Re: dynamic loading Factor

From: <Christopher>
Date: Thu Apr 28 2005 - 12:37:00 EDT

On Apr 28, 2005, at 10:54 AM, Tony Paulin wrote:

> Displacements are certainly
> lower as a function of the square of the frequency but stresses,
> because of
> the small radius of curvature associated with higher modes continue to
> add.
> This has been a problem with the approach used in CAESAR since 1987,
I'll defer to your judgment with CAESAR, but I've been doing dynamic analysis with ANSYS and COSMOS/M for about 25 years and I've never seen a problem diverge like that by increasing the number of modes. The mode coefficients all drop off at frequencies higher than the ZPA level. The accelerations remain constant at the ZPA and the modal mass drops off to zero, so the mode coefficients become vanishingly small. Stress certainly increases at with increasing number of modes considered at first, but after 4 or 5 modes it invariably (in my experience) converges as the mass participation gets up past 50%

What radius of curvature are you referring to? If you plot out mode shapes it may appear that you get increasing curvature, but once you apply the mode coefficients to get actual displacements, the displacements and radii of curvature drop off. I will try your problem, though. What kind of impact load do you use? Seismic loading cuts out at 33 Hz, but actual impacts have frequency content all over the map. I can do either.

Christopher Wright P.E. |"They couldn't hit an elephant at <a href="/group/PipingDesign/post?postID=WrxVMzfi1fR7WcseZCJIkoper-KctOikwaEd1sFtvrtyiLkHWlSi4ULIPgsCtSxdmShnUa5uQ0dI">chrisw@skypoint.com</a> | this distance" (last words of Gen.

.......................................| John Sedgwick, Spotsylvania

1864)
<a href="http://www.skypoint.com/~chrisw/">http://www.skypoint.com/~chrisw/</a> Received on Thu Apr 28 12:37:00 2005