Constructing the Model

In this section, we construct the D-Region Boundaries and the Strut-and-Tie Model. After that, we define and assign Bearing Plates (if any), Point Loads, and Point Supports.

2.1

We will use the Guidelines constructed in Step 1.5 to help us create our D-Region Boundaries. If you do not see the Guidelines on the screen, click Show Guidelines from the View menu. This creates a rectangle surrounding the icon next to it and works as a toggle to turn on/off the Guidelines. Make sure that the Snap Boundary Corners or STM Nodes to Guidelines button located in the Snapping toolbar is on by clicking it:   when on and when off. Click then the button (or click the Construct menu and then click Outer Boundary) to switch to the Draw mode. Next, create the D-Region Boundary by placing the Boundary Corners at the Guideline intersections as follows:

• Click on the intersection of Guideline X=-1000 and Guideline Y=1000
• Click on the intersection of Guideline X=-500 and Guideline Y=1000.
• Click on the intersection of Guideline X=-500 and Guideline Y=1500.
• Click on the intersection of Guideline X=500 and Guideline Y=1500.
• Click on the intersection of Guideline X=500 and Guideline Y=1000.
• Click on the intersection of Guideline X=1000 and Guideline Y=1000.
• Click on the intersection of Guideline X=1000 and Guideline Y=400.
• Click on the intersection of Guideline X=500 and Guideline Y=0.
• Click on the intersection of Guideline X=500 and Guideline Y=-600.
• Click on the intersection of Guideline X=-500 and Guideline Y=-600.
• Click on the intersection of Guideline X=-500 and Guideline Y=0.
• Click on the intersection of Guideline X=-1000 and Guideline Y=400.
• Click on the intersection of Guideline X=-1000 and Guideline Y=1000 to close the D-Region Boundary. This last step can also be accomplished by simply clicking the right button of your mouse.

Up to this point, we have finished drawing the geometry of the structure as shown in Figure 12.

Figure 12   CAST Window after Step 2.1
(Click here to view a larger image)

Notes:

• You can create openings within the D-Region, if any, in the same manner as you create the Outer Boundary, except that you must click the button (or click again the Construct menu and then click Inner Boundaries).
• You must first create the Outer Boundary before you can create Inner Boundaries and Strut-and-Tie Model.
• You are allowed to create only one Outer Boundary, but you can create as many Inner Boundaries (to represent openings) as you want.
• If you hold the Shift key while drawing, it turns on the orthogonal mode. In this mode, all lines you make will be parallel to either X or Y axis.
• If you make a mistake, for example, you draw an element at a wrong place, you can undo your work by clicking the button from the Standard toolbar or clicking Undo from the Edit menu (or by pressing Ctrl+Z key combination).
• You can also move/add/remove an Outer or Inner Boundary Corner by clicking the button to switch to Modify mode (or by selecting Edit Boundaries from the Edit menu), and then:
  • To move the whole D-Region Boundary, simply drag it to the desired position. If the (Snap Boundary Corners or STM Nodes to Grid Points) or the (Snap Boundary Corners or STM Nodes to Guidelines) option is turned on, the centroid of the D-Region Boundary enclosed area will be snapped to any Grid Points or Guidelines while dragging.
  • Similarly, to move a corner position, drag the corner to the desired position. If the (Snap Boundary Corners or STM Nodes to Grid Points) or the (Snap Boundary Corners or STM Nodes to Guidelines) option is turned on, the corner will be snapped to any Grid Points or Guidelines while dragging.
  • To add or remove a corner:
    • Right click on a corner or an edge. The pop-up menu similar to Figure 13 will be displayed.
    • Click Add Corner to add a corner. Click Remove Corner to remove a corner.
• Pressing the Esc key will terminate the Draw mode and take you back to the Select mode.
• You can adjust object sizes by using the object size adjusters located at the status bar. The most left control can be used to adjust the font sizes in the display window. The middle control is used to adjust the line thickness of D-Region Boundary, STM Element, and Bearing Plate objects and to adjust the diameter of STM Node objects. The most right control is used to adjust the length of Point Load and Point Support objects.

Figure 13   The Pop-up Menu for Adding or Removing D-Region Boundary Corners

2.2

Let’s now draw the geometry of the Strut-and-Tie Model. Again, we will again use the Guidelines to help us create the geometry. Click the button (or select STM Elements from the Construct menu) to switch to the Draw mode. Make sure that the Snap and Glue STM Nodes to Other STM Nodes option is on by clicking it: when on and when off. Also, make sure that the Snap STM Nodes to Boundary Edges option is on by clicking it: when on and when off. The (Snap STM Elements Perpendicular to Boundary Edges) option allows the STM Elements drawn to be perpendicular to Boundary Edges.

We also need to show STM Element and Node IDs for our later reference. This is done by clicking the button from the Standard toolbar or by selecting Set Object View Options menu from the View menu and then clicking the OK button. This will show the Set Object View Options dialog box as shown in Figure 14. In this dialog box, check Node IDs and Element IDs checkboxes in the Strut-and-Tie Model frame, and then click the OK button.

Figure 14   The Set Object View Options Dialog Box
(Click here to view a larger image)

Now, do the following:

• Click on the intersection of Guideline X=-910 and Guideline Y=900, and then click on the intersection of Guideline X=-250 and Guideline Y=900. This step basically draws STM Element ID E1 (see Figure 15).
• Click on the intersection of Guideline X=-250 and Guideline Y=900, and then click on the intersection of Guideline X=250 and Guideline Y=900. This step creates STM Element ID E2.
• Click on the intersection of Guideline X=250 and Guideline Y=900, and then click on the intersection of Guideline X=910 and Guideline Y=900. This step creates STM Element ID E3.
• Click on the intersection of Guideline X=-910 and Guideline Y=900, and then click on the intersection of Guideline X=-250 and Guideline Y=0. This step creates STM Element ID E4.
• Click on the intersection of Guideline X=910 and Guideline Y=900, and then click on the intersection of Guideline X=250 and Guideline Y=0. This step creates STM Element ID E5.
• Click on the intersection of Guideline X=-250 and Guideline Y=0, and then click on the intersection of Guideline X=250 and Guideline Y=0. This step creates STM Element ID E6.
• Click on the intersection of Guideline X=-250 and Guideline Y=-600, and then click on the intersection of Guideline X=-250 and Guideline Y=0. This step creates STM Element ID E7.
• Click on the intersection of Guideline X=-250 and Guideline Y=0, and then click on the intersection of Guideline X=-250 and Guideline Y=900. This step creates STM Element ID E8.
• Click on the intersection of Guideline X=-250 and Guideline Y=900, and then click on the intersection of Guideline X=-250 and Guideline Y=1500. This step creates STM Element ID E9.
• Click on the intersection of Guideline X=250 and Guideline Y=-600, and then click on the intersection of Guideline X=250 and Guideline Y=0. This step creates STM Element ID E10.
• Click on the intersection of Guideline X=250 and Guideline Y=0, and then click on the intersection of Guideline X=250 and Guideline Y=900. This step creates STM Element ID E11.
• Click on the intersection of Guideline X=250 and Guideline Y=900, and then click on the intersection of Guideline X=250 and Guideline Y=1500. This step creates STM Element ID E12.
• Click on the intersection of Guideline X=-250 and Guideline Y=900, and then click on the intersection of Guideline X=250 and Guideline Y=0. This step creates STM Element ID E13.
• Click on the intersection of Guideline X=-250 and Guideline Y=0, and then click on the intersection of Guideline X=125 and Guideline Y=-600. This step creates STM Element ID E14.
• Click on the intersection of Guideline X=-910 and Guideline Y=900. Click on the intersection of Guideline X=-900 and Guideline Y=1000. This step creates STM Element ID E15.
• Click on the intersection of Guideline X=910 and Guideline Y=900. Click on the intersection of Guideline X=900 and Guideline Y=1000. This step creates STM Element ID E16.

Figure 15   CAST Window Showing an STM Element and Guideline Locations
(Click here to view a larger image)

Up to this point, we have finished drawing the geometry of the Strut-and-Tie Model. Figure 16 shows how the CAST window should look like now.

Figure 16   CAST Window after Step 2.2
(Click here to view a larger image)

Notes:

• When we draw an STM Element, STM Nodes are automatically provided at each end of the STM Elements.
• Again, if you make a mistake, for example, you draw an element at the wrong place, you can undo your work by clicking the button from the Standard toolbar or clicking Undo from the Edit menu.
• You can also move the position of STM Nodes by clicking to switch to Modify mode (or by selecting Drag STM from the Edit menu) and then dragging the node. If the (Snap Boundary Corners or STM Nodes to Grid Points) or the (Snap Boundary Corners or STM Nodes to Guidelines) option is turned on, the node will be snapped to any Grid Points or Guidelines while dragging.
• The Modify mode also allows you to move the position of an STM Element by dragging the element to the desired position. However, the connectivity to the nodes at the ends of the element will be lost. If the (Snap Boundary Corners or STM Nodes to Grid Points) or the (Snap Boundary Corners or STM Nodes to Guidelines) option is turned on, the centroid of the STM Element will be snapped to any Grid Points or Guidelines while dragging.
• Pressing the Esc key will terminate the Draw mode and take you back to Select mode.

2.3

The next step is to assign bearing plates to STM Nodes we have just drawn. Do the following:

• Click on the button from the Drawing toolbar to be in the Select mode.
• Select STM Node IDs N12 (at X=-900 Y=1000) and N13 (at X=900 Y=1000) by either clicking on them or drawing a window completely around them. The selected nodes will become star-type shapes as shown in Figure 17.
• Click on the  button from the Assigning toolbar (or select Bearing Plates from the Assign menu). The Assign Bearing Plates dialog box (Figure 18) then appears. In this dialog box:
• Enter 600 in the Width text box.
• Enter 150 in the Length text box.
• Enter 25 in the Thickness text box.
• Click the OK button to confirm the input and close the window.

After this stage, the CAST window will look similar to Figure 19.

Figure 17   CAST Window Showing Selected STM Nodes
(Click here to view a larger image)

Figure 18   The Assign Bearing Plates Dialog Box

Figure 19   CAST Window After Step 2.3
(Click here to view a larger image)

Notes:

• You are only allowed to assign bearing plates to STM Nodes that are located on the Outer or Inner Boundaries.
• To clear all selected nodes, click on the button from the Selecting toolbar to be in the Select mode and click on them. We can also clear the selected nodes by selecting Deselect STM from the Select menu, clicking on Point/Window, and clicking on them (or drawing a window completely around them).
• If the bearing plates do not appear on the screen, click the button (or select Set Object View Options from the View menu). After the Set Object View Options dialog box (Figure 14) is displayed, check Bearing Plates checkbox in the Structure and Loading frame, and then click the OK button.

2.4

Now, let’s assign the Boundary Conditions, i.e., the Point Loads and Point Supports, to the STM Nodes as follows:

• Click the button from the Drawing toolbar to be in the Select mode.
• Select the previous selected STM nodes (IDs E12 and E13; see Figure 19 for ID reference) by either clicking on them or drawing a window around them. This step can also be done by clicking the button from the Selecting toolbar or by clicking Restore Previous Selection from the Select menu.
• Click the button from the Assigning toolbar (or from the Assign menu, click Boundary Conditions). The Assign Boundary Conditions dialog box (Figure 20) then appears. In this dialog box:
  • Click on Force option (radio) button.
  • Enter -1005 in the text box next to the Force radio button just clicked.
  • Click the OK button to confirm the input and close the window.
• Select all STM Nodes at the column bottom (IDs N7, N9, and N11; see Figure 19 for ID reference).
• Click again the button from the Assigning toolbar. In this dialog box:
  • Click on the Support radio button.
  • Click the OK button to confirm the input and close the window.
• Select the two STM Nodes at the column top (IDs N8 and N10; see Figure 19 for reference).
• Click again the button from the Assigning toolbar. In this dialog box:
  • Click on Force radio button.
  • Enter -3000 in the text box next to the Force radio button.
  • Click the OK button to confirm the input and close the window.

At this stage, we have finished assigning Point Loads and Point Supports to the STM Nodes. Figure 21 shows how the CAST window should look like now.

Figure 20   The Assign Boundary Conditions Dialog Box

Figure 21   CAST Window after Step 2.4
(Click here to view a larger image)

Notes:

• You may assign Point Loads to STM Nodes that are located on D-Region Boundaries (Outer or Inner) or in the D-Region under consideration, but you are only allowed to assign Point Supports to STM Nodes that are located on D-Region Boundaries.
• If the Point Loads or Point Supports do not appear on the screen, click the button (or select Set Object View Options menu from the View menu). After the Set Object View Options dialog box (Figure 14) is displayed, check Point Loads/Supports checkbox in the Structure and Loading frame and click the OK button.
• Use the object size adjusters (the most right one) located at the status bar to adjust the size of Point Load and Point Support objects.

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