Sagar

1 hour ago, Rana said:

Yes, put the dead and live loads manually to the columns to keep things simple and if resulting column reo is not that much.

I'll try doing it this way:

I will model and analyze the truss only with fixed supports in bottom. And then  input the support reactions to the two end columns (example: A2 and A5 columns in elevation A-A) of the truss ?

But the columns A3 and A4 are also providing supports to the truss. Should I provide supports in their positions while modelling truss? If not, they take at least the dead load of the truss. What should be done?

16 hours ago, Rana said:

When you remove the steel stress at top, your concrete columns become cantilever giving you 'satisfactory' or increased reo because of unbraced length. When you put in the steel structure, concrete columns become braced and hence less effective length.

I would not consider steel members to brace top story concrete columns.

Will modelling the top floor with cantilever columns give me an approximate result?

If yes, should I input dead load and live loads of truss to the model?

Thank you 

@Rana

Will modellung the top floor with cantilever columns give me an approximate result?

If yes, should I input dead load and live loads of truss to the model?

Thank you

I completed the modelling of the building in two models; one with concrete parts only and other with only the top floor modelled.

I exported the support reactions of column at the base to excel and calculated the maximum value of each value Fx, Fy, Fz, Mx, My and Mz for each joint at the bottom using Excel Macro.

Then I input those values in the joints of model with concrete part only at the top of story 2.

The results seem quite satisfactory.

Can I do it like I mentioned above?

I have modelled a RCC building with a steel roof truss atop. Now when I run analysis, the reinfforcement in columns etabs shows is always the minimum reinforcement required. I cannot figure out why this is happening.

Could anyone help me?

Also,

I remodeled it another time with all of steel components removed.

The analysis result was satisfactory then.

Why cant I get similar result with truss atop?