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# Re: Dynamic analysis

• To: <seaint(--nospam--at)seaint.org>
• Subject: Re: Dynamic analysis
• From: Christopher Wright <chrisw(--nospam--at)skypoint.com>
• Date: Mon, 20 Dec 2004 13:09:28 -0600

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On Dec 20, 2004, at 11:49 AM, grupocg(--nospam--at)cantv.net wrote:

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If the structure is well modelled, in a building, the first two modes are translational modes and the third mode is a rotational mode.
You want to be careful about this. It's true that if you have a regular structure uniformly loaded the first couple of modes are usually ordinary cantilever modes, but that's not necessarily true. If the planform isn't symmetric or the structure stiffness isn't symmetric or the mass including the mass of the building's contents isn't placed symmetrically, you'll end up with some combined torsional and bending modes. The effects make for odd-looking mode shapes but fairly high modal mass. Make sure your boundary conditions are correctly modeled, too. As Alberto mentioned, trash modes can usually result from the odd cantilever member with too much mass attributed. Trash modes may signal an improperly modeled detail or a member with a poor choice of moment of inertia. It's always a good idea to figure out what's causing any anomaly.
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If by 90% participation, you mean the the cumulative modal mass from all 20 modes is 90% of the 'sprung' mass that isn't unusual, but if you mean that the modal mass of the 20th mode alone is 20% of the sprung mass, you ought to look at the model very carefully. It's hard to comment without a little more detail. That much mass at the 20th mode means that you have 19 frequencies occurring at frequencies below the most energetic mode, which is a pretty good indication that you have a lot of little pieces floating around that are either too soft, too massive or not properly connected.
```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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