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Covered Bridge Truss Design

A truss is a structure that consists of members organized into connected triangles so that the overall assembly behaves as a single object. Trusses are most commonly used in bridges, roofs, and towers.

A truss is made up of a triangle or a web of triangles joined together to enable the even distribution of weight and the handling of changing tension and compression without bending or shearing. The triangle is geometrically stable when compared to a four-sided (or more) shape which requires that the corner joints are fixed to prevent shearing from excessive stress from use.

Trusses consist of triangular units constructed with straight members. The ends of these members are connected at joints, known as nodes. They are able to carry significant loads, transferring them to supporting structures such as load-bearing beams, walls, or the ground.

In general, trusses are used to achieve long spans, minimize the weight of a structure, and support heavy loads. Trusses are typically made up of three basic elements: a top chord which is usually in compression a bottom chord which is usually in tension, and bracing between the top and bottom chords.  The top and bottom chords of the truss provide resistance to compression and tension and so resistance to overall bending, whilst the bracing resists shear forces.

The efficiency of trusses means that they require less material to support loads compared with solid beams. Generally, the overall efficiency of a truss is optimized by using less material in the chords and more in the bracing elements.

Early trusses were designed without an understanding of the engineering dynamics at work.  It wasn’t until 1847, that American engineer, Squire Whipple, published the first correct analysis of the way a load is carried through the truss, which enabled him to design stronger bridges with fewer materials.

American-covered bridges use about 18 different types of trusses. Each consists of vertical members, called posts, and diagonal members. The vertical and diagonal members hold together the upper and lower chords, which serve as horizontal beams. These trusses, ranging in complexity from simple triangles and lattices to X-shapes, absorb the stress of vehicle weight and transfer it to the abutments. This construction technique made it possible for wooden pieces to span a distance much greater than the length of any single piece of lumber.King Post Truss

The king post truss is used for simple roof trusses and short-span bridges. It is the simplest form of truss in that it is constructed of the fewest truss members (individual lengths of wood or metal). The truss consists of two diagonal members that meet at the apex of the truss, one horizontal beam that serves to tie the bottom end of the diagonals together, and the king post which connects the apex to the horizontal beam below. A bridge would require two king post trusses with the driving surface between them. A roof usually uses many side-by-side trusses depending on the size of the structure. The oldest surviving roof truss in the world is a king post truss in Saint Catherine's Monastery, Egypt, built between 548 and 565.

Multi-king post truss

A multiple king post truss is a truss that can be thought of as a cross between a king post truss and a queen post truss. Installing this type of truss is an easy way to stretch the span capability of a truss. It allowed the construction of bridges about twice the length of a Kingpost truss design. 

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

 

Early North American bridge builders actively pursued patents for their designs in an attempt to gain more bridge construction contracts. A few of the very first patents involved general bridge construction, but by 1797, there were several that involved specific schemes for timber arches. Among others, Timothy Palmer received a and began construction of his Permanent Bridge only a few years after this, his initial patent.

Lewis Wernwag, born in Germany in 1769, and obtained a patent in 1812, which was used in another remarkable bridge crossing the Schuylkill River at Philadelphia’s Upper Ferry. The huge 104-m (340-ft) trussed arch span was quickly termed the "Colossus" and represented a major triumph in bridge construction, with its attractive and apparently efficient use of timber, supplemented with iron rod bracing members. Wernwag owned a metal works company and relied more on early forms of metal connections and components rather than on traditional timber joinery only. He received a second patent in 1829 for improvements in his structure. Regrettably, his “Colossus” bridge was lost to fire in 1838.

Burr Arch TrussTheodore Burr obtained the first of his many patents in 1804. His second patent was issued in 1817. Burr's trademark design dates from this patent. He extended curved lower ribs that had reached only bottom chords, up along the trusses, all the way to the top chord. This superposition of arch and truss forms seems to have been influenced by earlier bridges built in Switzerland.

The resulting structure has been described as a combination of conventional trusses (parallel chords with compression diagonals) and supplemental arches. One of Burr's early examples of this bridge form, and probably the basis for his 1817 patent, was his Union Bridge crossing of the Hudson River between Lansingburgh and Waterford, NY, circa 1804. This was a significant structure; 244 m (800 ft) long, with four spans. The structure was rebuilt after being in service for some time, including a roof and siding. This heavily braced and counterbraced structure exemplified what today is called a Burr arch.

Town Lattice TrussIthiel Town (1784--1844) of New Haven, CT, was a prominent architect known for designing many types of buildings. He also planned many bridges, initially experimenting with various truss arch combinations before he landed on his patented lattice style design. Town wanted to devise a structure that would require fewer carpentry skills than was required by the intricate joinery details of some of the early bridges. Using only planks joined with round wooden pegs, he began developing a lattice style of truss construction and obtained his first patent in 1820.

He was nearly as good a promoter as an inventor, and the lattice truss became very popular, although it has been criticized for its apparent waste of material. It could be “built by the mile and cut off by the yard,” was his sales pitch. This truss layout proved to be very adaptable. It could include heavier members for longer spans, and could even be doubled up to include two layers of web members and three layers of chords for heavy loads, such as those generated by the railroads. A few of his bridges were built with such heavy members that they became identified as a timber lattice, as compared with the more common plank lattice. The most famous of the surviving timber lattices is found in the Windsor, VT-Cornish, NH, covered bridge over the Connecticut River, which remains one of the longest two-span covered bridges in the United States.

 

Stephen Long (1784--1864) had a varied background and career. He gained his experience as a timber bridge builder while serving in the U.S. Army. Long was commissioned to locate, plan, and build the Baltimore and Ohio Railroad. 

He chose to use a standardized truss for all his spans, with timber counterbraces in all the panels. With the addition of timber wedges at the bearing joints between the posts and diagonals, he found that he had better control over the trusses' as-built geometry. He obtained his first bridge patent in 1830 and established himself as another wooden truss patentee, when he invented the Long truss design. It was the first wooden truss in America where a few mathematical calculations were entered. His truss design was a panel truss which needed no arches. When viewed from the side, the Long truss resembled a series of Boxed X’s. This truss design vied with Town’s lattice design truss for favor among the lands railroad networks, toll bridge companies, and town highway planners for about ten years. After 1840 though, both Town’s and Long’s patented wooden truss bridges were almost totally eclipsed by the introduction of the Howe truss design.

William Howe (1803--1852) made a major contribution to the evolution of timber-covered bridges by being the first to use metal components as primary members within an otherwise timber truss. He used parallel timber chords, with timber diagonals and counters in the panels, but he used round iron rods for the vertical tension members. The threaded rod ends allowed easy adjustment of the structure, to keep it tight both during and after erection. Many modifications were made over the years to Howe's original design to address various desired details, but his truss was quickly adopted to withstand the heavy loads on railroads. The popularity of the Howe truss continues today. It is often selected when constructing new covered bridges. Howe's modification was a major reason for the short life and reduced popularity of Stephen Long's truss-which was essentially the same, but without the iron rod verticals.  Howe’s truss design resembled Long’s truss design so much that Colonel Long claimed patent infringement for years, which he never got any satisfaction on his claim from the patent office.

Thomas Willis (T. W.) Pratt and his father Caleb were well-known engineer/architects in New England in the middle of the 19th Century. T. W. attended college in Troy, NY at the Rensselaer Institute, later Rensselaer Polytechnic Institute. He did not graduate, which was not uncommon in those days, as only about a third of those who matriculated graduated. He returned to Massachusetts to work on the Boston and Worcester RR and the Providence & Worcester RR in the mid-1840s. Pratt no doubt had seen Howe’s Springfield Bridge, patented in 1840, across the Connecticut River and was aware it was adopted by many railroads replacing the Long and Towne Trusses. The span of the Howe Truss was limited, as the diagonal compression members were susceptible to buckling as their length increased with an increase in span.

Pratt basically took the Long Truss and replaced the wooden diagonal members with two iron rods with threads and nuts, while keeping wooden verticals in compression, on both ends to make necessary adjustments to obtain the required camber and pre-stress. Since longer spans were possible with the long members in tension, the bridge appeared to correct some of the problems with the Howe Truss. T.W., along with his father, obtained a patent, #3,523, on April 4, 1844, for a TRUSS FRAME OF BRIDGES (Truss Bridge). Not many bridges in wood and iron were built to this Patent.

Developed in 1846 by Horace Childs, the Childs truss was used exclusively after 1883 by Ohio bridge builder Everett Sherman. The truss simply added diagonal iron rods to a multiple kingpost design. E.S. Sherman, a bridge builder from Vermont, won the contract to build 14 Childs Truss bridges and one Kingpost Truss bridge in Preble County between 1886 and 1896. He moved to Eaton right after the storm in 1886 and won the contract due to the fact that the bridges he built were so sturdy, economical and quickly erected. The following bridges are still standing today in different parts of Preble County, Ohio.

Robert W. Smith was born in 1833 in Miami County, He worked as an apprentice for several years before going out on his own. His very first invention, though never patented, was a self-supporting roof truss for large barns. Smith was living in Tippecanoe City, Ohio, in 1867 when he was granted a patent for a wooden truss bridge. Later that year, he formed the Smith Bridge Company and moved to Toledo. He built five bridges in Ohio and Indiana in 1867, 22 in 1868, and 75 in 1869. Also in 1869, a second patent was granted to Smith on an improved bridge truss. However, in his bridges, Smith never exactly followed his patented designs, but constantly made changes to improve strength and design. The Smith Bridge Company maintained a yard in Toledo where all timbers were cut and first fitted together for each bridge. They would then be disassembled and shipped by rail or boat to the bridge site for reassembly by either a company carpenter, a Smith Company agent, and a local crew of carpenters. So in a sense, Smith was the Henry Ford of the covered bridge business.

A typical Smith truss rarely had vertical posts. The timbers between the two chords were braces set at an angle. Smith truss bridges were four distinct types: Type 1 was the original patent with two posts and crossed timbers in the center panel; Type 2 had open V-timbers; Type 3 was a reinforced truss with two diagonal timbers; Type 4 was the strongest as it had further reinforcements of both double and single sets of diagonals.

Prior to 1977, there remained many original documents concerning Smith and his proposed bridges. Two handwritten bids gave the cost of proposed bridges. For a superstructure with the Smith Patent truss, the following costs were quoted: an 80-foot span, $11.50 per linear foot, plus $250 extra for siding and roof; a 100-foot span, $14 per linear foot, covering extra; a 120-foot span complete with siding and roof at $19.50 per linear foot; and a 150-foot span complete at $21.50 per linear foot. He eventually received contracts for two Howe truss bridges in Carroll County, Ohio: the Lancaster and Adams Mill. Both are still standing.

The Smith Bridge Company built 21 bridges in Indiana. Regardless of whether a bridge was built by Smith or an agent, Smith always guaranteed the structure. Bridges with Smith constructed trusses were built in Indiana, Ohio, Pennsylvania, and Oregon. Some 20 years after its inception in the early 1990s, the Smith Bridge Company was taken over by the Toledo Bridge Company. Smith then became interested and involved in real estate until his death in 1998.

Peter Paddleford (1785-1859) was a bridge builder from Littleton, New Hampshire. He was a user of the Long truss. He created his own truss by modifying the Long truss, stiffening it with a system of interlocking counterbraces. His work was challenged by the holders of the Long Truss patents because of its similarities.

The Paddleford resembles the Long truss in that the vertical posts are lapped, or notched, into the upper and lower chords; the braces set into notches in the vertical posts. Both designs use the counterbraces for a special purpose. In the Paddleford truss, channels are cut into the inner chord planks and into the faces of the braces and vertical posts where the counterbraces cross. The posts, braces, and chords are locked together into one unit by the counterbraces. An inside arch is often added.

The Paddleford truss superficially resembles the Long truss. They both use a parallel chord system with vertical posts, and they both use braces and counterbraces. But the designers each had something different in mind. Long designed his truss to implement his pre-stressing idea while Paddleford tried to make the braces work both in compression and tension by having the braces lap overall of the frame members. He tried to use wood like people later used iron rods. It's hard to make a tension joint with wood unless you have a lot of distance to do it in. It is complicated joinery.

Paddleford did it with the counterbraces by running them through the series of tight-fitting channels across members that are under loading stresses. The stresses tend to re-align the truss members, causing them to lock onto the counterbraces. While the Paddleford truss design was never patented because of court challenges threatened by the owners of the Long Truss patent, it was widely used, especially in New Hampshire where many exist today. It was also popular in Orleans and Caledonia counties where Vermont's last two Paddleford bridges stand.

In 1839, Haupt patented a bridge construction technique called the Haupt Truss. Two of his Haupt truss bridges, both built-in 1854, still stand in Altoona and Ardmore, Pennsylvania. The brace configuration in Haupt’s truss implementation distributes, at least for the first several truss panels, compression forces more directly to the abutments. This is a key feature of the Haupt patent. In 1840, Haupt was appointed a professor of mathematics and engineering at Pennsylvania College. He drew the attention of J. Edgar Thomson who became chief engineer of the Pennsylvania Railroad. Haupt returned to the railroad business in 1847, accepting a position as a construction engineer on the Pennsylvania Railroad, and then becoming its general superintendent from 1849 to 1851. Haupt and Thomson designed the Horseshoe Curve (now a National Historic Landmark) which enabled the Pennsylvania Railroad to cross the Allegheny Mountains and reach Pittsburgh, Pennsylvania.

From 1851 until 1853, Haupt was the chief engineer of the Southern Railroad of Mississippi, then became the Pennsylvania Railroad's chief engineer until 1856; in the latter position, he completed the Mountain Division with the Allegheny Tunnel, opening the line through to Pittsburgh. He was the chief engineer on the five-mile (8 km) Hoosac Tunnel project through the Berkshires in Western Massachusetts from 1856 to 1861.

The Warren truss was patented in 1848 by James Warren and Willoughby Theobald Monzani, and consists of longitudinal members joined only by angled cross-mebers, forming alternately inverted equilateral triangle-shaped spaces along its length, ensuring that no individual strut, beam, or tie is subject to bending or torsional straining forces, but only to tension or compression. Loads on the diagonals alternate between compression and tension (approaching the center), with no vertical elements, while elements near the center must support both tension and compression in response to live loads. This configuration combines strength with the economy of materials and can therefore be relatively light. The girders being of equal length, it is ideal for use in prefabricated modular bridges. It is an improvement over the Neville truss which uses a spacing configuration of isosceles triangles. A Warren truss or equilateral truss[1] is a type of engineering truss employing a weight-saving design based upon equilateral triangles.

Bridge historians and early textbooks generally call a truss with alternating compression and tension diagonals a Warren; however, sometimes it is called an equilateral truss since all panel lengths and diagonals are of equal length creating a series of equilateral triangles. When the panel lengths are shorter than the equal length diagonals, it was sometimes called an isosceles or isometric truss. James Warren and Willoughby Monzani's patent of 1848 in England, based on similar trusses that were built in France by Alfred H. Neville and a patent that was granted in England to William Nash in 1839 on a similar design. Warren and Monzani were well-known English engineers, and their design was for a truss that could be used as a deck or a through truss.

A Brown truss is noted for its economical use of materials and is named after the inventor, Josiah Brown Jr., of Buffalo, New York, who patented it July 7, 1857, as US patent 17,722. The Brown truss is a box truss that is a through truss (as contrasted with a deck truss) and consists of diagonal cross compression members connected to horizontal top and bottom stringers. There may be vertical or almost vertical tension members (which the patent application diagram does not show) but there are no vertical members in compression. In practice, when used in a covered bridge, the most common application, the truss is protected with outside sheathing.

The floor and roof are also trusses, but are horizontal and serve to give the truss rigidity. The bottoms of the diagonals tend to protrude below the sheathing. This type of truss is particularly suited for timber structures that use iron rods as tension members. The Brown truss is noted for the economy of materials as it can be built with very little metal. The Brown truss enjoyed a brief period of favor in the 1860s and is known to have been used in four covered bridges in Michigan, the Ada Covered Bridge, the Fallasburg Bridge, Whites Bridge, and one other.

Reuben L. Partridge (September 10, 1823 – July 17, 1900) was an American pioneer and engineer in Union County, Ohio, known locally as "The Bridge Builder." Partridge received a patent for a design that was remarkably close to the Smith's truss. Designed and patented in 1872, after local bridge designs proved ineffective against road traffic and heavy rains. It became the standard for covered bridges built-in central Ohio in the late 1800s and early 1900s. Due to the design of local bridges at the time, they would sometimes crumple under the sudden stress of heavy rains. Partridge saw this and worked to design a new truss and brace system to support additional stresses including vehicle traffic. In 1855, he built the first self-supporting bridge in Union County, Ohio. On June 11, 1872, he received a patent for a new U-shaped truss bridge design, the "Partridge Truss" (Patent #127,791). As of 1882, Partridge was responsible for having built 90% of the bridges in Union County, and by 1883 had built over 125 in total. In 1886, he moved to Columbus and worked as the Vice President of the Columbus Bridge Company for the next 10 years. While most of the bridges he built were wooden covered bridges of the "Partridge truss" design, he did design and build some iron bridges while with Columbus Bridge Company.

Partridge moved back to Marysville in 1896. He died 4 years later, on July 17, 1900, from complications after breaking his leg falling through a bridge he was supervising the removal of in Taylor Township north of Marysville. Over the course of his life, Partridge built over 200 bridges across Union County and central Ohio. He was also an active member of the first militia formed in Union County, a Marysville city council member, township clerk, township trustee, and avid supporter of the local veterans of the Mexican War and Civil War. He is buried at the Oakdale Cemetery in Marysville.

A Post truss is a hybrid between a Warren truss and a double-intersection Pratt truss. Invented in 1863 by Simeon S. Post, it is occasionally referred to as a Post patent truss although he never received a patent for it. (Iron rods indicated by dashed lines). The Ponakin Bridge and the Bell Ford Bridge are two examples of this truss and were one of the major bridge forms used in developing the national railroad network, including major crossings such as that of the Union Pacific Railroad over the Missouri River (the Bell’s Ford Bridge in Jackson County, Indiana, collapsed in 2006).

McCallum, 1867 patent. Posts are flared slightly. One example, the Powerscourt Bridge in Québec. The Powerscourt Covered Bridge was designated a national historic site of Canada in 1984 because it is the only known example of a McCallum inflexible arched truss bridge still in existence; it is one of the oldest covered bridges that exists in Canada.

The heritage value of Powerscourt Covered Bridge lies in its age and in its use of rigid (or inflexible) arch-truss construction technology. This technology, more commonly used for railway construction, was invented by New York bridge builder Daniel McCallum in 1851. Also known locally as Percy Bridge, it was built in 1861 to carry traffic on the First Concession Road over the Châteauguay River.

Key elements of the bridge include the linear footprint, the rectangular massing, and the curved roofline. The use of vertical wooden board and batten weatherboard siding are key to the McCallum rigid arched truss, as is an opening between the roof and side covering.

No Name Truss (neither a Haupt nor a modified Burr), two examples: Sayres Bridge in Orange County, Vermont, and Bath Village Bridge in Grafton County, New Hampshire.

 

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