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RE: wind
[Subject Prev][Subject Next][Thread Prev][Thread Next]- To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>
- Subject: RE: wind
- From: "Sherman, William" <ShermanWC(--nospam--at)cdm.com>
- Date: Thu, 27 Feb 2003 09:52:25 -0500
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 ******* ****** ****** ****** ******* ****** ****** ********
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