Cutline
This is a feature under “Mat” tab which gives option to cut a section and check different analysis results along that section.
Introduction
After a successful analysis, SFA receives post-analysis data from the analytical model (STAAD File), including plate stresses for all plates. Often, not all the plate data are useful for designing a mat. A common method of mat design is the strip method in which stresses for each strip are calculated. SFA has an automated process for defining strips and extracting the data automatically, but for the designer to justify and fine tune the design using the strips, it is necessary to be able to visualize the pattern of stresses at different locations. The most popular approach is the cutline method, where a line is drawn on the mat and the stresses along that line are calculated. The cutline method in SFA allows the designer to draw any number of lines on the mat and obtain and visualize the stresses with respect to an axis through each line.
When a mat is analysed and a cutline is drawn through the mat, the following forces are reported for the cutline – Ms, Mt, Mst, Ss, St, Sst, Sqs and Sqt.
Ms and Mt are the moments that produce stresses along cutline axis s and t respectively. The positive values of these moments will produce tension at the top of the surface. The top surface is defined as the surface that lies in the positive direction of the global vertical axis. The figure below describes the positive sense of moments on an element around the point on the cutline on which the stresses have been determined. These stresses are reported in force-length/length units.
Mst are the twisting moments which are reported in force-length/length units. The positive sense of the moments is also shown in the figure.
Ss and St are membrane stresses. The positive membrane stresses shown in the next figure will create tension in the surface element. These stresses are reported in force/ length^2 units.
Sst is the in-plane shear stress of the element isolated from around the point of the cutline. They are reported in force/length^2 units. The positive sense of in-plane shear stress is as shown in the next figure.
Sqs and Sqt are the out-of-plane shear stresses reported in force/length^2 units. The positive senses of the out of plane shear stresses are reported in the next figure.
Cutline Axis System
The axis system of a cutline on a horizontal surface (a surface in the XZ plane for a Y-up model and the XY plane for a Z-up model) is determined as follows.
The direction from the start point of the cutline to the end point of the cutline determines the direction of the positive direction of the s-axis.
The vector product of the s-axis unit vector with the global vertical axis unit vector determines the positive direction of the t-axis unit vector.
Thus, for a Y-up model, t is determined as s x GY.
And for a Z-up model, t is determined as s x GZ.
Creating Cutlines-
To use this feature, you must first analyse the mat successfully to provide the plate stress data. Next, select the mat where you wish to place the cutline to display the Mat tab in the menu bar.
Click the Mat tab and in the ribbon, click the Cutline icon. You can now directly click on the mat at two points to define the line or enter their coordinates in the Create Cutline window that is displayed. Next, enter the number of intervals into which the cutline is to be divided. If there is no opening or re-entrant corner and the line lies completely on the mat, number of points on the line is 1 plus the number of intervals. If there are holes or openings the points that fall into these zones are automatically left out by SFA. Click the Create button to finalize the cutline. You can define multiple cutlines.
Offset Option-
This feature allows you to extend the cutline at its start and end point. In the figure below, at left we can see a cut line is defined from its start node at (7 m,0 m,1 m) to end node (7 m, 0 m, 4 m).
In the next figure, a start offset of 1.5 m and an end offset of 1 m are applied to the line by entering these values in the Properties-Cutline window, shown by broken lines in the schematic. Clicking Create finalizes the operation.
Viewing results-
To view stresses for a cutline, first select it to display the Cutline Graph window. Selection of cutline can be done either by selecting the cutline by clicking it in the drawing area, or by going to “Cutline” in Project Explorer and selecting it.
On left side of the cutline window, six options are provided.
a) Loads -Click on the arrow beside the icon to expand it and select the load combination for which you wish to view the graph from the drop-down list.
b) Type- Click on the arrow beside the icon to expand it and select the types of stresses you want to view in the graph from the drop-down list. Multiple graphs can be drawn in a single location. Available types of stresses are Mt, Ms, Mst, Sst, Sqt, Ssq, Ss, and St. These are described below.
c) Vertical Grid- Toggle setting for switching Off or on the display of the vertical grid of the graph.
d) Horizontal Grid- Toggle setting for switching Off or on the display of the horizontal grid of the graph.
e) Series Line Type- This setting allows to you choose a line graph or a curve graph to represent your data. As shown below, a line graph consists of line segments that connect the data points, while a curve graph consists of a smooth curve that is calculated by interpolating the data points.
f) Save - Click here save the graph in .png format.
Description of Reported Items-
When the cursor hovers over any point of the line in the graph, the corresponding data displayed in an inset window. The inset can show the following (refer to the figure):
Global Coordinates -The global coordinates of that point (8.61,0,3.83 in the figure) on the cutline on which the data was extracted are referred to the global coordinate system defined for the mat.
Orientation Angle -The orientation angle of the plate in degrees (226.27 here in the figure) is taken with respect to the global coordinate system. You can go to the STAAD.Pro model to view plate orientation by using the keyboard shortcut SHIFT+T.
The figure below shows the plate orientation system and the global coordinate system.
Local coordinates of the point -The local coordinates of the point are its coordinates with respect its local axis whose origin is the centre of gravity of the plate. As illustrated in the figure below, the local coordinates (0.05, -0.15) of the point take into account the orientation of the plate.
Associate Plate Number -The program-assigned numerical label for the plate (1495 in our example shown).
Ms / Mt / Mst / Sst / Sqt / Sqs / Ss / St - Plate stresses along STU axis system (the local axis system of the cutline). The plate stresses that you select are displayed. More details are provided in the section “Significance of the Stresses” below.
Selecting Points on the Cutline-
The points at which data is calculated are the vertices of the line segments that arise from the program dividing the cutline into (n+1) parts, where n is the number of intervals that you specified. A larger value of n produces a smoother graph, If the cutline extends beyond the mat, then those points that are outside the boundaries are automatically left out by SFA. In the following figure, we see that SFA has dropped the points on the line where it lies outside the mat.
Hover over any point of the graph to highlight the corresponding plate and point.
Significance of the Stresses-
When a cutline is drawn, SFA creates multiple points upon the line. At each point, an associated plate is selected. Plates do not have the same orientation during mesh generation. You can view all all the plate orientations by opening the generated STAAD model and using SHIFT+T keyboard shortcut.
For each plate, the analysis provides stresses such as Mx, My, Mxy, Sx, Sy, Sxy, Sqx, or Sqy per STAAD.Pro conventions, as shown in the figure below. These data are given in the local coordinate system of the corresponding plate at location of the corresponding point.
STAAD.Pro results are passed automatically into SFA and are available for all load combinations; at all points, these include Mx, My, Mxy, Sx, Sy, Sxy, Sqx, and Sqy. SFA converts these to the axial orientation of the cutline to give Mt, Ms, Mst, Sst, Sqt, Ssq, Ss, and St.
Plate stresses in XYZ coordinate system are shown in the figure below. This XYZ coordinate system is the local coordinate system of the plate. These plate stresses are calculated and interpreted by STAAD.Pro as is. SFA imports this data and applies the corresponding transformation to provide them in the STU coordinate system associated with the cutline.
The STU System for a cutline is illustrated in the figure below.