رفتار سازه از اتصال تیر-ستون پیش استرس و به عنوان جایگزینی برای سیستم مفصل الوار سنتی

We present a new and alternative proposition for the traditional Asian timber column-and-beam connection, using the Kusabi–Nageshi (K–N) joint system. The mechanical behaviour is complex and relies on the embedment strength of the timber, pre-stress, friction, and elastic energy storage. Using a systematic comparative analysis, the advantages of the K–N joint over more traditional joints can be made clear. The comparison of the hysteresis from cyclic loaded cross-joint tests with varied pre-stress clarifies the enhanced mechanical behaviour of the K–N joint system.

Traditional Asian timber structures in which the stability is dependent on the moment rotation behaviour of column-and-beam joints, such as the Nuki joint (column centre penetrating beam) or Nageshi joint (column side penetrating beam) shown in Fig. 1 left and centre, have a number of shortcomings. One of most critical among these is the lack of a permanent tight fit between the column and beam, essential for an immediate load take up, which leads to an unpredictable behaviour of the structure. The reasons for the loss of this fit include wood shrinkage, dislocation of wedges, and non-recoverable deformations by seismic impacts. As a solution, a new joint has been developed, namely the Kusabi–Nageshi (K–N) timber joint system, details of which are shown in Fig. 1 right [1] and [2]. The two traditional elements, the Kusabi wedge of the Nuki system and the side penetrating Nageshi beam, have been combined to form a horizontal long wedge–beam system. A penetrating bolt that keeps the elements together can be tightened to make and maintain the pre-stress. A part of the pre-stress is passed on to the embedment areas to enable a firm and continuous fit in both the elastic and post-elastic deformation stages, before, during, and after the seismic loadings. Tight fittings are achieved with more certainty, leading to improved structural performance with immediate load take up, enhanced rotational stiffness, ductility, and energy dissipation.The use of pre-stress itself in timber structures is not new. For traditional structures, tight fittings achieved by striking-in a type of wedge can be described as applying a pre-stress. The degree of the applied pre-stress is uncertain and solely dependent on the carpenter’s judgment, which makes the evaluation of its effectiveness difficult even today. In the long-term perspective, many aspects, such as the moisture content variation, may affect the degree of pre-stress. Another active application of pre-stress and friction in timber structures can be seen in the production of laminated timber bridge decks. High strength steel bars that run through the timber deck laminations introduce those stresses where friction between the laminations transfers the transversal loads. In this application, stress relaxation losses are compensated by a preset re-tightening schedule. The appropriate scheduled re-tightening is reported to maintain 40% of the initial pre-stress value, even in an outdoor, rather harsh environment [3]. This may also support the effectiveness of the traditional joints mentioned previously, if the wedges are stroked-in periodically. (The investigation of pre-stress losses, and an effective solution to maintain a sufficient pre-stress level of the K–N joint are also critical concerns, but these will be examined in a future report.) For the K–N joint, the pre-stress and associated friction play a vital role. The major focus of this paper is in the clarification of the influences which the pre-stress, in association with friction, brings to the K–N joint’s structural performances. This report clarifies the complex mechanical behaviour of a pre-stressed K–N joint by the following: (1) Identification of the major resistive mechanisms, namely the embedment, friction, and pre-stress. (2) Systematic comparative analysis of the moment–deformation hysteresis loops.

The comparison analyses in structural performance of a K–N joint and a traditional Nageshi joint show a significant improvement in stiffness, strength, and energy dissipation capacity for the K–N joint. Considering the parameters such as pre-stress and friction forces, which contribute to the enhanced performance, it was concluded that: (1) Pre-stress, which activates the friction forces, is vital to make full benefit of the K–N system. (2) Pre-stress activates the gap elimination, which positively affects the stiffness of the system and results in a more uniform symmetrical moment–deformation hysteresis. (3) A convex embedment deformation shape works advantageously for friction force ff-1. (4) Friction force ff-2, created by applying pre-stress normal to the rotating direction of any joint, provides a favourable “horizontal band zone” to the moment–deformation hysteresis. (5) Friction force ff-3 is vital to improve the overall structural performance. (6) The bolt and washer of the K–N system act as an elastic energy storage, which stores seismic energy and brings pre-stress to the system.