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FW: Bracing connection design forces
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- Subject: FW: Bracing connection design forces
- From: "Vasu, Ravi" <Ravi.Vasu(--nospam--at)jacobs.com>
- Date: Tue, 15 Oct 2002 11:57:21 -0400
Rick, Thanks. But as you have mentioned, it is confusing. I have gone through the UBC, ch. 22 & the'omega factor discussion' few times. It is not clearly defined and still one could interpret(from the words)any way. I am sort of saying YES to you. One more question. Pl. look at UBC section 2213.8.3.1(pg. 2-258), In UBC section 2213.8.3.1(pg. 2-258), the forces for bracing connection has 'The maximum force that can be tranferred to the brace by the system'(3 rd item). Does it mean, the load combinations to use to find the force is from section 1612.3(pg. 2-4) ONLY? or any other load combinations required? If the load combinations are only from section 1630.2, then forces from items 1 & 2 in section 2213.8.3.1 may not GOVERN at all.? (since we have to take LEAST force only?) I appreciate your response & thanks. Ravi -----Original Message----- From: Rick.Drake(--nospam--at)Fluor.com [mailto:Rick.Drake(--nospam--at)Fluor.com] Sent: Monday, October 14, 2002 3:54 PM To: seaint(--nospam--at)seaint.org Subject: RE:Overstrength Factor The SEAOC State Seismology Committee has posted a position paper on the web regarding the Overstrength Factor. http://www.seaoc.org/Pages/committees/seismpdfs/UBC/OmegaFactorDiscussion.pdf Part of this discussion specifically addresses 1997 UBC Chapter 22, Division V, quoted as follows: "In Chapter 22, Division V (ASD version) the Omega factor is specifically identified in multiple locations, often with the phrase omega-sub-o times the design seismic forces. Note that design seismic forces is defined in Section 1627 as the minimum total strength design base shear, thus the base shear should be factored up by 1.4 prior to applying omega-sub-o" The above is part of a larger discussion that I recommend you read. All current codes calibrate the overstrength factor (omega-sub-zero) reduction factor (R), and seismic coefficients (C-sub-a and C-sub-v) with respect to a Strength level earthquake. When you choose to size your members using working stress methods, you enter the confusing world of conversion factors. In the 1997 UBC, you convert the Strength basis E to working stress by a factor of 1.4 in the ASD load combinations. This requires that you convert back by multiplying times the same 1.4 for the seismic design of Chapter 22, Division V. There is no conversion confusion if you choose to size your members using strength design methods such as LRFD or ultimate strength design. Regards, Rick Drake, SE Fluor Daniel, Aliso Viejo, CA > -----Original Message----- > From: Vasu, Ravi > Sent: Friday, October 11, 2002 8:58 AM > To: seaint(--nospam--at)seaint.org > Subject: ASD load combinations in UBC '97, vol. II ------------------------------------------------------------------------ -- > Your following message has been delivered to the list > seaint(--nospam--at)seaint.org at 07:19:49 on 11 Oct 2002. > ------------------------------------------------------------------------ > > I have two questions(UBC 1997, Vol. 2) here. > Thanks in advance. > > 1 a. Refer to UBC '97, section 2213.5(Allowable stress > design(ASD)), pg. 2-255(column requirements) > The load combinations have 'omega' to check column compression & > tension. AISC, '97 'seismic provisions for structural buildings', > pg. 135(section c4.1) says that for all Allowable stress design, > E(from UBC) should be divided by 1.4. > So, in the above UBC section 2213.5, I think, from section > 1630.2,(to calculate Pe) should be divided by 1.4(since that section > in in ASD). In fact, in LRFD(section 2211.6.4), for the same check > on columns, we do not need to divide E by 1.4(from AISC on ' > seismic...). > > b. Similarly, for bracing connection design > and moment connection(UBC section 2213.8.3.1 & 2213.6 under "ASD"- > pg. 2-258 & 2-256), in the combinations with 'omega' times the > 'design seismic force', the E (from section 1630.2 procedure) should > be divided by 1.4 or not?. Pl. verify. > > 2. In UBC section 2213.8.3.1(pg. 2-258), the forces for bracing > connection has 'The maximum force that can be tranferred to the > brace by the system'(3 rd item). Does it mean, the load > combinations to use to find the force is from section 1612.3(pg. > 2-4) ONLY? or any other load combinations required? If the load > combinations are only from section 1630.2, then forces from items 1 > & 2 in section 2213.8.3.1 may not GOVERN at all.? > > I appreciate your time & response very much. > > Thanks > > Ravi > Telephone:832-351-6856 > > ======================================================================== ====================================================================================== NOTICE - This communication may contain confidential and privileged information that is for the sole use of the intended recipient. Any viewing, copying or distribution of, or reliance on this message by unintended recipients is strictly prohibited. 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