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[SEAOC] Re: [SEAOC] ATC 19, "Structural Response Modification Factors"

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    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
    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