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Innovative joints in hardwoods

Coordinator: Carmen Sandhaas, Karlsruhe Institute of Technology/ Timber Structures and Building Construction (KIT), Germany
carmen.sandhaas (at) kit.edu

Other partners: AT, FR, SE
Project duration: 02/2019-10/2022

Project abstract:

The existing hardwood stocks in Europe’s forests are increasing, making them a significantly underutilised potential. Hardwood lends itself to high volume use in engineered structures. However, efficient mechanical joints for hardwood structures are widely missing which prevents the effective and economical use of structural hardwoods. To overcome this barrier, economic, reliable and innovative joint technologies for hardwood members are developed and design rules were derived, covering a wide range of fasteners and loading directions.

One of the most important fastener types are screws, which stood central in this project. Large studies covered all applications of joints with screws loaded in their axial direction. Both single screws and groups of screws inserted at different angles to the grain were investigated, and for the particular case of axially loaded screws inserted in end grain, the effect of duration of load was assessed. But also joints with screws loaded perpendicular to their axis were addressed, where due to the high densities of hardwoods, novel failure modes including steel failure can occur for which calculation methods were derived. Further investigated fastener types were staples and nails, where a simple and straightforward test setup was designed to first assess if these small diameter fasteners can be inserted in hardwoods at all.

Together with experimental tools, also different modelling approaches were developed to perform parameter studies that help to extend experimental findings and prepare valid design rules. In particular, an engineering model for joints with laterally loaded fasteners was established that is able to generate load-displacement curves containing information on the joints’ stiffness and capacity, which is crucial for the design of large and complex modern joints in engineered timber structures.

Finally, also optimised joint geometries were developed by using CNC-machining in order to increase friction in the joints’ shear planes through generating rough surfaces of the timber members to be joined. It could be shown that this increased surface roughness leads to significantly increased joint stiffness and capacity.

Project presentation at ForestValue kick-off seminar 23-24 May 2019: pdf
Stakeholder Article 1 hardwood_joint – Joints with staples and nails
Stakeholder Article 2 hardwood_joint – Joints with axially loaded screws; long‐term investigations
Stakeholder Article 3 hardwood_joint – Joints with laterally loaded fasteners
Stakeholder Article 4 hardwood_joint – Joints with shallow grooves
Stakeholder Article 5 hardwood_joint – Modelling of joints