A new classification system for the joints used in lattice shells

A new classification system for the joints in lattice shells is proposed. The stiffness and moment capacity of the joints together with the overall structural behavior of the lattice shells are considered in order to establish the classification system. According to this new system, joints in lattice shells can be classified into unique categories: rigid, semi-rigid or pinned. The rigid joints have both high bending stiffness and moment capacity; the semi-rigid joints have both moderate bending stiffness and moment capacity, and the pinned joints have either low bending stiffness or low moment capacity. Determination coefficients α and β are defined, based on the stiffness and moment capacity of the joints, and these are used to establish clear boundaries between the different categories. Some numerical examples are included to demonstrate the validity of the classification system. With the help of the classification system, an efficient process for practical engineering design is proposed, which can help designers choose the appropriate analysis method for lattice shells with different joints.     

基于柱坐标系的油井出砂三维数值模型设计与研究

根据实际储层砂岩的物性和射孔试验特征,从岩土力学的角度基于柱坐标系建立三维颗粒流（PFC^3D）数值模型。综合考虑流体压力梯度力和拖曳力产生的流固耦合效应,运用几何学方法研究油井出砂过程中的砂岩模型变化。数值计算与理论分析比较可知流体运动效应不可忽略,两者的应力分布曲线相吻合,验证了数值模型的可行性和适用性。另外,外界条件一定时,砂岩模型的应力分布、黏结颗粒性态、颗粒位移和流体对单颗粒作用力结果显示不同的单元划分对数值计算结果的影响不大,说明该数值模型的收敛性和稳定性。由此可见,提出的三维数值模型能较好地反映砂岩出砂过程中的宏细观力学特性,为准确理解砂岩出砂的力学机理提供了重要的研究思路和研究手段。