Articles - Girder Bridge

 



Girder Bridge

 

A girder bridge is perhaps the most common and most basic bridge. A log across a creek is an example of a girder bridge in its simplest form. In modern steel girder bridges, the two most common girders are I-beam girders and box-girders.

Typical Span Lengths

10m - 200m

World's Longest

Ponte Costa e Silva, Brazil
 

Total Length

700m

Center Span

300m

A Matsuo Example

Namihaya Bridge

Illustration #1: I-Beam Cross Section

 

If we look at the cross section of an I-beam girder we can immediately understand why it is called an I-beam (illustration #1.) The cross section of the girder takes the shape of the capital letter I. The vertical plate in the middle is known as the web, and the top and bottom plates are referred to as flanges. To explain why the I shape is an efficient shape for a girder is a long and difficult task so we won't attempt that here.

Illustration #2: Box Girder Cross Section

 

A box girder is much the same as an I-beam girder except that, obviously, it takes the shape of a box. The typical box girder has two webs and two flanges (illustration #2.) However, in some cases there are more than two webs, creating a multiple chamber box girder.

Other examples of simple girders include pi girders, named for their likeness to the mathematical symbol for pi, and T shaped girders. Since the majority of girder bridges these days are built with box or I-beam girders we will skip the specifics of these rarer cases.

Now that we know the basic physical differences between box girders and I-beam girders, let's look at the advantages and disadvantages of each. An I-beam is very simple to design and build and works very well in most cases. However, if the bridge contains any curves, the beams become subject to twisting forces, also known as torque. The added second web in a box girder adds stability and increases resistance to twisting forces. This makes the box girder the ideal choice for bridges with any significant curve in them.

Box girders, being more stable are also able to span greater distances and are often used for longer spans, where I-beams would not be sufficiently strong or stable. However, the design and fabrication of box girders is more difficult than that of I beams. For example, in order to weld the inside seams of a box girder, a human or welding robot must be able to operate inside the box girder.