The paper analyzes the span P3 of the Kolibki steel footbridge in Gdynia (Poland). The footbridge is composed of four simply supported spans, 5.81 m, 11.62 m, 27.62 m and 5.81 m long respectively and has a constant width of roughly 3.5 m. The deck rests on a steel grid, composed of longitudinal and transversal beams. In parallel, at both sides of the footbridge, a slightly inclined steel arch is connected to edge beams through a system of hangers. A preliminary experimental campaign detected the frequencies of the first modes. Even though the 1st natural frequency in bending was greater than 5 Hz, the span P3 showed a lively behavior, with large vertical accelerations. Hence, a new series of ambient vibration tests was performed. Their outcome pointed out the presence of two separate resisting mechanisms provided one by the deck slab/grid, the other by the arch, the latter being more difficult to be identified. To investigate their role, a few forced vibration tests under walking pedestrians, having a wired accelerometer positioned at their waist, were realized. In parallel, the finite element modelling of the bridge was pursued. The results of modal analysis confirmed the presence of the two mechanisms but also evidenced the difficult reproduction of experimental results. The analysis of experimental data in time and frequency domain allowed to investigate the role of the different normal modes in the vertical response to walking pedestrians. The possible reasons for the unexpected behavior in terms of vertical accelerations are addressed.