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Re: wind loads make me crazy

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> From: "Andrew D. Kester" <andrew(--nospam--at)baeonline.com>

> Enclosed- nothing below
> Open- All walls at least 80% open. This is pretty straightforward.
> Partially Enclosed:
> Area of openings in wall is greater then all of the other opening areas by
> 10%
> AND
> Area of said openings > 4sf or 1%, and the % of openings in the rest of the
> building do not exceed 20%.
> 
> OK, so someone used the analogy of a bag, open at one end, with holes cut in
> the rest of the bag greater then the opening plus 10%. So the wind goes in
> the large opening, and can go out the other openings. I can buy that. So if

> Now the problem is we have a rectangular metal building with several large
> openings. At each short end the walls are almost completely open, and the
> long ends there are a series of openings but not 80%. It does not meet the
> definitions of Partially Enclosed either. So despite numerous large

> PARTIALLY ENCLOSED. But we are the EOR and this is a pre eng MB.)
> 
> Some confirmation would do wonders to our mental health, as we thought we
> finally had this wind loading thing under control.

I find that classifying some structures as being strictly Enclosed,
Partially Enclosed and Open is more headache than worth. Sometimes words
affect our ability to think about the affects. Many buildings simply
don't meet the nice definitions in standards. That's when the art of
engineering over-rules the science.

ENCLOSED says that the small amount of transient air flow through the
building is insignificant w.r.t. the wind load on the building.

PARTIALLY ENCLOSED says that you have roof and walls that respond to
internal de/pressurization resulting from the air flow crossing the
building envelope. Dynamic variations within the envelope are ignored.

These first two assume that (nearly) all wind goes over or around.

OPEN says that you have a roof or airfoil and no significant
obstructions to wind flow patterns. Dynamic variations will be
significant.

Your case is none of the above, really. Most wind goes over or around
but lots of wind goes through.

The NBCC takes a slightly different nomenclature approach without the
mind-robbing numeric definition and I think that it helps to
conceptualize the real conditions:
Category 1 - nominal (near zero) internal pressure effects
Category 2 - significant internal pressure effects, no gusting
Category 3 - significant internal pressure effects with gusting.

The current low-rise wind load analysis procedures are based on the
results of testing done at University of Western Ontario, partially
sponsored by MBMA. There has been some significant research since then.
Notably, CSSBI sponsored a study by RWDI to determine the effects of
opening size and location in buildings. This report essentially
validated the NBCC approach but also provided some interesting
background for considering mitigating effects such as gust attenuation
and building size.

Using similar rationale, I have designed some components of buildings to
suit the highest wind factors when they are near a large opening but
have used reduced factors when the elements are further from the opening
(e.g. in large volume buildings, where internal gusting is mitigated).

I haven't seen anything that suggests that I can't have a dual
classification building by any North American code. Further, I can class
the building strictly per code and design specific portions for a
greater category.

The metal building manufacturer (typically) employs qualified engineers
but (rule of thumb) they do NOT play the role of consultants by making
judgement decisions about how the building will be used. They provide a
qualified quote/design to meet the strict interpretation of the codes.
They will design whatever the owner (owner's consultant) spec's above
minimum as long as the owner pays for it. You could go so far as to
identify each element, location and load.

If you interpret the requirements for the building design differently
than the code/standard, then that needs to be in the specs for
tender/quote. If you change requirements in mid-project and decide that
the building should be designed for a greater wind load scenario, expect
the manufacturer to ask for an extra if they have quoted using strictly
defined code load factors.

Suggestions:
1) Main frames at moderate factors, purlins/girts/cladding at high
factors. This is validated by the separation between components and
MWFRS by tributary area and number of tributary surfaces. AND the cost
difference is minimized.
2) Do a wind tunnel study of the building.

Will you sleep better, now?  :)

-- 
Paul Ransom, P. Eng.
Civil/Structural/Project/International
Burlington, Ontario, Canada
<mailto:ad026(--nospam--at)hwcn.org> <http://www.hwcn.org/~ad026/civil.html>

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