Wooden buildings: when and why do they rot? (Part 2 – Structural aspects)

As mentioned in the previous article  which was also connected to the survey, the second requirement of the ground attack detail is the structural safety.
The support of the perimeter wall on the substructure made by concrete must be carefully evaluated in order to prevent the wood exposed to moisture and / or water stagnation.

According to the italian law the building must be designed and built in order to ensure at least 50 years of life. To achieve this result is necessary to adopt the measures that protect wooden parts from moisture and it is not always sufficient to rely on bituminous waterproofing membranes.

We do not know how the regulatory framework in the rest of Europe is but you can refer to the German standard (DIN 68800-2: 2012) which offers some useful solutions to save the wooden parts from moisture and water stagnation.

We definitely suggest to keep the wooden walls higher than the outside floor quote where is it possible to have water infiltration; inside the house such phenomena rarely occur and when they occur they are easily identifiable.

German standards recommended to have the following situation: the lowest part of the timber wall must be higher than the external ground. 5cm, 10cm, 15cm? It depends on the drainage characteristics of the ground.

An Austrian document created by a carpenter association proposes not less than 30cm!

The ground attack definitely plays a vital role, in fact this is where all the forces are transferred from the building to the underlying structure. To ensure compliance with the design assumptions is essential to ensure a uniform wall support surface and for this purpose it is advisable to install a layer of special leveling mortar above the foundation.

Image from “Legno… Costruire… Abitare…” published by Eng. Franco Piva

We suggest not to use the so-called curb larch, especially in the case of multi-storey building. This solution, essentially used for mounting reasons, presents serious problems from the structural point of view in that this element is subject to local buckling phenomena due to the non-linearity of the distribution of forces at the base of the walls.

Furthermore, from the point of view of the mounting process, the use of larch requires double connections: concrete-larch and larch-wall.
The interposition between the wooden wall and the foundation of an insulating panel (foamglass or similar) should be evaluated carefully keeping in mind the following factors:
– Compression of the insulation resistance
– Brittle behavior of the insulating material
– Real contribution to reducing the risk interstitial condensation

The transfer of shear and tensile stresses resulting from horizontal actions on the building to the underlying structure, is essentially secured by metal brackets anchored using the cylindrical connectors (screws, nails, bolts, etc…).

It is very important to take into consideration the following factors:
– Distances from the edges for concrete and timber
– Distances between anchors
– Depth of the anchor in concrete

For this reason it is essential a very close collaboration between the designers of the foundations and the designers of wooden structures (if they are different). In this case a BIM approach can surely help.

It is necessary to respect the quota (±z) of the concrete structures during casting to avoid the errors shown in the following figures.

The building physic aspects (thermal bridges, interstitial condensation risk) will be analyzed next month with the third part of the article.

Do you want to discuss about the article and ask for other informations? You can do it here   .