Timber roof truss

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Two king post trusses linked to support a roof.
Key:1: ridge beam, 2: purlins, 3: common rafters. This is an example of a "double roof" with principal rafters and common rafters. Roof parts simplified.svg
Two king post trusses linked to support a roof.
Key:1: ridge beam, 2:  purlins, 3: common rafters. This is an example of a "double roof" with principal rafters and common rafters.

A timber roof truss is a structural framework of timbers designed to bridge the space above a room and to provide support for a roof. Trusses usually occur at regular intervals, linked by longitudinal timbers such as purlins. The space between each truss is known as a bay. [1]

Contents

Rafters have a tendency to flatten under gravity, thrusting outwards on the walls. For larger spans and thinner walls, this can topple the walls. Pairs of opposing rafters were thus initially tied together by a horizontal tie beam, to form coupled rafters. But such roofs were structurally weak, and lacking any longitudinal support, they were prone to racking, a collapse resulting from horizontal movement. Timber roof trusses were a later, medieval development. [2] A roof truss is cross-braced into a stable, rigid unit. Ideally, it balances all of the lateral forces against one another, and thrusts only directly downwards on the supporting walls. In practice, lateral forces may develop; for instance, due to wind, excessive flexibility of the truss, or constructions that do not accommodate small lateral movements of the ends of the truss.

History

Timber roof truss example Inside wboylston old stone church.jpg
Timber roof truss example

The top members of a truss are known generically as the top chord, bottom members as the bottom chord, and the interior members as webs. In historic carpentry the top chords are often called rafters, and the bottom chord is often referred to as a tie beam. There are two main types of timber roof trusses: closed, in which the bottom chord is horizontal and at the foot of the truss, and open, in which the bottom chords are raised to provide more open space, also known as raised bottom chord trusses. [3]

Closed trusses

Closed/open distinction is used in two ways to describe truss roofs.

Closed truss:

  1. A truss with a tie beam; or
  2. Roof framing with a ceiling so the framing is not visible.

Open truss:

  1. A truss with an interrupted tie beam or scissor truss which allow a vaulted ceiling area; or
  2. Roof framing open to view, not hidden by a ceiling. [4]

King post truss

King post truss.
Key: 1: king post, 2: tie beam, 3: principal rafters, 4: struts. King post truss 3D.svg
King post truss.
Key: 1: king post, 2: tie beam, 3: principal rafters, 4: struts.

A king post truss has two principal rafters, a tie beam, and a central vertical king post. [5] The simplest of trusses, it is commonly used in conjunction with two angled struts.

The king post is normally under tension, and requires quite sophisticated joints with the tie beam and principal rafters. In a variation known as a king bolt (rod) truss the king post is replaced by a metal bolt (rod), usually of wrought iron.

Queen post truss

Queen post truss.
Key: 1: queen posts, 2: tie beam, 3: straining beam, 4: principal rafters. Queen post truss.svg
Queen post truss.
Key: 1: queen posts, 2: tie beam, 3: straining beam, 4: principal rafters.

A queen post truss has two principal rafters and two vertical queen posts. [5] The queen post truss extends the span, and combined with spliced joints in the longer members extends the useful span for trusses of these types. As with a king post, the queen posts may be replaced with iron rods and thus called a queen rod truss. This truss is often known as a palladiana (Palladian truss) in Italy, as it was frequently used by the Renaissance architect Andrea Palladio. [6] [lower-alpha 1] Sometimes a Palladian truss is defined as a compound truss with a queen post and king post truss in the same assembly. [7]

The queen post truss and the king post truss may be combined, by using the straining beam of the queen post truss as the tie beam for a king post truss above. [8] Such combinations are known as compound trusses.

Liegender Stuhl

German illustration of a purlin roof with liegendem stuhl truss highlighted in blue. Kehlbalkendach mit liegendem Stuhl 1200px.png
German illustration of a purlin roof with liegendem stuhl truss highlighted in blue.

Liegender Stuhl is a truss of German origin, the German name is used in America. This truss is found in some 18th and 19th-century buildings where Germans settled in the U.S. The literal translation is "lying chair", lying meaning the top chords are angled or leaning (and chair in the sense of a support, in this case a post or truss). [9] Carpenters in the Netherlands also used this truss where it is spelled liggende stoel. [10]

The opposite being a "Stehender Stuhl", which is the common roof truss type where simple vertical posts replace the more elaborate support structure in the image (highlighted in blue).

Open trusses

Arch-braced truss

A single arch-braced truss.
Key: 1: principal rafters, 2: collar beam, 3: arch braces. Arch brace truss 3D.svg
A single arch-braced truss.
Key: 1: principal rafters, 2: collar beam, 3: arch braces.

Lacking a tie beam, [11] the arch-braced (arched brace) [12] truss gives a more open look to the interior of the roof. The principal rafters are linked by a collar beam supported by a pair of arch braces, which stiffen the structure and help to transmit the weight of the roof down through the principal rafters to the supporting wall. A double arch braced truss [12] has a second pair of arched braces lower down, from the rafter to a block or inner sill: This form is called a wagon, cradle, barrel or tunnel roof because of this cylindrical appearance. [13]

Hammerbeam truss

Chambers 1908 single hammerbeam truss Chambers 1908 Hammerbeam.png
Chambers 1908 single hammerbeam truss

The hammerbeam roof was the culmination of the development of the arch-braced truss, allowing greater spaces to be spanned. The hammerbeam roof of Westminster Hall in London, designed by Hugh Herland and installed between 1395 and 1399, was the largest timber-roofed space in medieval Europe, spanning a distance of just over 20 metres (66 ft). [14] It is considered to be the best example of a hammer-beam truss in England. [15]

Hammer beam trusses can have a single hammerbeam or multiple hammerbeams. A false hammerbeam roof (truss) has two definitions: 1) There is no hammer post on the hammer beam [16] [17] as sometimes found in a type of arch brace truss [18] or; 2)The hammer beam joins into the hammer post instead of the hammer post landing on the hammer beam. [19]

Scissor truss

A scissor truss Scissors truss 1.png
A scissor truss

The scissor truss gets its name from being shaped like a pair of shears (scissors). Two defining features of a scissor truss are: 1) the joint where the bottom chords pass (the hinge of a pair of scissors) must be firmly connected and 2) the rafter (top chord) feet must land on the bottom chords. If the bottom chords join to the under-side of the top chords the assembly is said to be "scissor braced" [20] rather than a scissor truss.

Bracing

It has been proven that permanent bracing designs can improve the lateral stability of roof trusses, specifically with j chords. Continuous lateral braces can prevent negative effects of lateral forces by being diagonally set in to brace the chord. However, there are variable effects of permanent braces on the truss web, given that the length of the chord would determine the number of brace locations. [21]

Pre-fabricated wood trusses

Pre-fabricated wood trusses offer advantages in building construction through machine-made accuracy and tend to use less timber. [22] However, this does not take into account site-specific design alterations that require customized truss design.

Historic wood trusses

The challenges of maintaining wood trusses are not limited to newer building projects. Visual grading can be used to conduct condition assessments for historic trusses. This along with load carrying tests, can be used to determine the best possible solutions to repairing older truss systems. Metal plates and correct, period material can be used to repair, although 100% recovery of the material may be hard if the wood truss has deteriorated. MPC (Metal Plate Connected) wood trusses are typically made from dressed wood (lumber), rather than timber (round-wood poles). [23]

The interior of the barn at Bartram's Garden with a view of the wooden trusses. Truss Bartram's Barn.jpg
The interior of the barn at Bartram's Garden with a view of the wooden trusses.

Connections in wooden trusses

The earliest wooden truss connections consisted of mortise-and-tenon joints and were most likely crafted at the construction site with the posts. Since most early trusses were made from unseasoned posts, the subsequent shrinkage would create cracking at the mortise-and-tenon joints. Additionally, the mortise-and-tenon joints in older trusses were located at the weakest point in the post, accelerating failure. Much of the early truss connection designs anticipated structural behaviour under loads. This is why holes were drilled slightly off-centre, allowing the peg to naturally pull the posts together with gravity. [24]

See also

Related Research Articles

<span class="mw-page-title-main">Mortise and tenon</span> Woodworking joint

A mortiseand tenon joint connects two pieces of wood or other material. Woodworkers around the world have used it for thousands of years to join pieces of wood, mainly when the adjoining pieces connect at right angles.

<span class="mw-page-title-main">Truss</span> Rigid structure that consists of two-force members only

A truss is an assembly of members such as beams, connected by nodes, that creates a rigid structure.

<span class="mw-page-title-main">Timber framing</span> Traditional building technique

Timber framing and "post-and-beam" construction are traditional methods of building with heavy timbers, creating structures using squared-off and carefully fitted and joined timbers with joints secured by large wooden pegs. If the structural frame of load-bearing timber is left exposed on the exterior of the building it may be referred to as half-timbered, and in many cases the infill between timbers will be used for decorative effect. The country most known for this kind of architecture is Germany, where timber-framed houses are spread all over the country.

<span class="mw-page-title-main">Joist</span> Horizontal framing structure

A joist is a horizontal structural member used in framing to span an open space, often between beams that subsequently transfer loads to vertical members. When incorporated into a floor framing system, joists serve to provide stiffness to the subfloor sheathing, allowing it to function as a horizontal diaphragm. Joists are often doubled or tripled, placed side by side, where conditions warrant, such as where wall partitions require support.

<span class="mw-page-title-main">Steel square</span> Flat tool used in carpentry to mark right angles and calculate angles

The steel square is a tool used in carpentry. Carpenters use various tools to lay out structures that are square, many of which are made of steel, but the name steel square refers to a specific long-armed square that has additional uses for measurement, especially of various angles. It consists of a long, wider arm and a shorter, narrower arm, which meet at an angle of 90 degrees. Today the steel square is more commonly referred to as the framing square or carpenter's square, and such squares are no longer invariably made of steel ; they can also be made of aluminum or polymers, which are light and resistant to rust.

<span class="mw-page-title-main">Treenail</span> Wooden fastener

A treenail, also trenail, trennel, or trunnel, is a wooden peg, pin, or dowel used to fasten pieces of wood together, especially in timber frames, covered bridges, wooden shipbuilding and boat building. It is driven into a hole bored through two pieces of structural wood.

<span class="mw-page-title-main">Hammerbeam roof</span> Type of English Gothic roof

A hammerbeam roof is a decorative, open timber roof truss typical of English Gothic architecture and has been called "...the most spectacular endeavour of the English Medieval carpenter". They are traditionally timber framed, using short beams projecting from the wall on which the rafters land, essentially a tie beam which has the middle cut out. These short beams are called hammer-beams and give this truss its name. A hammerbeam roof can have a single, double or false hammerbeam truss.

<span class="mw-page-title-main">Rafter</span> Supporting structural member in roof construction

A rafter is one of a series of sloped structural members such as steel beams that extend from the ridge or hip to the wall plate, downslope perimeter or eave, and that are designed to support the roof shingles, roof deck, roof covering and its associated loads. A pair of rafters is called a couple. In home construction, rafters are normally made of wood. Exposed rafters are a feature of some traditional roof styles.

<span class="mw-page-title-main">Framing (construction)</span> Construction technique

Framing, in construction, is the fitting together of pieces to give a structure support and shape. Framing materials are usually wood, engineered wood, or structural steel. The alternative to framed construction is generally called mass wall construction, where horizontal layers of stacked materials such as log building, masonry, rammed earth, adobe, etc. are used without framing.

A tie, strap, tie rod, eyebar, guy-wire, suspension cables, or wire ropes, are examples of linear structural components designed to resist tension. It is the opposite of a strut or column, which is designed to resist compression. Ties may be made of any tension resisting material.

<span class="mw-page-title-main">Purlin</span> Structural member in a roof

A purlin is a longitudinal, horizontal, structural member in a roof. In traditional timber framing there are three basic types of purlin: purlin plate, principal purlin, and common purlin.

<span class="mw-page-title-main">Howe truss</span>

A Howe truss is a truss bridge consisting of chords, verticals, and diagonals whose vertical members are in tension and whose diagonal members are in compression. The Howe truss was invented by William Howe in 1840, and was widely used as a bridge in the mid to late 1800s.

<span class="mw-page-title-main">King post</span> Central vertical post used in architectural or bridge designs

A king post is a central vertical post used in architectural or bridge designs, working in tension to support a beam below from a truss apex above.

<span class="mw-page-title-main">Queen post</span>

A queen post is a tension member in a truss that can span longer openings than a king post truss. A king post uses one central supporting post, whereas the queen post truss uses two. Even though it is a tension member, rather than a compression member, they are commonly still called a post. A queen post is often confused with a queen strut, one of two compression members in roof framing which do not form a truss in the engineering sense.

<span class="mw-page-title-main">Collar beam</span>

A collar beam or collar is a horizontal member between two rafters and is very common in domestic roof construction. Often a collar is structural but they may be used simply to frame a ceiling. A collar beam is often called a collar tie but this is rarely correct. A tie in building construction is an element in tension rather than compression and most collar beams are designed to work in compression to keep the rafters from sagging. A collar near the bottom of the rafters may replace a tie beam and be designed to keep the rafters from spreading, thus are in tension: these are correctly called a collar tie.

A bent in American English is a transverse rigid frame. Historically, bents were a common way of making a timber frame; they are still often used for such, and are also seen in small steel-frame buildings, where the term portal frame is more commonly used. The term is also used for the cross-ways support structures in a trestle. In British English this assembly is called a "cross frame". The term bent is probably an archaic past tense of the verb to bind, referring to the way the timbers of a bent are joined together. The Dutch word is bint, the West Frisian is bynt, and the German is bind. Compare this with the term bend for a class of knots.

A post is a main vertical or leaning support in a structure similar to a column or pillar but the term post generally refers to a timber but may be metal or stone. A stud in wooden or metal building construction is similar but lighter duty than a post and a strut may be similar to a stud or act as a brace. In the U.K. a strut may be very similar to a post but not carry a beam. In wood construction posts normally land on a sill, but in rare types of buildings the post may continue through to the foundation called an interrupted sill or into the ground called earthfast, post in ground, or posthole construction. A post is also a fundamental element in a fence. The terms "jack" and "cripple" are used with shortened studs and rafters but not posts, except in the specialized vocabulary of shoring.

<span class="mw-page-title-main">American historic carpentry</span>

American historic carpentry is the historic methods with which wooden buildings were built in what is now the United States since European settlement. A number of methods were used to form the wooden walls and the types of structural carpentry are often defined by the wall, floor, and roof construction such as log, timber framed, balloon framed, or stacked plank. Some types of historic houses are called plank houses but plank house has several meanings which are discussed below. Roofs were almost always framed with wood, sometimes with timber roof trusses. Stone and brick buildings also have some wood framing for floors, interior walls and roofs.

The Priest House is a Grade II* listed fifteenth century timber framed hall house in the centre of West Hoathly, in West Sussex, England. It is close to The Cat Inn and St Margaret's Church. It is now a museum, open to the public six days a week from March to October.

<span class="mw-page-title-main">Gothic-arch barn</span> Type of barn with curved rafters

A Gothic-arched roof barn or Gothic-arch barn or Gothic barn or rainbow arch is a barn whose profile is in the ogival shape of a Gothic arch. These became economically feasible when arch members could be formed by a lamination process. The distinctive roofline features a center peak as in a gable roof, but with symmetrical curved rafters instead of straight ones. The roof could extend to the ground making the roof and walls a complete arch, or be built as an arched roof on top of traditionally framed walls.

References

Notes

  1. The palladiana had been used in Italy for many years before Palladio's time, but has been given his name because of its extensive use in the buildings he designed. [6]

Citations

  1. Curl, James Stevens (2006), "truss", A Dictionary of Architecture and Landscape Architecture (online ed.), Oxford University Press, ISBN   978-0-19-860678-9 , retrieved 1 December 2012(subscription required)
  2. Steane (1984), p. 196
  3. Lewandoski, Jan, Jack Sobon, and Kenneth Rower. Historic American roof trusses. Becket, MA: Timber Framers Guild, 2006. 48. ISBN   0970664346
  4. Domestic Buildings Research Group (Surrey, England), http://www.dbrg.org.uk/GLOSSARY/Open%20truss.html
  5. 1 2 Glossary of Australian Building Terms – Third Edition.(NCRB)
  6. 1 2 Valeriani, Simona (August 2006), "The Roofs of Wren and Jones: A Seventeenth-Century Migration of Technical Knowledge from Italy to England". Working Papers on the Nature of Evidence: How Well Do 'Facts' Travel?" (PDF), Department of Economic History, London School of Economics, retrieved 22 December 2012
  7. Proceedings of the Third Annual Congress on Construction History, May 2009. Figure 1. based on Sachero, C., 1864: Sulla stabilità delle armature dei tetti. Giornale del Genio Militare 2. 250. http://www.bma.arch.unige.it/PDF/CONSTRUCTION_HISTORY_2009/VOL3/TURRI-Zamperin_Cappelletti_1_paper-revised_layouted.pdf accessed 12/24/2012
  8. Yeomans (2003), p. 42
  9. Anderegg, Jean. Holzbaukunst: Fachwerk, Dachgerüst, Zimmermannswerkzeug : systematisches Fachwörterbuch = Architecture en bois : construction en pan de bois, charpente de toit, outils du charpentier : dictionnaire spécialisé et systématique = Architecture in wood : timber-frame construction, roof frame, carpenter's tools : specialized and systematic dictionary. Munich: K.G. Saur, 1997. : 65. ISBN   3598104618
  10. Herman Janse, Houten kappen in Nederland 1000-1940. Delftse Universitaire Pers, Delft / Rijksdienst voor de Monumentenzorg, Zeist 1989.
  11. Harris (2003), pp. 75–77
  12. 1 2 Chappell, Steve. A timber framer's workshop: joinery, design & construction of traditional timber frames. Brownfield, Me.: Fox Maple Press, 1998. 115. Print.
  13. Whewell, William, and F. de. Lassaulx. Architectural notes on German churches; with notes written during an architectural tour in Picardy and Normandy.. The 3d ed. Cambridge: J. and J.J. Deighton; [etc.], 1842. 44. Print.
  14. Steane (1984), pp. 196–97
  15. Webber, Frederick G.. Carpentry & Joinery. Methuen and Co., London: 1898. 152.
  16. Davies, Nikolas, and Erkki Jokiniemi. Dictionary of architecture and building construction. Amsterdam: Elsevier/Architectural Press, 2008. 144.
  17. Alcock, N. W.. Recording timber-framed buildings: an illustrated glossary. London: Council for British Archaeology, 1989.
  18. Sharpe, Geoffrey R.. Historic English churches a guide to their construction, design and features. London: I.B. Tauris, 2011. 111. fig. 61.
  19. Wood, Margaret. The English Mediaeval House. London: Ferndale Editions, 1980, 1965. 319.
  20. Lewandoski, Jan, Jack Sobon, and Kenneth Rower. Historic American roof trusses. Becket, MA: Timber Framers Guild, 2006. 7. ISBN   0970664346
  21. Permanent bracing design for MPC wood roof truss webs and chords, Forest Products Journal, 51(7), 73-81
  22. "The benefits of pre-fabricated timber roof trusses." Civil Engineering (10212000) 24, no. 7: 47-48. Business Source Complete, EBSCOhost (accessed November 8, 2017)
  23. Non-destructive assessment, full-scale load-carrying tests and local interventions on two historic timber collar roof trusses. Engineering structures, 140.
  24. "Early Wooden Truss Connections vs. Wood Shrinkage: From Mortise-and-Tenon Joints to Bolted Connections" APT Bulletin, Vol. 27, No. 1/2, A Tribute to Lee H. Nelson (1996), pp. 11-23.

Bibliography