how would steel bars effect the modulus of elasticity of concrete section? I need the moment of inertia and modulus of elasticity of reinforced concrete section with effect of percentage of steel. actually I need it because I need stiffness of column to calculate the stiffness of whole story of the building. I modeled a RC building in etabs. etabs gives me stiffness of the story after I run the analysis. but i haven't even run design. How did it calculate the story stiffness even before the reinforcement percentage in column is determined. Is it calculating stiffness of columns with no reinforcement in it ? or is it calculating stiffness of column with minimum required percentage of steel? will it change the stiffness of columns and then the stiffness of whole story after I run design in accordance with the amount of steel it calculates for each column? is the stiffness of the whole story sum of stiffnesses of all the columns of that story?