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RE: EQ resistant Design

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Think about a square building with a single braced frame on each side, all
equal.  The frames are designed to handle 100% of the load in their
respective orthogonal direction (say V = 20 kips, or  10 kip / frame to make
the numbers easy).

Now apply 100% of the load at a 45 deg. angle.  It is true that the frames
are now only 70.7% effective because of the angle, but it is also true that
you now have four frames providing resistance instead of two.  4 *  0.707 *
10 = 28.3 kips capacity compared with 20 kips force (141% capacity).

This is a very basic scenario; a funny shaped building (i.e., one designed
by an architect) might need to be checked in non-orthogonal directions.

On this same topic, go back to the building above.  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.  Mathematically, the
column load is the same as when the EQ forces are applied orthogonally
(resistance = force = 100%).  The problem is that now the frames have a
capacity of 141%, while the column only has a capacity of 100%.  This makes
the column the weak point, which in generally considered bad.  I thought the
code addressed this issue, but after a quick look I couldn't find it in IBC,
UBC or AISC.

-----
Jason W. Kilgore, P.E.
Leigh & O'Kane, L.L.C.
(816) 444-3144
jkilgore(--nospam--at)leok.com



-----Original Message-----
From: Christopher Wright [mailto:chrisw(--nospam--at)skypoint.com]
Sent: Tuesday, March 19, 2002 12:43 PM
To: SEAOC Newsletter
Subject: Re: EQ resistant Design


>This would mean that you
>would analyze the brace frame system for 100% of the seismic load (since
>it is taking ALL of the seismic load in one direction) and likewise you
>would analyze the moment frame for 100% of the seismic load.
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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