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Re: More Drift Talk
[Subject Prev][Subject Next][Thread Prev][Thread Next]- To: seaint(--nospam--at)seaint.org
- Subject: Re: More Drift Talk
- From: "Mehdi M. Khabbazan" <MKhabbazan(--nospam--at)allott.co.uk>
- Date: Tue, 13 Feb 2001 11:16:22 +0000
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 Fax: -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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