In seismic design of upgraded structures composed of two different load-bearing construction systems divided by elevation it was found that separate seismic analysis is not allowed and that modal analysis of composed model of full structure has to be performed. It is important to limit the additional upgrade mass to keep the seismic forces in the range of the previous level on existing building. Seismic forces depend also on the natural period of the structure and they are decreasing with increasing of the natural period of the structure. It depends on the composition of the system where rigidity and weight of the upper structure is of the major importance. If the upgrade structure is too flexible a whip effect with amplified horizontal deformations at the top of the upper structure could appear. Regarding the EC 8 requirements, it was pointed out that there is a gap in determination of q factor without regulation for seismic design of composed structures along elevation.In conclusion, the main characteristics which should affect the decision of a type of upgrade structure are summarized.
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