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RE: Drift Control

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-----Original Message-----
From: raul labbe [mailto:rlabbe(--nospam--at)]
Sent: Tuesday, March 13, 2001 1:55 PM
To: seaint(--nospam--at)
Subject: Drift Control

Unfortunately, I have just an old version of the SEAOC
Recommended Lateral Force Requirements,1990.
Regarding seismic story drift limitation :
1  SEAOC ( 1990 ) specified that the ' calculated story drift shall not exceed . . . etc.'
    I assume that in here, this 'calculated story drift' is to be the elastic one. ?
*****Yes, it is the elastic drift calculated  using the design lateral forces (Rw>1) - that is why the drift limitations include an Rw factor in the denominator position.  See page 14, section 8.
2  SEAOC, when dealing with deformation compatibility, e.g  non-shear wall panels
    infilled in RC frames, specifies to provide the gap in between the two systems, by
    considering the calculated elastic story drift, amplified by the factor 3(Rw/8), assuming
    that this will allow for the actual deformations.
3  The philosophy underlying in other codes (e.g. the chilean codes ), considers that,
    since a response modification factor Rw is used reducing the actual loads to a design
    level, therefore for determining actual deformations, the calculated elastic deformations
    are to be amplify by the full Rw factor.
The actual expected deflections during the design earthquake are dependent on a number of offsetting factors.  See the 1990 SEAOC Bluebook, page 25-C for a discussion of the origin of the 3(Rw/8) factor (it is basically a carryover from previous codes, which were based on an educated guess as far as I can tell).  This section, as well as 1E.8.b on page 31-C, both state that 3(Rw/8) can be unconservatively low in some cases, and basically that the EOR has to use engineering judgement to determine the appropriateness of this ESTIMATE.
If you want a detailed discussion of the Bluebook bases and assumptions (essential for interpretation of the requirements contained therein, IMO), see Appendix 1E2a-Rw, pages 146-C through 155-C.  This section explains the demand curve/capacity curve approach assumed by the Bluebook (and echoed, I think, in ATC-40 to an extent) - as the earthquake shakes the structure, overstrength and participation of "nonstructural" elements increases the capacity for lateral force resistance, while lengthening period and damping lowers demand on the structure, and the two are supposed to meet at some point short of structural collapse.
One note concerning point #2 above - the 1990 Bluebook doesn't, as I recall, really hammer on the point of cracked sections, but the 1999 Bluebook and IBC both do.  It should be remembered that the designated ductile detailed members (the LLRS) will be cracked quite a bit during the actual EQ, while isolated elements such as your infilled walls, will NOT sustain the same degree of cracking.  This means that the "nonstructural" elements will have a higher RELATIVE stiffness (comparing pre-EQ to post-EQ) than the LLRS elements.  This should be accounted for by reducing the stiffness of the LLRS elements to make sure that the structure isn't going to soften to the point that the
3(Rw/8) estimate will be unconservatively low.
Hope this helps. 
T. Eric Gillham PE
Question :  In the SEAOC Provisions, where does the 3(Rw/8) factor come from ?
                    Why a so low factor to get the actual drifts ?
 NERHP 1997 Provisions recommend a factor close to 0.69 Rw for RC frames.
 This is higher than the SEAOC's, but we are still below the full Rw.
Any hint on this issue, from the people involved in these codes, will be truly
Eng. Raul Labbé