Girder bridge

Bridge built of girders placed on bridge abutments and foundation piers

A girder bridge is a bridge that uses girders as the means of supporting its deck. The two most common types of modern steel girder bridges are plate and box. The term "girder" is often used interchangeably with "beam" in reference to bridge design. However, some authors define beam bridges slightly differently from girder bridges. A girder may be made of concrete or steel. Many shorter bridges, especially in rural areas where they may be exposed to water overtopping and corrosion, utilize concrete box girder. The term "girder" is typically used to refer to a steel beam. In a beam or girder bridge, the beams themselves are the primary support for the deck, and are responsible for transferring the load down to the foundation. Material type, shape, and weight all affect how much weight a beam can hold. Due to the properties of the second moment of area, the height of a girder is the most significant factor to affect its load capacity. Longer spans, more traffic, or wider spacing of the beams will all directly result in a deeper beam. In truss and arch style bridges, the girders are still the main support for the deck, but the load is transferred through the truss or arch to the foundation. These designs allow bridges to span larger distances without requiring the depth of the beam to increase beyond what is practical. However, with the inclusion of a truss or arch the bridge is no longer a true girder bridge.

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. These types of bridges have been built by human beings since ancient times, with the initial design being much simpler than what we utilize 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 ancient 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 but very dependable and strong while serving a very important purpose in social life. As the Industrial Revolution came and went, new materials with improved physical properties were utilized; and wrought iron was replaced with steel due to steel's greater strength and larger application potential.

A rolled steel girder is a girder that has been fabricated by rolling a blank cylinder of steel through a series of dies to create the desired shape. These create standardized I beam and wide flange beam shapes up to 100 feet in length. A plate girder is a girder that has been fabricated by welding plates together to create the desired shape. The fabricator receives large plates of steel in the desired thickness, then cuts the flanges and web from the plate in the desired length and shape. Plate girders can have a greater height than rolled steel girders and are not limited to standardized shapes. The ability to customize a girder to the exact load conditions allows the bridge design to be more efficient. Plate girder can be used for spans between 10 metres and more than 100 metres (33 feet to more than 330 feet). Stiffeners are occasionally welded between the compression flange and the web to increase the strength of the girder. A box girder or "tub girder" is, as the name suggests, a box shape. They consist of two vertical webs, short top flanges on top of each web, and a wide bottom flange connecting the webs together. A box girder is particularly resistant to torsion and, while expensive, are utilized in situations where a standard girder might succumb to torsion or toppling effects.