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Re: I crack.

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There are a number of references you could check out, but the values you have been given are reasonably defensible.  If you have access to the seismic design of concrete book by Paulay and Preistley, you could get some more information to back you up with your assumption.  Also, outside of the hinge zone of the wall, you could make a convincing argument that the Ieff is much greater than that range once you are outside the zone where cracking is expected to be its greatest.  Alternately, you could calculate the actual values for two or three locations up the building, and the use those.  Your drift may change quite a bit as you vary Ieff, but if you are at or near code minimum forces levels the forces may not change much, so you may not have to iterate Ieff much or at all.  All this is assuming that your building is similar to the half dozen or so 15 story buildings I have been involved with, which it may not be.  Good luck.  

Paul Crocker, PE, SE

>>> "Alex" <seaosd(--nospam--at)> 09/20/04 12:08PM >>>
I am working on a 20 story with concrete shear walls. for calculation of
Ieff we use formula (9-8)(318/318/R-103) of ACI. which needs calculation
of Icrack.
Icrack is again a function of steel reinforcement in the section (which
is not known before analysis) and I think practically it is almost
impossible to calculate it for each section at each story with lots of
trial and errors. I have been advised to use [ Icrack=(0.35 - 0.5) x
Igross ] that I don't feel comfortable with it.
Any experience or advise on that would be really appreciated.
Alex M,PE

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