Determination of fasteners for structural joints in selected species of deciduous trees

Systems for mechanically joining solid wood are primarily developed for coniferous tree applications concerning material, design, geometry, and surfaces, which are predominantly used for structural load-bearing applications compared to deciduous species (Ramage et al. 2017). With a potential shortage of coniferous tree species in forest stands due to climate change-driven species composition alterations, the use of deciduous tree species may gain prominence (Taj et al., 2009). However, the intensity of their usage will hinge upon the rapid development of new solutions and technologies for applying deciduous wood in construction applications (Krackler et al. 2011). The strength of a mechanical joint is influenced by various factors (Ringhoffer et al. 2015), including the wood species used (Maleki et al., 2017), fastening methods themselves (Karaman, 2020), and application process factors (Stamatopoulos, 2016). Commonly available fastening methods on the market are usually developed for coniferous tree applications (Domínguez et al. 2022). Due to the diverse structure of deciduous tree species, particularly microscopic structure (Gonzáles et al., 2020), density (Koman and Feher, 2015), core tannin content (Mämmelä et al., 2000), the structure of different wood species is variable (Lichtenegger et al. 1999). Consequently, when applying these conventional fastening methods to deciduous tree species, issues like head breakage caused by excessive twisting moments (Palacio, 2023) or wood structure cracking leading to additional defects negatively affect the strength of the mechanical joint.

Methodology:

The scientific methods employed in this work systematically comprise several components. Existing fastening methods are primarily developed for constructing joints in coniferous tree species, and their use for potential connections in deciduous tree species has not been thoroughly verified. It is assumed that previously developed and used fastening methods primarily for connecting coniferous tree species will exhibit different effectiveness when applied to connect deciduous tree species. Test specimens will be prepared from selected types of deciduous trees with potential for construction applications common in Central European regions, namely beech, oak, black locust, and hornbeam. Spruce will be selected as a reference wood species. For preparing test specimens, timber produced according to the conditions of ČSN EN 14080 with parameters for construction applications and quality specified by ČSN EN 338 will be used, and wood selected according to one of the procedures specified by ČSN EN ISO 8970 will be chosen.

Fastening methods will consist of screws for structural joints with different dimensions, blade geometry, thread pitch, material, and coating type, and nails with different dimensions, tip geometry, material, cross-section, surface structure, and shank coating.

The influence of process parameters for applying the fastening method into wood, wood moisture, application with and without pre-drilling will also be investigated. The significant manufacturer Rotho Blaas stl. will supply the fastening methods for research and will also design the specific fastening method prototypes with defined parameters to be verified for joining deciduous tree species.

The first phase will examine the dependency of the course and deformation of wood fiber elements’ structure around the applied fastening method on the resistance of the method against pull-out. Wood structure will be dyed with a contrasting substance before, after penetration, and after pulling out the fastening method from the wood. Subsequently, the wood structure and its deformation will be displayed in 2D through image analysis and in 3D through CT analysis.

The second phase will determine the pull-out strength of fastening methods according to ČSN EN 1380 and ČSN EN 1382. Maximum tensile forces will be determined at a constant feed rate until the test sample fails; these tests will be conducted on the universal testing machine Tira Test 2850 (TIRA GmbH, Germany).

The third phase will involve tests to determine the characteristics of the strength and deformation of wood joints using mechanical fastening methods according to ČSN EN ISO 8970 and ČSN EN 26891. The strength tests will be performed in wood-wood interaction using the universal testing machine Tira Test 2850 (TIRA GmbH, Germany).

The risk in mechanical tests is the formation of cracks in the tested samples of deciduous tree species. Therefore, when creating test specimens, requirements for reduced areas of limited proportional deformation (ČSN EN 1995–1–1) and requirements for the mutual distance and distance of fastening methods from the edge of the test specimens will be considered. The determination of characteristic strength values for mechanical joints in deciduous tree species will be verified regarding the applicable design modification coefficient Kmod according to ČSN EN 1995–1–1 for wood for construction applications.

Literature and Sources:

ČSN EN 338 Konstrukční dřevo — Třídy pevnosti. Praha: Úřad pro technickou normalizaci, metrologii a státní zkušebnictví, 2016.

ČSN EN 1380 Dřevěné konstrukce — Zkušební metody — Nosné hřebíky, vruty do dřeva, kolíky a svorníky. Praha: Úřad pro technickou normalizaci, metrologii a státní zkušebnictví, 2009.

ČSN EN 1382 Dřevěné konstrukce — Zkušební metody — Únosnost na vytažení spojovacích prostředků. Praha: Úřad pro technickou normalizaci, metrologii a státní zkušebnictví, 2008.

ČSN EN 1995–1–1 Eurokód 5: Navrhování dřevěných konstrukcí — Část 1–1: Obecná pravidla — Společná pravidla a pravidla pro pozemní stavby. Praha: Úřad pro technickou normalizaci, metrologii a státní zkušebnictví, 2006.

ČSN EN 14080 Dřevěné konstrukce — Lepené lamelové dřevo a lepené rostlé dřevo — Požadavky. Praha: Úřad pro technickou normalizaci, metrologii a státní zkušebnictví, 2013.

ČSN EN 26891 Dřevěné konstrukce. Spoje s mechanickými spojovacími prostředky. Všeobecné zásady pro zjišťování charakteristik únosnosti a přetvoření. Praha: Úřad pro technickou normalizaci, metrologii a státní zkušebnictví, 1994.

ČSN EN ISO 8970 Dřevěné konstrukce — Zkoušení spojů s mechanickými spojovacími prostředky — Požadavky na hustotu dřeva. Praha: Úřad pro technickou normalizaci, metrologii a státní zkušebnictví, 2020.

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