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Fw: R-values in Seismic Provisions
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- Subject: Fw: R-values in Seismic Provisions
- From: "T. Eric Gillham" <gk2(--nospam--at)kuentos.guam.net>
- Date: Wed, 16 Sep 1998 08:06:39 +1000
Just to throw a little gasoline on the fire... Take a look at UBC97 Table 16-N for the R factor associated with Cantinlevered column elements. The table lists the factor as 2.2. 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? This of course is according to UBC97 (not necessarily in reality). Ron Hamburger has a good point in that there are factors which make the yield strength of a seismic resistant system higher than we assume for design: phi factors reduce capacity forcing us to increase the strength, the fact that I will likely put more reinforcement in the column than required (eg 16#9 bars as opposed to 15.2#9 bars), overstrength of the reinforcement (actual yield of say 66ksi as opposed to 60ksi). These would be the only factors which would increase the ACTUAL yield base shear (resulting in a decrease of R), since there is only one column so no redistribution of forces can occur, and since I am assuming there are no non-structural elements. One could expect to get period elongation, due to the decrease in Keff, over the course of the earthquake. There would also be some damping I imagine (which, interestingly, would lead to a higher maximum displacement). It would seem reasonable, in my mind, that the above 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. Anyone agree/disagree? T. Eric Gillham PE ---------- > From: Bill Sherman <SHERMANWC(--nospam--at)cdm.com> > To: seaint(--nospam--at)seaint.org > Subject: Re: R-values in Seismic Provisions > Date: Wednesday, September 16, 1998 4:36 AM > > Ronald O. Hamburger, thank you for your thoughts on this - but I am still > somewhat unconvinced. Your first comment relates to the "approximate period > formulae". But the noted R-value was not specified to necessarily be solely > for designs using the approximate period formulae. Some designs could use > more accurate period calculations or could be relatively rigid structures, > which are not impacted by the approximate formulae. (And as pointed out by > another responder, other features may actually reduce the actual period.) > > I do agree with your second comment that "Most structures incorporate > substantial overstrength." However, I am not sure this overstrength can be > generalized and I am not sure whether it is typically by as much as a factor > of 2. Thus I would concur that reality is that full elasticity generally > would occur at an R-value greater than 1.0 (but may not be as high as 2.0). > This can be a significant issue if one is attempting to keep a structure > "nearly fully elastic" in a design. Since yielding is permitted in ultimate > strength design, an R-value of 1.0 is certainly implied as the "theoretical" > value for full elasticity. > > (The speaker said that an Rw = 3.0 used to be the estimated limit of elastic > behavior using allowable stress design, now reduced to R = 2.0 for load > factor > design.) > > > > > > >
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