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RE: Quick/Easy Question for Seismic List
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- Subject: RE: Quick/Easy Question for Seismic List
- From: "SEConsultant" <seconsultant(--nospam--at)earthlink.net>
- Date: Thu, 23 Mar 2000 13:12:01 -0800
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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