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RE: Quick/Easy Question for Seismic List
[Subject Prev][Subject Next][Thread Prev][Thread Next]- To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>
- Subject: RE: Quick/Easy Question for Seismic List
- From: Steve Hiner <shiner(--nospam--at)folsom.ca.us>
- Date: Thu, 23 Mar 2000 15:28:18 -0800
I'm with you Ben. This may help, but then again ... consider steel SMRF w/ 13' story heights- 1994 UBC (T<0.7 sec.): max. drift = 0.005(13')(12"/') = 0.78" = (0.04/12)(13')(12"/') = 0.52" ... governs (at ASD "elastic" level drift) 1997 UBC (T<0.7 sec.): max. drift = 0.025 (13')(12"/') = 3.9" ... (at maximum "inelastic" level drift) To compare 1997 UBC drift to 1994 UBC drift, you need to divide by 0.7R to get to elastic level and also divide by 1.4 to get to ASD level, therefore: (3.9")/(0.7)(8.5)(1.4) = 0.47" ... approximately equal to 0.52" per 1994 UBC (unless I blew a digit some where, which wouldn't be the first time). Bottom line, drift should be in whatever units you multiply by the maximum allowable "drift ratio" (i.e. 0.025, etc.). Whew! Steven T. Hiner, SE Folsom, CA -----Original Message----- From: Yousefi, Ben [mailto:Ben.Yousefi(--nospam--at)ci.sj.ca.us] Sent: Thursday, March 23, 2000 1:42 PM To: 'seaint(--nospam--at)seaint.org' Subject: RE: Quick/Easy Question for Seismic List It's amazing how such a simple issue, which was clarified earlier, has turned into a huge discussion item. The drift is always a percentage of the story height, hence unitless. There are no changes from 94 to 97 UBC in the definition of building drift. The only change is the allowable limit, which by first inspection seems excessive. But that is only because the calculated deflection (at strength level) gets multiplied by 0.7R. I sure hope that I am not missing something here, otherwise we have been checking drift calculations incorrectly for almost 9 months now! Ben Yousefi, SE San Jose, CA -----Original Message----- From: SEConsultant [SMTP:seconsultant(--nospam--at)earthlink.net] Sent: Thursday, March 23, 2000 1:12 PM To: seaint(--nospam--at)seaint.org Subject: RE: Quick/Easy Question for Seismic List Paul, It's a shame that the evolution of the code equations have not been documented so that these issues are simply not questionable. I personally think this is a failing of the code writers (again). There is a general opinion that empirical formulas often use arbitrary constants and that the final units are assumed. For example, the first term of the diaphragm deflection formula (for blocked diaphragms) represents the chord deflection. We had a number of debates that this was an empirical formula and that it was assumed in inches to match the other three components of the formula. However, one or two engineers responded to the list and recreated the derivations of the formula from the basic deflection formula to the one used in the code. The constants assumed to be unitless were, in fact, conversion factors. I am not convinced that at first glance we should make assumptions as what the final units should be. It is the responsibility of the code writers to insure that the formulas and glossary of terms is written so that the engineer understand what units of measurement is expected as input. This way, the engineer knows whether or not he or she needs to convert the formula. This one short thread has had very strong opinions from each side. When we thought that it did not matter if the we were to consider the height in feet or inches, Shafat's opinion is that the formula is unit dependent and therefore it matters. I'm still confused and think the answer should come from the "horses mouth". Regards, Dennis S. Wish, PE -----Original Message----- From: Paul Crocker [mailto:PaulC(--nospam--at)ckcps.com] Sent: Thursday, March 23, 2000 10:44 AM To: seaint(--nospam--at)seaint.org Subject: Re: Quick/Easy Question for Seismic List Why do you assume that if you put feet into the equation that it yields something other than feet as its result? It is also acceptable to use meters for story height. Surely this could not be so if the 0.02 was calibrated to make a feet to inches conversion. If the EERI web site still has the field report from the earthquake in Columbia last year (?) posted, it might be worthwhile to read this, as it briefly discussed observed correlations between drift and damage/failure, which largely agreed with the pre-existing body of research. Depending on your anticipated loads and building use, getting too close to the 1.48' drift may not be appropriate, but I do not see it as a code violation. Also, if you get too close to the drift limit, drift will exceed the threshold beyond which P-Delta analysis cannot be ignored, which may cause problems of its own. Overall, though, if a maximum inelastic drift limit approach is taken, a limit of 1.48' for a 74' building is not surprising. Surely moment frames would never be possible if a 1.48" level of stiffness was code required. In fact, only the longest shear walls could possibly comply with that. Try to visualize 1.48" in a 74' building under extreme loading and imagine if that would be possible. Paul Crocker
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