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- Subject: Fw: Dual Systems
- From: "J+R" <jeffrandi(--nospam--at)adelphia.net>
- Date: Tue, 28 Jan 2003 22:50:02 -0500
Thanks for the input. Just to clarify: The reason I mention excluding the flexible system is because the criteria for a dual system states to 'resist total design base shear in proportion to their relative rigidities'. This then excludes a truly flexible system which would be distributed solely on tributary area. And to further clarify the previous question: What would be the advantage of classifying a system as a dual system (R=4.2), say for the case of masonry shear walls and Steel OMRF,when you could use the lower of the two systems (R=4.5) per combinations of systems along the same axis? Thanks Jeff Morris From: "Gerard Madden, PE" <gmadden(--nospam--at)attbi.com> To: <seaint(--nospam--at)seaint.org> Subject: RE: Dual Systems On the 25% rule, you take you base shear and multiply it by 0.25. Then you make your shearwalls or braced frames inactive in your model leaving only the flexible moment frame active in the analysis and design the frames for this force. You choose the R value from the classification in the UBC Table for the specific dual system. You do not exclude the flexible system in your model with 100% base shear. By the fact that it is much less rigid, it will get a very small amount of load.=20 HTH, -gerard Santa Clara, CA -----Original Message----- From: Peter Griem [mailto:griem(--nospam--at)slamcoll.com]=20 Sent: Friday, January 24, 2003 9:35 AM To: 'seaint(--nospam--at)seaint.org' Subject: RE: Dual Systems On a related note... In order to satisfy the 25% requirement, do you have to proportion your more flexible system to be stiff enough to carry 25% of the load, or do you design it separately for a value equal to 25% of your base shear? =20 Back to the original question, my take is that if you have say MF's and SW's in the same direction you must use the lower R value -unless- you specifically detail the building for the special requirements associated with dual systems. But once you choose that SW R value you'd have to design/detail so that only the SW's make up you lateral force resisting system. If you distribute the loads to some MF's based on rigidity and you count it in your analysis I think you technically have a "Shear Wall Frame System" which is only allowed in the lowest seismic zones and only applies to concrete shear walls. All of which leads back to my question about the 25% rule... Any clarification would be appreciated. Peter -----Original Message----- From: Yousefi, Ben [mailto:Ben.Yousefi(--nospam--at)ci.sj.ca.us] Sent: Friday, January 24, 2003 11:13 AM To: 'seaint(--nospam--at)seaint.org' Subject: RE: Dual Systems The only time you get penalized for choosing a dual system is when you utilize a more rigid system, such as shear walls or braced frames combined with moment frames. The rationale for that is that, the SMRF is a much more ductile system (or supposed to be!) so when you mix it with a system of less ductility you should use a smaller R value. However, if your dual system is comprised of EBF's and SMRF's you'll get the highest possible R, since both systems have substantial ductility. And, if you mix different systems along the same line of resistance you can either try to qualify it as dual or choose the lower R value for base shear calcs. Ben Yousefi, SE San Jos=E9, CA -----Original Message----- From: J+R [mailto:jeffrandi(--nospam--at)adelphia.net] Sent: Thursday, January 23, 2003 7:48 PM To: seaint(--nospam--at)seaint.org Subject: Dual Systems I am looking for some insight why dual systems are penalized in regards to R value over each individual system (I know this has been asked in the past but I could not find any explainations). It seems odd that a more redundant system is penalized with higher design loads than if that axis had all the same type of lateral resisting elements. Once classifed as a dual system the shear is distributed based on the rigidity of the elements regardless if the diaphragm is flexible or rigid per UBC and IBC. Is it required that the system be classified as a dual system if say moment frames and shear walls occur along the same axis OR can the lower R value of the moment frame/shear wall be used and the shear distributed based on the classification of the diaphragm? Thanks Jeff Morris ******* ****** ******* ******** ******* ******* ******* *** * 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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