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Re: More Drift Talk
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- Subject: Re: More Drift Talk
- From: "Mehdi M. Khabbazan" <MKhabbazan(--nospam--at)allott.co.uk>
- Date: Wed, 14 Feb 2001 17:50:29 +0000
Mike I think the confusion is due to the definition of the design seismic forces which has been used in 97UBC. In the first sentence of Section 1630.9.1, it has been quoted that the DeltaS should be calculated using E forces from section 1630.2.1 wherein the seismic base shear has been introduced with no load factor! Further down in the third sentence of the same section for load combination reference has been made to Section 1630.1.2 wherein again a load factor of 1.0 has been used for E. According to Section 1612.3 which relates to allowable stress design the seismic force should be divided by a factor of 1.4. As it is seen neither of these sections increase the seismic force from what has been given in Section 1630.2. It is becoming a Paradox isn't it!! Mike Valley wrote: > Despite an earlier erroneous ICBO erratum, ALL seismic drift > calculations must use strength-level loads. ICBO is issuing another > erratum to undo the one that is reflected in the Third printing. > > I reported this error to ICBO some time ago. Here is their response: > "A new errata that will soon be published in the Building Standards > Magazine and included on our website will revise the reference section > number in the third sentence of Section 1630.9.1 from Section 1612.3 > to Section 1612.2." > > Since the demands Chris is considering are "service level", his > application of a 1.4 factor to acheive strength-level loads for drift > checks is correct. > > - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - > Michael Valley, P.E., S.E. E-mail: mtv(--nospam--at)skilling.com > Skilling Ward Magnusson Barkshire Inc. Tel:(206)292-1200 > 1301 Fifth Ave, #3200, Seattle WA 98101-2699 Fax: -1201 > > -----Original Message----- > From: Mehdi M. Khabbazan [mailto:MKhabbazan(--nospam--at)allott.co.uk] > Sent: Tuesday, February 13, 2001 3:16 AM > To: seaint(--nospam--at)seaint.org > Subject: Re: More Drift Talk > > Chris > I did not quite understand that why you are using a factor of 1.4 when > calculating DeltaM. According to 97UBC1630.91 & 2, DeltaS should be > computed based on the load combinations of section 1612.2.2 > considering > PDelta effects wherein the seismic load factor is equal to unity. > > Mike Valley wrote: > > > Most engineers have long realized that 3Rw/8 x service level > > displacement grossly underestimates the real, expected deflections. > > Using Newmark's "equal displacement rule", the expected inelastic > > displacement is roughly equal to the unreduced elastic displacement. > > (Structures with very short periods displace even more--according to > > the "equal energy rule.") The 1997 UBC displacement calculations > are > > closer to the expected value (0.7 times the unreduced, elastic > > displacement rather than 3/8 of the unreduced, elastic > displacement). > > > > Also note that while the "maximum inelastic response displacement", > > delta_M, is larger than what was previously calculated, the > separation > > between adjacent buildings may be based on the SRSS of the > individual > > building displacements. The SRSS of the displacements is based on > the > > fact that two separate buildings are unlikely to acheive maximum > > displacements toward each other simultaneously. Using your example > > (and assuming that the two adjacent buildings have equal > > displacement), the change is really only 1.8/1.414 = 1.27 times the > > separation required using the 1994 UBC. > > > > - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - > - > > Michael Valley, P.E., S.E. E-mail: > mtv(--nospam--at)skilling.com > > Skilling Ward Magnusson Barkshire Inc. > Tel:(206)292-1200 > > 1301 Fifth Ave, #3200, Seattle WA 98101-2699 > x: -1201 > > > > -----Original Message----- > > From: croper(--nospam--at)bjgse.com [mailto:croper(--nospam--at)bjgse.com] > > Sent: Friday, February 09, 2001 11:03 AM > > To: seaint(--nospam--at)seaint.org > > Subject: RE: More Drift Talk > > > > Mehdi, > > > > I understand what you are saying. Take a braced frame system you > > spoke of > > as an example and I'll try to illustrate what I don't understand > using > > Building Separations requirements (94' UBC 1631.2.11 / 97' UBC > > 1633.2.11) > > 94' UBC: Separation = 3Rw/8 x service level displacement = 3 x > > service > > level displacement (for brace frames with Rw=8). > > 97' UBC: Separation = DeltaM displacement = .7R x DeltaS > displacement > > = > > 3.92 x strength level displacement (for brace frames with R=5.6) = > > 3.92 x > > 1.4 x service level displacement = 5.49 x service level > displacement. > > Compare the two and the actual separation required by 97' UBC is 1.8 > > times > > what it was in 94'. > > Am I wrong in my reasoning or understanding of the provisions? If > > not, why > > does an expansion joint now needs to be 1.8x what it used to be. > > > > -----Original Message----- > > From: Mehdi M. Khabbazan [mailto:MKhabbazan(--nospam--at)allott.co.uk] > > Sent: Friday, February 09, 2001 9:37 AM > > To: seaint(--nospam--at)seaint.org > > Subject: Re: More Drift Talk > > > > Chris > > The story drift according to 97UBC is calculated based on the > > amplified > > seismic > > lateral load and the amplification factor is 0.7R. This, for an > > ordinary > > steel > > braced frame will become approximately equal to 4, meaning that > > seismic > > lateral > > loads will be enhanced by a factor of about 4 when calculating story > > drifts. > > The > > drift limits on the other hand have been increased by a factor of > > about 5 in > > this code, and therefore the final result is not as bad as you > > thought. > > Mehdi Khabbazan > > PhD, CEng > > > > croper(--nospam--at)bjgse.com wrote: > > > > > Can someone explain why the drifts/building separations have > > increased so > > > much from 94' UBC to 97' UBC? Was research done to substantiate > the > > change > > > or is it simply a by-product of the change in methodology from > > service-level > > > to strength-level design forces? > > > The new DeltaM drifts are approx. 5x what they were in 94' UBC, > but > > then > > > again the new drift limits have changed correspondingly which > makes > > sense > > to > > > me. What doesn't make sense is the fact that now when you take a > > look at > > > the separation between two structures (for example) it is almost > 2x > > what > > it > > > was in 94'. Is there a reason an expansion joint (for example) > now > > needs > > to > > > be twice the size it used to be? > > > I know these questions and this thread may get old, but input from > > the > > more > > > knowledgeable (dare I say older) engineers is invaluable to us > > younger > > ones > > > trying to make some sense of the seemingly overcomplicated codes > we > > have > > > been talking about so much lately. > > > > > > TIA > > > Chris Roper >
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