History/Design

Burr Arch Truss


History

Why cover the bridges?

Today covered bridges are considered historical landmarks, romantic structures resembling the past, but what was the purpose of covering these bridges? There were thought to be many reasons why the bridges were covered although not all of these ideas were correct. There is a short answer. Wooden bridges with exposed superstructures are vulnerable to rot. Covering and roofing them protects them from the weather, and so they last longer. The following are more explanation for the covering of the bridges. The spans were built to resemble barns so farm animals would feel more at home and not stampede as they were driven across the streams and rivers. Another explanations were, to keep snow off the bridge, to keep the oiled planks of the roadbed from becoming dangerously slippery in the rain, to cover up unsightly trusses, to provide shelter to travelers caught in a storm, and to provide a place to court your lady and secretly give her a kiss (the name "Kissing Bridges originated). One real reason for covering bridges was to protect the trusses from the weather because the environment caused bridges to fail sooner. Bridge engineers pointed out that a housed timber truss span has a life expectancy at least three times greater than one unhoused. Another positive reason why bridges were covered was that the roof strengthened the entire structure.

 Design

Covered bridges are usually identified and classified by their truss construction. The truss design determines how long a span it can have and complex it is to build. Simpler designs, crossing small streams for example could often be constructed by local residents and may not have required  the use of experienced bridge builders. Multi-span bridges have supports embedded within the stream, allowing a particular truss design to be used over longer reaches. This type design is basically like putting multiple bridges end-to-end. 

The first known covered bridge in America was designed by the Massachusetts millwright, Timothy Palmer.  It crossed the Schuykill River at Philadelphia with a length of 550 feet Interestingly enough, the bridge was not originally intended to be a covered, but a suggestion from Judge Richard Peters, whose estate bordered the river at the bridge, was to handsomely roof, side and paint the structure.  The idea was well received and so the painted  and covered timber bridge became a common sight  on American roads from Maine to Florida to Oregon. These bridges were constructed in the half of  the 20th century for carriage and later auto use.

After the Civil War came in the age of Iron. At once covered bridges were thought to be old fashioned.  Gradually and sadly they were replaced, even though most were perfectly sound.  Pennsylvania, oddly enough, bucked this trend.  The iron-smelting giant felt that its wrought iron and steel beams were fine for other states bridges =, but at home, Pennsylvanians took pride in the romance and character of the timber covered bridges and continued to build them.  At the height of the bridge building period, 1830-1880, estimates show that Pennsylvania had the most in the country, with at least 1500, representing all of the major truss designs. Thus, today the stated has the most remaining covered bridges in the US, with 212 in 37 counties.

Multi Kingpost Truss

The  oldest covered bridge design is the Kingpost. The kingpost that forms the basis for this truss is found in the center two panels.  The multiple form is the simplest and by far, the most common type.  It's limitation is that it's length (thus width of stream it could cross) is about 40 feet. A variation on the Kingpost that allowed bridge lengths about twice that of a Kingpost truss design was the Multi-Kingpost.

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Queen Post Truss

This truss design forms an elongated , topless triangle with support posts at each end. Although it is still a simple design, the Queenpost truss permits greater bridge lengths than the Kingpost truss. The longest Queenpost bridge was 100 feet using a single truss system. There are 40 of this type bridge remaining across Pennsylvania.

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Burr Arch Truss

The Burr truss system incorporated great reinforced arches that tied directly into the bridge' abutments with a series triangular support posts. The design created great strength and allowed, for the first time, long spans averaging over 100 feet. Of the remaining covered bridges in Pennsylvania 122 have Burr trusses. Nearly all are in good to excellent condition. The Burr Arch truss was patented in 1804 by Theodore Burr of New York .

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Pratt Truss

In 1830 Col. Stephen H. Long of the U.S.Topographical Engineers became the first American to use mathematical calculations to develop a truss.  It became known as an "X" truss.

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Long Truss

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Town Truss

Connecticut architect Ithiel  Town received a patent for a truss of crisscrossed diagonals, or lattice, in 1820.

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Howe Truss

In 1840 Massachusetts builder William Howe introduced iron into wooden truss design by substituting adjustable iron rods for he vertical members of  Long's truss.

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Smith Truss

 Robert W. Smith received truss patents in 1867 and 1869. Three different variations of his basic design still exist in Bridges

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Partridge Truss

Reuben L. Partridge received a patent for a design that was remarkably close to the Smith's truss.

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Childs Truss

Developed in 1846 by Horace Childs, the Childs truss was used exclusively after 1883 by bridge builder Everett Sherman. The truss simply added diagonal iron rods to a multiple kingpost design

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Warren Truss

Patented in 1848 by two Englishmen, one of whom was named James Warren, it utilizes isosceles triangles.

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Warren Truss Plus Arch


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Wernwag Truss

This design has a historical story that  some may find very interesting.  It seems the great engineering projects of these and other covered bridge builders did not go without public skepticism and the fear that these long spans would collapse.  one famous story surrounds the building of a bridge in Fairmont Park, Philadelphia, by German designer extraordinaire, Lewis Wernwag.  In 1805  he constructed a single span of 340 feet across the Schuykill River, by far the record of the day, since Palmers bridge of 550 feet was actually made of 3 separate spans.  The bridge was so enormous that rumors spread widely across Philadelphia that the bridge would collapse as soon as the scaffolding was removed and on that day thousands of people lined the river to watch the supposed collapse.  Confident, Lewis Wernwag removed the supports himself, and to the surprise of  all viewing., the bridge, nicknamed "The Colossus" stood soundly for 26 years. before dramatically being destroyed by fire.

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