Seismic vulnerability of merlons in ancient fortified buildings

The paper investigates the seismic vulnerability of a typical element of the fortified architecture: the merlons. Indeed, the last seismic events in Italy have caused severe damage to a wide range of historical buildings, in some cases even for low values of peak ground acceleration. Among those, the crenellations typical of historical fortified buildings were particularly affected. Based on previous works, in which the seismic damage phenomena on the fortified building typology were collected and catalogued, the merlons proved to be particularly vulnerable elements. For this reason, the present work focuses specifically on the damage mechanisms suffered by these protruding elements, in order to understand their behavior in case of earthquake, to quantify their vulnerability and to provide instruments for their seismic protection. Different case studies referring to merlons, both on crenellated fortified walls and towers, were illustrated. Examples showed that free-standing merlons are mainly subjected to out-of-plane overturning; however if merlons support a roof, as often happens in castles which were subjected to a civil reuse in the course of time, also in plane shear failure can occur. Furthermore, some of the case studies analyzed presented seismic retrofitting interventions, whose behaviour under the seismic actions is described and discussed. Case studies showed that the collapse of merlons can occur even with low values of peak ground accelerations (PGA). Though the collapse of merlons is rather common, it received little attention in the literature. The present work focuses specifically on the out-of-plane overturning of merlons. The behavior of the merlons was studied by means of simple mechanical models. In particular, a simple linear elastic model was used to identify the activation of the mechanisms. The subsequent collapse by overturning was studied by means of a non-linear kinematic model. The seismic filtering effect, which is exerted by the supporting wall or tower, influences the behavior of the merlons. For this reason, proper filtering equations that modify the response spectrum at the ground were chosen. The proposed approach was used for a parametric analysis with merlons and supporting walls. Results permitted to plot curves that relate the slenderness of the merlons to the PGA that leads to their collapse. The proposed work thus defines a simple but reliable procedure than can be adopted by the practitioner for the seismic assessment of merlons and that could be used at a territorial scale in order to identify the most vulnerable cases and optimize the limited economic resources for the prevention of future damage on these elements, which might seem of minor importance but which are particularly meaningful both from the historical and aesthetical point of view.