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Re: seaint Digest for 14 May 2000

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Respnose to the questions from: Jake Watson on Sunday 5/14.

>Has anyone gone through the concrete shearwall provisions in detail? My
question is this.  If you follow the code, you are allowed to assume yield
curvature as Ec/(Length of wall).  This amounts to assumed maximum concrete
strain (0.003) divided by wall length. Two questions come from this:

>1)  Is the point to find the concrete or steel yield point?
>2a) If it is steel, shouldn't the curvature be (Es+Ec)/(Length of Wall)
>2b) If it is concrete, shouldn't the curvature be Ec/C`u?

"Flexural yielding" of concrete shear walls is generally controlled by yielding
of the steel, not crushing of the concrete.  Unless a wall has been designed or
carries large axial loads, it generally will yield as a result of the
reinforcing steel yielding.  This is why the code provsion limits the amount of
axial load that may be carried by the wall and requires confinement steel for
walls that carry large axial loads.

The real yield curvature should indeed be (Es+Ec)/Length of wall.  If you want
to, the code permits you to calculate what this curavture is.  It takes a
somewhat tedious trial and error process, in which you must iterate to a nuetral
axis location.  Because many bars along the length of a wall will typically
yield, Es in the above equation is going to be somewhat higher than Ey = yield
strength/ Young's modulus.  For Grade 60 rebar Ey = 0.002.  The 0.003/Lw
formulation is intended to be a lower bound approximation of what you would get
from a real analysis.

The curvature indicated in 2b) above is the "failure" curvature, not the yield
curavture.  Also, Ec is only 0.003 for unconfined concrete. For confined
concrete, it gets much larger.