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Re: R-values in Seismic Provisions
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- Subject: Re: R-values in Seismic Provisions
- From: "Bill Sherman" <SHERMANWC(--nospam--at)cdm.com>
- Date: 15 Sep 98 19:31:03 -0400
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 USD, then THEORETICALLY it would have yielded at a base shear value of the elastic 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 for. 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 actually "overdesigning" by a factor of 2 with respect to elastic strength? Note that in Table 16-O, the Rp for "rigid components with nonductile material" is only 1.5. This seems closer to what I would expect from overstrength.
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