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Re: Calculation of drift in Wood Frame Buildings

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Albert,

Sounds like you have run a foul of the difference between a Seismic
Performance Catergory (from 1996 BOCA code and other code based upon
pre-1997 NEHRP provisions) and a Seismic Design Catergory (from the 2000
IBC and other codes based upopn 1997 or later NEHRP provisions).  That
would be the inclusion of the site soil characteristics in the "equation".

So, it sounds like you may need to do some "edukating" of the architect.
You could do this yourself by explaining what has changed, or you could
lay your hands on an article that should explain it and maybe do it more
pursuasively.  Once such article is "New Model Codes and Seismic Design"
by S.K. Ghosh in the July 2001 Concrete International.  He does a pretty
good discussion of what has changed in the lastest model building codes
and why some areas of the country who were apart of "no seismic" country
will now find themselves with really seismic design issues.  If you can't
get a hold of that article, then I could forward you a short article I did
for our local ASCE newsletter that deals with the same subject, but in a
much abbreviated fashion and with a much less promenent name attached to
the article (i.e. mine).  My article was meant to inform my fellow local
Michigan engineers that "we aren't in Kansas anymore" <grin>.

The end result is that you may just be forced to point out to the
architect that in the past seismic issue were thought of only to a large
degree in California and western states close to California, but that a
national program established by Congress in the 80's (i.e. NEHRP) has led
to us realizing that seismic issues are much more far reaching than we
originally thought.  As a result of this national program, we now realize
that there are potential seismic issues even in areas of low seismic
activity IF a building is located on a site with rather poor soils.

I am afraid that you are running afoul of what more and more engineers
will discover once their areas adopt model codes based on 1997 or later
NEHRP provisions (i.e. 2000 & 2003 IBC and the new NFPA 5000 code).  There
will be a lot of engineers and architects and others in the construction
industry that will have to make some adjustments to their thinking.

HTH,

Scott
Ypsilanti, MI


On Thu, 16 Jan 2003, Albert J. Meyer, Jr. wrote:

> Gerard -
>
> Thanks for your reponse.  This is what I really was thinking that I would
> hear.  I have been doing a significant amount of wood frame design in the
> past five years and most of it has been two, three and four story
> multi-family housing.  For the most part it has been in the Eastern United
> States.
>
> What I try to deal with now is explaining to the architect that I need wood
> panel shear walls throughout the buildings I am designing, even though the
> OTHER buildings, that are four story wood frame buildings right NEXT to the
> spot where we are going to put our buildings didn't seem to need any
> shearwalls or holddowns.
>
> I have a done a number of designs where the building was in area of low
> seismicity (Maryland) and by BOCA 1996 no seismic analysis was required
> (other than ties and continuity considerations) where now per the IBC 2000 I
> need to do a lateral analysis for seismic forces, that really doesn't let me
> use gypsum shear walls like I did when the BOCA 1996 was the governing code.
> The architects and owners just can't seem to deal with this.  I've even told
> a few that if they can get the fire related provisions of the code waived,
> I'll ingore the lateral force provisions! (Just kidding).
>
> I don't mind doing the analyses, but I've had people VE me to death for
> actually doing a lateral force analysis for wind loads!  I can't compete
> with those who wave the magic wand and say that the building is OK for
> lateral forces by inspection.  I think I'm going to go for a new code
> provision to just post a building with signs like "NO LATERAL WIND OR
> SEISMIC FORCES", kind of like the "NO WAKE" signs near the marina and boat
> slips.
>
> Thanks again for your thoughts.
>
> ----- Original Message -----
> From: "Gerard Madden, PE" <gmadden(--nospam--at)attbi.com>
> To: <seaint(--nospam--at)seaint.org>
> Sent: Thursday, January 16, 2003 7:16 PM
> Subject: RE: Calculation of drift in Wood Frame Buildings
>
>
> > Sarcasm ON>
> >
> > What school did you go to? Didn't they teach you this stuff in your
> > Gypsum Engineering Course? This is why a masters degree should be
> > mandatory for structural engineering !!! What do you mean a diaphragm
> > could be flexible, prove it ... I DARE YOU.
> >
> > >OFF with the Sarcasm
> >
> > Al,
> >
> > This is half the complaining I and others have been doing on this list
> > in the past. Your situation is even more of an example of light frame
> > construction code gone awry.
> >
> > Trying to prove a flexible diaphragm is a waste of time and will not be
> > true in your case. The bigger problem you have is determining the
> > rigidity of the GWB shear panels. I supposed you could try to use the
> > same equations for plywood shearwalls with the different properties for
> > GYP instead of OSB or Ply, but I don't know where you would find that.
> >
> > Here is how everyone else has handle the drift calculations in
> > residential buildings ( YAAAWWWWWWWNNNNNNN ) and that is still how it is
> > done for the most part outside of seismic areas. In seismic areas -
> > simplified analysis and you don't have to do the drift calc - about 80%
> > of engineers are doing this if I had to guess.
> >
> > I strongly suggest that you avoid gyp shearwalls if there is a seismic
> > concern. Wind is probably okay. By the way, if you use wood sheathing,
> > most of your problems and questions are still valid. The whole shearwall
> > deflection/rigidity thing is not valid due to the fact that no special
> > inspection is required to make sure the wall is built as designed.
> >
> > HTH,
> > -gerard
> >
> > -----Original Message-----
> > From: Al Meyer [mailto:ameyer(--nospam--at)cagleyharman.com]
> > Sent: Thursday, January 16, 2003 3:25 PM
> > To: SEAINT(--nospam--at)SEAINT.ORG
> > Subject: Calculation of drift in Wood Frame Buildings
> >
> > List -
> >
> > I know that in recent times we are trending away from the use of gypsum
> > sheathed shear walls based on their performance in recent seismic
> > events.
> > The IBC 2000 does, however, provide values for shear resistance for
> > gypsum
> > sheathed shear walls.
> >
> > Essentially if your building is an SDC A you don't have to do a formal
> > lateral analysis of the building, although a minimum force level and
> > connection requirements still must be considered.  When a wood frame
> > building is such that it is in an SDC of B or higher and it does not
> > meet
> > the requirements for a simplified analysis (which allows story drift to
> > be
> > be computed as 1% of the floor to floor height) or the requirements of
> > section 2308, Conventional Light Frame Construction, it must be analyzed
> > as
> > specified by the code.
> >
> > In the past, wood sheathed diaphragms were generally considered
> > flexible,
> > and Breyer's "Design of Wood Structures" textbook even states this.
> > Now, a
> > wood diaphragm may be flexible or rigid depending on its deflection
> > relative
> > to its supporting shear walls.  Equations are available to calculate the
> > deflection of wood panel shear walls, but there is no rational basis
> > (per
> > information I have read in NEHRP documents) for calculating deflection
> > of
> > gypsum sheathed shearwalls.
> >
> > While it seems logical that in many cases a wood diaphragm would seem to
> > be
> > rigid relative to gypsum sheathed shear walls, I'm certain that this
> > isn't
> > always the case.  How has anyone handled this issue other than the
> > obvious
> > route of not using gypsum sheathed shearwalls?  I kn ow that
> > buildings have
> > been constructed all over the country which have used gypsum shear walls
> > but
> > don't know how anyone else has handled drift calculations.  Thanks in
> > advance for any input you can provide.
> >
> > Albert J. Meyer, Jr., P.E.
> > Cagley, Harman and Associates
> > Structural Engineers / Parking Consultants
> > 1015 West Ninth Avenue
> > King of Prussia PA 19406-1222
> > ameyer(--nospam--at)cagleyharman.com
> > (610) 337-3360
> > (610) 337-3359 Fax
> > www.cagleyharman.com
> >
> >
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