Why Percentage of Steel Higher in Top Floor Column?

Question 

Most of the time, for multistory building ETABS/STAAD Pro gives a percentage (%) of steel in terrace/top floor Column/Shear wall higher than that of the lower one. Is it correct or an errors?What may be the reason?

Answer 

First of all, it's not an error. It happens due to following reasons:

Hello Friends,

This is a blog for clarification of percentage (%) of steel is high in top floor than other floors. I will try to explain based on my understanding.

At Top story, all the moment from beam is transferred to bottom columns as there are no top columns to share the moment.

Top story columns have lesser axial force and moment from beams higher than bottom floors (as explained above). So they are subjected to higher tensile forces. An increase in moment demands more steel than an increase in compression. Hence Top story columns demand more steel than a bottom story.

This problem is very common for corner/edge columns of top stories.

Solutions can be following:

Solution 1-Specify simply supported beam for top floor. By giving MY and MZ release for all top floor beam. 

Solution 2- Keep number of bars same as bottom but increase diameter. 

Solution 3- For low rise buildings provide the higher percentage of steel right from bottom.