# Assigning densities in complex assembly

Hello,

I have a large assembly with ~1000 components and need to determine the center of mass.  Most of these components are steel, but a few large pieces are wood. If I treat them as steel I am concerned the center of mass will be far from accurate. How can I apply separate densities to different elements to be considered in the mass properties window?

Thanks,

Alex

• Not sure if you can apply "density" property to elements, as it seems the "Mass per Volume" is a tool setting rather than an element's property.

One work around I can think of, since you only mention 2 types of materials, is to get the centre of mass for each type of material, then manually work out the centre of mass of the combined centres of masses.
• Microstation doesn't support Density of different materials (yet - hint hint developer...) till then the only solution to find the center of mass of complex geometry is to tool the system into thinking it's all the same.

You've said most of the parts are steel so roughly 490 lb/cuft and you have some wood? Lets assume pine for the sake of easy math and i'm going to go with a avg of 26 lb/cuft which would be roughly that of white pine. The difference in density between these two objects is... 490/26 = 18.846.

First make a copy of your entire model and call it something else like CofG model. One at a time grab the solid that you have the least off (in this case wood) and proportionally scale them down about their own CofG by 1/18.846 or 0.05306. Now hopefully you didn't have to do this 100 times or if you did there were enough common "same sized" parts that you could easily cut your work down by copying them over and over.

Now you've scaled down all the wood to be essentially equal to that of the steel in terms of mass and the center of gravity of each element has not moved so you can select everything and find the CofG of it all.

If you have to change things in the future just document which items changed then copy those into your calculation model and re-do the steps above as required.

I've done this more times then I can count tho normally because i'm mixing steel and Aluminum or some other metal. I'm willing to bet if the "wood" portion of your project isn't significant you may find that it has a negligible effect on the CofG outcome, I would suggest you find the CofG with the wood included and the wood left out both to see how much effect it has. Keep in mind real world un-foreseen things will have an effect on this as well such as fasteners, welds, un-modeled hardware, fillets and features left out of the model to keep things simple. Often these are ignored with little or no negative effect.
• Not sure if this is going to help at all. It is for Bentley View but the Measure tools would be the same for MicroStation.

communities.bentley.com/.../7014

As you are saying there are both steel and wood this simple method might not work as you are wanting.

Regards
Andrew Bell
Technical Support
Bentley Systems