||It has been suggested that beam bridge be merged into this article. (Discuss) Proposed since April 2013.|
Two different girder bridges. The top is a plate girder bridge, while the bottom is a concrete girder bridge.
|Related||Trestle, truss bridge, moon bridge|
|Descendant||Box girder bridge, Plate girder bridge|
|Carries||Pedestrians, automobiles, trucks, light rail, heavy rail|
|Span range||Short, Medium|
|Material||Iron, steel, concrete|
A girder bridge, in general, is a bridge built of girders placed on bridge abutments and foundation piers. In turn, a bridge deck is built on top of the girders in order to carry traffic. A girder bridge is very likely the most commonly built and utilized bridge in the world. Its basic design can be compared to a log ranging from one side to the other across a river or creek when at its most simplified form. In modern girder steel bridges, the two most common girders are I-beam girders and box-girders. A girder bridge only requires a rigid horizontal structure, also known as a beam, and two supports at each end to rest upon it. These components are what allows the downward weight of the bridge and any traffic to travel across it.
Many girder bridges utilize concrete or steel beams to fortify the structure and properly handle the load. Both the size and height of the beam controls the distance that the beam can span. By increasing the height of the beam, the beam has more material and thus more easily able to dissipate any tension exerted on the foundation.
Engineers and architects will often add something known as “truss” to the beam of the bridge. This is intended to increase its ability to dissipate the tension and compression to the existing beam, however will aid the bridge to a certain distance before the strain becomes too large.There are several different subtypes of girder bridges:
- A rolled steel girder bridge is made of I-beams that are rolled into that shape at a steel mill. These are useful for spans between 10 metres and 29.5 metres (33 feet to 100 feet). Rolled steel girders are practically available with a web height of up to one metre (3 feet).
- A plate girder bridge is made out of (mostly) flat steel sections that are later welded or otherwise fabricated into an I-beam shape. Plate girders can have a greater height than rolled steel girders. Plate girder spans can be used for spans between 10 metres and more than 100 metres (33 feet to more than 330 feet). The web (vertical section) of a plate girder can be taller than that of a rolled steel girder, providing greater strength than a rolled steel girder. The thickness of the top and bottom flanges of a plate girder does not have to be constant; the thickness can be changed (typically at a field splice) to save on material costs. Stiffeners are occasionally welded between the compression flange and the web to increase the strength of the girder.
- A concrete girder bridge is made of concrete girders, again in an I-beam shape. The concrete girders can be either prestressed cast concrete or post-tensioned girders. Concrete girder bridges are best for spans between 10 metres and 50 metres (33 feet to 164 feet). Prestressed, precast concrete girders are readily available.
- A box girder bridge is built from girders in a rectangular box shape instead of an I-beam shape.
- An I-beam bridge is simple to design and build, and works well for straight spans. However, if the bridge needs to be curved, the beams are subject to twisting forces (torque). This can be alleviated by building several shorter, straight spans with a curved bridge deck, or by using box girders. Building metal box girders is more difficult, though, because the welding of the inner corners between the flanges and the webs has to be done either by a robot or a human, depending on which can fit inside.
In modern steel girder bridges, the most common girders are I-beam girders and box-girders. An I-beam girder is described by having the cross section of the girder taking the shape of the capital letter I. The vertical plate in the middle is known as the web, and the top and bottom plates are referred to as flanges. A box girder is much the same as an I-beam girder except that, obviously, it takes the shape of a box. The typical box girder has two webs and two flanges. 
Girder bridges have existed for millennia in a variety of forms depending on resources available. The oldest types of bridges are the beam, arch and swing bridges, and they are still built today. However, today we refer to the beam bridge as the girder bridge. These types of bridges have been built by human beings since ancient times, with the initial design being much simpler than what we enjoy today. As technology advanced the methods were improved and were based on the utilization and manipulation of rock, stone, mortar and other materials that would serve to be stronger and longer.
In Rome, the techniques for building bridges included the driving of wooden poles to serve as the bridge columns, and then filling the column space with various construction materials. The bridges constructed by Romans were at the time basic; however very dependable and strong while serving a very important purpose in everyday societal life.
As the industrial revolution came and went, new materials with improved physical properties would be utilized; and wrought iron was replaced with steel due to steel's greater strength and larger application potential.
- "Girder Bridge". Argentina: National Institute of Industrial Technology (INTI). Retrieved 26 October 2012.
- Design Technology. "Beam Bridges".
- Nova. "Bridge the Gap", section "Beam Bridge".
- Robert Lamb and Michael Morrissey. "How Bridges Work".
- Ohio Department of Transportation. "Bridge Terms Definitions"
- Leonardo Fernandez Troyano. "Bridge Engineering: A Global Perspective". 2003.
|Wikimedia Commons has media related to Girder bridges.|
- Structural Systems and Dimensions (PDF) - Montana Department of Transportation
- Structural Steel Superstructures (PDF) - Montana Department of Transportation
- Bridge Types - Girder[dead link]
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