Its been many years since I last used sap2000, but I thought you defined the reinforcement for design purposes in the same window where you define the geometry for the section.

Otherwise what did the manuals say?

Just input all the values in the data form. If it is for design purpose, you can set the values of the re-bars to zero but you have to include the total area of re-bar steel. Note, SAP2000 uses the full depth of the beam but not the effective depth as in the steel ratio.

Set the Live Load Pattern to zero. Then SAP2000 will treat the beam as an individual beam and moves the negative moments to mid-span.

You can begins with Options and then Preferences...

This is one book which all designers shouldn't miss whether they are using SAP2000 or not. I don't know whether it is still available from CSI.

It is called:

"BUILDING SUPPORT STRUCTURES, Analysis and Design with SAP2000 Software" by WOLFGANG SCHUELLER.

disclaimer: all calculations and comments must be checked by senior engineers before they are taken to be acceptable.