The main purpose of any bridge is to get any vehicle, walking person, or sometimes even railroad car from point A to point B safely and efficiently over a giant array of natural and manmade barriers. The bridge is the actual structure that supports the road that the vehicle, walking person, or railroad car needs to travel across. There are many different bridge types that all do this same job. A cantilever bridge is only one of many other types. Some of the others are suspension bridges, beam bridges, and arch bridges.
The architects who design the blueprints for a new bridge have to consider three different things before they come up with a final copy: cost, safety, and visual appeal. Architects are only coming up with the blueprints of bridges that might not even be built. Before any full sized bridge is made, many tests are conducted to ensure that the bridge is safe. These tests are made possible because whoever is conducting the tests can take the architect's blueprint and scale it down into a model bridge whose size is appropriate size for testing. If the tests conducted on this model bride show that it is unstable, than the full sized bridge surely wont be made. But in the case that the model is proven to be stable, than it is goes through the next set of tests.
Usually the people living in the area that the bridge is being built in a taxed somehow to help pa for the building of a new bridge. Many times bridges are made to help an area economy so it is almost like an investment of sorts. But in other cases the people aren't to happy about having to pay taxes on the bridge. Because of this main problem, bridges are made to be as cheap as possible. If and architect draws up a blueprint of a bridge that is projected to cost 3 billion dollars to build than naturally that bridge is going to be rejected and not build even if it would be a safe bridge. The government of the local area in which a new bridge is projected to be installed wants to pay the least amount of money as possible.
The last barrier that a bridge blueprint has to overcome is that that it has to be visually appealing. Remember, this is something that represents the place that the bridge is being installed in; if it is going to look like a hideous structure constructed out of scrap metal and it looks like a big pile of junk, then no one is going to want the bridge installed in their area. A great bridge could be denied at the last step just because it was not visually appealing. If a bridge blueprint passes these three tests, then it has a great chance of being constructed and installed.
Cantilever bridges are built using cantilevers--structures that stick out horizontally into space, supported by only one end. Cantilevers generally have two anchor piers on each side of the bridge that are pulling on each other an equal amount therefore making the outside parts of the bridge very strong. The next step to building a good cantilever bridge is to build cantilever arms toward the center of the span and also toward the supporting foundations at the ends of the bridge. This is called balanced cantilever construction. Every time these are made the force of the tension and compression balance each other out to make a very strong structure.
The next step in building the bridge is adding more piers around the giant anchor piers at each end. By doing this you add even more support to the overall structure of the bridge. All of these piers are most likely build off sight but carefully installed on sight. One thing the installers have to be careful of is the natural erosion forces from wind and water. Most of the time wind and water wont pose too much of a threat to the piers of the bridge but more so to the upper parts of the bridge that aren't anchored into the ground.
Once this is done, installers can put in the truss bridges. These are the giant humps that you can see on pretty much every single cantilever bridge out there. These truss bridges can be made on or off sight because once they are built they are just pulled up by cranes and dropped onto and attached to all of the piers that are all already anchored into the ground. Depending on the size of the bridge there can be between 1 and 9 truss bridge parts. You could make even more truss bridge sections but that means the overall length of the bridge would have to massive.
After the truss bridge parts are attached to the piers the roadway decking can be installed. After the roadway decking is installed the actual decking itself can be installed and the bridge will be ready for painting and antirust treating.
Painting is a huge part of maintaining the safety of a bridge. If there wasn't any paint on the steel and iron parts, then gradually over time those parts would deteriorate and become weaker and weaker. Once one of the parts gets rusted so bad that it snaps, the entire bridge could possibly fall to its inevitable death. Paint protects the steel and iron from rain and other weathering. Because bridges are such large structures painting is a never ending process. Once you have completed al of these steps than you have got yourself a perfectly function cantilever bridge ready for public use.
