Mr Surana

 I really appreciate your help.

Would you mind explain about ultimate displacement? does the intensity at which unstability or collapse occurs, show ultimate intensity? 

and about  multi-records IDAS, should I find yield and ultimate displacement for each record separately?

This depends upon the structure. For a simple SDOF (single-degree-of-freedom) oscillator, the yield displacement is simply the yield strength divided by the initial stiffness. The displacement ductility demand (to distinguish from the curvature ductility demand - they are very different) is then simply the maximum displacement divided by the yield displacement, and is different for each ground motion analysis (the yield displacement is not). For a MDOF, there is, strictly speaking, a displacement ductility demand on every member of the structure. The largest may or may not be the most critical. A well designed column might have a displacement ductility capacity of 6, while a poorly detailed column might have a displacement ductility capacity of 3.

It is possible to convert an MDOF into an equivalent SDOT in some cases. See:

Priestley, M.J.N. and Mervin Kowalsky, "Direct Displacement-Based Seismic Design of Concrete Buildings", Bulletin of the New Zealand Society for Earthquake Engineering, 33(4), December 2000, for example. Best of luck in your work.

Thanks for your kind help.