The Beam Type
The beam type of bridge is by far the simplest. The beam type of bridge is also the
most common. You probably see beam bridges often. A beam bridge is just a beam supported
by pillars or columns. (See figure.)
Materials and Construction
Beam bridges are commonly built from concrete. Click here for information on concrete.
Beam bridges are also made of steel, or a mixture of steel and concrete. They often
are made in sections, or boxes, where they are attached at the site of the bridge. The
boxes are made out of steel and concrete or just concrete.
Steel beams and boxes
Modern steel beam bridges are usually made section-by-section and then they are
transferred to the site of the bridge and welded together. They are attached to the
supports. It is very costly to maintain a steel beam bridge. The boxes are made in a trapezoid shape, so that wind harmlessly passes under the boxes.
Here are the steps used in building a steel beam/box bridge:
- The box is wheeled out next to the V-frame.
- The box is transferred to the V-frame, where it is brought out to the launching girder. (See Diagram 1.)
- The launching girder is lowered via rods, and the box is welded to the previous box (or connected to the ground.)
- The launching girder is moved forward.
- Step 3 and 4 repeat themselves until the next box will touch the support.
- The box that will touch the support will be put in place. The beam will most
likely be sagging, so the next beam must be fitted in place to eliminate the sagging before being welded.
- The box which touches the support is welded.
- The whole sequence will repeat if there are many supports on the bridge.
This is the most common type of bridge. The concrete beam is versatile and does not
require much maintenance. Concrete beams contain large amounts of steel inside of them.
None of this steel is exposed. The supports that hold up the sections are made at the
site of the bridge.
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