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RE: IBC 2007 Wind calcs.

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Paul,

In many ways, we are not really that far apart.

Part of my issue is that people seem to be making such a huge deal about
Method 1.  Maybe I am crazy (it is certainly possible), but Method 1 is NOT
hard.  It is basically looking up some pressures in a table with minimal
calculations required (you basically get a pressure from the table and then
apply some "adjustment" factors for height, exposure, importance).  For the
life of me, I don't see what the big deal is.

Now, I agree that it is completely stupid to have the what seems like 40
different pressures for different zones for a MWFRS calc, but then when you
know why/where Method 1 comes from, it starts to make some sense (in a weird
warped way).  Since it was developed primarily for PEMB use and the
"allowed" to be used for other structures, it makes sense to me why it is so
"fine tuned" and segmented to different zones...PEMB folks tend to like to
eek out ever little savings that they can.  Thus, it makes sense to me that
they want to have as "fine tuned" as possible the pressures that they use.

The other issue that I see when people encounter Method 1 is that they treat
it as a cook book method.  In otherwords, many  take it as gospel that those
are precisely the pressures that they must use.  Last time I check, those
are the MINIMUM pressures that you must use.  To my knowledge, there is
NOTHING that prevents an engineers from employing a little engineering
judgement and using a higher pressure.  The engineer can even choose to use
the worst pressure from the zones and apply that over that whole horizontal
surface, which likely will result in a "simplified" pressure that is rather
similar to the UBC pressures.  This would not be a problem for that vast
majority of the buildings out there.  It would have to be a rather
torsionally sensative building with a non-symmetrical geometry where such a
"simplification" might not be conservative...and there is a good chance that
such a building might not meet the limitations of Method 1.  And the result
is basically what you are arguing for anyways...the UBC is more simple
because it uses a more conservative wind pressure that is uniform.  To my
knowledge, there is nothing that prevents an individual engineers from doing
something similar using Method 1.  I have done it many times, mainly for
"first blush" pressures, but sometimes for final engineering pressures on
more simple buildings.

And my point about seismic was more to the fact that seismic design used to
be a whole hell of a lot more simpler for most of us East of the
Mississippi.  It was more simple than the equivalent lateral design method
that is predominately used for most projects that is in the code now.  Like
the "basic" wind methods, the basic seismic methods have gotten more complex
and frankly I did not hear nearly the griping from the "East coasters" that
I hear from the "West coasters" about wind.  Now, it could be that there are
a lot of East coasters that are in serious denial.  But, my point is that
what California engineers are facing with wind stuff is nothing new in
general...others have faces similar type issues with seismic and lived.

The last thing mention (which will likely be viewed as an attack, even
though it is not meant to be) is that I find it highly ironic that the
complaint seems to be that it will take more time, which you don't have, yet
you have more then enough time to debate the issue with me...and I have just
as much control over what is in the code as you do...i.e. zero control.
Some might ask what is the big deal if you have enough time to have a debate
over the issue in a forum that will likely not change a thing.  Personally,
I don't begrudge you an opportunity to vent.  Venting is good.  It relieves
stress and pressure.  But, frankly debating the issue here will likely have
little to no impact on whether or not ASCE 7 is changed for the better.

Regards,

Scott
Adrian, MI

-----Original Message-----
From: Paul Feather [mailto:PFeather(--nospam--at)se-solutions.net] 
Sent: Thursday, February 14, 2008 9:07 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: IBC 2007 Wind calcs.


Scott,

We use a static seismic procedure for most buildings as the common method.
If we have a particularly large or tall or unique structure we go outside
the realm of static analysis and must move up to a more rigorous analytical
approach using dynamic methods.

Under UBC, I have designed hurricane area structures, but even under UBC any
building in excess of 400 feet was outside the code limits and required a
wind tunnel analysis.  Kind of the wind equivalent dynamic analysis if you
will.  

There is a place for having complex analytical procedures at your disposal,
and there is a place for having common procedures that fit the majority of
conditions.  You feel the general ASCE method is that common procedure, I
feel the ASCE missed by quite a large margin and could have developed a much
more user friendly common method that would work economically for 90 percent
of the structures.  I am not against having the more complex procedures,
only against requiring their use to design a barn, or a big box store.  If
you are in a high wind region, then the option to use the more rigorous
analysis methods for economy and efficiency is fantastic, and I agree with
the need.  I would add that wind tunnel testing is still going to be the
primary method for significant structures, and that in high wind areas maybe
the bar could be lowered to trigger the use of theses methods sooner.  

But honestly if you are in a 140 mph region the difference between 143 psf
and 150 psf is only 5%, and that is a 7 psf spread.

I would also argue that in those high wind regions it is good detailing that
prevents damage much more so than refining the wind pressure the extra few
percent; the same way good detailing is what ultimately provides good
seismic performance rather than knowing the actual ground motion to within a
few percentage points.  


Paul Feather PE, SE
pfeather(--nospam--at)SE-Solutions.net
www.SE-Solutions.net
 




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