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Re: Code question on wind loading

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Not to be picky, but the only the windward loads of the primary frames
will vary with height.  And it actually is not stepped but a continuous
parabolic shape.  It is just easier to show "stepped" values in the chart.
Sorry to be a nit picker.

With regards to you question...while I don't know the answer for sure, I
believe the I do...others who are smarter than me can correct me if I am
off base.

The difference between the primary frame loading and components/cladding
is related to the relative area and the "averaging" of the wind pressures.
When the wind "hits" an object, there will generally be a variation of the
pressure along the entire surface.  For example, even though we might
determine by code that the wind pressure is 30 psf, the pressure on the
"exposed" surface could vary from 0 psf (or even negative pressure) to 60
psf or more a various points along the surface.  The code value is the
average value that we should expect to see on the surface.

Now thing about the size of the surface that we are considering.  If it is
relative small (i.e. 5 foot by 5 foot window on the  face of a
building...component/cladding), then the probability that the surface
could see high pressures is greater an thus the average will be higher.
If it is relatively large (i.e. the entire face of the building...primary
structure), then the higher local pressures will get "swallowed" up in the
average by the lower pressures, thus resulting in a lower average.  This
can even be illustrated in the components and cladding provisions.  THe
factor of CGp is a function of hte tributary area of the object that you
are evaluating.  THe smaller that area the larger CGp...the larger the
area the smaller CGp until you approach the primary frame values.  In
fact, under BOCA if the trib. area is 700 sq. ft. or larger, you are
permitted to use the primary frame values.

Hope this helps.

Scott

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Scott E. Maxwell, SE, PE
Structural Engineer

BEI Associates Inc.
601 West Fort Street
Detroit, MI  48226

Telephone: (313) 963-2300                              
Fax:       (313) 962-4269

Email for Business: 
Email for Personal: smaxwell(--nospam--at)engin.umich.edu
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On Wed, 22 Mar 2000, Jim Sadler wrote:

> In the 1997 UBC elements and components are designed using a wind loading
> based on a uniform loading calculated from the mean roof height. (Section
> 1622). On the other hand, primary frames and systems (method 2) uses a
> stepped loading going up the building (Section 1621).
> 
> Since the load path is from the exterior elements to the primary frame, why
> isn't primary frame subjected to and designed for the same wind loading as
> the exterior elements?
> 
> Jim Sadler
> 
> 
>