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Struggling with ASCE 7-02 Wind Loading

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Title: Struggling with ASCE 7-02 Wind Loading

Here's some information from a recent request like this to our helpdesk re: wind on gable vs. hip roofs. It's not the identical situation, but the solution from our wind guru has some insights that might help:

Figure 6-10... that's the basis of the simplified loads in Figure 6-2.  When the wind blows across a surface with zero roof slope (or in this case, parallel to the ridge on a gable roof), the GC_p values are:

 
Windward Wall (WW) = 0.40
Windward Roof (WR) = -0.69
Leeward Roof (LR) = -0.37
Leeward Wall (LW) = -0.29
 
So, the wall portion is 0.40-(-0.29)=0.69 in the wind direction.  The roof portion is 0.
 
When the wind blows across a surface with 30-45 degree roof slope perp. to the ridge (or in this case, against the sloped hip roof), the GC_p values are:

 
WW = 0.56
WR = 0.21
LR = -0.43
LW = -0.37
 
So, the wall portion is 0.56-(-0.37)=0.93 in the wind direction.  The roof portion is 0.21-(-0.43)=0.64.
 
Without multiplying out the tributary areas, it's pretty clear that the second case has higher loads than the first case.  However, note the differences in the coefficients for these cases.  If the roof slope were less than 30-45 degrees, say 20 degrees, the net load on the roof would be -0.21, REDUCING the total lateral load, probably below the first case.

 
This isn't intuitive... there should be a net positive load, right?  Well, in most cases the net load is positive... Figure 6-10 Footnote 6 and Figure 6-2 Footnote 7 ensure that the net load is positive.  However, you are also seeing some of the conservatism in the low-rise wind loads... These are "psuedo-loads" developed to provide a set of useable loads that envelop the major structural actions in a low-rise structure.  We sometimes refer to them as "enveloped loads" for this reason.  They don't give you the exact directional effect in all cases, but they are enveloped so that by checking a few cases, you design the MWFRS for the major structural actions.

 
The designer also has the option of applying the "directional loads" in Figure 6-6.  This will require checking multiple load cases and will generally yield higher lateral loads on the structure because they have not been enveloped to check to simultaneous effects on multiple surfaces.


HTH

Buddy

John "Buddy" Showalter, P.E.
Director, Technical Media
AF&PA/American Wood Council
1111 19th Street, NW, Suite 800
Washington, DC 20036
P: 202-463-2769
F: 202-463-2791
http://www.awc.org

The American Wood Council (AWC) is the wood products division of the American Forest & Paper Association (AF&PA). AWC develops internationally recognized standards for wood design and construction. Its efforts with building codes and standards, engineering and research, and technology transfer ensure proper application for engineered and traditional wood products.

*********************
>The guidance provided herein is not a formal interpretation of any AF&PA standard.  Interpretations of AF&PA standards are only available through a formal process outlined in AF&PA's standards development procedures.

*********************


From: "L. Pack" <Lloyd(--nospam--at)pecid.com>
To: seaint(--nospam--at)seaint.org
Subject: Struggling with ASCE 7-02 Wind Loading
<color><param>0100,0100,0100</param>Hello,

I'm struggling with the wind loading found in the ASCE 7-02. As I read th=
e
process to design a regular wood structure by the Method 1 procedure, I ge=
t
confused about the negative pressures in the horizontal zones B & D. I'm
looking at the 85 mph Basic wind speed and a 10=B0 Roof Slope. So, When
I read the numbers in Fig. 6-2 and Note 7 of that same figure, I get the i=
mpression
that I should be neglecting the negative or outward forces in zones B & D
so that the loading isn't less than if those zones were zero. This is cou=
nter-
intuitive to me. I'm used to designing for wind loading as prescribed in =
the
UBC's (94 & 97). With the UBC wind loading, I get an increase in pressure
as I ascend the building. The ASCE 7-02 seems to take the upper or roof
area as having no loading or a negative loading, which would reduce the ne=
t
loading on the projected area. This seems backwards.

When I go back to a fundamental idea of impact of the wind mass on the pro=
jected
area of the building, I get an increasing load as I ascend the building, d=
ue to the
increased velocity of the wind at that height. This makes sense to me fro=
m a
physics point of view. When we subtract out the uplift due to the Bernoul=
li effect
of the wind over the roof, it still seems that the uplift would be on the =
lee side
of the roof ridge and that the windward side of the roof would have a down=
ward
pressure. Because of this, it seems that the projected area would have a =
positive
pressure and not a negative pressure.

I'm wondering if the ASCE Publication: Guide to the Use of the Wind
Load Provisions of ASCE 7-02 is any help in sorting out the application
of these loadings? Does the book have example problems? Is it worth the
money?

Any help on this subject is appreciated.

Lloyd Pack
Thank you,
Lloyd Pack, P.E.
Project Engineering Consultants, LTD.
(208)466-7190
(208)466-7168 Fax
(208)250-2992 Cell