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54-547: The Knight's Ferry Bridge is a historic covered bridge spanning the Stanislaus River at Knights Ferry, California . Built in 1863, it is one of the best-preserved 19th-century wood-iron Howe truss bridges to survive. It was designated a U.S. National Historic Landmark in 2012. The Knight's Ferry Bridge stands at the eastern edge of the small community of Knights Ferry, spanning the Stanislaus River

108-436: A live load to dead load ratio of 2-to-1. If this ratio is 2-to-1 or greater, then a six-panel truss must have counter-braces and these must at least one-third as strong as the braces. The counter-braces in an eight-panel truss must be at least two-thirds as strong as the braces, and the counter-braces in a 10-panel truss must be at least equal in strength to the braces. If rapidly moving live loads of any ratio are expected on

162-612: A brace, connected from the top of the last vertical post to the end of the lower chord. Struts are used to connect the two parallels of the chords to prevent lateral bending and reduce vibration. Two diagonals, connecting to the top of the vertical posts, are used. One of the diagonals should be a single piece, while the other is framed into the first piece or made of two pieces connected to it. X-braces, usually made of slender metal rods with threaded ends, are installed between vertical posts to help reduce sway . Knee braces, usually flat bars with eyelets on either end, are used to connect

216-424: A bridge of six panels or less (about 75 feet (23 m) long) needs no counter-bracing. An eight-panel truss requires counter-braces in every panel but the end panels, and these should be at least one-fourth as strong as the braces. A 10-panel truss requires counter-braces in every panel but the end panels, and these should be at least one-half as strong as the braces. A Howe truss bridge can be strengthened to achieve

270-410: A chord are separated along their long side by a space equal to the diameter of the vertical posts, usually about 1 inch (25 mm). This allows the vertical posts to pass through the parallel in the chord. Batten plates are placed diagonally between the members of a chord, and nailed in place to reduce bending and to act as a shim to provide ventilation between chord members. The middle third of

324-468: A covered bridge can last over 100 years. In the United States, only about 1 in 10 survived the 20th century. The relatively small number of surviving bridges is due to deliberate replacement, neglect, and the high cost of restoration. Surviving covered bridges often attract touristic attention due to their rarity, quaint appearance, and bucolic settings. Many are considered historic and have been

378-416: A hole may be drilled in the lug and brace/counter-brace and a dowel inserted to hold the beam in place. Iron angle blocks should have a hole cast in the upper lugs so that a bolt may pass through the lug and brace/counter-brace, securing the braces in place. The lower lugs in an angle block also have holes cast in them, to permit the angle block to be bolted to the chord. Two or more holes are cast through

432-407: A level of redundancy which allows it to withstand excessive loading (such as the loss of a panel due to collision). Prestressing is critical to the proper function of a Howe truss. During its initial construction, the diagonals are connected only loosely to the joints, and rely on prestressing, done at a later stage, to perform correctly. Moreover, diagonals in tension can only withstand stress below

486-704: A mile in length, but was destroyed by ice and flooding in 1832. The longest, historical covered bridges remaining in the United States are the Cornish–Windsor Bridge , spanning the Connecticut River between New Hampshire and Vermont, and Medora Bridge , spanning the East Fork of the White River in Indiana. Both lay some claim to the superlative depending upon how the length is measured. In

540-545: A patent for an all-wood parallel chord truss bridge. Long's bridge contained diagonal braces which were prestressed with wedges. The Long truss did not require a connection between the diagonal and the truss, and was able to remain in compression even when the wood shrank somewhat. William Howe was a construction contractor in Massachusetts when he patented the Howe truss design in 1840. That same year, he established

594-446: A short distance upriver from the modern Sonora Road bridge. The bridge is almost 379 feet (116 m) long, with a total of four spans set on stone abutments and piers. In addition to spanning the river, the bridge crosses a historic millrace north of the river. The bridge sections consist of Howe trusses formed out of wooden planks bolted together, with wrought iron tension rods, all joined by wrought iron bearing blocks. The exterior of

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648-408: A surge in iron bridge building occurred throughout the United States. The most commonly used designs were the Howe truss, Pratt truss, Bollman truss , Fink truss , and Warren truss . The Howe and Pratt trusses found favor because they used far fewer members. The 1962 built, World's Longest, Single Span, Wooden Covered Bridge at Bridgeport State Park, California, uses a Burr-Arch in combination with

702-495: Is avoided in order to avoid buckling. In practice, most wood stringers are 16 inches (410 mm) in width due to limitations in milling. There are usually six stringers in a bridge. Building the deck for a railroad bridge requires that a stringer lie directly beneath each rail, and that a stringer support each end of the railroad ties . Ties are usually 6 by 8 inches (150 by 200 mm) in cross-section, and 9 to 12 feet (2.7 to 3.7 m) in length. They are set directly on top of

756-407: Is used to secure the vertical post to the chord. Special plates or washers of wood or metal are used to help distribute the stress induced by the vertical post onto the chords. Vertical posts are in tension, which is induced by tightening the nuts on the vertical bars. Braces are diagonal beams which connect the bottom of a vertical post to the top of the next vertical post. They are placed in

810-468: The Knight's Ferry historic district. Covered bridge A covered bridge is a timber- truss bridge with a roof, decking, and siding, which in most covered bridges create an almost complete enclosure. The purpose of the covering is to protect the wooden structural members from the weather. Uncovered wooden bridges typically have a lifespan of only 20 years because of the effects of rain and sun, but

864-608: The Towne lattice truss or Burr truss design. Some later bridges were McCallum trusses (a modification of the Burr truss). About 1840, iron rods were added to wooden bridges. The Pratt truss used wooden vertical members in compression with diagonal iron braces. The Howe truss used iron vertical rods in tension with wooden diagonal braces. Both trusses used counter-bracing, which was becoming essential now that heavy railroad trains were using bridges. In 1830, Stephen Harriman Long received

918-676: The 1820s. Extant bridges from that decade include New York 's Hyde Hall Bridge and Pennsylvania 's Hassenplug Bridge , both built in 1825, and the Haverhill–Bath Covered Bridge and the Roberts Covered Bridge , in New Hampshire and Ohio respectively, both built in 1829. The longest covered bridge ever built was constructed in 1814 in Lancaster County , Pennsylvania, and spanned over

972-617: The 1950s, peaking at about 400 covered bridges. These mostly used the Howe, Town, and Burr trusses. Today, there are 58 covered bridges in New Brunswick , including the world's longest, the Hartland Bridge . Ontario has just one remaining covered bridge, the West Montrose Covered Bridge . Roofed, rather than covered bridges, have existed for centuries in southern Europe and Asia. In these cases,

1026-771: The Howe Bridge Works to build bridges using his design. The first Howe truss ever built was a single-lane, 75-foot (23 m) long bridge in Connecticut carrying a road. The second was a railroad bridge over the Connecticut River in Springfield, Massachusetts . This bridge, which drew extensive praise and attention, had seven spans and was 180 feet (55 m) in length. Both bridges were erected in 1840. One of Howe's workmen, Amasa Stone , purchased for $ 40,000 ($ 1,220,800 in 2023 dollars) in 1842

1080-591: The Howe truss to be the best form of wooden truss bridge, and believed it to be the most commonly used truss bridge in the United States at that time. All-iron Howe trusses began to be built about 1845. Examples include a 50-foot (15 m) long iron Howe truss was built for the Boston and Providence Railroad and a 30-foot (9.1 m) long railroad bridge over the Ohio and Erie Canal in Cleveland. Iron, however,

1134-419: The Howe truss, the counter-braces used in the center panel should be equal in strength to the braces, and the panel next to the end panel should have counter-braces at least one-half as strong as the braces. Where diagonal braces and counter-braces meet, they are usually bolted together. Braces and counter-braces are held in place with angle blocks. Angle blocks are triangular in cross-section and should be

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1188-501: The Howe-Truss to achieve this over 210' span. The only maintenance a Howe truss requires is adjustment of the nuts on the vertical posts to equalize strain. The diagonals in a wooden Pratt truss proved difficult to keep in proper adjustment, so the Howe truss became the preferred design for a wooden bridge or for a "transitional" bridge of wood with iron verticals. Engineering professor Horace R. Thayer, writing in 1913, considered

1242-820: The Pratt and Howe trusses spurred Benjamin Henry Latrobe II , chief engineer of the Baltimore and Ohio Railroad , to build large numbers of iron bridges. After two famous iron bridge collapses (one in the United States, the other in the United Kingdom ), few of these were built in the North . This meant most iron bridges erected prior to the American Civil War were located in the South . About 1867,

1296-531: The U.S., although it states that only 670 of those were standing when the 1959 edition was published. The tallest (35 feet high), built in 1892, is the Felton Covered Bridge , just north of Santa Cruz, California . Between 1969 and 2015, the number of surviving covered bridges in Canada declined from about 400 to under 200. In 1900, Quebec had an estimated 1,000 covered bridges. Relative to

1350-412: The angle block. The upper lug may be a single flange that fits into a groove cut into the surface of the diagonal, or there may be two to four lugs which form an opening into which the brace and counter-brace are seated. The diagonals are kept in place by tightening the nuts on the vertical posts. Cleats can be nailed to a wooden angle block to help keep braces and counter-braces seated. Alternatively,

1404-675: The bridge is finished in vertical board siding, with a metal roof. The bridge, the second to stand on the site, was built in 1862–63, after the first bridge (built 1856) was swept away during the Great Flood of 1862 . Because of this flood, the new bridge was built on higher piers. The bridge was for a long time known as the longest covered bridge west of the Mississippi River , and since the 2011 destruction of Old Blenheim Bridge in New York State, which had approximately

1458-423: The center of the angle block, to allow the vertical posts to pass through and be anchored on the other side of the chord. End panels are the four panels on either side of the end of a Howe truss bridge. These should be the same height as the chords, but not more. The upper chord does not extend past the portal (the space formed by the last four vertical posts at either end of the bridge). The end panels need only

1512-509: The construction site, and sometimes even entire trusses could be manufactured and assembled off-site and transported by rail to the intended location. Some sort of falsework , usually in the form of a trestle , is required to erect the bridge. The development of the Pratt and Howe trusses spurred the construction of iron bridges in the United States. Until 1850, few iron bridges in the country were longer than 50 feet (15 m). The simple design, ease of manufacture, and ease of construction of

1566-480: The cover is to protect the users of the bridge rather than the structure. Examples include: In addition to being practical, covered bridges were popular venues for a variety of social activities and are an enduring cultural icon; for example: Howe truss The earliest bridges in North America were made of wood, which was abundant and cheaper than stone or masonry. Early wooden bridges were usually of

1620-425: The end of the bridge suffer the highest amount of stress. The stress affecting counter-braces depends on the ratio of live load to dead load per unit of length, and how the live load is distributed across the bridge. A uniform distribution of live load will put no stress on the counter-braces, while putting live load on only a portion of the bridge will create maximum stress on the center counter-braces. Because of

1674-403: The last strut and last vertical posts on both ends of the bridge. Individual panels may be prefabricated off-site. When panels are connected to one another on-site, shims are used to pack any spaces and bolted in place. Floor beams extend between the parallels of a chord and are used to support the stringers and decking. Floor beams may sit atop the chord below them, or they may be hung from

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1728-403: The lower chord beams are of wrought iron . A minimum of three small beams are used, each uniform in width and depth. Fishplates are usually used to splice beams together. (Lower chord beams may have eyes on each end, in which case they are fastened together with bolts, pins , or rivets .) In wooden trusses, cotters and iron bolts are used every 4 feet (1.2 m) to connect the beams of

1782-425: The lower chord is always reinforced by one or more beams bolted to the chord. This reinforcement is generally one-sixth the width of the cross-section of the lower chord. If a wood chord needs to be strengthened even more, additional slender beams may be bolted to the middle third of the each side of the lower chord. When construction is complete, the upper chord of a Howe truss bridge will be in compression , while

1836-414: The lower chord is in tension . Vertical posts connect the upper and lower chords, and divide the truss into panels. The Howe truss usually uses iron or steel verticals. These are straight and round, slightly reduced in circumference at the ends, and a screw thread added. The vertical usually passes through the center of the angle block and then through space left in the upper and lower chord. A nut

1890-525: The mid-19th century, the use of cheaper wrought iron and cast iron led to metal rather than timber trusses. Metal structures did not need protection from the elements, so they no longer needed to be covered. The bridges also became obsolete because most were single-lane, had low width and height clearances, and could not support the heavy loads of modern traffic. As of 2004 , there were about 750 left, mostly in eastern and northern states. The 2021 World Guide to Covered Bridges lists 840 covered bridges in

1944-509: The most popular designs was the Burr Truss , patented in 1817, which used an arch to bear the load, while the trusses kept the bridge rigid. Other designs included the King , Queen , Lattice , and Howe trusses . Early trusses were designed with only a rough understanding of the engineering dynamics at work. In 1847, American engineer Squire Whipple published the first correct analysis of

1998-633: The precise ways that a load is carried through the components of a truss, which enabled him to design stronger bridges with fewer materials. About 14,000 covered bridges have been built in the United States, mostly in the years 1825 to 1875. The first documented was the Permanent Bridge , completed in 1805 to span the Schuylkill River in Philadelphia . However, most other early examples of covered bridges do not appear until

2052-424: The prestressing level. (The size of the member does not matter due to the loose fitting of the diagonal to the joint.) Proper prestressing during construction is therefore critical in the correct performance of the bridge. Maximum stress is placed on the center of the chords when a live load reaches the center of the bridge, or when the live load extends the length of the bridge. Both the vertical posts and braces at

2106-571: The rest of North America, Quebec was late in building covered bridges, with the busiest decade for construction being the 1930s. Initially, the designs were varied, but around 1905, the design was standardised to the Town québécois , a variant on the lattice truss patented by Ithiel Town in 1820. The designer is unknown. About 500 of these were built in the first half of the 20th century. They were often built by local settlers using local materials, according to standard plans. The last agricultural colony

2160-561: The rights to Howe's patented bridge design. With his financial backer, Azariah Boody, Stone formed the bridge-building firm of Boody, Stone & Co., which erected a large number of Howe truss bridges throughout New England . Howe made additional improvements to his bridge, and patented a second Howe truss design in 1846. The Howe truss bridge consists of an upper and lower "chord", each chord consisting of two parallel beams and each chord parallel to one another. The web consists of verticals, braces, and counter-braces. Vertical posts connect

2214-442: The same height and width as the parallel of the chord. Angle blocks may be made of wood or iron, although iron is usually used for permanent structures. Angle blocks are attached upside down to the upper chord, and right side up to the lower chord. Angle blocks have lugs— flanges or projections used for carrying, seating, or supporting something. The ends of the braces and counter-braces should cut or cast to rest squarely against

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2268-759: The same length, it is the second-longest covered bridge in the United States, behind the Smolen-Gulf covered bridge in Ohio. It was documented by the Historic American Buildings Survey and by the Historic American Engineering Record . In addition to its status as a National Historic Landmark , it is also listed on the National Register of Historic Places as a contributing structure in

2322-469: The same plane as the chord. Unlike iron or steel braces which are built up, wooden braces are cut to length. Where the parallel in a chord has a thickness of X number of beams, each brace should have a thickness of X minus 1 beams. The depth-to-width ratio of each member of a diagonal brace should be no greater than that of the brace as a whole. Braces may be a single piece, or several pieces spliced together with fishplate. Braces are in compression due to

2376-487: The stress placed on the bridge, the Howe truss is suitable for spans 150 feet (46 m) in length or less. No provision is made in a Howe truss for expansion or contraction due to changes in temperature. The Howe truss was highly economical due to its ease of construction. The wooden pieces can be designed using little but a steel square and scratch awl , and the truss can be framed using only an adze , auger , and saw . Panels could be prefabricated and transported to

2430-404: The stringers, about 12 inches (300 mm) apart. Guard rails 6 by 8 inches (150 by 200 mm) in cross-section are set 20 inches (510 mm) from the center of the ties, and bolted to every third tie. The inner truss of a Howe truss is statically indeterminate . There are two paths for stress during loading, a pair of diagonals in compression and a pair in tension. This gives the Howe truss

2484-600: The subject of historic preservation campaigns. Typically, covered bridges are structures with longitudinal timber-trusses which form the bridge's backbone. Some were built as railway bridges, using very heavy timbers and doubled up lattice work. In Canada and the U.S., numerous timber covered bridges were built in the late 1700s to the late 1800s, reminiscent of earlier designs in Germany and Switzerland . They tend to be in isolated places, making them vulnerable to vandalism and arson. The oldest surviving truss bridge in

2538-415: The tightening of the nuts on the verticals. Counter-braces are diagonal beams which connect the bottom of a vertical post to the top of the next vertical post, and run roughly perpendicular to braces. They are placed in the same plane as the chord, are generally uniform in size, and should have a thickness one beam less than a brace. Unlike braces, counter-braces are a single piece. Generally speaking,

2592-418: The upper and lower chords to one another, and create "panels". A diagonal brace in each panel strengthens the bridge, and a diagonal counter-brace in each panel enhances this strength. Howe truss bridges may be all wood, a combination of wood and iron, or all iron. Whichever design is used, wooden timbers should have square ends without mortise and tenons . The design of an all-metal Howe truss follows that of

2646-402: The upper chord to one another. In the lower chord of a wooden bridge, clamps are used to couple beams together. Although generally of the same length, beams are positioned so that a splice (the point where the end of two beams meet) is near the point where two panels meet but not adjacent to the splice in an adjacent pair of beams. The individual small beams which make up a parallel in

2700-449: The vertical posts. Floor beams generally have the greatest depth of any beam in the bridge. Floor beams are usually placed where two panels meet. If they are placed somewhere mid-panel, the chord must be reinforced to resist bending , buckling , and shear stress . Stringers are beams set on top of the floor beams, parallel to the chords. A stringer may have a depth-to-width ratio anywhere from 2-to-1 to 6-to-1. A ratio greater than 6-to-1

2754-448: The wooden truss. The parallels in each chord are usually built up out of smaller beams, each small beam fastened to one another to create a continuous beam. In wooden Howe trusses, these slender beams are usually no more than 10 to 15 inches (250 to 380 mm) wide and 6 to 8 inches (150 to 200 mm) deep. In iron trusses, the upper chord beams are the same length as the panel. Upper chord beams are usually made of cast iron , while

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2808-638: The world is the Kapellbrücke in Switzerland, first built in the 1300s. Modern-style timber truss bridges were pioneered in Switzerland in the mid-1700s. Germany has 70 surviving historic wooden covered bridges. Most bridges were built to cross streams, and the majority had just a single span . Virtually all contained a single lane. A few two-lane bridges were built, having a third, central truss. Many different truss designs were used. One of

2862-714: Was founded in 1948, and the last bridge was built by the Ministry of Colonisation in 1958 in Lebel-sur-Quévillon . There are now 82 covered bridges in Quebec, Transports Québec including the Félix-Gabriel-Marchand Bridge , the province's longest covered bridge. In Quebec covered bridges were sometimes known as pont rouges (red bridges) because of their typical colour. Like Quebec, New Brunswick continued to build covered bridges into

2916-608: Was the preferred bridge for automobile and railroads, and the Howe truss did not adapt well to all-iron construction. The Pratt truss' single diagonal bracing system meant less cost, and its ability to use wrought-iron stringers under railroad rails and ties, led bridge builders to favor the Pratt over the Howe. Heavier live loads, particularly by railroads, led bridge builders to favor plate girder and Towne lattice bridges for spans less than 60 feet (18 m), and Warren girder bridges for all other spans. Trusses have been widely used in architecture since ancient times. The Howe truss

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