All solids have a natural frequency but how do you determine its value? How would you determine it for a large building? I'll guess that for each different material they apply a constant external force with a specific angular frequency. The force with an angular frequency that breaks the material will be the natural or reasonant frequency of that material. All the different reasonanct frequencies of different materials are tabulated and the material the building is made out of will determine its reasonant frequency. However buildings with many different materials may be more difficult. Would you just average the reasonant frequencies of the materials?