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RE: wind

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Your numbers appear to be correct, but please note the following:

The 2001 Supplement to the IBC does provide a little clarification to the
columns which read "Max Horizontal Wall Loads" in Table 1609.6.2.1(1). A
footnote states "Max. Horizontal Wall Loads are only for the design of wall
elements which also support roof framing." Thus, I don't think that these
loads should be combined for design of the overall lateral force resisting
system, and I would use the smaller overall wind pressure you calculated for
the lateral system design. (Unfortunately, I think the footnote for the last
columns still needs more explanation.) 

For the ASCE 7-98 Analytical Procedure, I would not include the internal
pressure coefficient in the overall net lateral wind pressure. If you keep
the windward and leeward pressures separate, the internal pressure would be
additive to one and would reduce the other - thus having no net effect on
the overall lateral force. But if you model the wind by applying it to
windward and leeward surfaces individually, it should be modeled with two
separate internal pressure assumptions - positive and negative internal
pressure. If you remove the internal pressure coefficient from your
calculation, the net effective pressure would be close to the 20.5 psf
derived from the IBC. 

For the ASCE 7-98 Analytical Procedure, I also looked at another alternative
- in lieu of using Figure 6-4 for coefficients, one could separate "G" and
"Cp" using Figure 6-3 for "Cp" and using G=0.85 assuming a rigid structure
per Section 6.5.8. However, this comes up with a much higher overall wind
pressure. 

I'm not sure why the 1997 UBC wind pressures are so much higher than IBC &
ASCE 7-98, but I have found IBC wind pressures to be less than 1997 UBC wind
pressures in some other wind comparisons I have made as well. 


William C. Sherman, PE
CDM, Denver, CO
Phone: 303-298-1311
Fax: 303-293-8236
email: shermanwc(--nospam--at)cdm.com


> -----Original Message-----
> From: Michael Hemstad [mailto:mlhemstad(--nospam--at)yahoo.com]
> Sent: Tuesday, February 25, 2003 8:31 AM
> To: seaint(--nospam--at)seaint.org
> Subject: wind
> 
> 
> I'm trying to get familiar with the IBC wind load
> provisions.  I have run a comparison between the IBC
> provisions, ASCE 7-98, and the 1997 UBC.  The results
> are a little dismaying, especially the IBC values. 
> Can someone tell me if I've made a mistake somewhere?
> 
> The analysis is for a fictitious 42 foot tall building
> in Minnesota, Exposure C.  Iw = 1.0; topographic
> constant 1.0.  The building is enclosed, with a flat
> roof.
> 
> IBC 1609.6:
> V = 90 mph
> Table 1609.6.2.1(1) (Interior Zone Wall) gives
> pressure = 8.5 psf
> Table 1609.6.2.1(4) gives Ht+Exp coeff = 1.51
> load factor = 1.6
> 
> w = 12.84 psf, x 1.6 = 20.5 psf
> 
> If instead I use the last columns of Table
> 1609.6.2.1(1), I get a pressure of 7.2 - (-5.8) = 13.0
> psf. Then,
> 
> w = 19.63 psf, x 1.6 = 31.4 psf
> 
> 
> ASCE 7-98 Simplified Procedure (shouldn't be used
> since ht > 30 feet)
> Fig. 6-1: v = 90 mph
> Table 6-2 give p = 14 psf
> Exp. C multiplier = 1.40
> load factor = 1.6
> 
> w = 19.6 psf, x 1.6 = 31.4 psf
> 
> 
> ASCE 7-98 Analytical Procedure
> Figure 6-1: v = 90 mph
> Table 6-6 gives Kd = 0.85
> Table 6-5, Case 1 gives Kh = 1.05
> Figure 6-4 gives GCpf = 0.40 - (-0.29) = 0.69
> Table 6-7 gives GCpi = 0.18
> qh = 18.51 psf
> load factor = 1.6
> 
> w = 18.51 x (0.69 + 0.18) = 16.1 psf, x 1.6 = 25.8 psf
> 
> 
> 1997 UBC
> Fig. 16-1: v = 80 mph (fastest-mile, not 3-second)
> Table 16-F:  qs = 16.4 psf
> Table 16-G: Ce = 1.32
> Table 16-H: Cq = 0.8 + 0.5 = 1.3
> load factor = 1.3
> 
> w = 28.14 psf, x 1.3 = 36.6 psf
> 
> 
> Factored loads vary from 20.5 psf to 36.6 psf.  Nice
> tight grouping.
> 
> Can anyone tell me if these values seem right?
> 
> In the IBC calcs, I assume the higher value is the one
> to use.  What is the lower value for?  Where does it
> say this?
> 
> In the ASCE 7 Analytical Procedure, footnote 3(ii) of
> Table 6-7 indicates GCpi is applied to all interior
> surfaces.  Thus, it should cancel out for the MWFRS. 
> However, Section 6.5.12.2 indicates it is additive to
> the exterior pressures for the MWFRS.  Can someone
> shed some light on that?
> 
> Thanks for any help.
> 
> Mike hemstad
> TKDA
> st. Paul, Minnesota
 

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