Temporary Support Method. This is perhaps the most traditional and common method. As its name implies, temporary supports are placed at midpoints between piers and abutments. The pieces of the bridge are then put in place, supported by the temporary supports, and joined together. When the entire girder (one span) is complete, the temporary support can be removed.
The temporary support method requires that the locations for the supports be clear and stable. Often the ground is not sufficiently stable and it is necessary to construct foundations for the temporary supports because of the weight they will bear. The assembly and removal of temporary supports require lots of equipment and labor. Even so, it is one of the cheapest and most commonly used methods.
Cable and Tower Method. Often conditions under the bridge are not suitable for the use of temporary supports. This can happen when the valley or ravine is too deep, the flow of the river is too rapid, or environmental reasons prevent the use of temporary supports under the bridge. If conditions on both sides of the bridge are suitable to erecting towers and placing cable anchorages (just like a suspension bridge), the cable erection method can be used.
There are two basic versions of the cable erection method; the diagonal cable method (much like a cable-stayed bridge) and the vertical cable method (much like a suspension bridge). In both methods, cables stretched between the two towers suspend a crane that carries the parts to their location for placement and attachment.
In the diagonal method, cables extended directly from the tower tops. The girder supports the semi-complete bridge. The tension in each individual cable must be carefully monitored and adjusted to maintain balance and keep the structure in its proper position.
In the vertical method, a structure much like a suspension bridge is created to support the bridge during erection. Support girders are hung from each hanger rope and serve as temporary supports. The main cable tension must be adjusted to keep the structure from sagging during erection.
Cable erection is a complex and difficult erection method. Steel cables stretch and elongate under heavy loads. As more weight is added to each support cable, it stretches changing the balance. This makes calculations and management of the erection quite difficult.
Cantilever Method. A cantilever generally refers to a beam or girder that is unsupported on one or more ends. A diving board is a good example of a common cantilever as it is supported on only one end.
In perhaps the most common cantilever erection, the side spans are erected first (often using temporary supports). These side spans then serve as counter weights as the girder is assembled toward the center. Once the girder is joined at the center the bridge is complete and there are no cantilever sections.
The longer the cantilever the greater the forces that occur at the support. Just as a diving board sags more and more the closer the diver gets to the far edge, greater and greater forces are at work the longer the unsupported section becomes. Special care must be taken to make sure the cantilevered section is stable and can withstand the forces at work on it.
Large Block Method. The large block erection method is perhaps one of the most simple and elegant. Very large sections of the bridge, in some cases the entire bridge, are put in place at one time. There are many advantages to being able to do this.
First, it reduces the amount of work that must be done at the site. Since the bridge or block can be assembled before shipping, it can be done in a safer and more controlled environment. If this work is at the production workshop, this can reduce costs, as there is no need to mobilize large amounts of people and equipment,
Second, there is less risk involved. Since the bridge is assembled on the ground workers are not placed at risk in high or dangerous places. There is also less risk of the bridge collapsing midway through the erection process or of damage to the individual parts during shipping or erection.
One other advantage is that large block erection can significantly shorten the length of time required for the project. The amount of time spent erecting temporary supports or scaffolding can be cut to almost zero.
On the other hand, large block erection can only be performed in certain special cases. The location must be one that will allow the entire bridge or large block to be transported from the fabricator to the job site. This method also requires the use of a crane or cranes large enough to lift the entire bridge or block. Sufficient heavy equipment and a route of travel for it must be available. In Japan this method is often used with bridges near the sea or on large rivers. The entire bridge or large parts of it are carried on barges. They are then put on place by huge floating cranes. The largest crane can carry 4,100 metric tons (9 million pounds).
