In unreinforced masonry structures, one of the most dangerous events that can occur during earthquakes is an out-of-plane mechanism. The use of tie rods can be a powerful tool in preventing these types of failure mechanisms, especially given the simplicity and the low cost of the strengthening intervention. When the wall is restrained to a horizontal element, such as a tie rod, its response significantly changes. The collapse, in this case, could take place for slipping / failure of the tie rod or for overturning of the wall, following the formation of a crack at an intermediate height between the base and the top. In the literature are present models accounting for geometric non-linearities in the case of a wall rigidly restrained at the top. The same can be stated for the model free at the top, which considers four different patterns. No model accounting for all previously mentioned phenomena while also presenting a non-rigid restraint, is available. Therefore, a model formed by two bodies of finite thickness and a flexible restraint at the top capable to capture the dynamic response of the system is proposed here. The model is characterized by four motion patterns, and it can detect the transition among them, as well as the collapse mechanisms which can occur for overturning of the entire wall or of the upper body alone. Explanatory time-histories describe the complex response of the system to recorded ground motion, pointing out its highly non-linear behavior. Finally, the investigation on the influence of the tie rods diameter confirms that the main advantages are obtained for tie rods characterized by a large cross-section.