Traditional Carpentry

Traditional carpenter-designers of timber framed buildings worked on a practical basis, solving problems through experience of success and failure and knowledge handed down the generations. When they arose, structural problems were solved without recourse to theoretical calculation and measurement based on proportions and their knowledge of geometry. Working with large, heavy timber sections, like Green Oak, Chestnut and Douglas Fir requires large, flexible floor areas as well as skilled craftsmen with a love for natural wood as well as the preservation of their centuries old crafts. Rationalised Traditionally Timber Frame Trusses But that does not mean that some of the processes and procedures used to design, mill and shape these elements into the beautiful features you and your clients want for their homes cannot be rationalised. That is what we have done in order to bring you the stunning interiors and exteriors that you and your clients are looking for at exceptionally reasonable prices. test test test Traditional Braced King Post Truss with mortice and tenon joints laid up ready for pegging Traditional Timber Joints Timber structures are weakest when joints or individual components are subject to tensile forces, i.e. either the joints are being pulled apart or the bending action in a timber member causes the wood fibres to be excessively stretched and twisted. Most of the components in timber framed buildings are therefore designed to act in compression. Where there was little choice but to have a member subject to bending and thus tensile forces, traditionally carpenters have devised means by which such a member could be propped to reduce and counteract the tension. Timber Cill Beams Jointing the posts on a cill beam rather than setting them into the ground removed much of the buttressing support both along and across the building. Timber Bracing The provision of triangular bracing was the principle means by which traditional carpenters avoid ’racking’ within frames. Timber Braces stiffen the frame. They help keep timber frames plumb, level and square and are found in each frame element i.e. cross frames, roof frames and along wall frames. To avoid failure by buckling, braces need to be relatively short and need to be used in opposing pairs, since structurally they operate in one direction only. To avoid failure by buckling, braces need to be relatively short and need to be used in opposing pairs, since structurally they operate in one direction only. Traditional Carpentry Joints Over the years, traditional carpenters have developed complex new joints and assemblies for dealing with new structural problems, although some of the pre 1200 joints continue to be used. Today the mortice and tenon joint – which has been used for thousands of years – has replaced older, simpler lap joints as the most common joint throughout timber framing.​ Traditionally Timber Pegged Joints In common with most timber frame joints, mortice and tenons are secured by timber pegs in the final erection of the building. Using ‘draw boring’ the hole for the peg in the sides of the mortice are slightly off-set on the tenon so that when the peg is finally driven through, the whole joint tightens up. ​Pegs provide considerable resistance to withdrawal, but over stressing can, and does, cause failure. Traditional Scarfed Joints Improvements in understanding geometry has enabled traditional carpenters to develop complex lengthening joints known as scarf joints (see below). Using these joints traditional carpenters are able to create and provide timbers longer than those which are naturally available, for example when these might be required for a long ridge beam or purlins to support the common rafters. The illustration below shows (left to right) a traditional mortice, tennon and pegged joint, a scarfed joint, and a complex joint where a post meets the wall plate, tie beam and roof structure. Traditional Structural Design in Trusses Joints in traditional carpentry often rely on compression to keep the faces of the timber in close contact without anything other than notches in connected members. ​A widespread classic connection at the heel joint in timber trusses is the front notched joint, with a single tooth in the strut, a notch in the top face of the bottom chord and sometimes metal fasteners. This joint directs the horizontal component of the internal compression force in the strut to the bottom chord, through compression of the front notch and a shear surface in the toe of the chord. ​These joints rely on the compression internal forces to keep facing surfaces in close contact. Metal fasteners are more common in Western cultures than in Oriental joint craft. Without these this joint relies on the friction developed between the timber elements in the front notch, which is highly dependent on the compression level in the strut, and cannot support alternating connection forces.​ Eurocode 8 [3] requires compression members of carpentry joints to “be designed in such a way that they are prevented from separating and remain in their original position”. This may mean that metal elements are necessary to comply with the safety requirements, assuring the contact between friction surfaces and stabilizing the performance under cyclic actions. On the other hand, often the metal parts are used to strengthen the joints, without adequate knowledge on the consequences of such actions on the joint and on the overall structural behaviour.​ The heel joint is usually considered as hinged and the internal forces acting in the connection are the axial compression in the strut, the tension in the chord and the vertical support reaction. The widespread method, commonly presented in the related literature to assess the strength of this type of joint relies on this assumption. This implies checking the compression stresses at an angle to the grain in the frontal notch surface (if the strut and the chord timber belong to different strength classes this must be done in both the strut and the chord) and shear stresses in the timber beyond the notch. Metal Fasteners: Stirrups &Tension Ties Metal fasteners have been widely applied in timber joints since the 19th century. They can improve the performance concerning out-of-plane actions in the carpentry joints and definitely change their mechanical response. Metal stirrups are a quite frequent reinforcement in timber trusses and are also widely applied to strengthen old joints. The metal stirrup illustrated here uses two welded 3×50 mm steel flat bars with 9 mm diameter holes and 8 mm threaded bar dowels.