Because of the way XTRACT meshes shapes added to cross sections, there are rules for section creation: • • • Shape boundaries cannot cross Sections are built from the outside in All parts of the section must be within the outer drawing boundaries
1) Shape boundaries cannot cross – When shape boundaries cross, XTRACT does not know which material belongs to whichshapes. If shape boundaries do cross, material laid upon existing material may give unexpected or incorrect results. Example:
If the user clicks the ‘Discretize’ button in the ‘Draw/Import Shapes’ dialog, the Discretizer will open. Selecting no cover, a confined concrete material model, and clicking the ‘Discretize/Overlay’ button
will result in the following warning:
Clicking ‘No’ willresult in material upon material as shown below:
Clicking ‘Yes’ will cancel the discretization. 2) Sections are built from the outside in – Because sections discretized within sections remove the material over which they are added, the program gets confused between which material belongs to which shape when larger sections are drawn upon smaller sections. Example: CORRECT – a smaller sectiondropped within a larger one:
INCORRECT – a larger section dropped on a smaller one:
By discretizing the larger section on top of the smaller one, all the corner points of the new shape are not contained within another shape – hence, the same error of overlaying material as described above will result. Clicking ‘No’ will result in material upon material. However, for this case, as long as theboundaries DO NOT CROSS, this problem can be resolved in the remesher. Clicking ‘No’ will allow material upon material. Then, to rectify, click the Reshape/Remesh Mode icon on the Builder Tools Toolbar to open the Remesh Section dialog box.
In the remesher, the shapes are organized from largest area to smallest area. By clicking the ‘Discretize All’ button, the larger shape will be discretizedfirst, then the smaller shape, thus resulting in the removal of the overlapping material. 3) All parts of the section must be within the outer drawing boundaries – XTRACT has an outer limit on the drawing area. These are the bounds defined such that all shapes must be within these defined bounds. These bounds can be set for individual sections in the Options dialog. The larger the user makes thesebounds as compared to the size of the section, the larger the memory demand on the system due to the auto mesh generation methodology. Example: The entire discretized cross section must be contained within the drawing boundaries (as shown below in yellow):
See Also: User Defined Cross Section Discretizer
The Discretizer is opened when: • • • The ‘Discretize’ button isclicked after an AISC Shape has been imported. When a User Defined Section shape has been closed (see snap to close shape). When a geometric shape has been Imported and the ‘Discretize’ button is clicked.
The Discretizer takes the defined shape and turns it into a triangular mesh. Shapes are discretized based on the mesh size entered, the size of the cover, and the core and cover materials.Discretized sections must conform to the Rules for Section Creation. To change the properties of a discretized shape (after it has been discretized) select the Reshape/Remesh Mode on the Builder Tools toolbar. Once a shape has been discretized, and the ‘Discretizer’ has been closed, the discretization can be only be undone by clicking the ‘Undo Discretization’ button on the Builder Tools toolbar.In this dialog, the user must enter the following information:
Input the mesh size in the given Units. The mesh is the average length of each side of a triangular fiber. Typically the mesh size should be at least the minimum section dimension divided by 16 for reasonable results. Smaller (tighter) meshes will give a more accurate analysis but at the cost of more computation time...