For design of reinforced concrete structures using sap2000 or Etabs, the cracked section properties should be used. this can be done by using Assign>Frame>property modifiers

Assuming beams and columns are modeled as frame then the stiffness modifier table is as follows: 

ACI                                                                      Sap2000 (or ETABS)

Beams........................................0.35*Ig               I22 = I33 = 0.35

Columns....................................0.70*Ig               I22 = I33 = 0.70

Walls-Uncracked.........    ........0.70*Ig               modeled as shell – f11, f22 = 0.70  

Walls-Cracked..........   ............0.35*Ig                similar to Walls-Uncracked (with                                                                                           modifiers of 0.35)

One can not represent cracks display as far as my knowledge is their. But what you can do is after constructing the column you can display the stresses. 1-1 direction 1-2 or maximum stresses. And by studying the stress lines you can study the development o cracks. 

Dear Ahmed

you can not represent cracks on sap2000. I can advise you to model it using ANSYS

You can not represent cracks. However if you do a 2D model you can make a mesh refinement and remove the elements in correspondence of your crack.It could be a rough geometric approximation and you could study stress concentration

In order design the column you can modify the stiffness of the columns.(this modification is a simple representation of cracks). To consider cracks sap2000 does'nt work and you have to use other FEA softwares like Ansys or Abaqus.

For design of reinforced concrete structures using sap2000 or Etabs, the cracked section properties should be used. this can be done by using Assign>Frame>property modifiers

Assuming beams and columns are modeled as frame then the stiffness modifier table is as follows: 

ACI                                                                      Sap2000 (or ETABS)

Beams........................................0.35*Ig               I22 = I33 = 0.35

Columns....................................0.70*Ig               I22 = I33 = 0.70

Walls-Uncracked.........    ........0.70*Ig               modeled as shell – f11, f22 = 0.70  

Walls-Cracked..........   ............0.35*Ig                similar to Walls-Uncracked (with                                                                                           modifiers of 0.35)

Giovanni Minafò , your answer is exactly the solution to a problem i am currrently facing in my thesis. However, I need a solid reference to some work which has used this before. Can you recommend me where i can find more on this ? It would help me a lot. Thank you very much

Check the following reference Please.. it might's assist you in your question … Regards…

http://www.vis-concretedesign.com/reinforced-concrete-design-for-sap2000-etabs/