Patuxent Branch Trail
Railroad expansion in the 19th century required strong, dependable bridges to carry trains over ravines and rivers. Early bridges were built of stone that was sturdy but expensive. Timber bridges often had to be replaced as wood weathered or was burned by hot cinders from steam locomotives. When cast iron, and later wrought iron, came into use, bridges could be built of great strength and longer spans. By the turn of the 20th century, steel had replaced iron for railroad bridges.
, Caleb and Thomas Pratt developed a bridge that was built with wood and diagonal iron rods. They patented the design, which was made up of sections called trusses. Soon, they built the bridge entirely of iron. This bridge had the advantage of low-cost construction, because the iron parts were made in shops, the parts were easily transported to the site, and the bridge could be quickly erected by semi-skilled labor. It was so popular, it became the standard American truss bridge for moderate spans (from 25 feet to 150 feet), well into the 20th century.
The Pratt Bridge in Guilford
is 83 feet long. The distance between the side trusses is 15 feet, 6 inches. It is a single-span structure designed to carry one set of train rails. Instead of crossing the river at right angles, the bridge has a built-in 35-degree left-hand skew. There are inclined end-posts at each end, and pin-connected eye-bars.
[Picture of Pratt Bridge in Guilford] Before:
The old bridge lies neglected and overgrown with vines. There is no flooring and the beams are covered with graffiti.
[Picture of pin connection] A Bottom-Chord Pin Connection
(Photo courtesy of GPI Greenman-Pederson Inc, designers of the bridge renovations)
This bridge was built in 1902
to carry the Patuxent Branch of the Baltimore and Ohio Railroad over the Little Patuxent River. The train carried heavy loads of granite stone from the Guilford quarries until 1925. The bridge abutments are made from this granite, which was used to construct many other bridge abutments and culverts on the B&O lines. After the railroad spur was abondoned, the bridge surface was planked to serve for a time as a local farm road.
Although hundreds of these bridges were built, only a few survive. Several miles downstream, a similar Pratt bridge, known as the Gabbro Bridge, once carried the quarry train over the Middle Patuxent River. It was washed away by floods.
Long abandoned, the Pratt Through-Truss Bridge was rescued by the Howard County Department of Recreation and Parks and adapted to carry the Patuxent Branch Trail across the river. Its reopening was celebrated with a ribbon cutting on November 2, 2002 - over 100 years after it was built.
[Diagram of a typical truss bridge] Diagram of a Typical Truss Bridge
(Courtesy of Historic American Engineering Record, National Park Service)
Caleb and Thomas Pratt
were father-and-son team from Boston. Caleb was an architect, and his son Thomas, born in 1812, became an engineer. By the age of 12, Thomas was preparing plans in his father's office. When he was 14, he attended Rensselaer Polytechnic Institute in Troy NY and then went to work for the railroads designing bridges and other structures. He designed his first truss bridge in 1842; then he and his father were granted a joint patent in 1844 for the Pratt Truss Bridge. When they made the bridge entirely of iron, the pioneered the age of iron railroad bridges.
[image of reopened bridge] After:
Flags and bunting decorate the bridge on opening day of the Patuxent Branch Trail November 2, 2002
is a structural triangle formed by three pieces of material (usually wood or metal) that are joined together to form a set of trusses called a web. This arrangement provides great strength and is relatively light. The main pieces may be either stiff, heavy struts or thin, flexible rods. How they are arranged determines the type of truss. Many wooden covered bridges were truss bridges.
work against structural members: compression
(pushing together) and tension
(pulling apart). Heavy, rigid members may resist both forces, but thin rods can only resist tension. One of the early bridge designs for railroads was the Howe turss, which had the vertical members in tension and the diagonal members in compression. The Pratt truss was just the opposite, with the diagonals in tension and the verticals in compression. This was a superior arrangement because it resulted in better distribution of stresses.
What is a Through Truss?
There are three basic arrangements of trusses, each carrying traffic in a different way:
[image of deck truss] The deck truss
is below the travel surface.
[image of pony truss] The pony truss
has trusses on the sides of the travel surface, but is not braced at the top.
[image of through truss] The through truss
has trusses on the sides, as well as cross bracing on the top and bottom. Traffic travels through it.
(Diagrams of trusses courtesy of the Historic American Engineering Board, National Park Service)