Responsible partners: SerQ (Portugal) and Cesefor (Spain)
Collaborative partner: Ademan (Spain)
Description: The main goal of the present experiment is to characterize local hardwoods (poplar, beech 2nd quality and chestnut), through an experimental campaign on sawn wood (boards), which is divided into 3 main tasks:
- Non-destructive characterization of raw material;
- Mechanical performance assessment (bending and tensile tests);
- Timber-to-timber connections behaviour (embedment strength tests).
More than 700 boards were non-destructively characterized to fulfil these goals: approx. 150 chestnut boards (PT); approx. 350 beech boards (ES) (33 from 1st quality and 320 from 2nd quality); and approx. 275 poplar boards. The characterization was based on recording defects such as knots and wane, as well as dimensions and weight, followed by the use of the longitudinal vibration method for the determination of natural frequency and dynamic modulus of elasticity (Fig. 1). The last presents significant correlations with mechanical properties usually determined through harmonized static tests (Fig. 2). Mechanical properties are crucial to assign a strength grade, which is the key to a correct design of timber structures, as well as to the reliable production of engineered wood products such as glulam and cross-laminated timber (CLT).
Figure 1 – Non-destructive characterization test
In the 2nd stage, static tests were performed to determine the modulus of elasticity and strength following EN 408, both for tension stresses (carried out at SerQ), as for bending stresses (carried out at SerQ and CESEFOR) (Fig. 3). Work is still under development, namely for the chestnut sample. However, the following results were achieved so far:
- Poplar can be assigned to T12 strength class (tension), equivalent to C20 (bending), being conditioned by the low characteristic value of density (333 Kg/m3);
- 2nd quality beech can be assigned to D30 strength class (bending), conditioned by bending strength (33.4 MPa). On the other hand, taking into consideration the hardwood strength classes proposed by Kovryga et al. (2020), beech can be allocated to DT22 (tension), conditioned by the average modulus of elasticity (13.2 GPa).
Figure 2 – Correlation between dynamic modulus of elasticity (Edyn) and static under tension (Et,0)
Figure 3 – Test layout for tension (up) and bending (down)
A topic of relevant interest in timber structure design is the connections which play an important role. Thus, embedment strength tests were carried out at CESEFOR both on Portuguese poplar and Spanish beech. Portuguese poplar was tested on both parallel and perpendicular to grain directions, with a 9 mm nominal diameter screw (Fig. 4). Beech was tested parallel to the grain direction with a 9 mm nominal diameter screw and a 12 mm bolt (Fig. 5).
Figure 4 – Test layout for embedment strength in poplar: parallel to the grain (up) and perpendicular to the grain (down)
Figure 5 – Test layout for embedment strength in beech: parallel to grain with screw (up) and parallel to grain with bolt (down)
Experimental tests were performed according to standard EN 383. The number of pieces tested was 23 for each test type in poplar, and 20 for each test type in beech. Load-displacement diagrams obtained for every test performed are presented in Figure 6 for poplar and Figure 7 for beech.
Figure 6 – Load-displacement diagram for poplar tests: parallel to the grain (up) and perpendicular to the grain (down)
Figure 7 – Load-displacement diagram for beech tests: parallel to grain with screw (up) and parallel to grain with bolt (down)
These results among others related to the project were considered within several outputs, such as international conferences (Sustainable and Digital Buildings; SHATIS’22; 22nd NDTE Symposium) and Master Thesis on Engineering (Cláudio Ferreira – Universidade de Coimbra and Gonzalo Cabrera – Universidad Politécnica de Madrid).