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Re: Is it a shearwall or not??? Need some help

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Dennis:

Sorry for this late contribution; I am only just now reviewing the mail
accumulated in the past week.

My humble input: don't forget to check for an existing foundation below this
existing interior wall. The existence - or non-existence - of a continuous
concrete footing below the wall can definitely resolve the original intent
of that wall. In any case, you will need a continuous footing to justify
considering this wall for lateral restraint. It would not make sense to
require a shear panel where there is no foundation. A little site
investigation might be the ticket on this one. (Like, been there, done
that.)

-Richard Flower, P. E.
RLFlower(--nospam--at)worldnet.att.net

----- Original Message -----
From: "Dennis Wish" <dennis.wish(--nospam--at)gte.net>
To: "Nicholas Blackburn" <nblackburn(--nospam--at)fdgoak.com>
Cc: "SEAINT Listservice" <seaint(--nospam--at)seaint.org>; <aec-residential(--nospam--at)polhemus.cc>
Sent: Thursday, November 02, 2000 5:54 PM
Subject: RE: Is it a shearwall or not??? Need some help


> Nick,
> Thanks for the reply. This was my point - after reviewing the existing
> "system" I don't believe I am removing shear. Starting with the footprint
of
> the building, the diaphragms are almost square - and not overly large (38'
> by 45'). A plywood diaphragm would certainly have the capacity to transfer
> shear to the perimeter in the strong direction and at each level. The
> weakness of the existing interior materials indicates that is highly
> unlikely that they would have been design - by flexible or tributary
> distribution - to accommodate 50% of the diaphragm shear at each level.
This
> is especially true at the first level since there is less than 30% of the
> shear capacity of the second floor wall alone. I make this assumption that
> the wall is shorter in length than the upper level wall and only sheathed
on
> one side with gypsum while the other side is a non-rated paneling.
>
> Digging into the problem a little further, the area of concern is on the
> first floor exterior shear elements since the floor cantilevers two
interior
> concrete or masonry shearwalls to created an 8-foot deep display window.
The
> significant weakness in the interior stud partition would indicated in my
> opinion, that the floor diaphragm has transferred the shear from the roof
> and second floor into the masonry or concrete piers at each end of the
> building and not into this stud wall.
>
> With this in mind, the issue is whether or not the second floor exterior
> walls were designed to take the full tributary load of the roof diaphragm.
> The roof diaphragm has the capacity to transfer the lateral roof load to
the
> exterior masonry or concrete walls by its stiffness. The length and
solidity
> of the second floor exterior walls appears sufficient to accommodate the
> shear demand from the roof.
>
> Balancing all of these observations, I can't see how the second floor
gypsum
> wall would have originally been designed to absorb shear from the roof. If
> the framing had been in the other direction, there would be no question.
The
> fact that it is a bearing wall is what complicates the issue as to what it
> was originally intended to be designed for.
>
> Still, the facts are overwhelming - the first floor wall is unable to meet
> the demand created by the  accumulation of shear through the roof and
second
> floor wall to carry the shear down into the foundation. At the very most,
> the second floor wall would have transferred roof shear into the second
> floor diaphragm which would have distributed it to the exterior shear
> element.
>
> Dave Merrick suggested I replace the shear removed and I think this is the
> best solution. I will not try to carry shear down to the foundation since
it
> is obvious that it was not designed originally for the capacity to carry
> shear from the roof to the foundation. I will assume that the shear may
have
> been brought to the second floor diaphragm through the wall and it is
> distributed through the more rigid second floor diaphragm to the exterior
> shear walls. This assumption would not present any more or less shear from
> the upper story than was in the design before I touched it.
>
> I will calculate the lateral load based on a tributary analysis from the
> roof to the second floor diaphragm using an 18.7% base shear as would have
> been appropriate prior to 1997. I have a strong feeling that the resulting
> shear in the second story wall will be sufficiently low and, with the dead
> load of the roof above, not create any major problem with uplift on the
> remaining panels (remember the roof contributes an 18' tributary dead load
> to the interior wall.
>
> Comments?????
>
> Regards,
> Dennis S. Wish, PE
> Structural Engineering Consultant
> mailto:structures(--nospam--at)engineer.com
> (208) 361-5447 E-Fax
> ICQ #95561393
>
>
> -----Original Message-----
> From: Nicholas Blackburn [mailto:nblackburn(--nospam--at)fdgoak.com]
> Sent: Thursday, November 02, 2000 3:43 PM
> To: 'seaint(--nospam--at)seaint.org'
> Cc: 'admin(--nospam--at)structuralist.net'
> Subject: RE: Is it a shearwall or not??? Need some help
>
>
> What about the ability of the floor and roof diaphragms to resist/transmit
> the forces that should be applied to each wall.  A review of the floor and
> roof systems may provide answers to what the original load path was and
lead
> to a determination of the adequacy (or lack thereof) of the walls.
>
> Since you are reducing the shear capacity of the wall and therefore its
> rigidity you should do a lateral distribution analysis to see what effect
> this change in rigidity makes to the presumed load path (unless you have a
> flexible diaphragm although I thought that the 97 UBC removed that
> distinction).  I would be wary of drastically changing the shear load path
> of a fifty year old building that has been modified (maybe) at unknown
times
> and with unknown care.
>
> Good luck
>
> Nick
>
>