In Europe, over the past decade, we have witnessed an intense evolution in timber construction, from simple roofs to roofs carved by hand at the construction site, and houses entirely made of wood with very complex geometries, designed with special software and cut at the factory. machine tools with an accuracy of less than a millimeter.
External metal plates are often used to make the joints, which are connected to the wood with nails, screws or bolts. The positive characteristics of this type are the ease of installation and the economic efficiency of the system, while the negative characteristics are associated with the fire safety of these elements, which must be provided with paints or coatings from other materials, as a result of which the aesthetic result is often not very pleasant.
Currently, there is a tendency to use hidden connection systems, the force of which is always transmitted mechanically through metal elements, but these latter must be placed inside a wooden beam, without going beyond it. The aesthetic result is definitely positive, because looking at the structure, you only see the wood surface, and fire resistance is ensured by adequate wood coating of the metal.
Industry research by Roto Blaas, in collaboration with excellent research centers such as the University of Trento, is moving towards the development of new 'standard' aluminum alloy concealed brackets that offer excellent static performance while maximizing the ease of on-site assembly of the structure.
After the tragic earthquakes that have struck in recent years southern Europe, and especially Italy, special attention is increasingly paid to a special wooden structure most suitable for seismic conditions; but what about the connection systems in this matter?
When implementing a project in seismic regions, the important parameters are the degree of plasticity and energy dissipation of the structure; these two parameters are obviously related to each other and, the greater their value, the more resistant the structure to seismic influences. Mechanical joining systems are of great importance in creating these properties of the structure, being made of metal, due to the intrinsic properties of the material and the excellent interaction with wood, they can provide good values of ductility and energy dissipation.
Obviously, this should always be based on good design of both the structure as a whole and the connecting systems. Taking into account the simple shear effect of a cylinder head element, which can be a self-tapping screw for wood (meaning that the screw is the most widely used system in the world in the assembly of medium-sized roofs and wooden buildings) for fixing joints to shear wood - wood, which can be seen in Figure 3, which shows how the optimal interaction between the screw and the wood occurs.
Indeed, it can be observed that there is significant destruction of the wood around the metal element, which led to the bending of the metal element to create two plastic hinges (one inside the wood element on the right and the other on the left). In addition, there is a deep penetration into the wood of the head, which is caused by the "cable effect", which occurs due to the presence of threads in the element, which has a resistance to pulling out more than the resistance to penetration into the wood of the screw head. When she displaces, the screw is extremely elastic, as you can see in the graph. All of this is considered on the example of one element; however, since connections are never made with one single element, close attention must be paid to the location of the various elements, carefully following the requirements set out in the normative documents, such as the minimum distance between the elements, the minimum fixing depth, etc. (going back to the concept where construction details like connections need to be well thought out).
As you can see from these few examples related to the behavior of the type of connection, the research goal of various companies and research centers is a comprehensive study of connection systems, not only in static type interactions, but also in cyclical ones.
However, there is another very important element related to the testing of connection systems and the introduction of technical standards in the field of construction: building materials and thus all timber fastening systems must bear the CE mark or the Certificate of Technical Conformity issued by the Central Technical Service. This applies to standard joint systems such as screws, perforated plates, staples, etc. And this is not a problem, as there are already relevant rules and directives to help manufacturers put the CE marking on their products; many systems on the market already bear the CE mark.
