Question

in SAP2000 if i want to model encased (steel tube filled with concrete) frame then how should i model? means how should i define material for that?

Answer 1

You can achieve this by using frame/line element. The section of that frame will be defined from section designer.

1. Open Section Designer.
2. Define steel and concrete material
3. In section designer,define a tube using rectangular solid shape.
4. Change its material to required grade of steel.
5. Add another rectangular solid shape with same center as that of the first solid shape.
6. Height and width of this second section will be that of first section minus the thickness of tube
7. Change the material of second tube as Opening by right clicking it. 
8. Add the third rectangle having size of opening and material of concrete.

Verify your section with a manual example that is already solved. Hope this answer you question