Steel bridge

File:Garabit.jpg

The Garabit Viaduct, a metallic arch bridge

A metallic bridge is a bridge with a structure made of metal, typically iron, cast iron, or steel.

History

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.

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.

Suspension bridges made of iron began to develop in the United States in 1810. 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.

Materials

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.

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. Other factors, such as temperature, stress corrosion cracking, and performance in saline environments, also influence material selection.

Profiles

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.

• 
  Unequal angle iron
• 
  I-beam
• Error creating thumbnail:
  U-shaped beam

Assembly methods

Steel assembly methods include bolting, riveting, and welding.

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.

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.

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.

Metallic beams

Metallic beams typically have an I-shaped profile, though U-shaped or box-section profiles are used when height is limited.

Solid web beams

Main article: Plate girder

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.

Flanges, with or without angle irons, form the beam’s chords in welded or rolled structures.

• Riveted solid web beam
  Riveted solid web beam
• Riveted U-shaped beam
  Riveted U-shaped beam
• Riveted box-section beam
  Riveted box-section beam
• Welded reconstituted beam
  Welded reconstituted beam

Truss beams

Main articles: Truss bridge and Truss

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.

Common truss systems include:

• Pratt truss
  Pratt truss
• Town truss
  Town truss
• Double Town truss
  Double Town truss
• K truss
  K truss
• Warren truss
  Warren truss
• Warren truss with verticals
  Warren truss with verticals
• Howe truss
  Howe truss
• Cross of Saint Andrew (Pratt and Howe combination)
  Cross of Saint Andrew (Pratt and Howe combination)
• Compound truss
  Compound truss
• Vierendeel truss (non-triangulated)
  Vierendeel truss (non-triangulated)

Beam connections

Riveted connections

File:Assemblage-métallique-rivé.jpg

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.

Welded connections

File:Viaduc-de-Briare (4).JPG

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.

Types of metallic bridges

Straight beam bridges

Depending on the beam structure, these include single box girder bridges (with voussoirs), twin-girder bridges, ribbed bridges, lenticular bridges, and truss bridges.

Suspension bridges

In a suspension bridge, the beam is called the stiffening girder, typically made of a metallic truss.

File:Pont-suspendu-Indicateurs.svg

Three parameters define a suspension bridge:

• Its span, equal to its length (L) for a single-span bridge without approach spans.
• Its sag (f), the distance between the midpoint of the chord connecting the pylon tops and the midpoint of the suspension cable.
• The height (H) of the stiffening girder, typically between L/80 and L/100.

For small to medium spans, the relationship between span and sag is generally $f=\frac{L}{9}$

Detailed classification

Family	Category	Image
Beam Bridges	Trestle	File:WilburtonTrestleEast.jpg
	Twin-girder composite	File:Bipoutre-mixte.svg
	Multi-girder composite	File:Multipoutre-mixte.svg
	Box girder composite	File:Caisson-mixte.svg
	Orthotropic deck	File:Dalle-orthotrope.svg
	Straight truss	File:BA-riverfront-17.jpg
	Cantilever truss	File:Pont de Québec vu du Parc aquarium du Québec.JPG
	Encased beams	File:Poutrelles-enrobées.svg
Arch Bridges	Suspended-deck metallic arch	File:Aggersundbroen1.jpg
	Intermediate-deck metallic arch	File:La Conner 32328.JPG
	Supported-deck metallic arch	File:Bourguignon-lès-Conflans - pont métallique sur la Lanterne 03.JPG
	Truss metallic arch	File:France Cantal Viaduc de Garabit 05.jpg
	Lenticular	File:Saltashrab.jpg
	Bowstring	File:Viaduc-de-Briare (11).JPG
	Strut-framed
Cable-stayed
Suspension Bridges	Chain suspension	File:Cavenagh Bridge 3, Dec 05.JPG
	Concrete deck with stiffening girder	File:Verrazano-Bridge-Dawn.jpg
	Orthotropic deck suspension	File:Akashi-kaikyo bridge3.jpg

See also

• Suspension bridge
• Cable-stayed bridge
• Steel
• Structural engineering
• Bridge
• Fatigue (material)
• Welding
• Iron Bridge (disambiguation)