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RE: EQ resistant Design

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Chris,

One potential problem with the approach of only apply loads in the buildings
weakest direction:  What is the building is weakest from east to west... one
would design the building for an e-w seismic event... theoretically now what
would happen now if the ground motion turned out to be north-south...  since
the building was not designed for north-south seismic loads then it may be
severally inadequate...

Yes there will be cases where non-orthogonal seismic events would control a
building design (vs. separate E-W and N-S designs) especially if both
restraint systems share common members.

Unfortunately the same reasons that you would state for doing a
non-orthogonal design also affect orthogonal designs.  Just because the
building can take the loadings at 40 or 45 degrees does not prove that the
building can take the loads at other angles or even orthogonally.

HTH,
Greg Effland, P.E.
KC, MO USA




-----Original Message-----
From: Christopher Wright [mailto:chrisw(--nospam--at)skypoint.com]
Sent: Tuesday, March 19, 2002 10:51 PM
To: SEAOC Newsletter
Subject: Re: EQ resistant Design


>Yes.  Under most codes in the US, you analyze the building separately in
>each orthagonal direction.
> you are infact achieving a building that has a lateral
>system that is capable of 100% in one direction AND 100% in the other
>direction at the same time
This sounds like a contradiction. How does assessing each load separately
prove that the structure can take both loads simultaneously?

>As you state, the result is that you are in
>effect designing the building for about 142% of the seismic loading if it
>were applied at a 45 degree angle to the primary orthogonal directions.
This would be the equivalent of a simultaneous application of 100% of the
load in one direction (the 'primary' direction, say) and simultaneous
application of the load applied normal to the primary direction.

I'm trying to make an association with nuke plant seismic analysis, where
we'll typically apply horizontal response spectra simultaneously in
orthogonal directions (or have separate EW and NS spectra) and combine
the directional responses on a RSS basis. I'd always assumed that the
horizontal ground motion is pretty much unidirectional but that EW and NS
spectra are found by resolving the ground motion along site EW and NS
directions for convenience in application.

That leads me to believe that building seismic analysis should  be done
applying the given seismic lateral load or acceleration in a 'worst case'
orientation found by iteration or by applying the load is the building's
'weak direction.'

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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