A through bridge has the road bed suspended below the trusses or arches, with the roadway passing through them. Truss, suspension, and arch bridges are common types. Through bridges can be further classified by design, such as through truss or through arch. The roadway on an arch bridge is usually suspended from the arches by cables. Suspension bridges are also considered through bridges.
A through bridge is designed so that the road bed is suspended below the trusses or arches which form the supporting structure of the bridge. The roadway then passes through the trusses or arches, rather than over them. Many different types of bridge designs qualify as through bridges, including some truss bridges, arch bridges, and others.
Bridge designs vary widely, and many types are still in use today. Truss bridges, suspension bridges and arch bridges are among the most common. While bridges can be classified in several ways, when classified according to deck or roadway placement, a bridge is either a through bridge or a bridge bridge. A bridge bridge is a type of bridge in which the roadway rests on top of the trusses and any supports. A through bridge has a roadway that runs between or through the trusses, arches, or other suspension towers and cables.
When describing a bridge design, the terms are combined to further distinguish it from other types of bridges. For example, a through truss bridge is different from a through arch bridge. Bridges that have trusses or arches that extend both above and below the deck are called mid-bridges. The trusses for a through bridge are often connected at the top, passing over the roadway, creating a tunnel effect. Historic wooden covered bridges are examples of through bridges where the tunnel effect has been maximized by completely enclosing the roadway. When the trusses are not connected, the bridge is called a pony truss bridge.
On an arch bridge designed as a through or semi-through bridge, the roadway is usually suspended from the arches, often by cables. This can be thought of as a type of suspension bridge, although the term is more often, and more correctly, applied to bridges with large cables running from anchor tower to anchor tower, with smaller cables extending to roadway for support. While through bridges are generally considered to necessarily be based on a truss or arch design, suspension bridges are technically also through bridges, as the roadway is supported by its lower horizontal members even though these are supported by suspension cables.
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