GNGTS 2013 - Atti del 32° Convegno Nazionale

Friction tests resulted in an extremely variable static coefficient, being defined as a function of the applied pre-compression and the cross section/number of notches of the logs. In conclusion, the average static friction coefficient resulted respectively equal to m= 0.520 for ‘Tirol’ timber logs (standard deviation 0.058) and m= 0.665 for ‘Schweiz’ timber logs (standard deviation 0.068). Compressive tests on ‘Standard’ joints. Further tests have been successively performed, at the Department of Engineering and Architecture of the University of Trieste, on small specimens representative of a ‘Standard’ joint typically used in Blockhaus structural systems (e.g. specimen S01). Due to in-plane seismic loads and to the interaction between main logs and orthogonal logwalls, these joints aremainly subjected to compressive forces acting orthogonally to timber grains. Therefore, experiments have been performed in accordance with the setup displayed in Fig. 3b, by applying a compressive force C to the 0.06 0.076 m 2 resisting surface of a single ‘Standard’ joint and leading the joint to collapse (e.g. ultimate vertical displacement d u ). The adopted test setup, should ideally reproduce the interaction between a main timber log under in-plane seismic loads and the orthogonal log in contact with it. A total number of 3 tests have been performed on ‘Standard’ joints, since experiments generally resulted in a high ultimate strength f u,90 of timber when compressed perpendicular to the grain (avg. f u,90 = 13.62 MPa, standard deviation 2.12) and in general in a large ultimate displacement at collapse d u (avg. d u = 13.17 mm, standard deviation 5.97). Advanced numerical modeling of Blockhaus shear walls under in-plane seismic loads. To investigate in detail the lateral response of the tested full-scale Blockhaus shear walls under seismic events, a numerical model was implemented in the software package ABAQUS/ Standard (Simulia, 2012). The lateral response of Specimen S01 under in-plane monotonic loads was analysed, and careful consideration was given to the FE geometrical description of ‘Standard’ joints. Only the 0.01 0.01 m 2 notches that characterize the top and bottom surfaces of timber logs were neglected, thus resulting in ‘Tirol’ logs composed of - with the exception of the ends - beams with a 0.09 0.16 m 2 rectangular cross section. To ensure the accuracy of results, a refined mesh composed of hex-dominated 8-node linear brick (C3D8R) and 6-node linear triangular prism (C3D6R) 3D-stress, reduced integration , solid elements was used and the FE-model consisted of 42200 elements and 75000 nodes. Concerning loads, the 10 kN/m vertical pre-compression was described by means of a uniformly distributed, constant vertical pressure q v applied to the upper surface of the top log. Similarly, the horizontal in-plane seismic load was applied to the edge surface of the top log as a uniformly distributed, quasi-static, time-varying pressure q h of amplitude similar to the experimental procedure. An important role in the adopted FE-model was assigned to the interaction between surfaces, which in ABAQUS/Standard can be implemented in the form of master-slave contact pairs . For all the contact-pairs, a mechanical interaction characterized by appropriate tangential and normal behaviors was taken into account. A penalty isotropic Coulomb formulation was used for the tangential behavior. Its input parameters are the static friction coefficient m and the slip tolerance F f . Based on experimental results previously discussed, m was set equal to 0.5. At the same time, for the non-dimensional slip tolerance F f , based on preliminary studies, the value 0.0005 was used. In ABAQUS/ Standard, F f is expressed as a ratio of the characteristic length of the surfaces in contact. This quantity is representative of possible “elastic reversible tangential slidings” between the surfaces in contact before the occurring of irreversible sliding motion (namely, for shear stresses t lower than the critical shear stress t crit ). If F f → 0, no elastic relative motion occurs. For the normal behavior, a default constraint, hard contact pressure-overclosure enforcement method, able to avoid interpenetration between surfaces in contact and to transmit any contact 27 GNGTS 2013 S essione 2.1