From: Christopher Wright <chrisw(--nospam--at)skypoint.com>
Date: Tue, 15 Feb 00 15:05:03 -0600
>first mode weight, which would be on the order of 0.7W" (under "Dynamic
>Response Modification"). What is the basis for this statement?
If the Blue Book is using the term properly (actually it should be
effective mass) the difference between the effective and actual masses
reflects the difference between a simple SDOF system with all the mass
moving at the same speed, and a distributed system where the movement
varies from zero at ground level to a maximum at the top. It's an
artifice which allows the balance between kinetic and potential energy to
be drawn correctly using a more accurate distribution of inertial forces.
The 0.7 factor applies to a cantilever with a more or less uniform mass
distribution, and would be higher for a structure like a water tower
where most of the mass is at the free end, and lower for a structure
where most of the mass is near the ground.
Christopher Wright P.E. |"They couldn't hit an elephant from
chrisw(--nospam--at)skypoint.com | this distance" (last words of Gen.
___________________________| John Sedgwick, Spotsylvania 1864)