GNGTS 2013 - Atti del 32° Convegno Nazionale

pressure p between them was used. A capability of this formulation is that if p → 0, two surfaces in contact can separate during the analysis. The timber was described as an orthotropic material. Based on values provided by standards for C24 spruce, its density was assumed equal to r= 420 kg/m 3 , whereas the corresponding mechanical properties were described by means of ABAQUS engineering constants ( E 0,mean = 11000 MPa, E 90,mean = 370 MPa, G mean = 690 MPa). In order to take into account the occurring of inelastic compressive phenomena in joints, the mechanical behavior of spruce was described by means of an elastoplastic characteristic curve, with f y the yielding strength. Several values were taken into account (2.5 MPa ≤ f y ≤ 7.5 MPa), and parallel simulations were performed. The analyses demonstrated that a good correspondence between monotonic test results and numerical calculations can be obtained if the yielding strength of spruce is assumed equal to f y = 7.5 MPa (Fig. 3d). If compared to suggestions given by standards for C24 spruce (UNI EN338:2009), the characteristic value for the compression strength f c,90,k is 2.5 MPa, which approximately corresponds to a mean strength value f c,90,mean = f c,90,k / 0.7 = 3.57 MPa, thus lower than the numerically calibrated yielding strength f y = 7.5 MPa. This difference could be justified by the anisotropy of timber or by the interaction between multiple logs. At the same time, the optimized numerical strength for timber ( f y = 7.5 MPa) is significantly lower than the mean value of compressive strength perpendicular to the grain f u,90,mean = 13.62 MPa obtained from experimental tests performed on small ‘Standard’ specimens. Also in this circumstance, this apparent underestimation of f y finds rational justification in the behavior of joints observed during full-scale experiments. When subjected to in-plane seismic loads, the shear wall S01 distributed the relative displacements and slidings along the entire elevation of the wall, thus resulting in small local deformations of single joints (e.g. no damage) and limited maximum pressures transferred from each main log to the adjacent perpendicular log. In contrast, small ‘Standard’ specimens were subjected to destructive experiments, thus resulting in f y < f u,90,mean . Apart from the mechanical properties of timber, major benefits were obtained by introducing in the numerical model of specimen S01 a series of geometrical gaps ( d gap = 0.5 mm) along the elevation of the shear walls, between each main log and the corresponding orthogonal logs. These gaps, although representing a numerical idealization, appeared realistically able to describe possible logs defects and imperfections, strongly improving the quality of experimental and numerical comparisons (Fig. 3d). Based on these exploratory results, it is thus expected that the ongoing research project could lead to a realistic seismic characterization of Blockhaus systems and to the identification of useful design rules for Blockhaus structures. Conclusions. In the paper, experimental and numerical results have been presented for full- scale Blockhaus shear walls subjected to in-plane lateral loads. Although native of forested areas not typically characterized by high seismic hazard, in the current practice Blockhaus buildings are frequently constructed in earthquake-prone regions, thus their seismic characterization currently requires investigation. Based on experimental and numerical results, these structural systems resulted characterized by: • high flexibility: cyclic tests performed on three different typologies of Blockhaus shear walls were stopped at the attainment of a maximum lateral displacement of the top log equal to 80 mm, and no damage was observed in the deformed specimens. Monotonic tests performed on the same specimens confirmed their high flexibility; • high strength: although directly dependent on the geometry of the adopted carpentry joints, the tested walls generally showed elevated shear resistance to in-plane seismic loads; • large dissipative capabilities: cyclic tests highlighted, for each specimen, significant energy dissipation of Blockhaus shear walls (equivalent damping ratio in the range 10% to 20% depending on the type of carpentry timber joint). 28 GNGTS 2013 S essione 2.1