structure shell create command

Syntax

structure shell create keyword

Primary keywords:

by-face    by-nodeids    by-quadrilateral    by-triangle

Create a shell. The four main keywords articulate four different techniques of creation: by creating a collection of elements from a group of zone faces (by-face); by creating a single element from four existing nodes (by-nodes); from a specified quadrilateral (by-quadrilateral); or from a triangle (by-triangle).

by-face [shellcreateoptions] <range>

Create new elements that are attached to the set of three- or four-sided zone-faces in the range. By default, only surface faces will be considered, and the internal keyword can be used to select internal faces. New nodes associated with the element will also be created (see the id keyword). The nodes of each new element will be ordered counter-clockwise with respect to the outside of the specified zone faces, thereby making the element z-axis point outward. The element will be attached to the grid at its nodes with link attachment conditions appropriate to the element type (see this table in the topic Structural Element Links). Note that after creating the elements with this command, the zones may be deleted and the elements may be positioned by moving its nodes with the structure node initialize position command.

by-nodeids i1 i2 i3 [shellcreateoptions]

Create elements given an existing set of nodes. The new elements will not be attached to the grid. If you wish to attach it to the grid, then after creating it, position its nodes with the structure node initialize position command (which will create links and set appropriate attachment conditions for all nodes that are moved into a zone).

by-quadrilateral v1 v2 v3 v4 [shellcreateoptions]

Create elements based on four points forming a quadrilateral, in order. The element local system will point in the opposite direction determined by using the right-hand rule. The new elements will not be attached to the grid. If you wish to attach it to the grid, then after creating it, position its nodes with the structure node initialize position command (which will create links and set appropriate attachment conditions for all nodes that are moved into a zone).

by-triangle position v1 v2 v3 [shellcreateoptions]

Create elements based on three points forming a triangle. The element local system will point in the opposite direction determined by using the right-hand rule. The new elements will not be attached to the grid. If you wish to attach it to the grid, then after creating it, position its nodes with the structure node initialize position command (which will create links and set appropriate attachment conditions for all nodes that are moved into a zone).

Keyword Block

The following keywords may be optionally provided on shell creation, using any of the major keywords: by-face, by-nodeids, by-quadrilateral and by-triangle.

id i

Specify the ID that will be assigned to all new elements created by this command. If this value is not given, then the next available ID will be used. The ID controls potential joining of the element nodes to an existing element, as follows. At each potential new node location, a new node is created unless all of the following conditions are fulfilled: 1) the ID is given; 2) an existing node is found “near” the potential new node location; and 3) the found node is part of an element of the same type with ID equal to i.

group s1 <slot s2 >

Assign the created elements to group s1. The group is assigned to the slot named s2 if supplied; it is assigned to the slot named Default if not. Use of the group logic is described in Group.

cross-diagonal <b >

If specified, a cross-diagonal mesh of four triangles will be created with a node lying at the centroid of all quadrilaterals; otherwise, a cross-hatch mesh of two triangles will be created dividing the quadrilateral.

When connecting elements to the grid, it is best to locate nodes at gridpoints. If the finite-element type can resist bending (DKT, DKT-CST, or DKT-(CST Hybrid) elements), then incompatibilities can develop along element faces and zone faces, because the transverse (out-of-plane) displacement field varies cubically across element faces, but varies only linearly across zone faces. Thus, when connecting bending-resistant elements rigidly to zone faces, it is best to utilize a cross-hatch mesh, because the middle node of the cross-diagonal mesh will be constrained to translate according to the linear displacement field of the zone face. This over-constrains the elements and can produce near-zero moments at these mid-nodes when present in a nonzero moment field.

distinct

Eliminate sharing of nodes among created elements. Every element will create a new set of nodes, even if they are at the same location in space and elements share the same ID number. This option is most commonly used with a later structure node join command to create links connecting nodes at the same location in order to apply custom inter-element behavior.

element-type keyword

Specify the finite element formation used by each structural element. If no element type is specified, then the element type will be DKT-CST. Note that the type of finite element is associated with each structural element when it is created and cannot subsequently be altered. The finite element type of existing structural elements is printed by the structure shell list element-type command. The value of the keyword supplied must be one of the following three-noded finite elements:

s

Attempt to match the string s to one of the available keywords below, and treats as that option.

cst

CST (6 degrees-of-freedom) finite element. This is a membrane element; it resists membrane, but not bending, loading.

csth

CST Hybrid (9 degrees-of-freedom) finite element. This is a membrane element; it resists membrane, but not bending, loading.

dkt

DKT (9 degrees-of-freedom) finite element. This is a plate-bending element; it resists bending, but not membrane, loading.

dkt-cst

DKT-CST (15 degrees-of-freedom) finite element. This is a shell element that combines the DKT and CST elements to resist both bending and membrane loading.

dkt-csth

DKT-(CST Hybrid) (18 degrees-of-freedom) finite element. This is a shell element that combines the DKT and CST Hybrid elements to resist both bending and membrane loading.

internal

This keyword only applies if the from-faces option was used. Specifies that all internal and surface faces in the range will have elements created on them. Normally only surface faces are considered.

maximum-length f

Break the element into segments no greater than length f. The segments setting, if provided, is overridden as needed by this setting.

segments i

Break the element into i equal segments, creating new nodes as needed. The default value is 1, indicating that only one element will be created.

size i1 i2

This keyword only applies if the from-quadrilateral option was used. Increases the number of elements created by subdividing the quadrilateral into i1 segments along the first edge and i2 segments along the second. By default, only one quad is created.