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RE: 97 UBC 1633.2.4 Deformation compatibility - is that a typo?

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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.


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