The effect of stress stiffening is investigated with reference to a 63.75m long suspension arch footbridge in Seriate (Italy); the arch rise is 1.3 m. The bridge model, based on the as-built design data, is implemented in ANSYS. A preliminary nonlinear static analysis is performed to account for the geometric stiffness matrix; the load configuration includes the gravity loads and the fictitious thermal loads necessary to produce the design prestress condition of the suspension system. Prestressing effects
are included through a negative temperature variation DT capable to induce a tension force in the elements. To match the design values of pre-tension, the value of DT is calculated using both an iterative procedure and a trial and error method. The modal properties of the structure, computed on the stressed condition, differ from those of the linear structure, pointing out the pretension effect. The model has been validated against the results of an experimental campaign of ambient vibration measurements. The dynamic characteristics of the model are in close agreement with the values identified from experimental tests.