自攻螺钉在胶合木中的拉拔破坏模型

MECHANICAL MODEL FOR WITHDRAWAL FAILURE OF SELF-TAPPING SCREWS IN GLULAM

随着木结构领域内力学紧固件的发展,具有足够长度和优化螺纹的自攻螺钉或螺纹杆为实现具有高强力学性能的木连接提供了一种新的技术方案。自攻螺钉在木材中的拉拔性能是影响螺钉连接力学性能的重要因素,为了充分利用自攻螺钉的轴向受力优势,应当避免埋置在木材中的自攻螺钉发生拔出破坏。不同于现有基于经典Volkersen理论的计算方法,该文提出一种适用于轴向受力螺纹紧固件的力学模型,首次在模型中引入一种包含螺纹的“装配单元”,用于研究自攻螺钉在胶合木中的拔出破坏机制,解决自攻螺钉的锚固长度计算问题。模型根据拔出破坏的典型现象重新定义了拔出破坏面,考虑了螺纹对木材破坏区域的影响,并在模型简化、公式推导和算法设计上体现出木材的局部受力状态和拔出破坏面上的剪应力、剪力不连续传递现象。理论计算与试验结果有较好的一致性,计算结果还表明,在发生拉断破坏以前,自攻螺钉和螺纹杆的拔出破坏荷载与埋置长度基本呈线性关系。

With the development of mechanical fasteners in timber structures, self-tapping screws or threaded rods with sufficient length and optimized thread provide a new technical solution for realizing the connections with high mechanical performance. Considering that the withdrawal property of self-tapping screws in wood has an important influence on the mechanical performance of connections with self-tapping screws as fasteners, the withdrawal failure of self-tapping screws in wood should be avoided to take the full advantage of the fasteners in axial load transfer. Different from existing calculation methods based on the classic Volkersen theory, a type of“ assembly unit”, which contains threads is first introduced in a new model, is utilized to research the withdrawal failure mechanism of self-tapping screws in glulam and to predict the anchorage length of self-tapping screws in glulam. The model redefines the withdrawal failure surface according to the typical withdrawal failure phenomenon, considers the effect of the thread on the wood failure region, and reflects the local stress of the wood and the discontinuous transfer of shear stress/shear force on the withdrawal failure surface in the aspects of model simplification, of formula derivation and, of algorithm design. An acceptable agreement is achieved between theoretical and experimental results, and a nearly linear relationship is predicted by the model, between withdrawal failure load and embedment length before self-tapping screw or threaded rod fracture.