated, whereas bolts are commonly used.

BRIDGES OF GREAT BRITAIN

Numerous bridges and one barrier span the River Thames in London. The London Bridge is the oldest in Great Britain and dates back to the time when the Romans invaded Britain and built the first bridge at the place of the wooden Celtic structure by AD 60. The Danes destroyed it in 805 but its replacement also crashed in 1091. Londoners rebuilt the bridge, but it burned down in 1136.

By 1209, a self-taught builder who proposed an arch bridge out of stone (fig. 9.1a) rebuilt the Old London Bridge once again. It was the first European masonry bridge. Its total length was of about 300 m with the spans of 20 m high above the water. The bridge consisted of 19 pointed arches, built on piers; a 20^th opening was a wooden drawbridge. The stone pier foundations were inside cofferdams with timber piles driven into the riverbed. The cutwaters, surrounded by loose stone fillings, protected the bridge piers. As a result, the bridge looked as if it were a dam because the reduced waterway was a quarter of its original width. It caused essential water level differences, and boat sailing was very dangerous and became one of the thrills of Londoners.

The Old London Bridge resulted into an overcrowded village with its chapel and passageways between the multistory houses, built above the shops lining both sides of the road between the fortified gates at either end of the bridge. The heads of traitors were stuck on the gate spikes until the 17^th century. From time to time, the bridge suffered from fires, though buildings and the bridge itself were made of stone. In 1282, five arches of the Old London Bridge collapsed under the pressure of winter ice.

 

a - the Old London Bridge
b – the Rennie’s London Bridge	c – the London Bridge today

 

Figure 9.1 History of the London Bridge

By 1762, Londoners removed the houses to widen the carriageway. Two central arches became one large arch, but it turned into a serious erosion problem for the riverbed. To protect the remaining piers they had constantly to pour gravel. In 1831a new granite structure with five semi elliptical stone arches was built several yards upstream by Sir John Rennie, but in 1971 its stone facing was dismantled and shipped stone-by-stone to the USA where Rennie’s bridge was re-erected in Arizona (fig. 9.1b). Prestressed concrete beams by Harold Knox King replaced Rennie’s stone arches in 1972. The modern London Bridge reaches 104 m in overall span length (fig. 9.1c).