Bridges & Trestles

Functionality and Architecture Meet

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This double span bridge is located in Glen, NH. Wendell Kiesman photo - used with permission

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                  Pratt Truss Bridge


Since its introduction in 1844, this bridge design became part of hundreds of bridges created up to Second World War. It was designed by the Thomas Willis Pratt (1812 – 1875) and his father Caleb Pratt, a pair of American engineers, just several years after William Howe patented his famous Howe truss design. This bridge design immediately became widely used during the period when many bridges moved from wood components toward all-steel construction designs. Its most compelling feature was the ability was to span great distances using simple construction methods. It was regularly used to span anchor points that are up to 250 feet (76 meters) apart. It was most commonly used in railroad bridge construction, although it was also a preferred choice for creating other types of bridges all around the world until early 20th century.

             What is a trestle Bridge?


With the increased use and development of railroads civil engineers had to deal with rough, unstable and often dangerous terrain and make sure that rails are adequately supported by trestle construction which was meant to be filled with solid material. When building railroad tracks across wide and deep valleys, trestles made of wooden timber were built to keep the track solid and safe high above the ground. Most trestles were meant to be temporary, allowing trains to transport materials necessary to create a solid fill beneath the tracks.

On the other hand, rather than temporary, trestles were used as permanent bridge support in sections of tracks where water flow or sudden flooding could cause solid fills to become unsafe. Despite the frail looks of trestle bridges, they remained a safe passage for freight trains around the still settling the United States while exploring and populating and developing western territories. In the United Kingdom, wooden trestles were used for a relatively short period of the main use of crossing deep valleys in mountainous areas and were soon replaced by stone, and concrete viaducts with only a few wooden trestles continued to be in use into the 20th century.

Frankenstein Trestle Crawford Notch about 1880.  Spindly trestle supports indicate built on initial opening of the track through Crawford Notch by the P and O 1875.


Frankenstein was strengthened for heavier trains during the summer of 1905 as Maine Central RR began a bridge upgrade program from Portland to St. Johnsbury. 


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