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# Re: Displaced Center of Mass Question

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• To: <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 20:50:21 -0500

```     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
>

```