# Calculate center of gravity (mass)

The only possibility to calculate the center of gravity is by means of the resultant of reactions & the position of the central point.

Then the resultant internal forces can be calculated back to find the center of gravity.

**STEP 1: create 3 load cases**

Create 3 load cases: 'Self Weight +X', 'Self Weight +Y' and 'Self Weight +Z'. The figure below shows the settings for load case 'Self Weight +X'. The other load cases are set similarly.

**STEP 2: calculation**

Execute the linear calculation.

**STEP 3: results**

Look at the result 'Resultant of reactions' (Main menu > Results > Supports) for the 3 created load cases.

The figure below shows the result for load case 'Self Weight +X'

You also need the central point. The coordinates of this point can be found as well in the results table (if this is not visible, you can right-click in the bar and choose for 'Add all columns').

You can copy these results from the results table immediately to MS Excel.

**STEP 4: calculate the center of gravity**

The equations for a 3D structure in the center of gravity are:

- Mx = Rz*dY - Ry*dZ
- My = Rx*dZ - Rz*dX
- Mz = Ry*dX - Rx*dY

For load case 'Self Weight +X' (Ry and Rz are 0) this becomes:

- My = Rx*dZ => dZ = My/Rx
- Mz = -Rx*dY => dY = -Mz/Rx

For 'Self Weight +Y' (Rx and Rz are 0) this becomes:

- Mx = -Ry*dZ => dZ = -Mx/Ry
- Mz = Ry*dX => dX = Mz/Ry

For 'Self Weight +Z' (Rx and Ry are 0) this becomes:

- Mx = Rz*dY => dY = Mx/Rz
- My = -Rz*dX => dX = -My/Rz

Calculate the center of gravity (Xc, Yc, Zc) with:

- Xc = X + dX;
- Yc = Y + dY
- Zc = Z + dZ

(X, Y, Z) are the coordinates of the central point.

Following Excel-file calculates the point of gravity automatically with the formulas that are described above: