Subject matter

The behaviour of timber joints is one of the most important topics in the field of timber structures, and one of the more studied, given their critical role and importance joints in the local and overall behaviour of timber structures. Given the complexity of their behaviour, research is often based on the experimental evaluation of joints, due to complexity of numerical analysis (anisotropic behaviour of wood, stress concentration around fasteners, variability of mechanical properties, moisture-dependency, size effects). Existing test standards and protocols are limited to the very simplistic nature of traditional connections.

Although experimental campaigns provide important information on the mechanical behaviour of modern connections, the non-standardization of the test procedures often precludes the comparison between the obtained results. In practice, different loading protocols are adopted and various methods are used to define yielding points, stiffness, and ductility. In order to allow for a better future evaluation and reusability of experimental data, existing testing protocols for timber joints should be discussed and reviewed.

The main objective of the proposed TC is to develop standardized and reliable procedures for characterizing the resistance, stiffness and deformation properties of timber joints that are appropriate for design provisions and guidelines. Two types of fasteners and loading will be addressed: i) axially loaded self-tapping screws (STS) under axial and lateral loads and ii) laterally loaded dowel‑type fasteners, under both quasi-static and cyclic loading.

The use of self-tapping screws is a state-of-the-art practice in connection and reinforcement technologies for timber structures. The high axial stiffness and load-carrying capacity of self-tapping screws, together with their easy handling, make them one of the most economical choices for applications as both fasteners and reinforcement. Hence, one of the specific objectives of the proposed TC is to collect data on the withdrawal strength and stiffness of STSs in different configurations (wood product, manufacturer, angle, length, diameter, …) and assess the influence of the test procedure on the obtained behaviour parameters.

Joints with dowel-type fasteners case have been extensively studied and are the most commonly use in practice. However, predicting their load-deformation behaviour, namely under cyclic loading, is still extremely complex. Current testing procedures for cyclic loading are quite flexible and the influence of fundamental testing parameters on the obtained load-deformation behaviour is often questioned (number and amplitude of the loading cycles, loading rate, number of specimens, etc.). On the other hand, different methods for data assessment have been used for the definition of stiffness, ductility, strength and stiffness degradation, as well as parameters for analytical and numerical models.

In practice joints comprise multiple fasteners and group effects (load redistribution, combined stress states) are often critical and compromise the behaviour observed in single-fastener tests. However, current testing protocols often do not take these parameters into account.

Moreover, most design codes, as Eurocodes 5 and 8, are under revision and significant improvements are expected in the methods available for structural design, namely the ability to use load-deformation curves in, e.g., push-over analyses. Therefore, reliable and well-established assessment methods are required, to support the safe and economic design of timber joints.

In summary, it is critical to re-evaluate existing testing protocols for timber joints and to establish reliable and unambiguous methods to accurately describe the load-deformation behaviour of timber joints.

Terms of reference

The global aim of the committee is to develop standardized, reliable procedures for characterizing the resistance and stiffness properties of timber joints that are appropriate for design provisions and guidelines. The committee is expected to run for 5 years with members recruited mainly from academia, industry and engineering practice. Considering the recent discussion at RILEM regarding PhD student involvement, their participation in the TC will be favoured.

The work will include: literature review of existing approaches; round-robin testing of promising experimental methods for timber joints with STS and dowel-type fasteners; comparison of test results of axially loaded self-tapping screws (STS) under quasi-static axial and lateral loads and dowel‑type fasteners under laterally cyclic loading, with available data; assessment of the influence of test parameters on the behaviour of joints with dowel-type fasteners under cyclic loading; development of tests methods for joints with dowel-type fasteners based on tests on single-fastener joints; recommendations on testing full-scale connections for design purposes; and recommendations for the most promising test methods and educational material to foster their use.

The timber industry is fostering the quick development of new fasteners and joints and using current solutions in more demanding applications. The development of new and more reliable procedures for characterizing the resistance, stiffness and deformation properties of timber joints will have a direct impact on evaluating new solutions, in further optimising the use of current solutions, and in better integrating the knowledge generated by studies performed in different laboratories. The guidelines and/or recommendations will directly support the industry this regard.

Detailed working programme

1. Initial state-of-the-art and TC planning [Year 1]

• Introductory on-line meeting (online, tentatively planned to November 2021);
• First in-person meeting (tentatively planned during RILEM Spring Convention in 2022);
• State-of-the-art workshop (in person, tentatively planned for RILEM Week in 2022);

2. State-of-the-art and planning of testing phases [Year 2]

• Preparation of a paper summarizing state-of-the-art tests methods to be submitted to Materials and Structures;
• Exploration of applicability of current experimental/modelling approaches for timber joints in what concerns resistance, stiffness and deformation properties;
• Selection of specific joints and corresponding test methods for Phase 1;

3. Phase 1 – Testing of axially‑loaded STSs and laterally-loaded dowel-type fasteners [Years 2-3]

• Assess influence of test parameters
• Evaluate the most promising approaches;
• Include hardwoods species;

4. Phase 2 – Round-robin testing of the most promising method for each selected timber joint [Years 3-4]

• Wide range of parameters - wood product, manufacturer, angle, length, diameter, number of fasteners, etc. - to ensure global applicability;
• Include the evaluation of multiple fasteners to cover practical applications;
• Assess repeatability and accuracy;

5. Summarizing and dissemination of results [Year 5]

• Recommendations for adoption of test methods for timber joints (and proposal of pre-standards);
• Final report and online tutorial/training/course for the further dissemination and adoption of test method;
• Workshop/symposium involving standardization groups (e.g., ASTM, ACI, CEN).