GNGTS 2013 - Atti del 32° Convegno Nazionale

Specimens S01, S02 and S03 were tested at CNR IVALSA Trees and Timber Institute, San Michele all’Adige (Italy), in quasi-static conditions (UNI EN 12512:2006), in temperature and relative humidity conditions of T= 20°C and RH= 65%, respectively. The shear walls were attached to the steel foundation by means of two 3mm thick steel angle bracket connectors, commonly used in practice by Rubner Haus. To investigate the effect of the gravity loads transferred on the wall, as a part of a real building, by floors and roof, each specimen was vertically pre-compressed (10 kN/m) by means of 4 Parker hydraulic jacks. The in-plane lateral force was then applied on the end-top surface of each specimen via a 500 kN hydraulic jack equipped with a load cell connected to a reaction frame. To avoid the overturning of walls, two 10 mm diameter steel cables were attached to the ends of the specimen upper log. During the experiments, a total number of 7 LVDTs (Linear Variable Displacement Transducers) with a resolution of 0.01 mm was used, in order to continuously register the main deformations (top lateral displacement, vertical uplift, etc.) of specimens. Cyclic tests were performed first, on specimens S01, S02, S03, according to UNI EN 12512:2006, in order to estimate the possible decrease of Blockhaus seismic resistance after a certain number of cycles. All the tests were stopped at the attainment of a maximum lateral displacement at the top log equal to d max = 80 mm. Subsequently, the same specimens were subjected to quasi-static monotonic in-plane lateral loads, to estimate their residual lateral resistance. After a first loading-unloading cycle up to the service load, the lateral load was linearly increased up to the collapse of each specimen, or to a maximum lateral displacement of the top log d max = 200 mm (maximum available displacement of the used hydraulic jack). Figs. 2a, 2b and 2c display the cyclic response for the shear walls S01, S02 and S03 respectively, in the form of horizontal in-plane load V vs. lateral displacement d curves. Specimen S01 showed an overall appreciable dissipative capacity, as can be deduced from the amplitude of the F -d cycles given in Fig. 2a. A good resistance to in-plane seismic loads and significant flexibility were found. No strength degradation was noticed in each series of 3 fully reversed imposed displacement cycles (UNI EN 12512:2006). The cyclic test was stopped at a maximum lateral displacement at the top log d max = 80 mm (load V max,C = 30 kN) and no sign of failure could be observed. Specimen S01 was then subjected to a monotonic lateral test, in which the collapse occurred at the attainment of a maximum lateral load V max,M = 43.28 kN (d max = 173.5 mm). Further investigations highlighted that the collapse, unlike the other specimens, occurred due to the sequential shear cracking of the five bottom logs in the main wall (Fig. 2d). In any case, the wall S01 showed high resistance to seismic loads. Based on formulas given by the Eurocode 5 (UNI EN 1995-1-1:2009) for the shear resistance calculation of Blockhaus shear walls, the design in-plane resistance of specimen S01 is in fact theoretically governed by the shear strength of ‘Standard’ halved joints, with an expected characteristic shear strength V k = 33.6 kN, significantly lower than the experimental ultimate load ( V max,M = 43.28 kN). The same test procedure was subsequently applied also to specimens S02 and S03. Also in this circumstance, both the walls demonstrated high capacity to dissipate energy and, at the same time, large flexibility due to relative sliding between overlapping logs. Results obtained from cyclic and monotonic in-plane tests are respectively showed in Figs. 2b and 2c. Concerning specimen S02, the cyclic test was stopped at a maximum lateral displacement at the top log d max = 80 mm ( V max,C = 18.48 kN) and no damage was noticed in the wall. Results obtained by the subsequent monotonic test confirmed the high flexibility of specimen S02. At the attainment of a maximum displacement at the top log d max = 200 mm, no sign of collapse was visible in the shear wall (Fig. 2e). Again, the ultimate experimental load ( V max,M = 27.12 kN) resulted higher than the the expected shear strength offered by ‘Rounded Dovetail’ shaped joints ( V k = 19.2 kN, Eurocode 5). Finally, also specimen S03 resulted very flexible and dissipative (Fig. 2c). At the end of the cyclic test (d max = 80 mm, V max,C = 20.02 kN), no visible damage was noticed in the shear wall. 24 GNGTS 2013 S essione 2.1