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RE: IBC 2007 Wind calcs.[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: <seaint(--nospam--at)seaint.org>
- Subject: RE: IBC 2007 Wind calcs.
- From: "Scott Maxwell" <smaxwell(--nospam--at)umich.edu>
- Date: Fri, 15 Feb 2008 11:21:59 -0500
Paul, In many ways, we are not really that far apart. Part of my issue is that people seem to be making such a huge deal about Method 1. Maybe I am crazy (it is certainly possible), but Method 1 is NOT hard. It is basically looking up some pressures in a table with minimal calculations required (you basically get a pressure from the table and then apply some "adjustment" factors for height, exposure, importance). For the life of me, I don't see what the big deal is. Now, I agree that it is completely stupid to have the what seems like 40 different pressures for different zones for a MWFRS calc, but then when you know why/where Method 1 comes from, it starts to make some sense (in a weird warped way). Since it was developed primarily for PEMB use and the "allowed" to be used for other structures, it makes sense to me why it is so "fine tuned" and segmented to different zones...PEMB folks tend to like to eek out ever little savings that they can. Thus, it makes sense to me that they want to have as "fine tuned" as possible the pressures that they use. The other issue that I see when people encounter Method 1 is that they treat it as a cook book method. In otherwords, many take it as gospel that those are precisely the pressures that they must use. Last time I check, those are the MINIMUM pressures that you must use. To my knowledge, there is NOTHING that prevents an engineers from employing a little engineering judgement and using a higher pressure. The engineer can even choose to use the worst pressure from the zones and apply that over that whole horizontal surface, which likely will result in a "simplified" pressure that is rather similar to the UBC pressures. This would not be a problem for that vast majority of the buildings out there. It would have to be a rather torsionally sensative building with a non-symmetrical geometry where such a "simplification" might not be conservative...and there is a good chance that such a building might not meet the limitations of Method 1. And the result is basically what you are arguing for anyways...the UBC is more simple because it uses a more conservative wind pressure that is uniform. To my knowledge, there is nothing that prevents an individual engineers from doing something similar using Method 1. I have done it many times, mainly for "first blush" pressures, but sometimes for final engineering pressures on more simple buildings. And my point about seismic was more to the fact that seismic design used to be a whole hell of a lot more simpler for most of us East of the Mississippi. It was more simple than the equivalent lateral design method that is predominately used for most projects that is in the code now. Like the "basic" wind methods, the basic seismic methods have gotten more complex and frankly I did not hear nearly the griping from the "East coasters" that I hear from the "West coasters" about wind. Now, it could be that there are a lot of East coasters that are in serious denial. But, my point is that what California engineers are facing with wind stuff is nothing new in general...others have faces similar type issues with seismic and lived. The last thing mention (which will likely be viewed as an attack, even though it is not meant to be) is that I find it highly ironic that the complaint seems to be that it will take more time, which you don't have, yet you have more then enough time to debate the issue with me...and I have just as much control over what is in the code as you do...i.e. zero control. Some might ask what is the big deal if you have enough time to have a debate over the issue in a forum that will likely not change a thing. Personally, I don't begrudge you an opportunity to vent. Venting is good. It relieves stress and pressure. But, frankly debating the issue here will likely have little to no impact on whether or not ASCE 7 is changed for the better. Regards, Scott Adrian, MI -----Original Message----- From: Paul Feather [mailto:PFeather(--nospam--at)se-solutions.net] Sent: Thursday, February 14, 2008 9:07 PM To: seaint(--nospam--at)seaint.org Subject: RE: IBC 2007 Wind calcs. Scott, We use a static seismic procedure for most buildings as the common method. If we have a particularly large or tall or unique structure we go outside the realm of static analysis and must move up to a more rigorous analytical approach using dynamic methods. Under UBC, I have designed hurricane area structures, but even under UBC any building in excess of 400 feet was outside the code limits and required a wind tunnel analysis. Kind of the wind equivalent dynamic analysis if you will. There is a place for having complex analytical procedures at your disposal, and there is a place for having common procedures that fit the majority of conditions. You feel the general ASCE method is that common procedure, I feel the ASCE missed by quite a large margin and could have developed a much more user friendly common method that would work economically for 90 percent of the structures. I am not against having the more complex procedures, only against requiring their use to design a barn, or a big box store. If you are in a high wind region, then the option to use the more rigorous analysis methods for economy and efficiency is fantastic, and I agree with the need. I would add that wind tunnel testing is still going to be the primary method for significant structures, and that in high wind areas maybe the bar could be lowered to trigger the use of theses methods sooner. But honestly if you are in a 140 mph region the difference between 143 psf and 150 psf is only 5%, and that is a 7 psf spread. I would also argue that in those high wind regions it is good detailing that prevents damage much more so than refining the wind pressure the extra few percent; the same way good detailing is what ultimately provides good seismic performance rather than knowing the actual ground motion to within a few percentage points. Paul Feather PE, SE pfeather(--nospam--at)SE-Solutions.net www.SE-Solutions.net ******* ****** ******* ******** ******* ******* ******* *** * 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. 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- RE: IBC 2007 Wind calcs.
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- RE: IBC 2007 Wind calcs.
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