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Re: EQ resistant Design

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Yes.  Under most codes in the US, you analyze the building separately in
each orthagonal direction.  There are provisions in the code for higher
seismic conditions (i.e. SPCs or SDCs of C or "greater if I recall
correctly) where you would analyze for 100% of the seismic load in one
othrogonal direction PLUS 30% of the seismic load in the other direction,
but I did not go into this so as to not complicate the issue further.

Thus, the end result is that even though you are designing the building to
take 100% of the seismic load in one direction without seismic load in the
other 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 (i.e. there is no way to "switch off" the
lateral system in the other direction just because the load may not be
coming from that direction).  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.

So, the result is that if a seismic event imparts load on a building in a
non-orthogonal manner, then the building should be fine (assuming that you
don't have something like columns that are shared by both orthogonal
lateral systems).

Of course, it is purdent many times to do more that what the code requires
as was true for the Citicorp building in NY (i.e. the check of wind
loading in a non-orthogonal direction).

HTH,

Scott
Ypsilanti, MI

On Tue, 19 Mar 2002, Christopher Wright wrote:

> Doesn't this imply that the structure was resisting a total seismic load
> of 1.414 times the design seismic loading at an orientation of 45 deg? If
> you meant that the seismic loading is to be applied separately in each of
> the two directions, is it also implied that seismic loading is to be
> applied in the 'worst' direction which might not in either the 'braced
> frame' or 'moment frame' direction, but at an angle to both.
>
> 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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