On Apr 28, 2005, at 2:36 AM, Tony Paulin wrote:
> When user's continue extracting eigenpairs beyond the range of the
> excitation time scale, and use a modal summation method, i.e. (SRSS,
> DSRSS,
> ABS, etc.) they will find that stresses can increase without bound,
> although
> displacements do not.
This isn't likely. The kinetic energy of higher modes (reflected in the
modal mass) drops off real fast in the higher modes. Even though you
use a dynamic amplification of unity the stresses at these modes are
pretty much negligible. You'll find that SRSS or CQC and most other
modal combination methods make the higher modes vanish inherently in
the arithmetic.
> stop extracting frequencies much beyond the point
> in the DLF curve where the multiplication factor returns to 1.0, and
> to make
> sure that in all cases the system is suitable for a static application
> of
> the dynamic load
I don't know about CAESAR but you can account for the contribution of
frequencies above the ZPA cutoff by checking the total dynamic mass
(equivalent mass and modal mass are other terms) agaisnt the total mass
of the structure. If you've included enough modes so that the dynamic
mass is 75% or more of the structural mass (in other words 75% mass
participation), you've got enough modes. In some cases you'll find that
mass is concentrated close to the supports and it's not possible to
obtain 75% mass participation, you can make a somewhat phony correction
by imposing a static acceleration to include the acceleration on the
'missing' mass.
Christopher Wright P.E. |"They couldn't hit an elephant at <a href="/group/PipingDesign/post?postID=O9OZd6NAXVE-VFg2JDPVmSp--zDYnFrrCBX0Se-lslrdugY751vt7LH0nhwi67XSbKMdqql7x6MQlA">chrisw@skypoint.com</a> | this distance" (last words of Gen.
.......................................| John Sedgwick, Spotsylvania1864)
This archive was generated by hypermail 2.1.8 : Tue Mar 04 2008 - 11:40:41 EST