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Analysis of the moment-rotational stiffness of dowelled timber joints
Abstract
Timber joints are usually considered as perfectly pinned or stiff against rotation during the calculations of engineering timber structures. However the semi-rigid behavior the joints cannot be avoided in case of accurate calculations. In this paper the rotational stiffness and the full moment-rotational behavior of dowel-type timber joints were analyzed using computer algorithm validated by finite element analysis. Based on the numerical calculation and the experimental results the paper describes the influences of the joint geometry, the grain direction and the number and position of connectors on the moment-rotational behavior.
Traditional timber joints behavior remains badly known. Recent studies have shown that some kind of carpentry joints can be considered as semi-rigid connections. Their rigidity (bending stiffness) in general sense plays an important role in computation of the global deformations and force distribution of timber roofing frames. Developments in computer-controlled manufacture of timber members enable a revival of traditional timber connections without steel fasteners. This study will focus on deformation, failure processes and bending stiffness of rafter-tie beam connection and modern tenon (dovetail) joint as secondary beam-main beam connection (both joints are made by CNC wood-working machine).
Dowelled joints, widely used in timber structures, are designed to transfer shear forces and bending moments between timber members. The anisotropic non-linear behaviour of the timber beneath the fasteners controls the stiffness of these joints. At the ultimate load-carrying capacity, the failure modes result from the shear stresses induced by the load distribution among the fasteners. The paper presents the experimental results obtained for beam to column joint with or without reinforcement using glued plywood plates. Based on these results, a two-dimensional finite element model was developed in two stages to describe the three-dimensional behaviour of the joint. At the single fastener scale, the model considers the non-linearity induced by the timber embedding and the fastener bending. At the structural scale, the modelling approach considers the timber as an elastic orthotropic material whereas each fastener is modelled by two non-linear springs. The elastic-plastic behaviour of each spring element is defined by the local scale model defined in two perpendicular directions. The load distribution among the fasteners is compared to the analytical results according to design rules. Considering the global load displacement curves, the results show that the modelling approach provides a good estimation of the structural response.
This paper deals with several items concerning the restoration of historical wooden structures. Firstly general problems related to the identification of the material and of the whole structural complex are faced. They are mainly influenced by either wood defects and degradations or past technologies for construction. An overview of the possible upgrading intervention is also introduced. Therefore the paper has been focused on a study case of historical roofing structure. In particular the detailed geometrical and mechanical surveys are presented. The main aspects of the structural modeling are evidenced and the results of the structural analysis are discussed. On the bases of the structural behavior pointed out, the appropriate retrofitting intervention has been illustrated.
Modeling timber joints it is essential to know the stiffness of the connections. In some cases the prescriptions of the standards contain only general approach without considering significant effects. The presented paper analyzes the load-slip behavior of dowel-type timber joints. A computer calculation method is presented, which is able to take into account the geometrical and stiffness parameters as well as the different joint arrangements.
Bolted cross-lapped joints (BCLJs) are one of the basic jointing methods used in Japan and European countries. There are, however, some problems in the design of BCLJs. With increasing use of large-scale wooden frame structures in Japan, it is necessary to establish proper estimating methods for predicting actual characteristics. A new approach was developed, using Saint Venant torsion theory, to estimate the performance of bolted timber joints in a more practical manner than using computer simulations. The calculated values were compared with the experimental results, indicating that the rotational stiffness and yield moment of BCLJs would be precisely predicted using the proposed theory. It was also found that the rotational stiffness calculated using the design method rooted on Coulombs torsion theory is about two times higher than the experimental results in the case of a rectangular arrangement of bolts.