high rise structure material

huangyhg

high rise structure material
most of our experiences are in 20 story max building design.
i was wondering, when it comes to over 40 story building, what kind of material would be suitable?  
reinforced concrete - with pt flat slab system, it works good in long span and better clearance for mech.  but when it comes to high rise, it seems to be a lot of unnecessary dead load to be carried by the columns and foundation.
steel - light and good for modium span length.  
question:
1.  at what height does steel becomes a better material to be used?  is there a rule of thumb for this?
2.  for a building with 40-50 stories, would concrete floor and concrete columns become uneffective due to heavy dead load of concrete vs steel?
i would appreciate if you could provide some advices.
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here are some pros and cons according to industry organisations of the two protagonists: