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# Re: Dynamic Analysis Question / Visual Analysis

• To: <seaint(--nospam--at)seaint.org>
• Subject: Re: Dynamic Analysis Question / Visual Analysis
• From: Christopher Wright <chrisw(--nospam--at)skypoint.com>
• Date: Fri, 2 Sep 2005 10:29:39 -0500

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On Sep 2, 2005, at 8:00 AM, refugio rochin wrote:

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It is about the properties that can be attributed to a node.  In Visual Analysis it allows
``` a rotational mass to be attributed, with lb-ft-s^2 / deg as units.

```
What would this mean?  It has been along time since dynamics course in BS. studies. Anyone able to apply this in dynamic analysis?  How would it have an effect? Do we not also have to account for 5% accidental torsion in dynamics?  I assume
``` this could be useful here if necessary.
```
The 'rotational mass' is the mass moment of inertia. From your basic physics, the torque required to change the angular velocity of an object equals the mass moment of inertia times the angular acceleration. The mass moment of inertia of an object is the sum of the mass of each particle times the square of its distance from the reference axis. The dimensions are mass-length^2 or force-length-time^2, which is the same thing. The units you've cited lb-ft-s^2 / deg aren't rotational inertia--you might want to check your program documentation carefully fro that.
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Most of the time you won't need to add a lumped moment of inertia, but it happens. I don't know about Visual Analysis, but all the other FEA software I've ever used calculates rotational modes just like it calculates translational response, so if you do have any eccentricity it will appear in the results. I think the 5% offset needs to be applied when you have a nominally symmetric structure. You won't really know the CG in such a case and it's likely to be offset about 5% from the symmetry planes. The possibility of an offset needs to be accounted for.
```Christopher Wright P.E. |"They couldn't hit an elephant at
chrisw(--nospam--at)skypoint.com   | this distance" (last words of Gen.
```
.......................................| John Sedgwick, Spotsylvania 1864)
```http://www.skypoint.com/~chrisw/

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