Re: dynamic stress analysis .

On Dec 27, 2004, at 10:54 PM, gautam wayse wrote:

> if some one working on dynamic analysis and having
> expertise in same .please solve my fallowing problems
You're going to have to solve your own fallowing problems, but I can tell you a little bit about dynamics. The difference between dynamic and static loading is the presence of inertia loading and damping. If you go back to your mechanics text, you'll see that a static problem involves setting the sum of the forces equal to zero and calculating the loading necessary to make it so. For dynamic problems the sum of the external forces equals the mass times the acceleration plus any damping forces. Dynamic loading is sometimes applied as an equivalent static load equal to the static loading, such as the weight of he structure times a dynamic amplification factor.

Depending on a great many things, time varying loads require dynamic analysis. For example a mass dropped suddenly on a beam produces twice the stress or more than the same mass placed slowly. Modal analysis is needed because the dynamic response in characterized my the natural frequencies and mode shapes of the structure, which are much different than the static displaced shape. If you have a non-linear problem, you may need to use time-history methods to figure the response because non-linear structures don't actually have mode shapes.

Christopher Wright P.E. |"They couldn't hit an elephant at chrisw@skypoint.com | this distance" (last words of Gen.

...................................| John Sedgwick, Spotsylvania 1864)