The constructive characteristics of the Gothic style are unique in the history of Architecture. Gothic cathedrals pushed structure to the limit associating their structural elements to a linear frame that supports forces in a delicate balance. In order to understand this fragile equilibrium we must create a precise geometric model of the building, simple conceptually but substantially representative of the structural and constructive system. Each part of the building has specific constructive and structural characteristics that must be represented in the model. The mathematical model of the building will be the result of combining a geometric model with a mechanical model containing the physical properties obtained from experimental measurements.