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    Mini Research Report——Metal Connections In Timber Structures

    Metal Connections

    In Timber Structures

    ——A Nail Connection

    091039 Shehong Liu(刘射洪;

    ?.Background

    When constructing timber structures, it is important that structural members such as beams, columns, and trusses be designed to carry the specified loads, be-cause no chain is stronger than its weakest link. So connection design is one of the most essential aspects in timber structures. However, as we know, wood is easily drilled or shaped to facilitate a connection. So a wide range of connectors and con-nection methods is available. On the other hand, a mechanical fastener is any device, metallic, plastic or timber, which transfers load from one piece of timber to another piece of timber. And the most common types of fasteners are metallic and include: nails, dowels, screws, bolts, coach-screws, toothed ring connectors, split rings, nail plates, proprietary or patented fasteners and so on. Most fasteners transfer forces through bearing on the timber and shear in the connector.

    Of course, the most widely used connector is the common nails. In engineering, joinery, and the building, nail refers to the point of shape hard metal (usually steel), which has a very long history. Meanwhile, according to the different uses, nail also has many types, such as round nail, manual twist nail, spell nail, cement steel nail, wood screw, shoot nail, bolt and so on. For instance, round nail are also called nail, its head is round flat and body is smooth cylindrical, and bottom is a sharp shape. And its common specification is from 10 to 220 mm about 20 types. Common round nail wooden structure is mainly used for the connection. Manual twist nail, its body is such as manual twist shape, head also is round flat look, and the base is pointed bottom. This nail force particularly is strong. So it is applied to in some places where need very strong force, such as drawer, wooden ceiling of the place such as the boom, its common specification is from 50 to 85 mm and has a variety of specifica-tions. Spell nail is a kind of nails which two heads are pointed, and the middle for smooth surface. Spell it was easier than other nail merger and fixed wood, especially suitable for collage board when make pin with, its common specification has 25-120 mm. Cement steel nail is very similar with round nail in shape, but its head is a bit thick. And the cement steel nail with high quality steel, with a hard, bending of ad-vantage, can be directly into the concrete and brick wall nail in. Its common specifi-cation is 7- 35 mm. Wood screw is also called teeth wood screw, which is more easily than other nails with wood union, multi-purpose in metal and other material and the combination of wooden material. Tapping screw, which has high hardness, cheap prices, better than any other nails to connect with two metal parts. It is used more for metal components connection fixed, such as the production of aluminum alloy

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    Mini Research Report——Metal Connections In Timber Structures

    doors and windows. Shoot nail, is very similar with cement steel nail, but it is shot out of the shot gun. Relative to character, shoot nail is better and more economic than artificial construction. At the same time, it is easier for construction than other nails. It is used to shoot to nail wooden of construction of the project, such as fine wood making to confront and wooden engineering, etc. Bolt, which is used in deco-ration in general engineering, is divided into two kinds, plastic bolt and metal bolt, and they are both used to replace embedded bolt.

    Basing on so many reasons, a nail connection in timber structures is the most common and very important. Though with the development of technology, materials are more and more and also better and better, of course, the types of connection in timber structures are also more and more and also better and better. However, as the simplest connection, a nail connection shows strong vitality. In another word, if you

    want to know timber structures very well, you must know and learn a nail connec-tion in timber structures.

?.Major application

    The major application of a nail connection in timber structures is very wide. On one hand, a nail connection is very simple and cheap. On the other hand, the perfor-mance of a connection is pretty good in mechanical property aspects.

    Nails provide a simple and quick way of connecting timber members in con-struction. Common nails are available with smooth, spiral or ring shanks. By shear mode, nails are ductile connectors capable of resisting many cycles of seismic load-ing. Nails are most commonly used in wood-frame shear walls, the main lateral load resisting system in North American platform frame residential construction. Numer-ous static, cyclic and shake table tests on nailed shear walls showed their favorable energy dissipating characteristics and superior seismic performance. Large dis-placements can be achieved in nailed connections without failure. Increased nailing density (smaller spacing) in nail connections can lead to formation of a brit-tle block-tearing failure mode and reduced load capacity.

    As another main type of nails, bolted connections are frequently used in heavy timber frames. They are placed in predrilled holes in the wood members that are normally 1 to 2 mm larger than the bolt shank diameter. Washers are useful in bolted connections as they spread the compression load by the tightening of the bolts over the face of the wood member. Seismic behavior of bolted connections is influenced by wood properties, bolt slenderness (member width to bolt diameter ratio), connection end distance, and fastener spacing parallel- and perpendicular to-grain. The connection capacity increases if the end distance, edge distance and the spacing between the bolts increase. If the number of bolts in the connection in-creases, the ultimate load per bolt decreases. Small diameter (slender) bolts tend to behave more like nails or dowels as their shaft can bend without inducing wood frac-ture, thus exhibiting ductile behavior and relatively large energy dissipation. When slender bolts yield, they also allow for better redistribution of the load among all the fasteners in the connection, resulting in improved load sharing. When larger diame-

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    Mini Research Report——Metal Connections In Timber Structures

    ter bolts are used, the inelastic behavior and energy dissipation of the connection depends on the embedding behavior of the wood alone, which often leads to brittle failures.

    In addition, timber rivets, also known as glulam rivets and as another example of a nail-type of connector, are high-strength nail-type fasteners developed in Can-ada for use in heavy timber construction. Although they are made of high-strength steel, they are pliable enough to provide very ductile connections. Capacity of a ri-veted connection is governed either by rivet yielding or a wood failure mode. The rivet yielding failure mode, where the rivets bend and yield while the wood under their shanks fails in crushing, is the desirable failure mode for seismic applications. Rivet spacing usually controls the failure mode. Larger spacing results in a rivet yield-ing failure mode, while smaller spacing leads to brittle wood failures, usually in the form of block-shear around a group of rivets. Connections with glued-in rods have potential for use in timber construction. Usually, holes are drilled in the end-grain of a glulam or structural composite lumber member, parallel to the member length. Steel rods are then placed into the holes and fixed using a structural glue. A com-prehensive research program was undertaken in New Zealand to determine the static and seismic behavior of these connections (Madsen, 2000). Some of the con-nections investigated during the research program were found to be less efficient due to the reduction of the strength at the joint as a consequence of the holes drilled through the beam. Best results were obtained with connections using steel brackets and glued-in rods. Several buildings have already been designed and built in New Zealand with this connection type.

?.Advantages and disadvantages

     Why a nail connection is widely used in timber structures? And it is really very perfect? Whether it has some shortcomings? It is granted that the answer is sure.

    (1) Advantages

    First, a nail connection is very simple. Compared with dowels, screws,

    bolts, coach-screws, toothed ring connectors, split rings, proprietary or pa-

    tented fasteners, a nail connection doesnt need too much work to made it

    or fix it, we can easily buy any types of nails in market.

    Of course, it is because of this that a nail connection is very cheap. And

    it may be the second reason why it is so widely used in timber structures.

    For example, if you use toothed ring connectors or split rings, you must or-

    der in advance, because there is no this types you want. So the costs of con-

    structions are high usually, however, which are not what bosses want.

    Third, a nail connection can behave in a very ductile mode. The nail is

    relatively slender and its stiffness level combines well with the stiffness of

    wood. Under lateral loading, a nail connection behaves like a beam support

    in a foundation (wood medium surrounding the nail). Under relatively light

    load levels, the wood behaves elastically; upon higher loads, a nonlinear in-

    elastic response takes place.

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    Mini Research Report——Metal Connections In Timber Structures

    Fourth, as for another example of a nail-type of connector, the timber

    rivets suit heavy timber construction very well in general. It is like a

    steel-to-wood connector. It was designed to be installed into a wood mem-

    ber through a steel side plate with predrilled round holes. The resulting

    connection is much stiffer and stronger than a similar connection using

    common nails.

    Finally, it was also reliable. Because the force transmission path of the

    nail connection is very clear, we can easily analysis the construction struc-

    ture, and as we all know, more simple more reliable in the mechanical

    structures.

    Of course, every coin has two sides. In the same way, a nail connection also is not perfect, and also has some disadvantages.

    (2) Disadvantages

    First, it is difficult to use in the super heavy timber structures. On one

    hand, a nail is not too long, which can limit its uses. On the other hand, we

    have better connectors in the super heavy timber structures, such as

    toothed ring connectors, split rings, nail plates, proprietary or patented fas-

    teners and so on.

    Next, the entirety of timber structures that use a nail connection will

    be not very good. As we know, a nail is too small, so in order to satisfy the

    requirement of strength of constructions, we usually use many nails, even

    finally use the timber rivets directly. However, the complicated and weak

    connection will decrease the entirety of timber structures.

    Besides, the types of nails are various, and their specifications and

    strength also are various, which is difficult and must be so cautious for de-

    signers to choose which types they really need. Meanwhile, the pattern of

    how to fix them and distance of them are also a good topic that designers

    and researchers need to study.

    From the above we can see, a nail connection is widely used in timber struc-tures, and it has many advantages. But it has its limitations and disadvantages. So as a civil engineer, we must aware these and learn to foster strengths and circumvent weaknesses, and make full use of various materials.

?.Conclusion

     Connections in heavy timber construction are one of the most important but least understood components. Connections provide continuity to the members, and strength and stability to the structural system. Often key to a successful design is to ability to detail safe, cost effective and elegant solutions that require low mainten-ance.

    Generally, wood is easily drilled or shaped to facilitate a connection. So a wide range of connectors and connection methods is available. For instance, the most common types of fasteners are metallic and include: nails, dowels, screws, bolts, coach-screws, toothed ring connectors, split rings, nail plates, proprietary or pa-

    Page 4 of 5

    Mini Research Report——Metal Connections In Timber Structures

    tented fasteners and so on. However, they all have their own advantages and disad-vantages. Among them, a nail connection is the most common and simplest.

    Nail has a very long history in timber structures. Meanwhile, according to the different uses, nail also has many types, such as round nail, manual twist nail, spell nail, cement steel nail, wood screw, shoot nail, bolt and so on. Of course, they have their own characters and various specifications.

    A nail connection in timber structures is very wide. Nails provide a simple and quick way of connecting timber members in construction. And they are most com-monly used in wood-frame shear walls, the main lateral load resisting system in North American platform frame residential construction. As another main type of nails, bolted connections are frequently used in heavy timber frames. In addition, timber rivets, as another example of a nail-type of connector, are high-strength nail-type fasteners developed in Canada for use in heavy timber construction.

    Of course, every coin has two sides. A nail connection not only has many ad-vantages, but also has its limitations and disadvantages. So as a designer or a civil engineer, we must know them and aware these, so that we can build more beautiful and more reliable timber structures.

?.Reference

    1. 何敏娟,Frank LAM,杨军,张盛东. 木结构设计. 北京:中国建筑工业

    出版社,2008.

    2. CWC 2004. Canadian Wood Council web site: www.cwc.ca.

    3. Madsen, B., 2000. Behavior of timber connections. Timber Engineering Ltd., North Vancouver, B.C., 2000.

     4. Blass H. J. 2003. Joints with dowel-type fasteners. Timber Engineering. Edited by S. Thelandersson and H. J. Larsen. John Wiley and Sons, Chicester, England. ISBN 0470-84469-8.

    5. Johansen K W, Forsogmed Traforbindelser. Danmarks tekniske hojskole medd[ R ]. Laboratoriet for bygningsteknik ( in Danish), 1941, No. 10.

    6. Web site: http://2010.cqvip.com/qk/98159X/200904/31477543.html

     http://www.google.com.hk/search

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