رفتار سازه دیوارهای الوار کنده چوب تحت بارهای جانبی سطح تراز

The present works intends to represent a further step in the knowledge of timber log-houses through an experimental approach, from which only few information is available. The main part of the experimental work is based on in-plane static tests conducted on timber log walls with distinct transversal stiffness, two vertical compression levels and different values of slenderness. Monotonic and cyclic tests were performed according to EN 12512:2001. The former were performed to define the elastic slip values and assessment of the failure mechanisms while the latter allowed for the evaluation of the impairment of strength, the measurement of the ductility and the quantification the energy dissipation. Previously to the full-scale tests of walls, an extensive characterization of the timber logs was made. Due to its importance, the connection between the first timber log and the basement was also evaluated through tests. In a second step of the research, a case study was used to develop a numeric analysis. Using FEM, the in-plane stiffness of timber logs walls was quantified, thus allowing to compare the result of distributing the horizontal loads by the walls according to their area of influence or their in-plane stiffness. Finally, improvements to the log system analyzed were suggested.

The construction of timber houses using logs is an ancient practice in many regions of the globe. Overlapped logs were used, covering the gaps between logs with moss. With the emergence of other construction materials, the use of timber decreased considerably and the log system lost importance. Nevertheless, timber log constructions are still popular in many forest regions of the world, especially in North America and Scandinavia. One of the main disadvantages of log construction is the lack of sound understanding of the structural behaviour of these structures, in particular, under seismic loads [1] and [2]. Log buildings rely on the walls built staking horizontal layers of logs, for resistance to both vertical and horizontal loads. The resistance to vertical loads depends mostly of the contact area between logs and on the compression strength perpendicular to the grain. While, horizontal loads are supported by transverse walls, depending strongly on the friction between slots. Lateral loads in log shear walls depends on the (1) interlocks between logs, (2) wood or steel dowels, (3) vertical through bolts and anchor-bolts, and, (4) frictions between logs due to vertical loads [1]. However, current codes only consider the influence of dowels and vertical through bolts [3], as a result of the significant variability and inexistence of accurate models for the other resistance mechanisms. In this paper, the resistance of a standardized log construction technology considering all mentioned mechanisms (Fig. 1) is evaluated experimentally. Wall panels under vertical and horizontal loads are tested and, lastly, a timber log house is studied under seismic loading.

Despite this being a traditional system used in timber constructions, Rusticasa produces a construction system based on timber log that, stemming from certain particularities, requires a series of experimental and numeric studies to apply to the European Technical Approval [19]. In this work, the main resistant mechanism of the timber log walls was analyzed, in particular the ones concerned with the in-plane resistance to horizontal loading. Timber logs used to make the walls were characterized and both connections between logs and also between walls were studied by means of numerical and experimental studies. Considerable friction stresses are developed in the connection between logs, which are, as expected, function of the vertical pre-compression level. Special attention was paid to the connection of the walls with the foundation since such connection is manufactured by Rusticasa in a quite unusual manner. This connection was tested and its influence on the global behaviour of walls subjected to in-plane displacement was assessed. The connection between orthogonal walls, namely the interlock between the logs of exterior walls is the main resistant mechanism of timber log walls under in-plane horizontal loads. Inside the halved joint used to materialize this intersection, shear stresses as well as compression stresses perpendicular and parallel to the grain occur. In the tests performed on full-scale walls, the localized failure was obtained always by compression perpendicular to the grain. Such tests aimed at evaluating full-scale timber log walls under different vertical pre-compression levels, in addition to distinct connection between the first log and the foundation, and also two types of stiffness of the orthogonal walls, besides assessing the effect of the slenderness of the wall. The experimental results obtained show a good capacity of these walls to dissipate energy, without any impairment of strength being the monotonic response normally enveloped to the behaviour obtained on the cyclic tests. The connection between the first log and the foundation, as far as the wall geometry evaluated is concerned, is not important to the global behaviour, which is function of: (a) the stiffness of the orthogonal walls; (b) vertical pre-compression value; and, (c) wall slenderness. Assuming a case study, the effectiveness of the simplistic method of distribution of the horizontal loads (seismic) over the walls according to their area of influence was assessed. The numeric analysis performed shows that this distribution must be based on the in-plane stiffness of the walls. Moreover, this analysis also showed that an inter-connection between logs should be implemented. According to actual standards and codes, friction can not be considered as a resistant mechanism and therefore, the in-plane behaviour of the system analyzed is totally ensured by the connection between orthogonal walls, where compression perpendicular to the grain and shear stresses are developed. Results also indicated that the connection between the first log and the foundation must be improved. Consequently, improvements to the Rusticasa system were suggested for the inter-connection between logs and also the connection of the first log to the foundation. Those suggestions are based on the test results and attendant to the conclusions drawn and supported by the results of the numeric analysis undertaken.