Seismic events have shown the significant risk of collapse faced by traditional masonry churches during earthquakes, highlighting the need for a reliable structural assessment and effective strengthening interventions. For historical churches, one of the most commonly used approaches for analyzing damage and seismic vulnerability consists in the evaluation of the potential local mechanisms. Among the recurrent local mechanisms observed in multi-aisled churches, there is the collapse of the nave arcade. This architectural feature is typically used to separate the central nave from the side aisle and is composed of arches placed at the top of columns or slender piers, creating a sense of openness and enhancing the architectural quality of the interior. The arches are usually characterized by large spans and limited restraints (both in the longitudinal and transverse directions), which makes the nave arcade highly vulnerable to seismic actions. This research investigates the seismic vulnerability of the arcade in a three-aisled, case study church characterized by a traditional open timber truss roof (i.e. without cross-vaulted roofing).The local failure mechanism is examined through non-linear kinematic analysis, based on the estimation of the collapse multiplier, and Finite Element numerical analysis. The effect of a roof bracing intervention using diagonal steel ties for the prevention of the local collapse of the arcade is also analyzed. The results provide valuable insights into the seismic behavior and vulnerability of these complex monumental structures.