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Re: Wind pressure

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Javier,

Here is the way that I understand the difference between you use MWFRS
pressures and C&C pressures.

You use the MWFRS pressures for design of the building's lateral SYSTEM.
This means that typically speaking the wind pressures are not directly
applied to the member in order to determine the force in that member due
to the MWFRS.  After all the 'S' in MWFRS is SYSTEM.  The wind pressure in
this case is applied to the whole structure, which then by the appropriate
load path would result in wind load going to various members that
participate as part of the wind lateral system, such as braces, columns in
the lateral systems (which can include columns in the braced frame or
moment frame as well as some interior column that INDIRECTLY sees uplift
or downward force from the MWFRS acting on the roof), beams in lateral
frames (i.e. moment frames or braced frames), etc.  The key point is that
the wind pressure from the MWFRS is NOT intended to be used as a direct
pressure applied to an individual element.

This is not only "keyed" to the 'S' in MWFRS (i.e. system) but by the
difference in how the MWFRS pressure is determined compared to C&C
pressure.  The MWFRS pressure is NOT a function of tributary area of the
item in question...or more precisely, the tributary is considered to be
so large that it no longer effects the code determined wind pressure.
Think of the code determined wind pressure as being a wind pressure that
best likely to represent the an "average" wind pressure over the item in
question.  In reality, the wind pressure is going to vary over the
surface of the building.  Thus, over the windward wall, for example, you
will have in reality some areas of higher pressure and areas of lower
pressure and everything in between.  And this can change as the wind
blows...in otherwords, the an area of higher pressure at one moment can
change to an area of lower pressure in the next moment.  Now when you look
at the whole wall, the "average" pressure can be the average of all these
difference pressures on the difference areas.  But when you look at an
item with a smaller tributary area, there is a greater chance that that
smaller area may be exposed to a higher "average" pressure.  This is
because your smaller area could be dominated by one of the areas of higher
pressure.  So, from a statictical and probability point of view, the
smaller the tributary area of the item in question, the higher the
"average" pressure will be.

So, this means that single elements (i.e. rather small tributary area
especially when compared to the whole structure) that have a DIRECT
exposure to the wind should be designed with the C&C loading.  This does
not mean that a single element could not end up with two different wind
loading cases...one from a DIRECT loading of the member due to C&C loading
and one from an INDIRECT loading of the member due to the MWFRS loading
being applied to the lateral system (i.e. the member is a part of the
lateral system).  The point is that if the member in question is DIRECTLY
exposed to the wind pressure, such as members (girts, columns, beams,
walls, windows, doors, etc) in the exterior walls and roof, then C&C
pressure should be used.

So, in the case of your question, I respond with a "depends".  If the
column in question is an exterior column and the structure is "designed"
such the those exterior columns would be DIRECTLY exposed to the wind
(i.e. the column supports girts or wall panels in bending), then they
should be designed with the C&C loading.  If the column is an interior or
exterior column that is really taking some wind load in a direct fashion,
but rather only taking it due to uplift or downward force due to wind on
the roof, then you could realistically have a good ol' debate as to which
to use.  But, the reality is that the debate would likely be entertaining
but a waste of time, since most codes allow you to use MWFRS pressures as
the C&C pressures for members over a certain tributary area (I believe
many use 700 sq ft as the cutoff point), which means that it is possible
that you could be in a position to use the MWFRS pressures any way if the
column grid/spans area great.

For the base plates and footings, I would be using the same basic criteria
except that obivously the wind pressure cannot be directly applied to
either.  So, I would be looking at the column that the base plate or
footing supports.  If I am using the C&C on the column in question, then
use the same for the base plate and footing.

And keep in mind that the exterior columns could have two wind load
cases...one with C&C loading bending (and potentially some axial load too)
the column and one with the MWFRS just likely adding or subtracting axial
load as part of the lateral system.

And if worst comes to worst, keep in mind that you can always look at what
is the most conservative case if you are not sure of which of the two to
use (which is likely the C&C case since C&C pressures should be greater).

HTH,

Scott
Ypsilanti, MI


On Mon, 20 Jan 2003, Javier Encinas wrote:

> In the design of a light warehouse located in a hurricane-prone zone, I'm faced with an important issue: It is clear that the joists and beams must be designed using the C & C pressures, but should the columns, base plates and footings be designed for the MWFRS pressures or for the C & C pressures? Please advise.
>
> Thanks in advance
>
> Javier Encinas
> Encinas & Assoc.
>


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