IDA curves in large sclaes

[mortezarazi]

in many articles and trusty works I'v seen that the shape of ida curve tends to be horizontal in scope of large scale factors (large Sa)
specialyy in rgular moment frames
as you know ida curve is in form of Sa versus maximum drift
seismostuct always gives an rising(ascending)form curve even in very large scale factors.
that means mechanisms dont form in stucture any way.
what's reason?

[seismosoft]

1) we are not sure that there is a “problem”; the results may well be perfectly justified

2) if there is a problem, on the other hand, it could stem from a modelling error, a results post-processing flaw, an erroneous plotting of response quantities, etc

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