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RE: Wind Loads

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I basically agree.  For the lateral portion of the wind loads (i.e. the
wind that hits the front projected surface of the canopy and sucks on the
rear projected surface of the canopy), you should be using a MWFRS wind
pressure.  And since you will not have an "internal" pressure acting on
those same surfaces, it will not matter whether you call the canopy
"open", "enclosed", or "partially enclosed".  You will only be applying
the lateral pressures from the "external" wind pressures.  The
classifications of "open", "enclosed", and "partially enclosed" to my
knowledge only effects the (GCsubpi) value which is strictly for internal
pressures, which would not exist for a canopy.  Thus, if you really want
to get technical, you would use "open" which means that (GCsubpi) is zero
(i.e. no internal pressures).

Now, the issue of uplift/vertical pressures for a canopy is an entirely
different manner.  This would still mainly be a MWFRS pressure (for the
overall canopy), except that some local elements will see higher pressures
from C&C as a local element.  To me, the distinction between MWFRS and C&C
goes back to whether or not you are looking at the system (i.e. canopy as
a whole) or specfic elements within the canopy.  If the former, then MWFRS
and if the latter than C&C pressures would likely apply.  This type of
loading to a certain degree has nothing to with "lateral".  It will be the
wind pressure on the upper surface of the roof that typically acts in
suction pulling the roof up (unless the canopy has a steep pitch to it)
PLUS some wind pressure on the bottom of the surface that is more than
likely pushing upward on the structure.  The results is that your uplift
force on a canopy should likely be about twice that of a typical building.
I agree that the building codes don't really address this to well.  The
closest that they really get to dealing with this portion of the wind
pressures is the overhang provisions (section 6.5.11.4 in ASCE 7-98).
This essentially creates the "double" uplift force on the structure.  This
may be conservative in some cases, however, depending on the configuration
of the canopy.  Basically this provision is to a certain degree treating
the overhang/canopy as if it is an airplane wing, which will have both a
negative pressure on the top surface (suction) and a positive pressure on
the bottom surface (pushing).  To what degree that this effect would
happen would become a function of shape and dimensions of the canopy.  The
end result is that I would likely be using the overhang provisions for the
vertical portions of the wind pressures, which essentially means that you
would get about double the uplift pressures when compared with an enclosed
structure of the same size.

As far as wind research goes, part of the disparity (at least in my
opinion) is because for a long time, we knew much more about wind than
seismic.  That gap has closed significantly.  But wind is still something
that in general is easier to predict and understand since it is such a
more frequent thing (at least "normal" wind stuff...hurricanes and
tornados are still much less frequent thing).  While there are certainly
lessons to learn in both wind and seismic applications, the truth (in my
mind at least) is that high wind situations have happen with so much more
frequency than serious seismic events that we have likely been "taught" a
lot more lessons in wind design than we have to date in seismic.  While we
seem to know a lot more about seismic design, it is still rather likely
that we will be "taught" a major lesson in a future seismic event.  After
all, everyone thought that we had the whole seismic design with steel down
pat, until Northridge happened that is.  Turns out we did not know as much
as we thought we did.  So, while we certainly can be "taught" a major
lesson like that in wind design, the odds are still in favor of such
another lesson coming in seismic design first.

Having said that, there was a push within Congress at one time (and I
don't know what happened to it) to create a National Wind Hazard
Reduction Program (NWHRP).  This would have created an federal government
based initiative for wind hazard damage reduction similar to the NEHRP
provisions.  While this would likely not directly mean research money, it
could indirectly lead to more money for research as such needs are better
defined.  For more information, I did find these web pages:

http://www.asce.org/pressroom/publicpolicy/ka0100_windhazard.cfm

http://www.windhazards.org/caucus.cfm

HTH,

Scott
Ypsilanti, MI



On Thu, 20 Feb 2003, Keith Fix wrote:

> Not just coastal areas.  A lot of "near miss" tornado events cause failures in
> the midwest.  Such wind loads are no different from coastal wind loads, both
> being essentially horizontal for low-rise buildings.  Like ice loads, it's a
> rarely considered load condition, but no less common in the midwest than an
> earthquake.
>
> I'd like to see more in the wind code.  At the same time, it'd be nice if it
> didn't become the headache seismic has become.
>
> As to the question, I use a MWFRS load for the lateral design.  I'm very
> conservative with uplift, and usually include some kind of load to simulate
> suction on the underside.  If I have no reason to do otherwise, I'll use the
> same load underneath the canopy as above.  For MWFRS, I check uplift and
> downward suction as separate load cases, as well as in combination.
>
> -Keith Fix, PE
>


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