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Re: Dynamic Analysis

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Jake's advice of looking at the model carefully and verifying by equivalent static methods is a good one. An additional gut check can be done by creating a simple dynamic model you think captures the basic elements of the response (unfortunately, this takes experience developed by doing analyses and carefully understanding the results). If there are differences with your more complex model, UNDERSTAND WHY before proceeding.
I find young engineers are frequently given powerful tools in the form of modeling software, whether it be for structural analysis, hydraulic networks or whatever but are generally given much too little guidance on understanding the results and how to verify that their model represents the physical situation they are trying to model. They run A MODEL and assume the results are correct without truly understanding what the model is portraying. Remember that a model is simply a mathematical attempt to understand a physical reality. Assumptions are made to simplify the analysis and it is usually these assumptions that make the model be a representation of the physical situation OR NOT. Too often I see people trying to use results they don't understand based on assumptions whose effects they also don't understand.
Christopher Wright has lectured this list for years on getting help from some one experienced in modeling and analysis. That is probably the best advice that can be given.
Bill Cain, S.E.
Berkeley CA
In a message dated 11/22/2006 4:21:47 A.M. Pacific Standard Time, jake.watson1(--nospam--at) writes:
I picked it up under the supervision of two other senior engineers.  The basic concepts are relatively easy.  CSIBerkely has a some great white papers on their website.  If you have never done this before, I strongly suggest your read them.  You can find them at

My two cents for dynamic are the same as for static analysis.  Garbage in, garbage out.  In addition to needing a realistic stiffness model (similar to a static analysis) you also need a realistic mass model.  The few times I have done dynamic models, I started with a static model.  Get that model working then work towards the dynamic model.  The final dynamic results should be confirmed with static results for a gut check.

Best of luck,
Jake Watson, P.E.
Salt Lake City, UT

On 11/21/06, Will Haynes <gtg740p(--nospam--at)> wrote:
How many of you have done a dynamic analysis due to earthquakes?  Did you pick up doing this on your own or did you take a dynamics class? I have not ever had to do structural dynamics but I know it can be very difficult and I am just wondering if it is something you can pick up on your own if you are required to do it. I figure that most people are using software (SAP etc.) to do the majority of the work but I would like to be able to do it all by hand if I had to. Do you think that most engineers that doing earthquake dynamics really know what they are doing or are they just relying on the computer? 

Bill Cain, S.E.
Berkeley CA