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[SEAOC] Re: [SEAOC] ATC 19, "Structural Response Modification Factors"[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: "INTERNET:fmcclure(--nospam--at)ccnet.com" <fmcclure(--nospam--at)ccnet.com>, seaoc <seaoc(--nospam--at)power.net>
- Subject: [SEAOC] Re: [SEAOC] ATC 19, "Structural Response Modification Factors"
- From: Richard Phillips <74562.212(--nospam--at)CompuServe.COM>
- Date: 22 Sep 96 20:31:14 EDT
- Cc: seaoc <majordomo(--nospam--at)seaoc.org>
In reference to Frank McClure`s comments about the recently published ATC 19 (a funded study on the "R" factors), SEAOC Seismology Committee established in 1993 a subcommittee to study the same matter, which I chaired. Our work resulted in1997 UBC code change #178 which was approved by ICBO last week. Although many compromises and alterations were made to the original proposal, one of the original concepts remains, albeit in a somewhat altered form. We concluded, just as the authors of ATC 19 did, that the "Structural Response Reduction Factor", R or Rw, consists of three parts: Rd, the portion related to ductility; Ro, the portion related to overstrength; and P (rho),the portion related to redundancy. These concepts will also, hopefully, be incorporated in the 1997 NERHRP. The first part ,Rd, unarguably causes a global reduction in response. The only disconnect is for structure periods less than Ts/2 ( Ts is the charactistic period of the soil), where the linear reduction starts to fall apart. Fortunately, very few real structures would be stiff enough, and the soil column soft enough, where this effect is significant. The Ro factor, overstrength, is not global, but varies from element to element. In the present code this term is represented by 3Rw/8. It is also is handled in the present code by special "phi" factors used when considering seismic forces. Frank is entirely correct in stating that this factor can, in moment frames, be very sensitive to D+L/E (dead +live to seismic) ratios, and therefore to Seismic Zones. Hopefully, now that the concept is in the codes, we can work on consensus to have this factor include the effect of these other variables. The most mysterious factor, redundancy, P (rho), in the 1997 provisions will be a multiplier, rather than a reduction, to the foce used for member design. This factor varies from 1.0 to 1.5, and is presently only an encouragement to the designer to use a structure with the amount of redundancy that the Seismolgy Committee considers preferable. As I attempted to explain to Andy Whitaker, this ain`t science, it`s engineering. My big problem with AITC 19 is with it`s timing, they started long before we did and we could have used their help, and that their conclusions, as stated by Frank McClure, don`t in my opinion match the text of the work. Dick Phillips ...
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