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Fwd: R-values in Seismic Provisions

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T. Eric Gillham wrote:

> If I were to design a single cantilevered column, supporting a mass, fixed
at the base, EXACTLY for the Vb associated with an R factor of 2.2, using
then THEORETICALLY it would have yielded at a base shear value of the
base shear/2.2, right? <

Yes, or you could say that the expected fully elastic seismic shear value
would be 2.2 times greater than the shear you have designed for.

>It would seem reasonable, in my mind, that the ... factors (overstrength,
period elongation, damping) would drop the R factor down from 2.2 to 2.  So,
at least according to UBC97, it would seem that an R factor of about 2 is
appropriate for an "elastic" response, at least the way I see it. <

Sorry you lost me here.  Allowing for a reduction in R from 2.2 to 2.0 still
means the fully elastic base shear is 2 times the value you have designed
 So the wall will yield?

It is true that in the definition of "R", the code lists both "inherent
overstrength and global ductility" as factors.  I'm just surprised that when
designing an element such as a cantilevered column using USD, we are
"overdesigning" by a factor of 2 with respect to elastic strength?  Note
in Table 16-O, the Rp for "rigid components with nonductile material" is
1.5.  This seems closer to what I would expect from overstrength.

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