A metallic bridge is a bridge with a structure made of metal, typically iron, cast iron, or steel.
A metallic bridge is a bridge with a structure made of metal, typically iron, cast iron, or steel.
The first metallic bridge was constructed from cast iron in England. Known as the Iron Bridge, it was built in 1779 by Abraham Darby III over the River Severn at Coalbrookdale. The bridge has a span of 30.5 metres (100 ft) and a total length of 60 metres (200 ft), standing 30 metres (98 ft) above the river. [1]
In France, the first metallic bridge was the Pont des Arts in Paris, constructed in 1803 by Louis-Alexandre de Cessart and Jacques Dillon. The pinnacle of cast iron bridges was reached with the Pont du Carrousel, built in Paris in 1834 by Antoine-Rémy Polonceau. [2]
Suspension bridges made of iron began to develop in the United States in 1810. [3] The widespread use of metallic bridges grew with advancements in steel production techniques, coinciding with the expansion of railway networks. This golden age of metallic bridges continued until World War I, despite the emergence of reinforced concrete in France by 1898. [4]
The steels used in bridge construction are low-alloy iron-carbon alloys. For aesthetic or safety reasons, other steel types, such as Corten steel or stainless steel, may be used. [5]
| Steel Grade | Yield Strength (MPa) | Elongation at Break (%) |
|---|---|---|
| Mild Steel | 235–355 | >15 |
| High-Strength Steel | 355–690 | >15 |
| Ultra-High-Strength Steel (for cables) | 1200–1400 | 1.5–2.5 |
For safety, steel in bridges is designed to operate well below its yield strength. Material fatigue limits stresses to approximately half the yield strength, around 120 megapascals (17,000 psi) for mild steel and 180 megapascals (26,000 psi) for high-strength steel. Fatigue strength is a critical factor in structural calculations. [5] Other factors, such as temperature, stress corrosion cracking, and performance in saline environments, also influence material selection. [6]
Steel profiles used in bridges include:
| Designation | Width (mm) |
|---|---|
| Flat | 30–180 millimetres (7.1 in) |
| Wide Flat | 200–1,000 millimetres (39 in) |
| Sheet | 800–3,600 millimetres (140 in) |
Common profiles include angle iron, U-shaped beams, and T-beams. [7]
Steel assembly methods include bolting, riveting, and welding. [2]
Bolts and rivets secure components through clamping force. Bolts, installed cold, are used for temporary assemblies or in cases where rivets are unsuitable. A bolt consists of a forged head, a threaded shank, and a movable nut screwed onto the threaded portion. [2]
Rivets, installed hot, were historically the primary assembly method in structural steelwork. A rivet has a factory-made head and a shank; the second head is formed by forging the protruding shank while hot, creating a strong clamping force upon cooling. [7]
Welding joins steel by melting and fusing components using coated steel rods (electrodes) that melt under the high temperature of an electric arc. Modern metallic bridges are typically welded, with rivets largely obsolete. Bolts remain in use for emergency bridges, which are assembled rapidly from prefabricated parts. [6]
Metallic beams typically have an I-shaped profile, though U-shaped or box-section profiles are used when height is limited. [5]
Solid web beams consist of one or more vertical webs and horizontal flanges (or wings) on either side. These beams can be hot-rolled (I-beams for smaller sizes) or assembled cold from flat plates through welding (welded reconstituted beams, or PRSs) or, historically, riveting with angle irons. [2]
Flanges, with or without angle irons, form the beam’s chords in welded or rolled structures. [2]
Truss beams, or triangulated beams, consist of chords connected not by a web but by vertical or inclined bars forming a triangulated framework. The arrangement of bars varies depending on the triangulation system used. [6]
Common truss systems include:
Riveted connections were standard before welding became prevalent. Both straight beam and truss bridges used rivets. A typical truss connection includes vertical and horizontal members made of angle irons and plates riveted together, with inclined members using U-shaped beams. Cover plates, or gussets, are added at joints to enhance rigidity. [7]
Modern metallic connections typically involve welding, as seen in solid web beams. A transverse beam, or cross-girder, is welded to a longitudinal beam, or stringer. Vertical stiffeners, often terminating in a gusset, reinforce the assembly. [6]
Depending on the beam structure, these include single box girder bridges (with voussoirs), twin-girder bridges, ribbed bridges, lenticular bridges, and truss bridges. [3]
In a suspension bridge, the beam is called the stiffening girder, typically made of a metallic truss. [3]
Three parameters define a suspension bridge:
For small to medium spans, the relationship between span and sag is generally [8]
| Family | Category | Image |
|---|---|---|
| Beam Bridges | Trestle |  |
| Twin-girder composite |  | |
| Multi-girder composite |  | |
| Box girder composite |  | |
| Orthotropic deck |  | |
| Straight truss |  | |
| Cantilever truss |  | |
| Encased beams |  | |
| Arch Bridges | Suspended-deck metallic arch |  |
| Intermediate-deck metallic arch |  | |
| Supported-deck metallic arch |  | |
| Truss metallic arch |  | |
| Lenticular |  | |
| Bowstring |  | |
| Strut-framed | ||
| Cable-stayed | ||
| Suspension Bridges | Chain suspension |  |
| Concrete deck with stiffening girder |  | |
| Orthotropic deck suspension |  |
