Basically, materials and sections are already assigned into the model by the previous modeler, if your model is directly from SACS model. In this case, things got a little bit easier and you just need to check generally whether your elements have their sections and material and in many cases they’ll have it.  If you are planning to construct your model from scratch, then materials and sections need to be assigned properly for each element/member. This can be done using both SACS and PREFRAME or GENIE. Design drawing or as built drawing is crucial for you in assigning sections and materials.  Assigning the materials and sections sometime can be difficult especially if you are working with unfamiliar software. No materials or sections will make the software terminate the analysis and you need to debug what’s happening. So, always keep in mind that you do have all the elements with their own assigned material and sections before start running the analysis.