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Drift Control

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Hello,
 
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. ?
 
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.
 
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
appreciated.
 
Regards,
 
Eng. Raul Labbé