Addition to existing reinforced concrete building

[vasilios13]

Hello,
We are proposing to add a partial third story to a existing concrete building. The existing building was built around 1950 when there were no seismic design provisions (I am in a heavy seismic region). The entire building is concrete except for the roof, it is framed with wood joists. We propose to remove a portion of the wood roof, drill dowels through existing second and first floor slabs and install new anti-seismic columns all the way down to foundation level and frame the third story above. The wood roof will be replaced with a reinforced slab which will act as a diaphragm (as the existing ones do now). My concern is with the existing wood roof. There will be a discontinuity in the concrete third floor and the wood roof. During a earthquake, the two buildings will oscillate at different frequencies. Should I be concerned of this? I have attached a sketch.
Thank you

 

[youngstructural]

I would think you should be concerned; There is certainly the possibility of the two dissimilar top "sections" behaving badly if not fastened composite.

I would treat this by solving for which of the two has a lower frequency, then use this lower frequency with the TOTAL weight considered (ie: both stiff and softer parts together)to determine my forces), and fasten the two sections together composite.

Also, you can consider the loading imposed by the more stiff structure as that which needs to be resisted to keep the two halves together (should be conservative).

I would be quite keen to hear is anyone else has any other idea...

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[csd72]

If the existing lower section collapses what is to stop it causing the new section to collapse?