Bridges are a piece of engineering which take months and sometimes years to build with many hands in the building process. Here's an overview of the basic steps they follow when engineers design those enormous hunks of steel and concrete.

Design and Placement
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Step 1

Determine what type of bridge you need for the situation. The common types are arch, suspension, truss, cable-stayed and beam types. If you're crossing a short span, you probably only need to build a beam or a small truss bridge. If you're crossing a deep ravine, an arch bridge might be the best choice. A bridge to span an ocean bay requires the very expensive suspension bridge.
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Step 2

Identify an ideal place to build the bridge. A lot of legwork goes into determining the ideal site to build anything, and when you're talking about something as expensive as a bridge, engineers are even more careful. The engineering team will send out surveyors, people who use electronic measuring devices to make a map of the general area. From this map, they will make calculations and determine the length and placement of the bridge.
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Step 3

Design the bridge. The engineer will take into account the potential traffic load of the bridge and use ASTM steel design manuals, computer programs and drafters to aid in the bridge designing process. The materials chosen should also take into consideration the inspection and upkeep costs after the bridge is built.

Bridge Construction
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Step 1

Break ground. Once the design has been finalized and stamped with approval, the actual building can begin. This will involve excavation or filling the banks on either side, stabilizing the ground where the supports will be placed and, depending on what type of bridge is going to be built, pouring concrete.
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Step 2

Assemble the steel. Some bridges are built on site and some are put together in pieces small enough to ship to the location and then assembled. Using the anchors placed in the concrete, the steel is put together like tinker toys using nuts and bolts.
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Step 3

Build the decking for the bridge. This is usually concrete, but can also be aluminum and new engineered composite materials which resists corrosion, decreasing bridge replacement costs.
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Step 4

Pave the new bridge. This is done with commercial paving machines. The government agency sponsoring the project might also include a repaving of the surrounding roadway as well.
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Step 5

Have the ribbon cutting ceremony. Building a bridge is a tremendous accomplishment. After months or possibly years of building this bridge the dust can settle and the public can finally use it.