Bohai Steel Structure Co., Ltd.

Bohai Steel Structure Co., Ltd.
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    author:Bohai Steel Structure Co., Ltd.

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    time:2019-10-23 11:19:58


    In practical engineering, because the structure of joints often determines that the joints are in complex stress state, and generally cannot be in thin film stress state or simple tension-compression state, shell elements or entities should be adopted in the selection of elements according to the shape of joints, the cross-section form of the joints connected with the joints, and the variation degree of wall thickness. Unit. Usually, shell element can be used when the node form is regular and the wall thickness of the member connected with the node is constant. For example, the intersecting joints consisting of tubular or box-shaped cross-section trusses; if the joint form is more complex, it obviously becomes a solid form, or, although the joint form is relatively simple, but the components constituting the joint are unequal wall thickness, in this case, it is necessary to select the solid element to simulate. In shell element, triangular shell element and quadrilateral shell element are usually used. Quadrilateral shell element is usually used in the case of regular node topology, which is limited in use, because in the case of complex topology, the use of quadrilateral element may require finer element subdivision.


    Relatively speaking, triangular shell element is more applicable and can be applied to complex structural topology. In many cases, quadrilateral can be used as the main triangle supplemented by the method of element division. In practice, according to the different finite element software selected, the requirements of shell element for modeling are also different. For example, in ANSYS software, the geometry of shell element division must be surface (AREA), not a solid with thickness. Then the thickness of shell element is defined in real constants, so that the geometric thickness characteristics of the analyzed object can be simulated truly. However, for some meta-software, such as ABCOSMOS, they not only provide a conventional shell element type, but also provide a solid shell element type, which can be applied to the solid surface. That is to say, the shell element can be used to divide the shell element by using the geometry of plate and shell modeled by entity, and the software can automatically detect the thickness of the shell element.

    For triangular shell element, there are usually three nodes and six nodes in finite element software. Four nodes and eight nodes are commonly used in quadrilateral shell element. Among them, 3-node and 4-node shell elements are linear elements, 6-node and 8-node are high-order non-linear elements with high accuracy. In use, software and unit type can be selected according to the actual situation of the problem. Figure 3 shows several typical examples of shell element subdivision. Fig. 4 is a solid shell element used in the analysis of a node. As mentioned above, in the analysis of complex nodes, shell element can not be reasonably divided and calculated, and solid element must be used for simulation. The advantage of entity element is that it can be applied to model analysis of almost arbitrary geometric topology. Moreover, with the encryption of element division, the accuracy of analysis is relatively high. Similar to plane element, solid element has two main forms, namely tetrahedral element and hexahedron element. According to the difference of integration points, tetrahedral elements are divided into 4-node elements and 10-node elements, and hexahedron elements are 8-node elements and 20-node elements.

    Tetrahedral elements have better adaptability and are suitable for solid models of arbitrary geometric topology, while hexahedral elements are suitable for relatively regular geometric topology, and hexahedral elements may not be possible for special complex situations such as intersection of different surfaces. Generally, the method of combining hexahedron with tetrahedron is used for complex entity analysis, hexahedron element is used in most areas, while tetrahedron element is used in local transition areas. It is necessary to pay attention to the fact that the entity element is easy to form stress concentration phenomenon, so it must be avoided when applying load and boundary constraints. Large nodal forces and improper boundary conditions are applied. In addition, in order to simulate the force characteristics to be analyzed, it is necessary to distance the boundary and the position of the applied load from the center of the analysis target, which is based on Saint Venant's principle to eliminate the effect of stress concentration. Fig. 7 is a case in which solid elements are used to partition geometric models in Fig. 6, and Fig. 8-10 is a typical case in which solid elements must be used to partition geometric models.

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