# Structure analysis

I am currently designing a setup for an experiment which involves loading a weight to its extension arm, and by design this structure is not bolted to any surface so its free to move. I need to simulate whether this structure will tip over because of the torque caused by this weight. I looked at some FEA softwares such as patran but I can only get static stress analysis, what kind of technique/software should I use to simulate this kind of motion in computer.

• You have to verify the center of mass is located within your structure base limits. This is a much easier task than calculating stresses with a FEA tool. Jul 21, 2019 at 18:40
• I can determine the result with pen and paper using center of mass as you said I just want to learn about how to make a computer calculate and visualize this problem, and learn what kind of analysis is this called. Jul 21, 2019 at 18:46
• @YanivBenDavid If there is any dynamics involved in the system, finding the position of the COM (even allowing for the fact that it moves) is not necessarily the complete answer to the problem. Jul 21, 2019 at 20:43
• The easy way to make a model is restrain the base and look at the reaction forces. If they are in tension not compression, the block will "lift off" at those points. Jul 21, 2019 at 20:45
• @alephzero: And then remove the supports "under tension" and run the model again. Check the remaining supports for tension. Repeat until you either reach a stable configuration (no supports under tension, meaning the load wouldn't tip over the structure) or you remove all the supports, which implies you probably tipped the structure over.
– Wasabi
Jul 22, 2019 at 2:52

As @kamran shows, you don't really need software for this - it's a fairly simple calculation.

However, if you wish to use a computer, I would select all the parts you have and look where the center of gravity for that combined object falls, laterally.

If it is within the base's reach, your setup stands. If it is outside, it falls.

In Fusion 360, you find the center of mass under Inspect > Center of Mass. Do realize that you must set the material values of the bodies right - that should be obvious.

See: Center of mass (Fusion 360 documents)

• N.B. You need to be sure that you have set the correct material properties for this to work Jul 22, 2019 at 10:07
• You’re right - I somehow skipped right over that when reading. - my bad. Jul 22, 2019 at 13:14

Let's call the mass of weight W and the mass of base B, the arm length extending outside of the base, L and the base width D.

Let's assume 3/4 of mass of horizontal arm called A1 is outside of the base and the rest called A2 is over the base. And call the mass of the mast Ma.

Now all we have to do is check the balance of this machine about the edge of the base.

Let's assume the right edge of base D lies on the origin and the left side of the base on negative side of x axis.

$$IF \ M_{origin} = [B*D/2+ Ma*D/2+(A2*D/4) -\ [W*L +A1*3/8L] >0$$ the moment is counterclockwise and the object is stable, if not it will tip over.

@kamran described how to analytically find the location of the C.G in your case. However, according to your comments I understand you want to play a little and gain experience with engineering programs. Am I Right?

Following @alephzero and @ Wasabi♦ comments, here you can find a way for implementing their approach on ANSYS WORKBENCH:

A simplified model was created - containing two structures having different geometries. Each of these geometries is a one piece item made of a single material. This was done just for simplicity, of course you can apply the same approach on any super complicated assembly. To make the examination even easier each item was simply supported to the ground as shown in the following screenshot:

Now, what you would like to do is to examine the support reactions. As long as the center of gravity is located between the two pin joints (no tip over), the two reactions will push the structure upwards. This is the case of the left scenario as shown on the following screenshot. In case the center of gravity location falls out of the lower base limits, one of the reactions will be directed downwards - which means a tip over in your case. This is illustrated on the right side of the following screenshot:

As already discussed, Using a FEA tool just for analyzing the center of gravity location is quite a wasteful procedure. Generally, examining the reactions is used as a sanity checks for the finite element model.

• Exactly! That's what i want to do. Thanks for great answer. Jul 23, 2019 at 15:16
• No problem. You can download a free copy of ANSYS academic. If you will have further questions regarding using it, feel free to ask... Jul 23, 2019 at 15:36
• @YanivBenDavid, where can I download free student version. thanks. Jul 23, 2019 at 18:14
• Jul 23, 2019 at 18:16