Manuel Guaita

The interest in fast-growing hardwood species for structural applications has increased across Europe due to the need for sustainable and locally sourced solutions. In this context, black poplar ( Populus × euramericana (Dode) Guinier) shows significant potential owing to its low density and rapid growth. This study presents the tensile mechanical characterization of the MC and Luisa Avanzo cultivars, which are widely cultivated in Spain, with the aim of assessing their structural suitability and supporting their normative inclusion. A total of 804 specimens from six plantations located in the Duero, Ebro, and Guadalquivir river basins were analyzed to determine their modulus of elasticity, tensile strength, and density, in accordance with current standards EN 408 and EN 384. The results enabled the assignment of strength class T10 to timber visually graded as MEF, meeting the regulatory requirements. The volumetric shrinkage coefficient was also determined and has been included in standard UNE 56546 as a parameter for dimensional stability. This characterization represents the first national implementation of the normative approach based on tensile testing parallel to the grain for hardwood species, using the French standard NF B 52001-1 as a reference.

In Spain, Eucalyptus nitens is distributed in the West of the Cantabrian Cornice, especially in the autonomous community of Galicia. Despite being mainly destined for paper pulp, in the last 15 years, eucalypts ( Eucalyptus globulus & E. nitens ) have been targeted as sawn timber for structural purposes in the country. In Europe, structural timber is graded according to the European standard EN 14081-1:2016+A1:CEN (2019). Timber structures – strength graded structural timber with rectangular cross section – part 1: general requirements (EN 14081-1), either visually or by machine, in order to determine the characteristic mechanical properties, as required by design standards. This study aimed to propose visual strength grading criteria for Spanish shining gum ( E. nitens ) and to assign its bending strength classes according to European Standards, based on four-point bending tests performed on full-size specimens. Considering the strength, stiffness, and density profile of the tested specimens – with characteristic values of 41.6 MPa for bending strength, 11.60 GPa for modulus of elasticity, and 453 kg/m ³ for density- the assignment of the strength class C35 was deemed adequate for the Spanish provenance shining gum timber graded to the “MEF” visual grade (structural timber from hardwood species). Furthermore, the relationship between global and local moduli of elasticity for the species was analyzed, and a formula was proposed to adjust the global to the local value.

III Congreso Ibero-Latinoamericano MAD de la Madera en la Construcción (CIMAD 24) : Libro de Actas de Resúmenes Extendidos

In timber structures, knowledge of the splitting capacity of beams loaded perpendicular to the grain by dowel-type connections is of primordial importance since brittle failure can occur. In the present work, single- and double-dowel-type connections following different loaded edged distance arrangements are experimentally investigated to derive the splitting behaviour of Eucalyptus globulus L., which is a hardwood species of increasing interest for structural use due to its high mechanical performance, fast growth, and good natural durability. The correlation of experimental failure loads with those theoretically predicted by the expression included in Eurocode 5 and by eight analytical models based on an energetic approach is discussed. Most of the analytical models studied overpredict the splitting capacity. However, the code splitting expression, derived from softwoods, proves to be very conservative in predicting the eucalyptus splitting failure load.

Elastic timber gridshells are lightweight structures whose stiffness is highly dependent on multiple factors, such as boundary conditions and the semi-rigidity and eccentricity of the joints. Their structural analysis requires calibrated numerical models that incorporate all aspects influencing stiffness. Unfortunately, very little research on experimentally verified numerical models can be found. This paper focuses on the structural behaviour of a novel concept of triaxial elastic long-gridshells supported only on their short sides, called by the authors TEL-gridshells. First, the most relevant details of the construction process and the load test of a full-scale laboratory prototype are presented. Then, two finite element models for structural analysis and form-finding are proposed. Both are based on the modelling of the joints using a series of aligned couplings that allow the integration of the actual joint eccentricity and the interlayer slip by means of springs in all shear planes. The first model replicates the geometry of the prototype built from experimental measurements, focusing on stiffness calibration. The results of the load test are used to verify the proposed model and to analyse the most influential aspects on the stiffness of the structure. The second is a form-finding model that reproduces the construction process of the laboratory prototype, focusing on the residual stresses generated during the deformation process of the structural elements. From the numerical results, the structural behaviour of the prototype is discussed and some of the main aspects to be considered in the design and structural analysis of TEL-gridshells are established.

Eucalyptus globulus Labill stands out as one of the hardwood species produced in Europe with prominent mechanical properties, which is undergoing a growing interest in extending added value. The development of engineered wood products with this species and its application in timber structures involving numerical finite element simulations requires knowledge of the mechanical properties for the different orthotropic material directions. The aim of the present study is to determine the main mechanical properties of E. globulus from small clear specimens, necessary for the development of finite element models. The work provides experimental results on the ultimate capacity and modulus of elasticity considering different stresses: tension parallel and perpendicular to the grain, compression parallel and perpendicular to the grain (in radial and tangential directions), shear and longitudinal static bending. The work is complemented with experimental data on timber-to-timber friction coefficients for 0°, 45°, and 90° orientation angles, which are useful in the modeling of traditional joints. Very high values of ultimate stress and modulus of elasticity for the different mechanical properties were obtained, highlighting the great potential of this species for structural applications.

The double-step joint is among the most frequently used layouts, within carpentry joints, for transmitting higher forces that would allow a single notch. They are especially effective in heavy timber structures. Nowadays, computer-aided manufacturing is being used more often, demanding further progress in its understanding. The conventional design of these joints is conducted by using simplifying assumptions, in particular regarding the shear stress distribution. This is overcome by the use of strength reduction coefficient, which is currently under study. Numerical simulation and experimental tests were carried out with heavy timber cross-sections for rafter to tie-beam truss joint. They were manufactured in glue-laminated timber owing to the large cross-sections tested. Experimental load-strain and load-displacement diagrams were compared with numerical results. This allowed observing the great shear stress concentration produced in the failure by shear crack, which suggests the application of conservative shear strength reduction coefficients.

Early selection of trees allows acceleration of genetic improvement, as well as processes related to forest management, to improve the quality of the wood produced; however, to reach this objective, it is necessary to know which parameters can be used as predictors of a tree’s aged condition. The objective of this research was to study parameters that are measurable in nursery seedlings and that could be used in prediction models of basic density (BD_t), modulus of elasticity (E_Mt), and strength (f_mt) of wood from trees. The tests were performed in 240 seedlings (3 and 6 months old) and in 52 trees (72 months old) from seven genetic units of two species: three Eucalyptus clones and four Pinus pinaster progenies. In the seedlings, measurements of longitudinal velocity of ultrasonic waves (VL_s), basic density (BD_s), height (H_s), diameter (D_s), strength (f_ts), and modulus of elasticity (E_ts) in tension parallel to the grain were obtained. The E_Mt and f_mt can be predicted by parameters obtained in seedlings of the same genetic unit. Thus, the use of these parameters, in association with others already used in selection programs, may increase the positive results of the early selection, with economic gains and time reductions in forest management.