RE: STAAD pro design query1) I just randomly designed a building by assuming the dimensions of sections and then analyzing it ; how can i design a real building from starting like professionals.......if some one can help me by suggesting some references or

There are lot of useful training materials on modeling buildings using STAAD.Pro on the Bentley Learn Server. For example

Designing Steel Buildings with STAAD.Pro
learn.bentley.com/.../ViewLearningPathDetails

Designing Concrete Buildings with STAAD.Pro
learn.bentley.com/.../ViewLearningPathDetails

You will need to log in with your Bentley credentials to access these.