Designing for the Worst - Modal Frequency in Autodesk Fusion 360

by James Herzing23 May 2016

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Whether you’re designing for earthquakes or giant lizards, wolves and apes, it is imperative that you understand the natural frequencies of your structure. The same can be said for anything that has a motor attached undergoing constant vibration loads, but I don’t see George pushing a lawn mower, so let’s stay focused on buildings for today.

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We’re starting off in Fusion 360 where we have modeled the bank building from the first level of Rampage on Nintendo (which does NOT include the wolf…fyi). If you want to grab this model and follow along, you can download it on the Fusion Gallery with the rest of my 8-bit simulation models. With the building constructed, we have to move from the model environment to the simulation environment by clicking on the left most button on the ribbon and choosing simulation.

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Once you get into the simulation environment you’ll have to start a new analysis and define the analysis type. Fortunately there aren’t a lot to choose from at the moment, so Modal Frequencies is an easy choice. Depending on your needs, you might need to go a step further and do a random vibration, frequency response or some other dynamics analysis, but this will be sufficient for determining the natural frequencies of our structure.

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You’ll really only have to define a couple of things to be ready for your first analysis. The first item on the list is to define materials for the building. This bank looks like an old limestone building to me, so press the select all button and change your material to limestone, and then click OK.

After this you need to define your boundary conditions. Rotate the building around so you can see the bottom, choose Constraint and then apply fixed boundary conditions to the base of the building. Click on automatic contact generation and you’re all set to solve your analysis.

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And just like that, we have our first results. As you can see, by default Fusion 360 provides us with 8 natural frequencies and mode shapes. In this analysis, the frequencies range between 2.37 Hz and 7.455 Hz, and the mode shapes are all over the place as you would expect. Check out 4 to see the floors bounce, and 8 to see some crazy torsion activity.

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Now what about that giant ape climbing up the side of the bank? Does he factor in at all?  Let’s use a Point Mass under the Loads button to represent our friend George. You are easily able to move the mass to the slide of the building by dragging the arrows around it, and Geometries selected you can pick the side of the building that he is hanging on. Assuming he weights around 200,000 kg, press OK and you’re all set to analyze once again.

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With just that one small, giant ape sized change, you can see that the results have shifted to between 2.346Hz and 7.39Hz. It might not seem like much, but on a building of this size it is really rather dramatic! It also represents how the natural frequencies of your designs could be altered by changes you make, whether that is removing material or addition additional stiffness.  It also might say a little bit about why punching holes in the side of the bank causes it to collapse…

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