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RE: EQ resistant Design

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>Imagine that the frames
>on two sides are at the corner and "share" the corner column.  If the loads
>are applied orthogonally, everything is fine.  If applied at 45 deg.,
>however, the column now has forces from both frames.
I don't think I'd've put it quite this way, but I believe that's the
point I was making. There's another factor when ultimate loading is
considered. A frame which goes plastic from loading in one direction is
plastic for loading in all directions, so if the failure mode is yielding
instead of brittle fracture in a botched weld, it makes things a little
more complicated. I think, using LRFD-speak, the demand stays the same in
the stronger direction but the resistance drops.

There's some mitigation in actual service because the actual building
response us dynamic with different frequencies in the 'principal' and
normal directions. Consequently the two motions won't be in phase except
if the stiffnesses are within 10-15% of each other. With a true dynamic
analysis the responses from orthogonal excitation combine on a RSS basis
to reflect the fact that each response peaks at different instants. The
fact remains that loading in both directions should be considered
simultaneously.



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)
http://www.skypoint.com/~chrisw


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