For a detailed investigation of a building structural drawings are required for modeling in software, but if it is not available how to make the process???
To prepare architectural drawings you can measure dimensions of spaces and elements. Preparing structural drawings is more challenging as you need to know placement, number, diameter of rebars that requires X-ray photography. You also need to test the tensile and compressive strength of samples of all types of the reinforcement as well as concrete used in the structure.
If possible, a tridimensional scanning would provide all the necessary information. If not possible, there are several empirical seismic vulnerability assessment methodologies in the scientific literature. I can send you my Ph.D. thesis if you are interested, as it contains an entire chapter regarding the state of the art in the field of the seismic vulnerability assessment.