flat slab stiffness

huangyhg

flat slab stiffness
hello,
i am looking into an 1917 five stories concrete building. it is a flat slab construction with column capitals. what is the best way to figure out the lateral stiffness of the flat slab? seismic forces govern and we ahve to figure out the existing lateral capacity of the building before we can add 2 more storeis to the building.
thanks in advance for the replies.
i'd be a little afraid to take lateral load into a flat slab of that age unless you know the reinforcing layout. back in those days flat slabs were typically proprietary systems with some strange reinforcing layouts. i'd personally push to add shear walls or other bracing. also, if you're in a seismic area this may not even be permitted.
the lateral capacity of a 1917 reinforced concrete building is probably near zero, per code numbers.  the seismic detailing is not there, at all (laps, ties, hoops, etc.)   adding two stories will increase your lateral and gravity forces more than 5% which will trigger ibc 3403.2.
i would talk to your building official and see what they want to see.  
thanks for the reply. can we use the equivalent frame method to arrive at some sort of stiffness values for the slab and columns?