Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

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

[Subject Prev][Subject Next][Thread Prev][Thread Next]
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