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# Re: Displaced Center of Mass Question

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• To: "Greg Smith" <strusup(--nospam--at)gte.net>, <seaint(--nospam--at)seaint.org>
• Subject: Re: Displaced Center of Mass Question
• From: "Greg Smith" <strusup(--nospam--at)gte.net>
• Date: Thu, 29 Jul 1999 21:40:34 -0500

```     The book by Breyer - Design of wood structures addresses "seismic
irregularity on pgs.785-794.  Re-thinking my statement on the eccentricity
defining an axis when the eccentricity is a scalar measure of how much the
structure will rotate (I can visualize what I am saying but sometimes I get
ahead of myself and put my foot in it wordwise), anyway, to explain what I
am saying in a direction away from what is leading to the center of
rotation, a very applicable analytical method would be the ellipse of
inertia which leads to the ellipse of elasticity with elastic centers and
instantaneous center.  This method orthogonalizes the oblique axis that I
mentioned which, unproven by me, relates the COM, CORig and CORotation
(instantaneous center).  The method is as powerful as any FE method and is
well suited for arches.

Greg
-----Original Message-----
From: Greg Smith <strusup(--nospam--at)gte.net>
To: seaint(--nospam--at)seaint.org <seaint(--nospam--at)seaint.org>
Date: Thursday, July 29, 1999 8:54 PM
Subject: Re: Displaced Center of Mass Question

>--------------------------------------------------------------------------
>Your following message has been delivered to the list
>  seaint(--nospam--at)seaint.org at 18:57:38 on 29 Jul 1999.
>--------------------------------------------------------------------------
>
>
>     I think that each portion of a multi-wing structure will undergo a
>rigid body movement PLUS the torsional displacement of the whole from
>eccentricity.  This eccentricity has axis and the structure has axis and
the
>amount of displacement of any part is a parametric function of : the
>direction of the earthquake on the two oblique axis.
>     The torsion is typically accounted for in shear wall design but the
>rigid body forces can be from any angle the normal yielding the most
>displacement assuming that wood stud walls are braced in their longitudinal
>direction and or designed for shear in that direction.
>     When an event happens, their will be many different periods all
>interacting where the "wings" are connected like the pistons in a radial
>engine except not likely to be in order.  If you could Williot diagram this
>and brace each wing to a central point of displacement and control the
>torsion at that point then you would have a stable structure.  Sounds like
a
>job for some very powerful software.
>
>Greg
>-----Original Message-----
>From: Seaintonln(--nospam--at)aol.com <Seaintonln(--nospam--at)aol.com>
>To: seaint(--nospam--at)seaint.org <seaint(--nospam--at)seaint.org>
>Date: Thursday, July 29, 1999 6:09 PM
>Subject: Displaced Center of Mass Question
>
>
>>The design examples from the Feb 1998 SEAOC Wood Seminar for the 97 UBC
>(and
>>the draft of the ICBO Design Manual Volume II) note "by displacing the
>center
>>of mass by 5% can result in the C.M. being on either side of the C.R. and
>can
>>produce added torsional shears in all walls."
>>
>>However, the Reinforced Masonry Engineering Handbook by James Amrhein (5th
>>edition 1994 UBC compliant) simply adds the 5% of the diaphragm depth
>>perpendicular to the direction of loading and adds it to the difference
>>between the C.M. and C.R.. - leaving the displaced C.M. in only one
>location.
>>
>>Which is considered the standard of practice in rigid diaprhagm analysis?
>If
>>the more involved method is applied in wood construction, has anyone been
>>able to calculate a significant difference in added shear from torsion?
If
>>so, how many of these buildings were residential (single and mulitple
>>residential) etc.
>>
>>Dennis Wish PE
>>
>
>

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