Need a book? Engineering books recommendations...
Return to index: [Subject] [Thread] [Date] [Author]
Re: ASCE 7 Gable Wind Loads
[Subject Prev][Subject Next][Thread Prev][Thread Next]- To: seaint(--nospam--at)seaint.org
- Subject: Re: ASCE 7 Gable Wind Loads
- From: Scott Maxwell <smaxwell(--nospam--at)engin.umich.edu>
- Date: Fri, 17 Mar 2006 06:25:00 -0500 (EST)
Brian: I forgot to mention that all my numbers where based upon Exp B. Presumably, going to Exp C or A or D would not significantly change the relative differences but would change the actual Kh/Kz values. Regards, Scott Adrian, MI On Fri, 17 Mar 2006, Scott Maxwell wrote: > -------------------------------------------------------------------------- > Your following message has been delivered to the list > seaint(--nospam--at)seaint.org at 03:16:49 on 17 Mar 2006. > -------------------------------------------------------------------------- > > > 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 ¼" / 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 > > > > > > > > > > ******* ****** ******* ******** ******* ******* ******* *** > * 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 ******* ****** ****** ****** ******* ****** ****** ********
- References:
- ASCE 7 Gable Wind Loads
- From: Brian S Bossley
- Re: ASCE 7 Gable Wind Loads
- From: Scott Maxwell
- ASCE 7 Gable Wind Loads
- Prev by Subject: Re: ASCE 7 Gable Wind Loads
- Next by Subject: RE: ASCE 7 Gable Wind Loads
- Previous by thread: Re: ASCE 7 Gable Wind Loads
- Next by thread: RE: ASCE 7 Gable Wind Loads
- About this archive
- Messages sorted by: [Subject][Thread][Author][Date]