Dowel-type fasteners. Timber Connections. Academic resources. Introduction. Deferent types of dowel-type fasteners. Version 1

Transcription

1 Academic resources Timber Connections Dowel-type fasteners Version 1 This unit covers the following topics: Deferent types of dowel-type fasteners Introduction There are four criteria designers should take into account when selecting and detailing connections for a particular application: load transfer (stiffness, ductility, robustness and differential movement) appearance fire resistance durability. The traditional mechanical fasteners for structural timber connections are divided into two groups depending on how they transfer the forces between the connected members doweltypes and metal connectors. Metal fasteners have negligible fire resistance and absorb and transfer heat quickly. Where required, fire protection with plasterboard is the most common solution. Friction within the interface between the two connected members Axial pullout resistance. Joints made with dowel-type fasteners are usually single or double shear. Usually large dowels and bolts have much higher load-carrying capacity compared with nails, screws or staples. When screws and nails are compared, nails generally have better lateral loadcarrying capacity than the same diameter screws. This is due to the uniform cross section of nails throughout their length, whereas screws have a reducing cross section along their threaded length. However some modern large screws also have uniform cross section. Screws have much higher axial withdrawal resistance (demountability) compared with nails. Figure 1 shows the relative load-carrying capacity for a selection of fasteners, based on Eurocode 5 design principles. Metal fasteners are efficient but the relative weakness of the timber perpendicular to the grain remains the governing factor. The appearance of the metal fasteners is not very important as most of them are concealed within the connection. However, careful detailing is needed if the architect wishes to expose the connection. Deferent types of dowel-type fasteners The most popular dowel-type fasteners are staples, nails, screws, bolts and dowels. The magnitude of the load transfer depends on following factors: The bending behaviour of the dowel The bearing stresses developed in the timber along the shank of the fastener Figure 1: Typical design load-carrying capacity per fastener for a timber-to-timber single shear joint, parallel to grain. Assumes a strength class C24 (to which SS grade European redwood and whitewood belong), medium-term load duration class and timber member thicknesses typically used with the various fasteners TRADA Technology October

2 Staples Staples are mainly used to fix thin panels or pieces of timber to solid timber frames. They are most commonly used in factory fabrication with pneumatic tools. Standards have been changed to reflect the production of new products by manufacturers. Staples for pneumatic tools are supplied as collated strips. In these strips staplers are held together by resin. This resin coating is not only for collating the staples but also reduces the friction on insertion so that the staple is less likely to buckle. Staples are also available as loose fixings for manual insertion. In order to use staples for structural purposes they should comply with EN Staples are also used for fencing. Staples should be driven at right angles to the grain as with nails. If possible, to reduce the risk of splitting, the crown of the staple should be at an angle to the grain direction. Staples should not be driven into end grain timber. Most collated staples are not suitable for higher density timbers as they are likely to buckle. Figure 2: Staples Photo: Young Black Industrial Stapling Nails Nails are the most widely used dowel-type fasteners. They are used for many forms of structural timber components, such as timber frame stud walls and floor diaphragms, as well as for connecting timber or wood-based panel products together, and for connecting metal plates to timber. The use of nails in large numbers, such as in nail plates, spreads the load more evenly and for this reason has an advantage over bolts. There are a number of different types of nails available in terms of sizes, shapes and materials. The most common is probably the round wire nail which is produced from a steel wire with a minimum tensile strength of 600 N/mm 2 as specified in BS EN Steel wire and wire products. General. Test methods. Twisted and annular shank nails have better pullout resistance than round nails. Nails for structural use are specified in BS EN A nail s performance under both lateral and withdrawal loading can be improved by modifying its surface from smooth round nail to threaded nail. Any nail with a modification of the shank profile is called a threaded nail. Common threaded nail types are annular ringed or helically threaded shank nails which have greater withdrawal strength. Square twisted nails are often specified for fixing three-dimensional nailing plates, such as joist hangers. These nails have high yield strength as the square cross section has been twisted into a helical pattern. Eurocode 5 specifies that the nails used to resist permanent or long-term axial loading shall be threaded. Figure 3: Annular ringed nail and round wire nail Photo: Simpson Strong-Tie For pneumatic nail guns, nails come in collated coils or collated strips, as in factory fabrication nails are usually driven in with these tools. The coils are collated with plastic or wire which breaks off as the nail is inserted. Some nails are specially designed with a segment of the head cut away, such as clipped head, D-head nails and T-head nails, which allows the nails to be collated in close-packed angled strips for cartridges for pneumatic tools. Collated strips are usually held together with adhesive paper tape that tears as the nail is driven in, so some paper is carried into the wood with the nail. Generally, nails should be inserted at right angles to the grain. For slant nailing Eurocode 5 guidance should be followed. End grain nailing is not usually recommended. However, for secondary structures only, Eurocode 5 has a simplified method to calculate the end grain capacities of nails. It also states that there should be at least two nails in a connection. Pre-drilling holes for larger nails may be necessary when driving into dense timbers and also when the connected members are so thin that the risk of splitting is high. Screws For plain timber-to-timber joints, wood screws can be used. Metal screws are often used for steel-to-timber and panel-totimber joints. As with nails, there are a number of different types of screws available. The most common types are countersunk head screw, round head screw and coach screw. Although nails are more commonly used for fixing joist hangers and framing anchors, screws can also be used for this purpose. Coach screws are suitable for large connections and are also capable of replacing bolts for single-sided access. They are a specific type of structural screw commonly produced to the German standard DIN 571. The thread is turned down from the original rod diameter, leaving a full diameter shank, and the TRADA Technology October

3 screws have hexagonal heads, like bolts. They are used in engineered timber structures, particularly for fastening metalwork to timber. Coach screws require a washer and must always be inserted into a pre-drilled hole. Apart from the screws mentioned above there are many screws designed for specific purposes for example the self-tapping Assy screw is intended for high strength and the Topix CC screw is intended for joining together large timbers into a tightfitting joint. A number of proprietary screws are also available that offer specific advantages they may be longer or stronger than typical screws, the thread geometry may vary along the length, the design of the head or tip may have special features. The advantages claimed by the manufacturers of these screws might not be reflected in standard tests, in which case they should provide a basis for their claims. However these improved screws can offer benefits that are independently verified and genuinely innovative. not more than 24mm in accordance with BS EN Screwed joints usually function in single shear. In order to avoid wood splitting, screws with a diameter greater than 6mm should be inserted into pre-drilled holes in accordance with Eurocode 5. However smaller diameter screws may be more closely spaced with pre-drilled holes than without. Bolts and dowels Bolts and dowels are used to join two or more components together and are generally loaded in shear. Bolts are threaded for nuts while dowels are plain or ribbed. Bolts are threaded dowel-type fasteners, with hexagonal or square heads or nuts. The cross section of the threaded part of the bolt is circular. Common types of bolts are the hexagonal head bolt, the square head bolt and the carriage bolt. The heads often remain visible on the surface of a joint. Figure 4: Coach and countersunk screws Figure 7: Hexagonal head and carriage bolts Dowels are plain or ribbed rods usually circular in cross section but sometimes deformed in rectangular cross sections as well. Usually dowels have a smooth surface but are also often fluted to ease insertion. Their plain ends are neater in appearance than bolts and are also stiffer. The dowel ends can be plugged which improves both the appearance and the fire performance. Figure 5: Assy screw Figure 8: Plain dowel Photo: Simpson Strong-Tie Figure 6: Topix CC screw Photo: Heco In order to gain the maximum load-carrying capacity, screws should be inserted by turning and not by driving with a hammer. Most industrial screws no longer have their traditional slotted head and proprietary head recesses allow screws to be machine driven. Screws with these recesses should only be driven with the correct matching bit. Wood screws are specified in BS EN For structural use, screws should have a nominal diameter of at least 2.4mm and In a connection, dowels are tightly fitted compared with bolts: bolt holes may be up to 1mm larger in diameter in timber than the bolt and 2mm or 0.1 times the diameter of the bolt larger in steel plates, whichever is the greater, in accordance with Eurocode 5. This is the principal difference between dowels and bolts. As a result, bolted joints slip a little before bearing lateral load. Dowels in timber-to-timber joints slip very little because the dowel is a snug fit, but dowels may slip in timber-to-metal joints. TRADA Technology October

4 In a bolt connection, a given bolt diameter achieves a lower strength in timber than in steel because the bearing strength of timber is lower than steel. Bolts can carry axial loads as they are threaded with nuts and fixed heads, whereas dowels cannot carry axial loads. Dowels and bolts are specified in BS EN and are usually made from mild steel, but may also be made from stainless steel. Dowels and bolts have a constant cross-section, usually circular. Most bolts and dowels are available in diameters from 6mm to 30mm. Timber is pre-drilled when inserting a bolt, allowing clearance for easy insertion. This also reduces the timber splitting possibilities if drying out occurs. In order for members to be fitted closely, bolts should be tightened properly and retightening may be required when the timber members reach equilibrium moisture content. BS EN 26891:1991 ISO 6891:1983 Timber structures Joints made with mechanical fasteners General principles for the determination of strength and deformation characteristics BS EN 28970:1991 ISO 8970: 1989 Timber structures Testing of joints made with mechanical fasteners Requirements for wood density BS EN 409:2009 Timber structures Test methods Determination of the yield moment of dowel-type fasteners BS EN 912:2000 Timber fasteners Specifications for connectors for timber BS EN ISO Fasteners Non-electrolytically applied flake coatings Summary of key points Whenever a bolt is used, either on its own or through a connector, a washer should be installed in order to avoid the head and nuts being in contact with the timber. Eurocode 5 gives the washer specifications to reduce the bearing stress on the timber. The diameter of the pre-drilled holes for dowels should be not greater than that of the dowel itself. In large dowel connections, such as with mechanically laminated bridge beams, it may be necessary to replace some of the dowels with threaded bolts to stop the joint from opening laterally. In dowelled joints that include steel members, allow a clearance in the steel members and allow for the slip that may occur as a result. Depending on the joint design, it may be possible to weld dowel ends to steel plates, in which case there should be no slip. Bolted and dowelled connections can be strengthened by incorporating a resin into the joint. Relevant Standards BS EN A1:2008 Eurocode 5: Design of timber structures Part 1-1: General Common rules and rules for buildings BS EN 1382:1999 Timber Structures Test methods Withdrawal capacity of timber fasteners BS EN 1383:1999 Timber Structures Test methods Pullthrough resistance of timber fasteners BS EN 14566:2008+A1:2009 Mechanical fasteners for gypsum plasterboard systems Definitions, requirements and test methods BS EN 14592:2008 Timber structures Dowel-type fasteners Requirements The most popular dowel-type fasteners are staples, nails, screws, bolts and dowels. The magnitude of the load transfer depends on: 1. The bending behaviour of the dowel 2. The bearing stresses developed in the timber along the shank of the fastener 3. Friction within the interface between the two connected members 4. Axial pullout resistance. Large dowels and bolts usually have much higher load carrying capacity compared with nails, screws or staples. The most common nail is the round wire nail which is produced from a steel wire with a minimum tensile strength of 600 N/mm 2 although twisted and annular shank nails have better pullout resistance than round nails. Any nail with a modification of the shank profile is called a threaded nail. Square twisted nails are often specified for fixing threedimensional nailing plates, such as joist hangers, as they have high yield strength. Eurocode 5 specifies that the nails used to resist permanent or long-term axial loading shall be threaded. End grain nailing is not usually recommended for primary structures. However end grain nailing can be used for secondary structures. Wood screws can be used for plain timber-to-timber joints. Metal screws are often used for steel-to-timber and panelto-timber joints. The most common types of screws are countersunk head screw, round head screw and coach screw. When screws and nails are compared, nails generally have better lateral load-carrying capacity than the same diameter screws. TRADA Technology October

5 TRADA Academic resources / Timber Connections / Dowel-type fasteners version 1.0 Bolts can carry axial loads while dowels cannot carry axial loads. Bolted and dowelled connections can be strengthened by incorporating a resin into the joint. References 1. Ross, P., Hislop, P., Mansfield-Williams, H. and Young, A., Concise illustrated guide to timber connections, 1st edition, ISBN , TRADA Technology, Wood Information Sheet 2/3-52: Fasteners for structural timber: nails, staples, screws, dowels and bolts, TRADA Technology, BS EN A1:2008 Eurocode 5: Design of timber structures Part 1-1: General Common rules and rules for buildings, BSI 4. NA to BS EN :2004+A1:2008 UK National Annex to Eurocode 5: Design of timber structures Part 1-1: General Common rules and rules for buildings, BSI TRADA Technology 2012 All rights reserved. All information included in this module is provided by TRADA as lecturer support material. It may be downloaded and reproduced by lecturers for course delivery purposes only, and up to 100 pages may be printed per year as student hand-outs, providing this is not for financial gain. No part of this information may be stored in a retrieval system, repackaged, adapted or transmitted in any form, by any means, electronic, mechanical, recording or otherwise without the prior permission of the copyright owners. Whilst every effort is made to ensure the accuracy of the advice given, the company cannot accept liability for loss or damage arising from the use of the information supplied. 5