Nowadays, interest in the preservation of cultural built heritage is globally increasing. The assessment of seismic vulnerability of existing masonry buildings is a complex task as their morphological evolution is often characterised by transformations, aggregations, addition of structural portion to existing ones and modifications developed over centuries.

In this paper the seismic vulnerability of the Zoological Station Anton Dohrn in Naples has been evaluated. It is a complex masonry building that includes six parts (the central part, West wing, East wing, two connecting parts and the library) built in different periods. Preliminary analysis, as the evaluation of geometric parameters (for example the ratio between the area of masonry walls and the geometric area), and nonlinear static analysis, by using the equivalent frame model, of central part and connecting parts, part A and B, respectively, have been carried out. Due to the complexity of the building, two hypothesis have been considered. The first hypothesis involves the analysis of the central part and the connecting parts as isolated structures, neglecting the interaction between the parts; the second hypothesis considers a perfect interaction between the parts that are analysed as a single structure.

By discussing the results obtained, a proposal for the evaluation of the seismic vulnerability of complex masonry buildings with additions of structural portion is formulated.