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Re: ASCE 7 Gable Wind Loads

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Brian:

One first thing to keep in mind is that the windward pressure determined
per section 6.5.12.2.1 varies with height, while the windward pressure
determine per section 6.5.12.2.2 will be based upon the roof's mean
height.  Thus, the "average" wind pressure for the first case will be
lower than the pressure used for the second case ASSUMING that you use the
same velocity pressure exposure coefficients (Kz and Kh).  Note, however,
that you DO NOT use the same Kz/Kh values for the two methods.  The second
method requires the use of the "Case 1" Kz/Kh values while the first
method requires the use of the "Case 2" values.  And the Case 2 values are
LESS THAN the Case 1 values.  So, the end result is that while the GCp
values might be lower for the second case, the Kh/Kz values will be larger
for the second case.

In addition, keep in mind that for the second method (6.5.12.2.2), the
GCpf value changes (increases) at the ends of the walls, while this does
not happen for the more "rigorous" method when doing MWFRS.  So, in
reality your effective GCpf value that you should use for comparison
purposes would be slightly higher than what you stated.

Based upon your situation, I get this:

For section 6.5.12.2.1 case:

windward:

G = 0.85
Cp = 0.8
Kz = 0.57 for 0-15 of height
   = 0.62 for 20 ft of height

ignore all other factors as they will be the same for both methods

thus, "effective" or average Kz = +/- 0.576

Thus, combined factors are about 0.85*0.8*0.576= 0.39

leeward:

G = 0.85
Cp = -0.5 (this assume your L/B ratio is less than 1 but you did not give
the other dimesion, but you implied this by saying you were using the .5
value)
Kh = 0.62

Thus, combinded factors are about 0.85*-0.5*0.62= -0.26

For section 6.5.12.2.2:

windward:

GCpf = 0.40 for most of the wall
     = 0.61 for the last 8 ft at each end of the wall (this assumes that
192 ft is the least horizontal dimension)
Kh = 0.70

Thus, your effective GCpf is about 0.45

Thus, your combined factors are about 0.45*0.7= .32

Leeward:

GCpf = -0.29 for most of the wall
     = -0.43 for the last 8 ft at each end of the wall
Kh = 0.70

Thus, your effective GCpf is about -0.3

Thus, your effective combined factors are about -0.3*0.7= -0.21



So, the results for windward is about 0.39 for section 6.5.12.2.1 vs.
0.32 for section 6.5.12.2.2.  Thus, the second method is about 82% of the
more rigorous method.  For leeward is it about -0.26 vs. -0.21.  This
results in the less rigorous method being about 81% of the more rigorous
method.  And both those are within in your stated tolerance of 80% to 90%.

The point is that you need to look at the WHOLE picture.  You focused in
on the differences between the obvious G/Cp/GCpf differences, but
neglected the other subtile differences in those values as well as the
difference between determining the Kh/Kz values in the two methods.

HTH,

Scott
Adrian, MI

On Thu, 16 Mar 2006, Brian S Bossley wrote:

> I have a question about wind loads on a gable roof.  The building is 20' tall and 192' wide with a " / ft roof slope.  Using the method described in 6.5.12.2.1, I would have a windward Cp of 0.8, a leeward Cp of 0.5.  Using the method described in 6.5.12.2.2, I have a  windward GCpf of 0.4, and a leeward GCpf of 0.29.  This translates to a windward Cp of 0.47, and a leeward Cp of 0.34.
>
>
>
> Basically, this means that the lateral wind load is 62% of what it would have been had I used the tried and true formulas for every other type of building out there.
>
>
>
> Does anyone know why this is?  I wouldn't question if the lateral load was 80 to 90% of the other formulas, but when it gets this low, I tend to think I'm missing something.
>
>
>
> Brian S Bossley, PE
>
> Ventura Engineering
>
> 7610 Olentangy River Rd
>
> Columbus, OH 43235
>
>
>
>

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