Plate girder bridge
Encyclopedia
A plate girder bridge is a bridge
supported by two or more plate girder
s. The plate girders are typically I-beam
s made up from separate structural steel
plates (rather than rolled as a single cross-section), which are welded
or, in older bridges, bolted or rivet
ed together to form the vertical web and horizontal flange
s of the beam
. In some cases, the plate girders may be formed in a Z-shape rather than I-shape.
The first tubular wrought iron
plate girder bridge was built in 1846-47 by James Millholland
for the Baltimore and Ohio Railroad
.
Plate girder bridges are suitable for short to medium spans and may support railroads, highway
s or other traffic. Plate girders are usually prefabricated, and the length limit is frequently set by the mode of transportation used to move the girder from the bridge shop to the bridge site.
Generally, the depth of the girder is no less than 1/15 the span, and for a given load bearing capacity, a depth of around 1/12 the span minimizes the weight of the girder. Stresses on the flanges near the centre of the span are greater than near the end of the span, so the top and bottom flange plates are frequently reinforced in the middle portion of the span. Vertical stiffeners prevent the web plate from buckling under shear stresses. These are typically uniformly spaced along the girder with additional stiffeners over the supports and wherever the bridge supports concentrated loads.
bridge deck is supported on top of two or more plate girders, and may act compositely
with them. In the case of railroad bridges, the railroad ties themselves may form the bridge deck, or the deck may support ballast
on which the track is laid. Additional beams may span across between the main girders, for example in the form of bridge known as ladder-deck construction. Also, further elements may be attached to provide cross-bracing and prevent the girders from buckling
.
In the half-through bridge, the bridge deck is supported between two plate girders, often on top of the bottom flange. The overall bridge then has a 'U'-shape in cross-section
. As cross-bracing cannot normally be added, vertical stiffeners on the girders are normally used to prevent buckling (technically described as 'U-frame behaviour'). This form of bridge is most often used on railroads as the construction depth (distance between the underside of the vehicle, and the underside of the bridge) is much less. This allows obstacles to be cleared with less change in height.
serve as intermediate abutments between the end Abutment
s of bridge. Separate plate girder bridges span between each pair of abutments in order to allow for expansion joint
s between the spans. Concrete
is commonly used for low piers, while steel trestle
work may be used for high bridges.
Bridge
A bridge is a structure built to span physical obstacles such as a body of water, valley, or road, for the purpose of providing passage over the obstacle...
supported by two or more plate girder
Girder
A girder is a support beam used in construction. Girders often have an I-beam cross section for strength, but may also have a box shape, Z shape or other forms. Girder is the term used to denote the main horizontal support of a structure which supports smaller beams...
s. The plate girders are typically I-beam
I-beam
-beams, also known as H-beams, W-beams , rolled steel joist , or double-T are beams with an - or H-shaped cross-section. The horizontal elements of the "" are flanges, while the vertical element is the web...
s made up from separate structural steel
Structural steel
Structural steel is steel construction material, a profile, formed with a specific shape or cross section and certain standards of chemical composition and mechanical properties...
plates (rather than rolled as a single cross-section), which are welded
Welding
Welding is a fabrication or sculptural process that joins materials, usually metals or thermoplastics, by causing coalescence. This is often done by melting the workpieces and adding a filler material to form a pool of molten material that cools to become a strong joint, with pressure sometimes...
or, in older bridges, bolted or rivet
Rivet
A rivet is a permanent mechanical fastener. Before being installed a rivet consists of a smooth cylindrical shaft with a head on one end. The end opposite the head is called the buck-tail. On installation the rivet is placed in a punched or pre-drilled hole, and the tail is upset, or bucked A rivet...
ed together to form the vertical web and horizontal flange
Flange
A flange is an external or internal ridge, or rim , for strength, as the flange of an iron beam such as an I-beam or a T-beam; or for attachment to another object, as the flange on the end of a pipe, steam cylinder, etc., or on the lens mount of a camera; or for a flange of a rail car or tram wheel...
s of the beam
Beam (structure)
A beam is a horizontal structural element that is capable of withstanding load primarily by resisting bending. The bending force induced into the material of the beam as a result of the external loads, own weight, span and external reactions to these loads is called a bending moment.- Overview...
. In some cases, the plate girders may be formed in a Z-shape rather than I-shape.
The first tubular wrought iron
Wrought iron
thumb|The [[Eiffel tower]] is constructed from [[puddle iron]], a form of wrought ironWrought iron is an iron alloy with a very low carbon...
plate girder bridge was built in 1846-47 by James Millholland
James Millholland
James Millholland , railway master mechanic, is particularly well known for his invention of many railway mechanisms. His association with the Philadelphia and Reading Railroad Company as master machinist spanned fifty years in the early development of the American railroad...
for the Baltimore and Ohio Railroad
Baltimore and Ohio Railroad
The Baltimore and Ohio Railroad was one of the oldest railroads in the United States and the first common carrier railroad. It came into being mostly because the city of Baltimore wanted to compete with the newly constructed Erie Canal and another canal being proposed by Pennsylvania, which...
.
Plate girder bridges are suitable for short to medium spans and may support railroads, highway
Highway
A highway is any public road. In American English, the term is common and almost always designates major roads. In British English, the term designates any road open to the public. Any interconnected set of highways can be variously referred to as a "highway system", a "highway network", or a...
s or other traffic. Plate girders are usually prefabricated, and the length limit is frequently set by the mode of transportation used to move the girder from the bridge shop to the bridge site.
Generally, the depth of the girder is no less than 1/15 the span, and for a given load bearing capacity, a depth of around 1/12 the span minimizes the weight of the girder. Stresses on the flanges near the centre of the span are greater than near the end of the span, so the top and bottom flange plates are frequently reinforced in the middle portion of the span. Vertical stiffeners prevent the web plate from buckling under shear stresses. These are typically uniformly spaced along the girder with additional stiffeners over the supports and wherever the bridge supports concentrated loads.
Deck-type plate girder bridge
In the deck-type bridge, a wood, steel or reinforced concreteReinforced concrete
Reinforced concrete is concrete in which reinforcement bars , reinforcement grids, plates or fibers have been incorporated to strengthen the concrete in tension. It was invented by French gardener Joseph Monier in 1849 and patented in 1867. The term Ferro Concrete refers only to concrete that is...
bridge deck is supported on top of two or more plate girders, and may act compositely
Composite material
Composite materials, often shortened to composites or called composition materials, are engineered or naturally occurring materials made from two or more constituent materials with significantly different physical or chemical properties which remain separate and distinct at the macroscopic or...
with them. In the case of railroad bridges, the railroad ties themselves may form the bridge deck, or the deck may support ballast
Track ballast
Track ballast forms the trackbed upon which railway sleepers or railroad ties are laid. It is packed between, below, and around the ties. It is used to facilitate drainage of water, to distribute the load from the railroad ties, and also to keep down vegetation that might interfere with the track...
on which the track is laid. Additional beams may span across between the main girders, for example in the form of bridge known as ladder-deck construction. Also, further elements may be attached to provide cross-bracing and prevent the girders from buckling
Buckling
In science, buckling is a mathematical instability, leading to a failure mode.Theoretically, buckling is caused by a bifurcation in the solution to the equations of static equilibrium...
.
Half-through plate girder bridge
Cross section (geometry)
In geometry, a cross-section is the intersection of a figure in 2-dimensional space with a line, or of a body in 3-dimensional space with a plane, etc...
. As cross-bracing cannot normally be added, vertical stiffeners on the girders are normally used to prevent buckling (technically described as 'U-frame behaviour'). This form of bridge is most often used on railroads as the construction depth (distance between the underside of the vehicle, and the underside of the bridge) is much less. This allows obstacles to be cleared with less change in height.
Multi-span plate girder bridge
Multispan plate-girder bridges may be an economical way to span gaps longer than can be spanned by a single girder. PiersPier (architecture)
In architecture, a pier is an upright support for a superstructure, such as an arch or bridge. Sections of wall between openings function as piers. The simplest cross section of the pier is square, or rectangular, although other shapes are also common, such as the richly articulated piers of Donato...
serve as intermediate abutments between the end Abutment
Abutment
An abutment is, generally, the point where two structures or objects meet. This word comes from the verb abut, which means adjoin or having common boundary. An abutment is an engineering term that describes a structure located at the ends of a bridge, where the bridge slab adjoins the approaching...
s of bridge. Separate plate girder bridges span between each pair of abutments in order to allow for expansion joint
Expansion joint
An expansion joint or movement joint is an assembly designed to safely absorb the heat-induced expansion and contraction of various construction materials, to absorb vibration, to hold certain parts together, or to allow movement due to ground settlement or earthquakes...
s between the spans. Concrete
Concrete
Concrete is a composite construction material, composed of cement and other cementitious materials such as fly ash and slag cement, aggregate , water and chemical admixtures.The word concrete comes from the Latin word...
is commonly used for low piers, while steel trestle
Trestle
A trestle is a rigid frame used as a support, especially referring to a bridge composed of a number of short spans supported by such frames. In the context of trestle bridges, each supporting frame is generally referred to as a bent...
work may be used for high bridges.
See also
- Beam bridgeBeam bridgeBeam bridges are the most simple of structural forms being supported by an abutment at each end of the deck. No moments are transferred through the support hence their structural type is known as simply supported....
- the ancestor of the plate girder bridge - Box girder bridgeBox girder bridgeA box girder bridge is a bridge in which the main beams comprise girders in the shape of a hollow box. The box girder normally comprises either prestressed concrete, structural steel, or a composite of steel and reinforced concrete. The box is typically rectangular or trapezoidal in cross-section...
- an evolution of the plate girder bridge - TrestleTrestleA trestle is a rigid frame used as a support, especially referring to a bridge composed of a number of short spans supported by such frames. In the context of trestle bridges, each supporting frame is generally referred to as a bent...
- some modern steelSteelSteel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...
trestles are composed of a number of girder bridge segments. - Pin and hanger assemblyPin and hanger assemblyA Pin and Hanger assembly is used to connect two plate girders of bridges. These assemblies are used when the space between two bridge piers is too wide to be spanned by a single set of girders. To overcome this, steel beams are set with one end resting on a bridge pier, while the other end is...