The History of the Railroad Article 14

The Wooden Trestle

     The wooden trestle was an indispensable part of the railroad in the 1860’s. It bridged the gaps when the tracks crossed over valleys, rivers and navigated the United States rugged terrain

     Bridges were used when the terrain could not support a level roadbed. Bridges made up to 3% of a railroad system. ¹

     Since timber was relatively cheap and abundant in this young country, it was used to construct the trestles in the 1860’s.  Durability was of the utmost importance for the trestle construction.      Untreated wood would only last for approximately twenty years before it had to be replaced due to rotting.

     The trestle is an opened, braced wooden framework. It’s purpose is to support the railroad above ground level. They were supported by a series of vertical posts that hold up a series of short spans.

        The timber trestles which were built in the 19^th century were constructed as temporary bridges.

     They did this to get the railroads into operation faster. Then after iron and steel came into use, the trains would get up and running; they would transport the raw materials needed to build the permanent trestles. ²

1864 US Military Railroads bridge across Running Water creek near Whiteside, Tenn.

                                                                                                                        George N. Barnard, American, 1819–1902, Public domain, via Wikimedia Commons                                                                                                                                                                                           

    The trestle pictured is a Four-tiered, 780-foot-long railroad trestle bridge built by the 1st Michigan Engineers with guard. The picture was taken on January 8, 1864.

      The wooden trestle bridge uses a rigid type of framework used in short spans that support the trestle table.

       Each framework is a tripod used to support a pair of isosceles triangles. Each supporting framework is called a bent. ²

Trestle Diagram

Large Scale Central Print PDF For Free

  

     The deck of the trestle is supported by the bents.     

 

 

Street-railroads Electric rilroadsTransporttation
Internet Archive Book Images, No restrictions, via Wikimedia Commons

                                                                                                                                         

     There are two main types of bents. The first is a pile bent. This type is not usually higher than thirty feet. Their length was assigned due to the length of the available timber of the local area.

     The maximum height is no longer than sixty feet due to safety reasons.

     The lengths are a single piece of wood with most of it driven easily into the ground. Adapted to construction the of a tall trestle. This is due to their round configuration. They are a round post of 12 to 16 inches in diameter. They are directly driven into the ground using a pile driver (pictured below). ³

 

 

Construction of a railroad trestle across a northern arm of Lake Tapps,

June 1, 1910 (SPWS 322).jpg

University of Washington, Public domain, via Wikimedia Commons

 

     The previous picture is of a construction photograph of the White River Power Plant, a hydroelectric facility on the White River. Built by the Pacific Coast Power Company, 1909-1911. This company was acquired by Puget Sound Traction, Light & Power Company in 1912.

     The second type of bent is called a frame bent. They are of five feet to a maximum of 30 feet. Again, to what even timber is available in the local area.             

     So, we know some of the wooden trestles were over a hundred feet tall. When a greater height was required, the bents are divided into stories that are ach thirty feet tall. They are separated by horizontal intermediate twelve-inch sills.

     If the trestle were to be between sixteen to twenty feet in height, only one story was used.

     When the trestles reach the taller heights. Bracing was of the utmost importance. ⁴

     Sway braces are angled at between 30 to 60 degrees. There are no more than two sway braces per story.

     Horizontal braces, called sashes, can be used if needed. They are mostly for pile bents. They are hardly used for the frame bents.

     To help make the trestle more ridged, a horizontal timber called a girts. They are placed on top of the sill or sashes and connect each bent to the next.

      The foundation for each frame bent has what is called a mudsill sitting on top of the footer or pier.

     For a pile bent, they sit on top of the piles driven into the ground. They seldom sit on the ground themselves.

     The vertical post of both types of bents has a bent cap on the ends of each post. Then three stingers are bolted together as a spacer between the cap and the ties.

     A galvanized plate is placed on top of the bent caps and atop of the bent caps and a top the stinger. This protects the wood from falling cinders. ⁴

    Some trestles were works of art. Stretching over deep valleys, ravines, and the occasional river.

     With the increased development of railroad technology, civil engineers had to deal with the roughest of terrain. The trestle needed extra support due to the extreme heights and lengths. Swaying would be a huge problem if the support was not constructed correctly.

     The wooden trestles were only meant to be only temporary, but some trestles were used as a permeant bridge support when areas where tracks were subject to flowing water or sudden flooding where a solid fill would become unsafe.

(Old No. 139) Santa Fe Railroad bridge over Canyon Diablo, Arizona,

National Archives and Records Administration,

 Public domain, via Wikimedia Commons

 

     Even though some of the trestles looked very frail and could have collapsed under the stress of having a fully loaded train go across them. They mostly remained safe. ⁴

     Wooden, steel, and concrete trestle bridges were safe to transport needed supplies and essential items needed in rural areas for everyday life and living.

     Even though today, wooden, steel, and concrete trestles bridges remain safe and reliable for traffic.

     There are some that are obsolete and are not used for service but remain well deserved. Remnants of the past remain well preserved and remain a nostalgic place of interest.

     Wooden trestles, if not designed right, maintained correctly or the early untreated wooden posts of bents placed into rivers could cause derailments, injuries or loss of life. ⁵