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RE: IBC 2007 Wind calcs.

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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
 

-----Original Message-----
From: Scott Maxwell [mailto:smaxwell(--nospam--at)umich.edu] 
Sent: Thursday, February 14, 2008 5:39 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: IBC 2007 Wind calcs.

I will offer a similar response to you as I did to Bill.  While the
differences might not be much for typical jobs that you design, do you
do
much work in high wind/hurricane areas?  Those differences between a
conservative approach in 140 mph wind zones and a more "fine tuned"
method
can have some rather big differences.  You can get to points with the
little
change in the 3 to 5 psf can have a big difference.

And I will say it again, I understand that it is more complex, which
might
mean it takes longer.  And I agree that there are some complexities that
are
a waste (the seemingly 40 different wind pressure zones for MWFRS
pressures
under Method 1...I can see the need for all the different zones for C&C,
but
not for MWFRS).  But, there is a difference between it being more work
and
tougher to understand.  I would argue that it is not that tough to get,
but
I will agree that it might take longer.  Personally, having used ASCE 7
wind
provisions for years, I don't find that it takes me all that long...and
find
that there are other stupid crap in the codes that are much more worth
griping and battling over than differences between UBC wind provisions
and
ASCE 7 "typical" wind provisions.  Now, if you want to gripe about the
flexible building provisions, then I will be right there with you...they
seems to require a PhD to deal with.

Regards,

Scott
Adrian, MI

-----Original Message-----
From: Paul Feather [mailto:PFeather(--nospam--at)se-solutions.net] 
Sent: Thursday, February 14, 2008 7:38 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: IBC 2007 Wind calcs.


It's not so much we are having trouble with the wind provisions, it's
just
the effort isn't warranted.  I agree the general method is not
over-whelming, despite my earlier post, but why do I want to go through
all
the machinations to derive a wind pressure of 17.4 psf net instead of 20
psf
I can calculate in 30 seconds and apply in a straight horizontal
projection?
The conservatism is non-existent for most structures, the wind forces
are
simply not controlling over seismic, and the average structure can
absorb
that conservatism with ease.  I have worked with both methods in
multiple
states for many years, so it isn't some new experience for me; the real
level of conservatism is not that great a difference.  

You may get pressures for method one in a minute, but you will be
applying
12 extra load cases to get design forces.  

And the new load case provisions (relevant to UBC)requiring .6DL for
seismic
is absurd.

I for one will take "dumbed down" unless I really need the more
analytical
method.  For the majority of applications be a little conservative and
spend
the time on the rest of the design.  Even seismic design, you don't try
and
perform highly analytical dynamic methods for simple structures.

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

-----Original Message-----
From: Scott Maxwell [mailto:smaxwell(--nospam--at)umich.edu] 
Sent: Thursday, February 14, 2008 4:23 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: IBC 2007 Wind calcs.

I don't disagree.  The current ASCE 7 methods are more complicated than
what
was in the last few versions of the BOCA code...and those last few
versions
of the BOCA code were more complicated than what has been in the 1997
UBC. A
lot of the added complexity that now appears in ASCE 7 compared to older
versions of ASCE 7/BOCA code do seem as overkill to me.

But, I will note that the UBC wind provisions were rather "dumbed down".
Note I don't say this to offend anyone, but it is a good simple way to
put
it.  UBC provisions are VERY simplified versions of the basic intent of
ASCE
7, which a to a large degree a crap load on conservatism in them.  And
that
is likely why Stan is finding pressures that are less than what he is
used
to...the UBC method is conservative, which means that it applies to more
situations without having to do much additional work.  A lot of the
"complexity" in the ASCE 7 is that you have more "branches" in the path,
which means the loads can be more talyored to your specific building and
also means you can end up with less conservative loads.  Personally, I
think
that have take it too far with the available "branches", but I have also
traditionally used versions that are more complex than the UBC.

My point is that still with that complexity, neither method 1 or method
2
are that complex in my opinion, unless you get into the flexible
building
scenario.  Method 1 is more of a pain since it produces more "zones"
with
different pressures.  As a result, I do tend to use Method 2 for my
calcs,
but I will use Method 1 to get some down and dirty pressures to get a
feel
for where things are.  I can literally get pressures out of Method 1 in
about a minute or so.  Besides, topograpihcal effects tend to bump you
out
of Method 1 right darn quick.

While I like seismic design as well, I will take having to deal with the
wind provisions LIGHT YEARS before messing with seismic.  And that is
why I
am kind of surprised from all the grumbling from California.  I have to
admit that I would not think that engineers who can handle the
complexities
of the seismic provisions should have any trouble with the wind
provisions.
Frankly, to me, the wind provisions are cake next to the seismic stuff.

Regards,

Scott
Adrian, MI

-----Original Message-----
From: Haan, Scott M POA [mailto:Scott.M.Haan(--nospam--at)usace.army.mil] 
Sent: Thursday, February 14, 2008 5:58 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: IBC 2007 Wind calcs.


Scott.  

Compared to the UBC, all the ASCE 7 methods are more complicated.  I
agree
with people who said use the analytical method for a rigid building is
the
easiest way because there aren't 10 different zones etc... etc... but
you
still have to have a spreadsheet to calculate the pressures.

I think if I had to design a flexible building I would send chocolate
cupcakes with turds in the middle to the ASCE7 wind committee have a
supercomputer to calculate the gust factor.

Scott.

-----Original Message-----
From: Scott Maxwell [mailto:smaxwell(--nospam--at)umich.edu] 
Sent: Thursday, February 14, 2008 9:58 AM
To: seaint(--nospam--at)seaint.org
Subject: RE: IBC 2007 Wind calcs.

I want to say that this method is more or less based off the simplified
method that Washington has produce and has been mentioned by others.
 
Personally, while I find there to be some complexities that I don't feel
are
necessarily needed in the current ASCE 7 wind provisions, I don't find
them
that difficult to use...including Method 1.  I find that I can pump out
wind
pressures in method 1 in very short order.  It does take more time to
use
those pressures to analyze stuff in MWFRS since they now have corner
pressures and such...but you don't really gain that much compared to
older
more "uniform" pressures except for some buildings that might be rather
succeptible to torsional effects.  But it does help that I have been
using
the ASCE 7 methods for a LONG time, while engineers in CA are more used
to
only using the simplified methods that were in the UBC.
 
I would be the first to agree that ASCE 7 has gone of the deep end to
some
degree in "sharpening the pencil" for wind provisions, but I am not sure
that I would liken them to a doctoral thesis (unless you are talking
about
the wind provisions for signs or flexible structures or dynamically
sensitive structures and have to start calculating gust coefficients).
 
Regards,
 
Scott
Adrian, MI

	-----Original Message-----
	From: Matthew [mailto:sandman21(--nospam--at)gmail.com] 
	Sent: Thursday, February 14, 2008 12:48 PM
	To: seaint(--nospam--at)seaint.org
	Subject: Re: IBC 2007 Wind calcs.
	
	
	You can also try using
http://www.documents.dgs.ca.gov/dsa/pubs/IR-16-7_WindLoad_12-18-07.pdf
	 
	Matthew
	
	
	On Thu, Feb 14, 2008 at 9:34 AM, Paul Feather
<PFeather(--nospam--at)se-solutions.net> wrote:
	

		Stan,
		
		First off, the simplified method is anything but simple.
We
use the
		general method (method 2) for everything and get more
consistent results
		easier.  The simplified method is derived from metal
building
		manufacturer methods, and for anything but a metal
building
results in a
		complete book keeping atrocity.
		
		You are looking at 25 degrees area B.  The way the
simplified method
		works this is just one small area that cannot be applied
in
the same
		thinking as the UBC horizontal projected area.  You have
to
add the area
		B to the Area E uplift, basically all areas A through H
get
applied
		simultaneously as one load case.  Then you rotate the
building reference
		corner and apply the whole thing again for all four
reference corners.
		
		Get away from the simplified methods and you will
simplify
your life,
		while getting something closer to what you are used to.
I
don't believe
		the ASCE wind provisions could be any more convoluted
and
difficult to
		apply to real world engineering if we tried.  The UBC
methods were
		derived as a conservative simplification of the ASCE
provisions years
		ago, and we desperately need to achieve something
similar
again.
		Spending three days on a doctoral thesis to develop
simple
wind
		pressures as opposed to working on load path and quality
engineering is
		counter-productive, and saving 1.4 psf in wind pressure
only
matters to
		mass produced square boxes trying to be paper thin.
		
		Paul Feather PE, SE
		pfeather(--nospam--at)SE-Solutions.net
		www.SE-Solutions.net <http://www.se-solutions.net/> 
		

		-----Original Message-----
		From: sscholl2(--nospam--at)juno.com [mailto:sscholl2(--nospam--at)juno.com]
		
		Sent: Thursday, February 14, 2008 9:09 AM
		To: seaint(--nospam--at)seaint.org
		Subject: IBC 2007 Wind calcs.
		
		
		After 40 yrs. of doing UBC calcs. I am attempting to do
my
first IBC
		calcs. and need help, even after attending a seminar,
which
seemed to
		cover lots of things but not this.
		
		For a simple house, using 6.4 Method 1 Simplified
Procedure,
I cannot
		get a reasonable wind pressure of something between 15
psf
and 25 psf.
		
		From 6.4.2.1 <http://6.4.2.1/> , I get p s= 1.0 (1.0)
1.0
(2.3) = 2.3 psf which is
		unrealistic. This is using Fig. 6-2, exposure B, h=30
ft.,
Kzt =1
		and I=1
		
		Can someone point out my omissions/errors?
		
		Stan Scholl, P.E.
		Laguna Beach, CA
	
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