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RE: wind
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- Subject: RE: wind
- From: Scott Maxwell <smaxwell(--nospam--at)engin.umich.edu>
- Date: Thu, 27 Feb 2003 10:11:42 -0500 (EST)
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 > > > ******* ****** ******* ******** ******* ******* ******* *** > * Read list FAQ at: http://www.seaint.org/list_FAQ.asp > * > * This email was sent to you via Structural Engineers > * Association of Southern California (SEAOSC) server. To > * subscribe (no fee) or UnSubscribe, please go to: > * > * http://www.seaint.org/sealist1.asp > * > * Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you > * send to the list is public domain and may be re-posted > * without your permission. Make sure you visit our web > * site at: http://www.seaint.org > ******* ****** ****** ****** ******* ****** ****** ******** > ******* ****** ******* ******** ******* ******* ******* *** * Read list FAQ at: http://www.seaint.org/list_FAQ.asp * * This email was sent to you via Structural Engineers * Association of Southern California (SEAOSC) server. To * subscribe (no fee) or UnSubscribe, please go to: * * http://www.seaint.org/sealist1.asp * * Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you * send to the list is public domain and may be re-posted * without your permission. Make sure you visit our web * site at: http://www.seaint.org ******* ****** ****** ****** ******* ****** ****** ********
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