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RE: 97 UBC 1633.2.4 Deformation compatibility - is that a typo?
[Subject Prev][Subject Next][Thread Prev][Thread Next]- To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>
- Subject: RE: 97 UBC 1633.2.4 Deformation compatibility - is that a typo?
- From: "Swingle, Mark" <Mark.Swingle(--nospam--at)dgs.ca.gov>
- Date: Tue, 16 Nov 1999 19:38:13 -0000
- Cc: "'mswingle(--nospam--at)earthlink.net'" <mswingle(--nospam--at)earthlink.net>
Dennis, let me give it a try: Let's say you have a standard occupancy (I=1) building with 12' story heights. Let's say that under the 1994 UBC it has a drift of 0.72" under code base shear, which equals 0.005H (the max allowed under 94 UBC 1628.8.2). Let's say it is in Zone 4 and Rw=8. If it is a short period building, then C=2.75, and under 94 UBC we get V=ZICW/Rw=0.14W. Remember those days? Remember: 94 UBC max drift, V=0.14W. Now, let' look at the 1997 UBC. SAME BUILDING. SAME ELASTIC STIFFNESS PROPERTIES. Under 1997 UBC we have V=2.5CaIW/R. Not within 15km of fault. Now Ca=0.44 and R=5.5, so E=V=0.20W. Same building, higher force (factored now), so the drift now is 0.72"x(0.20/0.14)=1.03". This is delta(S) per 1630.9.1. Now find delta(M). Delta(M)=0.7Rdelta(S)=0.7(5.5)(1.03")=4.0". Compare this to the max allowed by 1630.10.2 which is 0.025H which in this case is 3.6". No good. Drift too large by 10%. FOR THIS CASE the 97 UBC criterion is 10% stricter than the 94 UBC. Here's another way to think about it: In the old code (94 UBC) the drift limit was for base shear forces which are "actual" forces REDUCED by Rw. It was known that the real drifts were larger. Where knowledge of ACTUAL drift was important in the old code, this drift was multiplied by 3Rw/8. Now in the new code (97 UBC) we try to look at real drifts by multiplying the drift due to base shear by 0.7R. Hence in the deformation compatibility section (1633.2.4), there is no need to further increase the drift because it is already similar to the "3Rw/8" level. The drifts have not been increased, but the terminology has changed. FOR THIS CASE, the calculated drift has increased by a factor of R=5.5 (since the 0.7 factor cancels the 1.4 increase in base shear), but the max ALLOWABLE drift has also increased, but only by a factor of 5 (0.025/0.005), again a difference of 10%. The new criterion is stricter. Other situations may lead to slightly different results. Mark Swingle, SE Oakland, CA --------------------------------------------------- Dennis Wish wrote: Hey, I'm still confused. I know I got into this discussion late, but I am attempting to work through it now and don't see how increasing the interstory drift benefits the structure. The code sets Delta-M as 0.025 times the story height. The Seismic Design Manual for ICBO gives this figure in inches so that for a 12-foot story height of 3.6-inches. This seems extremely high considering that past codes set the limit for story drift at 0.005H or less than 0.72 inches for a 12-foot story height. What am I missing here? Can you explain it to me in simplistic terms (since I must be missing something). I appreciate the help Dennis
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