Because each bridge is custom-built for a certain location and function, the construction procedure differs from one bridge to the next. The stages outlined below illustrate the primary processes in building a fairly typical reinforced concrete bridge over a shallow river, with intermediate concrete column supports positioned in the river.
The first step in building a reinforced concrete beam bridge is to select a site for the bridge. If possible, choose a site where there are already some kind of support structures such as fences or walls. This will make it easier to determine how much reinforcement is needed for the new bridge and also allow you to use the existing structure as a reference point when positioning bridge components like curbs and sidewalks. Next, measure the width of the stream bed just upstream of where the bridge is to be built. With a metal tape measure, take several measurements across the channel at different points. Use these measurements to calculate the required depth for the base of the bridge. For example, if the measurement between two points is 20 feet and the desired overall height of the bridge is 30 feet, then the required depth of the riverbed under the bridge is about 10 feet.
Now that you have a good idea of how deep the riverbed is, you can start planning the actual layout of the bridge. First, determine where the center of the stream will be located using compass directions as your guide.
In contrast to the more complicated and varied design of bridges, viaducts consist of a primary bearing surface supported by arches of equal spans, which are often made of stone or concrete. Bridges, on the other hand, are typically constructed across bodies of water. The primary difference between the two types of structures is that viaducts are used for roads that do not cross bodies of water, whereas bridges are used for crossings.
Both bridge and viaduct designs have their advantages and disadvantages. Viaducts are known for their stability but they can be difficult to build because multiple foundations must be dug before the structure is erected. This can be problematic if soil conditions change or more than one road needs to be bridged at once. Also, some people find viaducts to be visually unappealing because they lack the charm of a curved bridge window or the drama of a suspension bridge. Finally, due to their uniformity, viaducts tend to be boring from an aesthetic perspective; architects usually add some sort of variation in height or texture as a diversion from the monotony of the bearing walls.
Bridges are generally considered to be more elegant than viaducts because they incorporate natural elements into their design such as windows, piers, and arches. This is especially true of modern bridges which are designed to match the local landscape rather than being flat surfaces with little variation in height.
When a bridge over a body of water requires piers, foundations are formed by lowering caissons into the riverbed and filling them with concrete. The anchorages are then constructed on both ends, typically of reinforced concrete with embedded steel eyebars to which the cables will be anchored. The central part of the bridge is then built up out of decking material, which is usually wood but can also be steel or concrete. The final steps include adding horizontal boards to the top of the piers and vertical boards between each pair of piers to make a walkway for pedestrians or vehicles.
Bridges are used to connect two points that cannot be connected by road, such as when one wants to cross large bodies of water. There are several types of bridges: footbridges, suspension bridges, cable-stayed bridges, and truss bridges. Footbridges are simply overpasses for pedestrians, often made of wood planks laid across two rows of posts. They are easy to build and inexpensive, making them popular around the world. However, they are not safe for traffic, so do not carry any form of motor vehicle. Pedestrians can access both sides of the road from footbridges, so they are useful for small towns where there is no bus service or if one does not want to drive on major roads.
Suspension bridges are exactly what they sound like: a framework of wires or cables holding up the bridge.