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Re: Time History Scaling

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On Jan 30, 2008, at 6:59 AM, David Francis Caballero wrote:

I'm very curious with thism thing, and i don't have any idea how complicated it is.
If you're talking about time history seismic analysis, it's very complicated from determining the loading all the way through to figuring out which load step is actually the critical one. If you really don't know how complicated it is, you should start out very simple with textbook problems whose results you know and try to duplicate the results. Or better yet, avoid it altogether, until you've done some in-depth study.

If this is a practical problem you're trying to solve for a client, your best bet is to give him back his retainer and admit you got in over your head. You're not going to be able to produce usable results in a reasonable time. If it's homework for a continuing ed course, go see your professor and get help. If you're doing it for cultural value, start in with learning some basics and get into it slowly. Structural dynamics isn't totally rocket science, but you must know your first principles, or you're wasting your time.

 Can anyone teach how to do this stuff?
If you know enough about structural dynamics, you can teach yourself, but you have to start out slow with easy problems. If you don't know your first principles, find yourself some books and start in. My favorites are _Structural Dynamics for the Practising Engineer_ by H.M. Irvine and _Vibration Problems in Engineering_ by Timoshenko, Young and Weaver.

You haven't mentioned the software you're using or the problems you're trying to solve--the approach really depends on both. There are several approaches in general, and all software packages tend to approach them differently. If your problem is linear, it's generally easier to solve since there are fewer convergence issues.

What parametersa nd/or  are needed?
Depends on the problems and the software. Loading is one of the biggest issues, so you'll need to know about response spectra and how they relate to the earthquake motion. Chances are if you're doing time domain work, you'll need to generate a synthetic earthquake record which matches some given response spectra. There's software which does that, but it's not for the faint-hearted.

What is the best guide in doing it?
First principles and experience, a willingness to experiment and the ability to troubleshoot and validate computer results. Start learning with simple linear problems with simple dynamic response you can get your arms around. Non-linear problems involving uplift or sliding get real complicated real fast because of convergence issues.

What formula i should study.
You don't study formulas. Study principles, then learn how the math quantifies the principles. Study what's behind simple harmonic motion and dynamic equilibrium and learn how to draw free-body diagrams. When you know this you can start writing equilibrium equations and solving them. Don't even boot up FEA software until you understand the basics.


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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