 |
Typical Span Lengths | 10m - 200m | |
|---|---|---|---|
| World's Longest | Costa Silva Bridge, Portugal | ||
| Total Length | 700m | ||
| Center Span | 300m | ||
|
Typical Span Lengths | 10m - 200m | |
|---|---|---|---|
| World's Longest | Costa Silva Bridge, Portugal | ||
| Total Length | 700m | ||
| Center Span | 300m | ||
Basics
The two main types of girder bridges can be split into I-beam girders and box-girders. In addition there are simple girders and continuous girders.
An I-beam girder (plate girder) takes the shape of the capital letter I. The vertical plate is called the web and the horizontal plates on the top and bottom are the flanges. A box girder is much the same except that with two or more web plates between the flange plates it take the shape of a box.
I-beam girders are generally simple to design and fabricate and are perhaps the most common steel bridge. However they are not very stable under twisting forces and thus are not well suited to curved bridges. The box girder on the other hand is a much stronger design, especially for twisting forces, and is often used in longer as well as curved bridges. However, design and fabrication are more difficult than the I-beam girder.
A simple girder is one that is supported only on the ends of the bridge. These supports are called bearings and in most cases allow one end of the bridge to move to compensate for expansion/contraction due to temperature changes. A continuous girder is one that is supported by bearings not only on the ends but in one or more places in the middle as well. Continuous girders are more economical, but the bending forces that occur above the middle bearings can cause cracking in the concrete slab, so special measures must be taken to prevent this.
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