Display Center of Mass and Weight ( Modeling )

Tool summary :

• Reports one or more members' center of mass with respect to a point that you locate. It also reports the total weight. If you don't locate a point, it measures from the global 0, 0, 0 point.
• The center of mass of a group of members is that global coordinates point around which the members' mass is evenly distributed. Shop bolt weights are taken into account when determining the center of mass.
• The center of mass and total weight of a truss or members can be used to evaluate the optimal lifting point for a crane.
• See (on this page):
  
  
  • Step-by-step instructions:
    • Introduction
    • Step 1: Preselection (optional)
    • Step 2: Invoke Center of Mass
    • Step 3: Takes place only if you did not preselect
    • Step 4: Optionally locate a reference point for measuring
    • Step 5: An ok dialog reports the information
  • To find the point shown in the ok dialog (alternative 2 only):
    • Script to put construction lines through the center of mass

Also see :

• User and Site Options > General > Point selection by area box (affects selection for Center of Mass )
• Global coordinates (center of mass point is reported in)
• Report Writer (can output weight and center of mass of individual members)

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   Step-by-step instructions :

The following instructions assume that you are using a 3-button mouse. You can do step 1a (preselection) or step 3 (in-tool selection) to perform this operation.

1 . Before using this tool:

Select Items bindings

1a (optional) : In Select Items mode with an appropriate selection filter selected, use Select or Select + (or draw an area box or Select Members by Number ) to select the member(s) whose mass and center of mass you want to find. When selected, the members will be displayed in the User and Site Options > Modeling > " Primary selection color ," which by default is green. Use Select- if you want to deselect members.

2 . Invoke Center of Mass using any one (1) of the following methods:

Method 1 : Click the Center of Mass icon, which is pictured above. The icon can be taken from the group named ' Model ' and placed on a toolbar (classic) or the ribbon (lightning).

Method 2 : If " Modeling layout style " is ' Classic ', you can use the menu system to choose Model > Center of Mass .

Method 3, 4 or 5 : Center of Mass can also be configured to be invoked using a keyboard shortcut , the context menu , or a mode . For the lightning interface, this configuration is done using Customize Interface .

3 . Skip this step if you preselected members in step 1a.

Select Item(s) bindings

3a (if you didn't preselect) : Center of Mass prompts you use Select Item(s) mouse bindings to select the member(s) you want. Press the Enter key (or right-click and choose " OK " on the context menu ) when you are done selecting members.

4 . The status line prompts, " Locate point ."

mouse bindings

Alternative 1 : Left-click ( Locate ) a point. If you do this, Center of Mass will (in step 5 ) report the center of mass relative to that point.

Alternative 2 : Right-click ( Return ) instead of selecting a reference point. If you do this, Center of Mass will (in step 5) report the center of mass relative to the 0, 0, 0 global coordinate .

5 . An ok dialog opens. It reports the center of mass and total weight of the selected members.

	Possibility 1 : If you located a point in step 4 , the center of mass is reported with respect to that point.
	Possibility 2 : If you did not locate a point in step 4 , the center of mass is reported with respect to the 0, 0, 0 global coordinate .

• For the examples shown above , a single, perfectly vertical column was selected to make it easier for you to interpret the results. The point selected for Possibility 1 was the top EXPT of the column. Since the EXPT shares the same X and Y global coordinates as the center of mass of the column, the center of mass relative to that point is a difference in elevation of -4-11 7/8 .
• The center of mass is the point at the gravimetric center of the members you selected. The numbers in parentheses are global coordinates (X, Y, Z) expressed in the primary dimension " Units ."
• The total weight is the actual shipping weight of all materials that make up the members you selected, including shop bolts. The weight of the materials is calculated with cut-off material (such as copes and holes) removed. The weight is reported in kilograms if the primary dimensioning " Units " are ' Metric '. The weight is reported in pounds if the primary dimensioning " Units " are ' Imperial.... '
• Press the " OK " button to dismiss the dialog.
• The following fields are available in the Report Writer for obtaining data on individual members:

Report Writer: Member.Weight
Report Writer: Member.CenterOfMassX
Report Writer: Member.CenterOfMassY
Report Writer: Member.CenterOfMassZ

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   Script to put construction lines through the center of mass :

This applies only if you did Alternative 2 in step 4 : When the ok dialog appears for step 5, possibility 2 , write down the coordinates that are reported as the center of mass with respect to the 0, 0, 0 point. Then Run this script and enter those coordinates in the appropriate fields on the parametric dialog. The script will place a pair of construction lines through the point. The defaults in the dialog are the coordinates of the point reported for possibility 2 above (95-0, 20-0, 124-1 5/8).

# Adds construction lines through a point. . . .          # insert startup code # user enters global coordinates of a point in the dialog dlg1 = Dialog( "Enter Global Coordinates" ) dlg1.entry('xdim', "95-0", "X dimension") dlg1.entry('ydim', "20-0", "Y dimension") dlg1.entry('zdim', "102-8 1/8", "Z dimension") dd = dlg1.done() # Dialog end # adds a vertical construction line through the point cl2 = ConsLine() cl2.Point1 = (dlg1.xdim, dlg1.ydim, dlg1.zdim) cl2.Rotation = 90 cl2.PenNumber = "Blue" cl2.Add() # construction line end # adds a horizontal construction line through the point cl3 = ConsLine() cl3.Point1 = (dlg1.xdim, dlg1.ydim, dlg1.zdim) cl3.Rotation = 0 cl3.PenNumber = "Blue" cl3.Add() # construction line end

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