TRUSS BRIDGE

A 'truss bridge' is a bridge composed of connected elements (typically straight) which may be stressed from tension, compression, or sometimes both in response to dynamic loads. Truss bridges are one of the oldest types of modern bridges. This type of bridge structure has a fairly simple design and is particularly cheap to construct owing to its efficient use of materials. For purposes of analysis most truss bridges may be considered to be pin jointed where the straight components meet. A more complex analysis may be required where rigid joints impose significant bending loads upon the elements.

In the bridge illustrated in the infobox at right, vertical members are in tension, lower horizontal members in tension, shear, and bending, outer diagonal and top members are in compression, while the inner diagonals are in tension. The central vertical member stabilizes the upper compression member, preventing it from buckling. If the top member is sufficiently stiff then this vertical element may be eliminated. If the lower chord (a horizontal member of a truss) is sufficiently resistant to bending and shear, the outer vertical elements may be eliminated.
The inclusion of the elements shown is largely an engineering decision based upon economics, being a balance between the costs of raw materials, off-site fabrication, component transportation, on-site erection, the availability of machinery and the cost of labor. In other cases the appearance of the structure may take on greater importance and so influence the design decisions beyond mere matters of economics. Modern materials such as prestressed concrete and fabrication methods, such as automated welding, and the changing price of steel relative to that of labor have significantly influenced the design of modern bridges.

Contents

Truss types

History in the United States

External links

See Also

References

Truss types

The truss may carry its roadbed on top, in the middle, or at the bottom of the truss. Bridges with the roadbed at the top or the bottom are the most common as this allows both the top and bottom to be stiffened, forming a box truss. When the roadbed is atop the truss it is called a 'deck truss' (an example of this was the I-35W Mississippi River bridge), when the truss members are both above and below the roadbed, a 'through truss' (an example of this application is the Pulaski Skyway), and where the sides extend above the roadbed but are not connected, a 'pony truss' or 'half-through truss'.
Sometimes both the upper and lower chords support roadbeds, forming a double-decked truss. This can be used to separate rail from road traffic or to separate the two directions of automobile traffic and so avoiding the likelihood of head-on collisions.

History in the United States

Early truss bridges would typically use carefully fitted timbers for members taking compression and iron rods for tension members, usually constructed as a covered bridge to protect the structure. In 1820 a simple form of truss, Town's lattice truss was patented, and had the advantage of not requiring high labor skills nor much metal.

A few iron truss bridges were built in the United States before 1850. Bridges based on the Bollman truss (patented in 1852) were used successfully by the Baltimore and Ohio Railroad. Truss bridges became a common type of bridge to see built from the 1870s through the 1930s. Examples of these bridges still remain across the United States, but their numbers are dropping rapidly, as they are demolished and replaced with new structures. Wrought iron bridges in the U.S. started being built on a large scale in the 1870s. Bowstring truss bridges were a common truss design seen during this time, with their arched top chords. Companies like the Wrought Iron Bridge Company of Canton, Ohio and the King Bridge Company of Cleveland, Ohio became well-known companies, as they marketed their designs to different cities and townships. The bowstring truss design (photo) fell out of favor due to a lack of durability, and gave way to the Pratt truss design, which was stronger. Again, the bridge companies marketed their designs, with the Wrought Iron Bridge Company in the lead. As the 1880s and 1890s progressed, steel began to replace wrought iron as the preferred material. Other truss designs were used during this time, including the camel-back. By the 1910s, many states developed standard plan truss bridges, including steel Warren pony truss bridges. As the 1920s and 1930s progressed, some states, like Pennsylvania continued to build steel truss bridges, including massive steel through truss bridges for long spans. Other states, like Michigan, utilized standard plan concrete girder and beam bridges, and only a limited number of truss bridges were built.
On August 1, 2007 a forty year-old truss bridge on Interstate 35W in Minneapolis collapsed into the Mississippi River.

External links

★ Bridge Basics - A Spotter's Guide to Bridge Design - from Pghbridges.com - Illustrates many of the various types of truss arrangements used in bridges.

★ Historic Bridges of Michigan and Elsewhere - Many photos of truss bridges are available on this informative and mainly truss-focused bridge website.

★ Historic Bridges of the U.S. - An enormous database of historic bridges. Over 1000 truss bridges are listed here.

★ Iron and Early Steel Bridges of Ohio - A comprehensive inventory of all remaining truss bridges in Ohio. Includes maps, photos, and invites visitor assistance in identifying extant or demolished bridges.

★ Matsuo Bridge Company: Bridge Types - Truss

★ Historic Bridges of Iowa - An illustrated list of different architectural bridge types found in Iowa, USA. Many of these are truss bridges.

★ structurae.de The Structurae database on bridges.

See Also

References