The components of a prefabricated timber frame structure kit and how they are put into practice
About timber frame
Timber Frame is the most widespread wood constructive system in our country as it is extremely versatile and efficient in terms of material consumption and cost. Other possibilities for wood construction are CLT ( Cross Laminated Timber, timber houses and girder traditional houses).
Not only are they suitable in residential construction works, such as dwellings, but also in commercial construction works, such as boarding houses or commercial areas. The maximum height that a timber frame structure can reach, according to the norms in Romania is P+2, but the structure can easily carry more floors, proof being this timber frame system apartment building that was tested for earthquakes. A timber frame structure is mainly made of prefabricated elements for walls, floors and roof. I have written more about timber frame in this article.
In wood construction works, it is highly important that the raw material, the wood, comes from sustainable sources and that it meets the technical and mechanical characteristics necessary for wooden load-bearing structure. It needs to have a strength class of C24 ( or GL24 for lamellar sections where we have bigger openings), it has to be dried at a maximum humidity of 18% and properly handled in accordance with use. For new households, we use an improved fireproof insecticide-fungicide, according to the authorization given by the National Authorities.
The timber frame structure kit
An aspect worth mentioning is that the structure kit is manufactured based on the architectural project or on the technical resistance draft, if it has been manufactured following the manufacturer’s specifications. Hence, the structure kit respects the initial architecture and rarely do changes in shape and volume happen. Our timber frame structure kit is made of the following elements:
1.External walls of 45x145mm framing plated with OSB3 of minimum 12mm/ Vidiwall 12,5mm/DHF 15mm on the outside or on the inside. When the structure is plated on the outside, it is very important that the plywood is steam-permeable. Otherwise, the steam barrier on the inside has to have a variable sd (steam permeability factor)- when there are humidity liberations on the inside, the sd factor is high and the steam does not penetrate the structure and vice-versa, when there are high liberations of humidity on the outside, the sd factor is low and the barrier becomes more permeable. If you are interested in the principles of energy efficiency, we have written a separate article on this subject. The internal walls of 45x100mm framing for non-load bearing walls and 45x145mm for load-bearing walls with plywood on a single side with additional bracing.
2 .Posi Strut beam floor system with widths varying from 200 to 350mm, more rigid and resistant to vibrations with openings up to 6m without protective casing. Moreover, ventilation ductworks and electrical wiring can be pulled through metallic lattice work without the need of beam perforations. More on Posi beams in this article.
3.Roof structure from differently configured prefabricated trusses: simple roofs and 2 or more ascents open-space attics. Roof trusses are prefabricated elements with MiTek connectors and they can adjust to any architecture, from rectangular or curved elements, water spillage or big openings, up to 25m, without protective casing.
Designing the timber frame structure
When we receive a project, it needs to be processed and adjusted for production, in accordance with our technical specifications. The manufacturing project comprises each and every structural element with all its components, wood and connectors. We perform the design and the model of the structure in various softwares: roofcon trusscon, cadwork și pamir.
The manufacturing process of the structure
The first step is wood cutting, which can be carried out either mechanically, by a CNC ( a numerically controlled machine) or manually. The walls are manufactured on the work bench with the internal posts at a distance of 625 mm between each other and the wall’s dimension is determined according to the assembly conditions. If the assembly is manual, the length of the walls is of maximum 2,5m in order to be easily handled. Otherwise, if the assembly is done mechanically, the walls can be of 10 or 12 m length, depending on the transport possibilities. The height of the wall respects the architectural project.
On the other side, the Posi beams and roof trusses are manufactured with a special hydraulic press, according to MiTek specifications. Joining MiTek connectors can not be performed through thursting. No matter the elements in question, the designing is made according to the national and EUROCODE 5 regulations.
Assembly of the structure
Besides the structural elements mentioned above, the following elements and fixings are needed for the assembly:
waterproof marking sole, made of wooden posts that are fixed on the concrete foundation, all over the outline of the internal and external walls. The marking sole breaks the capillarity between the concrete slab and the wooden structure, hence preventing humidity from reaching the wooden structural elements;
wooden belt bands, namely post of 45mm width that are fixed parametric over the walls, for reinforcement;
wooden, metallic or wooden-metallic bracing, in the case of latticed beams with MiTek connectors;
hardware and pinning fittings: anchors or chemical anchors fixing the marking sole to the foundation, seismic corners and simple corners that fix the walls to the marking sole to the foundation, screw spindle with which walls are fixed, screws of various sizes depending on use;
Advantages and disadvantages
Why would a building contractor or a customer opt for a structure kit instead of a traditional brickwork or wooden structure ? Firstly, even though the structure is briefly equipped, it comes as prefabricated elements and the production lead time is considerably reduced. A house of 220 square meters on 2 floors ( as the one presented above) can be assembled with a crane in less than a week. More than that, a part of the manual labour on the construction site can be moved to the factory, hence the walls can be insulated and internally plated, reducing even more the assembly time.
Another advantage of the prefabricated wood structures is that they do not entail unexpected costs during the assembly. Before production, when the exact quantities are calculated and the execution contract is signed, the quantities of wood, connectors and fittings are very well known and they cannot be modified over time, except for changes in the project and requirements on the construction site.
A disadvantage could be the materials’ cost. Putting aside the recent price rise, wood as material is more expensive than traditional brickwork, however if we take into account the production lead time and the respective manual labour costs on the building site ( higher for brickwork and lower for wood), the final cost, material and manual labour, balances out.
Furthermore, wooden structures, especially the timber frame constructive system also has other benefits and advantages, that we've been talking about in other blog articles.