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RE: wind

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William,

One difference to keep in mind is that the 1997 UBC is based off of the
"fastest-mile" wind speed (ala ASCE 7-95 I believe or maybe even ASCE
7-93...I don't have volume 1 of the UBC to verify which is permitted per
section 1604), while the IBC is based off of the "3-second gust" wind
speed (ala ASCE 7-98 and ASCE 7-02).

Now it was my understanding that in theory, both should have resulted in
the same pressures (at least at the translation time from ASCE 7-95 to
ASCE 7-98) since the new method was I believe correlated to the old
method.  But, then again I am getting old and senile and could be
imagining things! <grin>

HTH,

Scott
Ypsilanti, MI


On Thu, 27 Feb 2003, Sherman, William wrote:

> 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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