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Re: I crack.

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You might want to look also at the concrete text by Park and Pauly (from the 70's I think) where you can look at moment curvatures to determine the Ieff.  From the moment curvatures you can get the Ieff, but you need to know the reinforcing and cross section of the wall.  I cracked for the walls can easily be in the range of 10%-30% of Igross. If its a shear wall type building, Ieff will vary as you go up the building. maybe 20$ at the base, 45%-60% at the midheight, and somewhere between 70%-90% at the upper floor levels.  But Ieff has a lot to do with the reinforcing and the confinement being provided.  The moment curvatures can be applied to beams and columns as well. 
Hopefully the reinforcing is symmetrical, other wise you have alot of calculations to do to determine Ieff since you will have to consider the direction of the seismic forces acting on the elements (coming from one direction, and then in the opposite direction and the variation in seismic axial loads (example: Axial + seismic, Axial - seismic).
You may be able to get a program from someone that does moment curvatures calculations.  This can be time consuming. Hopefully there isn't alot of variabililty in column, wall, beam types in your building.
Mike Cochran
In a message dated 9/20/2004 1:15:57 PM Pacific Standard Time, pcrocker(--nospam--at) writes:
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