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A girder bridge, in general, is a bridge that utilizes girders as the means of supporting the deck. A bridge consists of three parts: the foundation (abutments and piers), the superstructure (girder, truss, or arch), and the deck. A girder bridge is very likely the most commonly built and utilized bridge in the world. Its basic design, in the most simplified form, can be compared to a log ranging from one side to the other across a river or creek. In modern girder steel bridges, the two most common shapes are plate girders and box-girders. The term "girder" is often used interchangeably with "beam" in reference to bridge design.〔 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" ) 〕 However, some authors define beam bridges slightly differently from girder bridges.〔 Leonardo Fernandez Troyano. ("Bridge Engineering: A Global Perspective" ). 2003. 〕 A beam may be made of concrete or steel - many shorter bridges, especially in rural areas where they may be exposed to overtopping and corrosion, will utilize concrete box beams. 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 inertia, 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. ==Design== All bridges consist of two main parts: the substructure, and the superstructure. The superstructure is everything from the bearing pads, up - it is what supports the loads and is the most visible part of the bridge. The substructure is the foundation, what transfers the loads from the superstructure to the ground. Both parts must work together to create a strong, long-lasting bridge. The superstructure consists of several parts: *The deck is the roadway or walkway surface. In roadway applications it is usually a poured reinforced concrete slab, but can also be steel grid or wood plank. The deck includes any road lanes, medians, sidewalks, parapets or railings, and miscellaneous items like drainage and lighting. *The supporting structure consists of the steel or concrete system supporting the deck. This includes the girders themselves, diaphragms or cross-braces, and (if applicable) the truss or arch system. In a girder bridge this would include only the girders and the bracing system. The girders are the primary load support, while the bracing system both allows the girders to act together as a unit, and prevents the beams from toppling. *The job of the bearing pads is to allow the superstructure to move somewhat independently of the substructure. All materials naturally expand and contract with temperature - if a bridge were completely rigid, this would cause unnecessary stress on the structure and could lead to failure or damage. By fixing the superstructure at one end, while allowing the other end of a span to move freely in the longitudinal direction, thermal stresses are alleviated and the lifespan of the bridge increased. The substructure is made of multiple parts as well: *An abutment is the foundation that transfers the bridge structure to the roadway or walkway on solid ground. A pier is an intermediate support. *The cap is the part that supports the bearing pads. Depending on the type of support structure, there may or may not be a cap. Wall piers and stub abutments do not require a cap, while a multi-column, hammerhead, or pile-bent pier will have a cap. *The stem or stub is the main body of the foundation. It transfers the load from the superstructure, through the cap, down to the footer. *The footer is the structure that transfers the loads into the ground. There are two primary types of systems: a spread footer, which is a simple concrete slab resting on bedrock; or a piling cap, which utilizes steel piles to reach sound bedrock that may be deep underground. Another system utilizes caissons or steel-reinforced concrete "pillars" below the stem. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Girder bridge」の詳細全文を読む スポンサード リンク
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